Quasi-aromaticity—what does it mean?

Quasi-aromaticity—what does it mean?

Tetrahedron 71 (2015) 4895e4908 Contents lists available at ScienceDirect Tetrahedron journal homepage: www.elsevier.com/locate/tet Tetrahedron rep...

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Tetrahedron 71 (2015) 4895e4908

Contents lists available at ScienceDirect

Tetrahedron journal homepage: www.elsevier.com/locate/tet

Tetrahedron report number 1083

Quasi-aromaticitydwhat does it mean? Tadeusz M. Krygowski a, *, Barbara Bankiewicz b, Zbigniew Czarnocki a, Marcin Palusiak c, * a

Department of Chemistry, Warsaw University, Pasteura 1, 02-093 Warsaw, Poland Department of Theoretical Chemistry, University of Białystok, Hurtowa 1, 15-399 Białystok, Poland c dz, Pomorska 163/165, 90-236 Ło dz, Poland Department of Theoretical and Structural Chemistry, Faculty of Chemistry, University of Ło b

a r t i c l e i n f o Article history: Received 23 March 2015 Available online 29 May 2015 This paper is dedicated in memory of our friend Professor Alan R. Katritzky and in recognition of his outstanding contribution to organic chemistry

Keywords: Aromaticity Quasi-aromaticity Hydrogen bonding p-Electron delocalization Resonance effect

Contents 1. 2. 3. 4. 5. 6. 7. 8.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4895 How do the criteria of aromaticity operate in quasi-aromatic molecules? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4897 Energetic and structural aspects in quasi-aromatic systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4897 The quasi-aromatic ring in polycyclic aromatic systemsdmimicking the aromatic cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4900 Geometry-based quantitative estimation of the p-electron delocalization in quasi-aromatic rings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4902 Do the magnetic properties describe properly p-electron delocalization in quasi-aromatic rings? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4902 Chemical reactivity criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4904 General conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4905 References and notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4906 Biographical sketch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4908

1. Introduction Aromaticity is one of the most popular and useful concepts in organic chemistry. Every day more than 30 papers appear in which the term ‘aromaticity’ is used either in the title, key words or in the

* Corresponding authors. E-mail addresses: [email protected] (T.M. Krygowski), [email protected] (M. Palusiak). http://dx.doi.org/10.1016/j.tet.2015.05.074 0040-4020/Ó 2015 Elsevier Ltd. All rights reserved.

abstract.1 However, apart from applying this term in its original form and sense, the term ‘aromaticity’ has recently been used with a lot of different prefixes2e5 such as anti-,2 pseudo-,2 hetero-,6 homo-,7 s-,8 spherical-,9,10 Mobius-,11 all-metal-,12 three-dimensional-,13 and many others. A common feature of these prefixed-aromaticities is that some (but not all) typical properties of the ‘proper’ (in other words, classical) aromatic compounds appear in many, sometimes very different, kinds of molecules. Hence it is necessary to apply prefixes to specify a particular kind of various applications of the term ‘aromaticity’.

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Quasi-aromaticity is probably the only one of these prefixedaromaticities that has not yet been reviewed, despite a very long history of its existence. Almost seventy years ago Calvin and Wilson14 documented an increased stability of chelating substances of the kind presented in Fig. 1 in which hydrogen was replaced by the metal cation. This observation was related to the resonance effect, which now we could call an increased p-electron delocalization in the chelating chain.

Fig. 1. Schemes of quasi-aromatic systems with chelating proton (I) and metal cation (II). CH bonds are not shown.

For the first time the term quasi-aromaticity was applied to describe15 the chelating structure of phenylosazone in which ‘sharply defined azo- and hydrazo structures disappear’. In other words, due to the resonance effect within the chelating chain some kind of p-electron delocalization occurred and no typically single or double bonds were observed. Finally, at The Jerusalem Symposia on Quantum Chemistry and Biochemistry ‘Aromaticity, PseudoAromaticity, Anti-Aromaticity’16 the following definition of quasiaromaticity was suggested: ‘. the molecules should be called quasi-aromatic only if they contain an acyclic conjugated pelectron system and show chemical properties typical of aromatic compounds, especially reaction by substitution with retention of type. The significant mesomeric stability is implied.’17 In the lecture by Lloyd and Marshall16 many examples of this kind of reactivity are presented.18e24 Thus, one of the most important criteria of aromaticity was already fulfilled at that time. In order to have a deeper insight into the problem of relations between different behaviour of chelating systems and aromaticity it is advisable to start from the definition of aromaticity. It is enumerative in nature and hence there are a few criteria, which have to be fulfilled to accept a given molecule as aromatic. Its classical definition was presented in Tetrahedron Report 520:25 aromatic are planar, cyclic p-electron compounds, which fulfill the following qualitative criteria: i) they are more stable than their acyclic unsaturated analogues, ii) they have less alternated bond lengths than their acyclic unsaturated analogues, iii) the external magnetic field induces in them diatropic ring currents, iv) they have a tendency to retain p-electron structure in chemical reactions The purpose of this review is to discuss how the above criteria of aromaticity are fulfilled in chelated systems named quasi-aromatic. From the formal and structural viewpoint cyclic p-electron systems in which three units as CHeCHeCH are replaced by eY/M(þ)/ Xe, where X and Y are electronegative atoms that are able to chelate atoms/ions as proton or cations of the first and second group of the Periodic Table behave as if they were aromatic (see Fig. 1). It should be noted that quasi-aromaticity differs from metalloaromaticity26 by the nature of the coordination centre involved: in quasi-aromatic systems it is either hydrogen or the

atoms, which do not participate in p-electron delocalization that operate in the chelate part of the system. A good example of formation of the quasi-aromatic system is shown in Fig. 2, where open and closed conformers of malonaldehyde exhibit completely different properties: the closed conformer is more stable by 12.96 kcal/mol27 (estimated at B3LYP/6-311þG(d,p) level of theory), and its bonds are less alternated than in the open one. The absolute values of differences between CO and CC bond lengths in the closed form are 0.082 and 0.074  A as compared with 0.129 and 0.118  A in the open form. Obviously, a smaller alternation in the closed form is observed, and hence the name of quasi-aromatic ring may be used here.

Fig. 2. Schemes of malonaldehyde in ‘closed’ (I) and ‘open’ form (II). DE for I and II equals 12.96 kcal/mol.27

In general, p-electron delocalization is observed in a sequence of bonds between atoms, which are p-centres whereas the chelated ion does not participate in this process except some electrostatic influence on chelating atoms and then further on the whole ring.28 As fundamental ideas, the criteria (i)e(iv) are clear. Nevertheless, it should be noted that: (a) none of the above-presented criteria and numerical indices resulting from them is sufficient in their description of aromaticity, therefore it is most useful to apply many of them to get the most reliable result.29e31 (b) in many cases of various series of the analyzed compounds a statistical multidimensionality is observed. This means that the applied indexes of aromaticity do not always speak with one voice.32e37 (c) Finally, it should be noted that in many cases the application of the above-mentioned criteria and the indexes resulting from them have limited applications. In what follows some aspects of these limitations will be presented, to warn the reader that in many cases no absolute conclusion can be drawn. Now let us look at and present shortly some details showing the limitations of the application of criteria (i)e(iv). Ad (i) The concept of resonance energy (RE) introduced by Pauling et al.38 and Kistiakovsky et al.39 works for simple hydrocarbons, but it is difficult to be applied in the case of more complex systems.40 Apart from the complexity of the systems in question, (too big systems, strain, various heteroatoms involved), there are a few sources of limitation. Firstly, in a virtual reaction defining aromatic stabilization energy (named ASE) it is necessary to find a way to compensate additional effects, which appear in the reference system and, which are not due to the lack of aromaticity itself. Depending on the kind of reference systems and nature of the hypothetical reaction stabilization energies computed for benzene are in the range 18.4e66.9 kcal/mol40dif all side effects are eliminated (protobranching, hyperconjugation, strain etc.)41 the stabilisation energy lies in the range 64.9e69.0 kcal/mol. This result is very important for any use of the energetic criterion of aromaticity, but the procedure is difficult to be applied for complex systems particularly those, which contain heteroatoms. Nevertheless, it is an interesting field for further exploration, and recently great progress was made,42 but unfortunately only in the field of pelectron hydrocarbons. Despite the above-presented reservations, the energetic criterion of aromaticity seems to be accepted as the most important one.

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Ad (ii) Julg et al.43 took as an important feature of aromaticity low (or none) bond length alternation and introduced the index of aromaticity based on the variance of bond length in a perimeter of a given cyclic p-electron hydrocarbon. Replacement of the mean bond length in the Julg model by an ‘optimal bond length’ (i.e., that which exists or should be existing in a purely aromatic system) for a given type of bond leads to the aromaticity index HOMA44,45 (acronym for Harmonic Oscillator Model of Aromaticity), since the optimal bond lengths assumed to be in a fully aromatic system are estimated by using harmonic potential) defined as a function of squared differences between n given bonds (Ri) in a molecule and the optimal bond lengths, Ropt for a given type of bonds:

HOMA ¼ 1 

aX n

Ropt  Ri

2

(1)

where a is a normalization factor to get HOMA¼1 for benzene and HOMA¼0 for its Kekule’ structure. HOMA may be applied to the whole molecule, to an individual ring and also to any fragment of a molecule describing the degree of p-electron delocalization.46,47 HOMA may be applied for the systems containing heteroatoms with bonds:46 CN, CO, CS, CP, NO, NN and BN,48 CSe,49 BB,50 CB.51 Recently, some revision for the values of a and Ropt was proposed.52 Limitation for using HOMA is associated with a situation in which the equalization of bond length is due strictly to the sigma electron structure. Planarization of 1,3,5,7-cyclooctatetraene,53 as a result of a partial rehybridization at carbon atoms due to pairwise forcing the CCH angle to be less than 120 54,55 (see Fig. 3)leads to an increase of the value of HOMA by to 0.5. However, magnetic criteria, NICS56 and magnetic susceptibility exclude the aromatic character of the planarized molecule of cyclooctatetraene modelled in this way.

Fig. 3. Effect of angular CCH-variation on geometry of the ring in non-planar COT.53

Ad (iii) One of the earliest quantitative magnetic characteristics of aromaticity was defined as an ‘increment de delocalisation’57,58, which was later replaced by using the term anisotropy of magnetic susceptibility59 or exaltation of magnetic susceptibility.60 Both characteristics may serve as aromaticity measures for whole molecules and are accessible experimentally and by means of quantum chemical computation.61,62 Another way of using magnetic characteristics is computation of the so-called ring current.63 Diatropic currents are assumed to be typical of aromatic systems whereas paratropic currents are considered to be typical of anti-aromatic ones.61 To some extent NICS (abbreviated from Nucleus Independent Chemical Shift)56 and its modifications as NICS(1), the perpendicular component of tensor NICS(1)zz or others64 have served in the last decades as a sui generis measure of the consequences of ring current and are assumed to be aromaticity indices of individual rings in polycyclic p-electron systems. Recently it was shown that local magnetic characteristics describing ring currents for individual rings (for 129 rings) in benzenoid hydrocarbons determined by means of graph theory correlate very well with the NICS values.65 Ad (iv) The tendency of aromatic systems to retain the p-electron structure upon reaction is difficult to be presented in a numerical way and in a unified scale. Usually this is considered experimentally in a qualitative way as a realization of electrophilic substitution in a system in question. However, in more complex

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p-electron systems there are many possible positions of attack and of competition with e.g., the addition reaction, and hence another source of problems appears. Recently it was shown by use of graphdtopological approach66 that the topological index of reactivity (TIR) for 35 nonisoarithmic molecules of benzenoid hydrocarbons correlate linearly with the experimentally derived67 position constants sþ68,69 as well as aromaticity energetic characr teristics REPE.70 In older definitions of aromaticity still another criterion was taken into account, namely that the aromatic p-electron systems should be planar.71e73 In the last few decades it has been shown that small deviations from planarity do not affect p-electron delocalization, thus the aromatic character is still retained.45,74e76 Interestingly, it had been found that the deformation of aromatic system from its natural planar geometry strongly affects classic aromaticity indicators, such as stability, kinetic inertness and symmetric structure, while it leaves the magnetic indicator fully aromatic (ring current remains unaffected as revealed by estimates of NMR chemical shifts).77e79 One more remark should be added here. In the case of intramolecular H-bonding a concept of Resonance Assisted H-bond (RAHB) appeared, which was introduced and thoroughly documented by Gilli et al.80e89 The main idea of this concept is that due to the H-bond formation, the p-electron chelating chain enhances the strength of this bond and this is a dominant factor for the energy of H-bonding. Contrargumetation to the original Gilli’s concept was also formulated.90e92 According to its authors the indications of stronger H-bonding in unsaturated (p-conjugated) moieties in respect to their saturated counterparts result not necessarily from resonance assistance, but rather from other effects, including geometrical (steric) conditions associated with intramolecular H-bond formation. It has to be clearly stated that in our review we will not discuss this kind of problems, but will concentrate our attention on the properties of the chelating chain, which together with H-bond or metal ion complexation form quasiaromatic rings, which simulate the properties observed in the truly aromatic rings. 2. How do the criteria of aromaticity operate in quasiaromatic molecules? The first chemical condition for aromaticity itself were purely chemical: a greater stability and tendency to substitution in competition to the addition. Then other criteria have appeareddas mentioned in the Introduction. A similar situation seems to be associated with quasi-aromaticity. Our purpose is to present, which criteria working well in the case of aromatic compoundsdalso work well in the case of quasi- and alternatively fail in the quasiaromatic ones, and to attempt to understand the reason for failing. It is important to note that our review will concentrate on properties of the chelating chains and their properties as well as its similarity to the typical aromatic characteristics. 3. Energetic and structural aspects in quasi-aromatic systems The most simple model system of quasi-aromatic ring is the closed (H-bonded) form of malonaldehyde. When unsubstituted, it may be represented with two isoenergetic ground-state forms, as shown in Fig. 4. The dynamic isomerization between GS1 and GS2 may proceed along the proton transfer path, through the transition state (TS). As schematically shown in Fig. 4, the TS structure is significantly delocalized, with full CC and CO bonds equalization (dCC¼1.399  A, dCO¼1.278  A, B3LYP/6-311þþG(d,p) level of theory) in the case of unsubstituted malonaldehyde (with R¼H in Fig. 4).93 The delocalization within chelate ring leads to lowering of the TS energy,

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Fig. 4. Two ground-state forms of malonaldehyde (GS1 and GS2) and the transition form (TS) linking them via proton-transfer process.93

leading in turn to lowering of the energetic barrier of proton transfer reaction, as shown e.g., by analysis of malonaldehyde itself (the energy corresponding to proton transfer barrier does not exceeds the value of 10 kcal/mol, depending on substituent),93 but also for ketoenamine-enolimine tautometic equilibrium studies performed for a series of various hydroxyaryl, alkyl Schiff and Mannich bases.94 As a consequence, the stronger H-bonds are observed for more delocalized chelate rings, which is in good agreement with Leffler-Hammond postulate, according to which systems of stronger H-bonds are structurally closer to transition states corresponding to proton transfer reaction.95,96 When the chelate ring is substituted in position 1 (or 3), the symmetric relation between both ground state energies and geometries is perturbed and one of ground state structures is energetically preferred. The energetic preference depends on the character of substituents, with GS2 in general more stable than its counterpart. Although the direct relation between the influence of the type of substituent and the energetic preferences were not specified, the relation between the role of substituent and the H-bond strength was explained, as illustrated in Fig. 5, with H-bond in GS2 stabilized by electronwithdrawing substituents and H-bond in GS1 stabilized by electron-donating ones.

becomes less delocalized and simultaneously less stable.97 From the other hand such observation is in agreement with earlier mentioned Leffler-Hammond postulate, since tautomer being structurally closer to transition state structure (most delocalized along proton transfer path) is in the same time energetically closer to it, thus less stable than its more localized counterpart. It is worth mentioning that the structural and electron density-topological parameters of H-bridge in malonaldehyde derivatives were found to be relatively well correlated with indications of various aromaticity indices, including those based on structural and electronic properties of the chelate ring.93,94,98 The case of malonaldehyde substituted in position 2 was also investigated and it was found that also in that case the transition state structure is fully delocalized, and the energy corresponding to the proton transfer barrier was in the range of 2.7e5.1 kcal/mol, thus being slightly lower than in 1(or 3)-substituted malonaldehyde. For electron-withdrawing substituents the H-bond within the chelate ring is stronger and the chelate ring itself is relatively more delocalized, as shown by indications of various aromaticity measures. (For instance HOMA equals 0.659 for electron withdrawing NO2 substituent and 0.588 for electron donating NH2 substituent) Also the proton transfer barrier was lower for that case. Obviously opposite effect of electron-donating substituents was observed. The whole situation can be explained by considering the contribution of canonical structures shown in Fig. 6.

Fig. 6. p-electron effects in malonaldehyde substituted in position 2.93

Fig. 5. Schematic representation of resonance (mesomeric) effect in malonaldehyde systems substituted at position 1 (3).93

Interestingly, it was found that for malonaldehyde and its substituted analogues from Fig. 4 the more stable are those GSs, which were characterized of weaker H-bonds, thus, which were more localized. This is clear difference between quasi-aromatic chelate ring and aromatic benzene ring, which when substituted,

As can be seen, the effect of electron-withdrawing substituents cooperates with resonance effect within the chelate ring, thus increasing the degree of delocalization along the sequence of covalent bonds. It is also worth stressing that the effect of resonance within the quasi-aromatic ring is strongly directed, and that the direction of delocalization is strongly related with H-bonding stabilization, as shown in Fig. 7. Thus, delocalization leading to charge outflow from the proton-donating group to the proton-accepting group (Fig. 7a) additionally stabilizes proton-bridge. The opposite effect can be observed when charge outflow leads to decrease of electron charge on proton-accepting center (Fig. 7b). This effect was nicely illustrated by Mandado et al.99 for various H-bonded quasi-aromatic rings. For instance, they had shown that

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of p-electron communication between them, leading to clear changes in p-electron structure along the sequence of conjugated covalent bonds. Therefore, it was postulated that the H-bonding in quasi-aromatic ring can be considered as the assisting effect that amplifies the substituent effect in ortho-substituted system from Fig. 9 and its analogues.

Fig. 7. The direction of resonance effect along quasi-aromatic ring and its influence on H-bonding strength: the strengthening (a) and weakening (b) of the H-bond.

NHS3 system (original nomenclature is used) from Fig. 8 (corresponding to situation illustrated in Fig. 7a) is thermodynamically more stable, with stronger H-bond and more delocalized bonds along NCCCS sequence of atoms. The NHS5 (corresponding to system as shown in Fig. 7b) is less stable, with weaker H-bond and lower delocalization along covalent bonds. Similar conclusion can be formulated for OHN system from Fig. 8.

Fig. 9. TOC graphic from Krygowski et al.100 showing schematically the increase of pelectron delocalization due to H-bond formation. Reprinted with permission from Krygowski, T. M.; Zachara-Horeglad, J. E.; Palusiak, M. J. Org. Chem. 2010, 75, 4944. Copyright 2010 American Chemical Society.

When the chelate ring is fused with the benzene ring, e.g., like in the case of aryl Schiff bases, the p-electron effects within the quasiaromatic ring are conjugated with p-electron effects within the aromatic part of the molecule. If assuming the resonance effect as the one, which stabilizes the H-bonding, it is possible to virtually separate the total p-electron effect acting in referred system onto three individual effects (Fig. 10):

Fig. 10. Virtually separated p-electron effects acting in the chelate ring fused with the benzene ring; the resonance effect assisting H-bond formation, (a), the cyclic aromatic delocalization in the benzene ring, (b), and the substituent effect, (c).

Fig. 8. Energy relations between various isomers of quasi-aromatic H-bonded rings. Original figure from Mandado et al.99 modified for purposes of discussion.

Such relation between the direction of p-electron delocalization and H-bond strength can be discussed in the context of substituent effect (between proton donating and accepting group treated as two ortho-placed substituents), which in specific conditions can be affected or even assisted by H-bonding. It was stated on the basis of study on salicylaldehyde, o-hydroxyl Schiff base, o-nitrosophenol, and their lithium analogues100 that the formation of a hydrogen or lithium bond leads to changes in electron-donating and electronwithdrawing power of substituents, thus affecting the efficiency

(1) the effect of resonance, which, according to Gilli’s original concept,81 assists the H-bonding formationdthe larger is the p-electron delocalization within the chelate ring, the stronger is the H-bond (Fig. 10a) (2) the effect of p-electron delocalization within the benzene ringdbenzene ring actually tends to keep its aromatic character, thus any external factors interacting with its p-electron structure should be considered as a perturbation (Fig. 10b) (3) the substituent effect present in the system regardless of the presence of H-bondingdboth the substituents interact with pelectron structure of the benzene ring, disturbing its delocalization, or in other words, localizing its structure97,101 (Note that hydroxyl group is known as electron-donating substituent, whereas carbonyl group acts as electronwithdrawing group102dthus, both substituents interact with themselves through the benzene ring and the chargeseparated canonic structure contributes to the overall superposition of all canonic forms of substituted benzene ring, see Fig. 10c) As one may suppose, effects (1) and (3) cooperate with each other against the effect (2). Accepting the classic point of view on pelectron aromaticity, the effects (1) and (3) may be considered as

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a specific perturbation of aromatic system, which leads to its destabilization. In fact, the presence of quasi-aromatic ring system in molecular entities being analogue to that from Fig. 10, changes properties of the aromatic moiety, changing even its preferences in reactivity, as it will be presented in Section 7 devoted to chemical reactivity of quasi-aromatic systems. When Hþ cation in chelate ring is substituted with Liþ counterpart, the TS structure from Fig. 4 becomes the ground state.28 This, however, does not mean that the delocalization within such quasi-aromatic ring is cyclic like in the typical aromatic ring. For instance, it was reported, on the basis of analysis of ring currents induced in related Li-bonded systems,28 that in fact there is no cyclic diatropic ring current in the quasi-ring formed due to Libonding. Thus, it may be expected that the Li-bonding mainly leads to the formation of partially ionic structure with negative charge surplus formally located within chelating system, which interacts with Liþ and forms the cyclic motif. Although the delocalization along formally covalent bonds is in that case more effective than in H-bonded counterpart, it should be explained rather as tendency of the system to delocalize the surplus of the electron charge within the anion, than the aromatization of the quasiaromatic system itself. Thus, there is no cyclic delocalization within quasi-aromatic rings, as deduced from ring currents analysis. This conclusion can be supported by recent studies on DNA base pairs.103,104 The hydrogen bonding in e.g., adenine-thymine pair (AT) can be classified as resonance-assisted, thus, such in which p-electron structure is of crucial importance for H-bond formation. The interaction energy decomposition shows that the contribution of p-bonding in the

H-bonding. The loss of HB energy due to switching off the p-conjugation was of about 50%, with w16 kcal/mol for p-conjugated system and w8 kcal/mol for nonconjugated system. Still, the stabilizing role of p-conjugation was not direct and the additional stabilization was derived from the better HOMO-LUMO spatial overlapping and the smaller HOMO-LUMO gap within the s-skeleton sp2 conjugated complex. It is also worth mentioning that some QTAIM-based parameters (e.g., electron density, density of total electron energy)105 measured in quasi-rings critical points may be intercorrelated with classic aromaticity indexes.106 Interestingly, such close to linear relations were observed rather for H-bonded quasi-rings, and not for Libonded ones. Interrelations between QTAIM-derived parameters were better when tested against values of structural aromaticity index HOMA. In case of magnetism-based index NICS such relations were evidently worse, if any. What is more, NICS index was not correlated even with HOMA, although both are the indexes of aromaticity. This observation may indicate that magnetism-based quantifiers of aromaticity rather fail in case of quasi-aromaticity. This aspect will be thoroughly discussed in the Section 6. 4. The quasi-aromatic ring in polycyclic aromatic systemsdmimicking the aromatic cycle Let us consider a simplest polycyclic aromatic hydrocarbon (PAH) that is, the naphthalene. When we measure the aromaticity of its individual rings, it appears that it is lower than in isolated benzene (see e.g., values collected in Table 1). This phenomenon may be explained with the concept of Clar’s aromatic sextet.107e109

Table 1 Values of selected aromaticity indexes estimated for basic native PAHs. Data estimated using DFT-B3LYP/6-311þþG(d,p) level of theory. (Numerical values taken from Dominikowska and Palusiak110) Ring

HOMA44,45,111,112

EN111,112

GEO111,112

PDI113

FLU114

HRCP103105,106

A

0.989

0.011

0.000

0.103

0.000

7.766

A B

0.783 0.783

0.082 0.082

0.134 0.134

0.075 0.075

0.009 0.009

A B C

0.629 0.72 0.629

0.127 0.168 0.127

0.244 0.112 0.244

0.065 0.065 0.065

A B C

0.868 0.459 0.868

0.056 0.296 0.056

0.076 0.245 0.076

A

0.901

0.041

B

0.097

C D

NICS(1)115

NICS(1)zz116

8.060

10.228

29.247

7.357 7.357

8.392 8.392

10.477 10.466

29.091 29.032

0.015 0.009 0.015

7.259 7.166 7.259

7.281 11.053 7.281

9.444 12.682 9.438

25.996 34.889 25.972

0.081 0.047 0.081

0.005 0.019 0.005

7.364 7.027 7.364

8.512 5.461 8.512

10.714 8.209 10.705

28.851 20.609 28.812

0.058

0.085

0.003

7.372

7.433

9.595

25.684

0.758

0.145

0.028

0.023

6.626

1.928

5.301

10.378

0.901

0.041

0.058

0.085

0.003

7.371

7.510

9.866

25.795

0.901

0.041

0.058

0.085

0.003

7.373

7.593

9.895

25.945

interaction energy is meaningless, thus, no cyclic p-delocalization should be expected in extra rings formed due the H-bond complexation of AT pair. However, the analysis of modified AT pair analogue, in which the p-conjugation between H-donors and acceptors was eliminated by the replacement of sp2 carbon atoms with sp3 ones, shows that p-conjugation in covalently bonded part of the extra rings is essential for additional stabilization of

NICS56

In the shortest way it can be stated that according to this concept the most stable is that structure of PAH, which contains the largest number of localized sextets. In naphthalene there are two rings, so there are two possible equivalent situations, as illustrated in Fig. 11. Since no one is privileged, we observe its superposition (structure I in Fig. 11) and, thus, the same degree of aromaticity for both

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Fig. 12. Structures with localized sextets possible for naphthalene system fused with quasi-aromatic ring. HOMA values for benzene and quasi-rings are given in red, d(H/O) distance giving information on H-bond strength (shorter means stronger) are given in blue.27

Fig. 11. Simple PAHs in view of Clar’s concept of aromatic sextet.

individual rings (still lower, than in isolated benzene ring).110 Situation is relatively similar for anthracene, with the difference that in this case some aromaticity indexes indicate slightly more aromatic lateral rings, while others do in an opposite way. Nevertheless, none of the anthracene ring is evidently more aromatic comparing with others (system II from Fig. 11).110 Situation changes dramatically when we move from straight to kinked PAHs.117,118 In the case of phenanthrene lateral rings are evidently more aromatic than the inner one, since Clar’s structure III(b) from Fig. 11 predominates over the other possible. Analogous situation may be found for triphenyl, in which lateral rings are again clearly more aromatic than the inner one (structure IV from Fig. 11). Thus, following Clar’s concept it is possible to define four types of benzenoid rings present in PAHs (see also Fig. 11):107,110,117,119,120 e The ring with the localized sextet, usually exhibiting strongly aromatic character. As an example of this type of ring we can mention a lateral ring in phenanthrene. e The ring with the migrating sextet, usually exhibiting less aromatic character than the ring with the localized sextet. As an example of such a ring we can mention any of the rings in naphthalene, but also in anthracene and larger linear acenes. e The ring with the localized double bond, usually exhibiting significantly reduced aromatic character. As an example of such a ring we can mention the inner ring in phenanthrene. e The empty ring, which formally does not possess p-electrons. An example of this type of ring can be the inner ring in triphenylene. The above characterization of individual rings in PAHs is supported by numerical data collected in Table 1. Now let us discuss the situation in which one of typical aromatic rings is replaced with a quasi-aromatic one. The simplest situation corresponds to e.g., salicylaldehyde (Fig. 9) and its analogues. As it was already stated in previous Section 3, the aromaticity of benzene ring in that molecular system is always lower than in unsubstituted benzene. This can be explained by, for instance, the substituent effect or the perturbation of the aromatic system due to partial contribution of benzene ring p-electrons in resonance effect within the quasi-ring. (See Fig. 10) When we pass from benzene analogue to naphthalene analogue, we have two possible situations, with quasi-aromatic ring fused such that it forms phenenthrene-like and anthracene-like systems, as shown in Fig. 12. Recent detailed studies on such systems revealed27 that in case of anthracene-like situation addition of the quasi-ring does not change too much local aromaticities of individual benzene rings. Situation is much more evident in case of phenanthrene-like derivatives. In this case

the evident lowering of aromaticity in substituted benzene ring that is, the one fused with quasi-ring, can be observed. In the same time the increase of aromaticity in lateral, unsubstituted ring was observed. This observation was generalized and it was concluded that the position of the extra quasi-aromatic ring influences both the strength of the H-bonding and the local aromaticity of the PAH skeleton. HBs are relatively stronger and the entire system is energetically more stable when a kinked-like structure is generated by the addition of the quasi-aromatic ring. Systems with a straightlike topology are relatively less stable and have weaker H-bonds.27 Interestingly, similar results were observed by direct comparison of molecular geometry for systems containing quasi-rings with Hþ replaced by Liþ. The lengths of C9C10 bond in phenanthrene and C3C4 bonds in its analogs with quasi-rings (d in Fig. 12) with H- and Li bonds are 1.365, 1.371 and 1.363  A, respectively.121 This is an impressive support for the mimicry effect of benzene ring by quasi rings. In that case the interaction between the quasi-ring and the rest of the molecular system was even more efficient.121 As it was already mentioned in previous Section 3, in systems as these shown in Fig. 12, the three independent p-electron effects can be virtually separated (see Fig. 10 and the corresponding text). In order to clarify situation and, as far as possible, to remove from consideration the mutual interrelation between these effect, the analysis of PAHs fused with quasi-rings has been enlarged on systems like these shown in Fig. 13.122 The direct comparison of structural and energetic parameters led to conclusion according to which, again, H-bonds are relatively stronger and the entire systems are energetically more stable, when the topological situation corresponds to that shown in Fig. 13b, thus for the phenanthrenelike topology.

Fig. 13. PAHs substituted in such a way that two possible positions of quasi-aromatic rings are topologically possible, with the same substitution type kept in both cases. HOMA values for benzene and quasi-rings are given in red, d(H/O) distance giving information on H-bond strength (shorter means stronger) are given in blue.122

Summarizing this part of review it can be said that quasiaromatic ring may in proper conditions adopt in part the role of typical aromatic ring, interacting with the rest of PAH system in the way topologically analogous to that observed for native PAHs. This is

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important property of quasi-aromatic ring, showing that the term ‘aromatic’, even, if prefixed by ‘quasi’, is still justified for such cycles. 5. Geometry-based quantitative estimation of the p-electron delocalization in quasi-aromatic rings Gilli et al. in an important work on resonance-assisted hydrogen bonding (RAHB)81 applied geometry of the chelating chain for estimation of resonance effect between hydroxyl- and keto-groups enhanced by H-bond formation. Following notation as in Fig. 14 they defined symmetry coordinates q1¼R4R1 and q2¼R2R3. Then q1 and q2 were found to be strongly correlated, hence as the overall characteristics was suggested Q¼q1þq2, which followed rational relationship with the distance between oxygen atoms dO/O. In this report have been also considered not only typical intramolecular H-bonding, but also intermolecular RAHB as e.g., thymine-adenine and cytosine-guanine coupling.

As a measure of p-electron resonance (delocalization) in Hbonded quasi-aromatic-rings Grabowski123 has chosen a difference in p-electron delocalization in the open and closed forms of malonaldehyde system (Fig. 2) In this way the influence of Hbonding on the structure of a chelating chain can be concluded from the comparison of geometry characteristics between the closed and the open conformer of the system, for instance, the enol of malonaldehyde (see Fig. 2).124 Moreover his approach allows a direct comparison of total energies of the open and the closed forms of systems presented in Fig. 2 giving an information on stabilizing energy due to the H-bond formation. Following this idea, structural changes associated with the transformation of the open form to the closed one may also provide information on the consequences of the H-bond formation on the quasi-aromatic ring structure. Hence q1 and q2 values from Gilli’s Q parameter definition can be obtained for both closed and open forms of the system presented in Fig. 2. Therefore, for the open conformation we have:

qo1 ¼ Ro3  Ro2

and

qo1 ¼ Ro4  Ro1

(2)

and similarly for the closed form:

qc1 ¼ Rc3  Rc2 Fig. 14. The H-bonded chelating-ring with Rn bonds used in the definition of Q parameter; arrows denote direction of bond length changes.81

Important to notedif there was no assistance of the resonance effect between hydroxyl group and the carbonyl one due to the Hbonding, then q1 and q2 would be close to a maximum value. As shown by the arrows in Fig. 14 that due to the resonance effect, these indicators become smaller and in the limit case of full resonance, they become equal to zeroth. It was shown also that q1 and q2 follow nice linear correlation despite of the fact that all data applied were taken from X-ray diffraction measurements and apart of experimental errors and some intermolecular interactions could function. Additionally, the H-bridge was always arranged by keto and hydroxyl groups, thus even after tautomerization (proton transfer) the groups remained the same. Q¼0 for fully delocalized system and Q¼0.320 or 0.320  A to the completely localized enol (EK) or keto (KE) forms. Since conjugated chain mixes EK or KE geometries the mixing parameter l is in use. l¼1, 0.5 and 0 for the p-localized EK and a fully delocalized and localized EK forms, respectively. This is nicely illustrated by Fig. 1581 where localization/ delocalization parameter l is related to O/O distance i.e., approximate strength of H-bonding.

and

qc1 ¼ Rc4  Rc1

(3)

where R values correspond to covalent bond lengths in OCCCO sequence (in the original paper the bond length was denoted by d and the bond length differences as Dd, the above notation is consistent with Fig. 14). On that basis the following parameter can be defined as a measure of resonance in a given quasi-ring:

rrp ¼

. i   1 h o qo2 q1  qc1 qo1 þ q02  qc2 2

(4)

rrp equals 0 if there is no difference between the closed and the open conformation, in other words, if the formation of the intramolecular H-bond does not cause any further p-electron delocalization. Later it was shown that rrp parameter correlates well with HOMA,125 Gilli’s l parameter126 and also with several energetic and electron-density-based measures of RAHB strength.123 Fazli et al.127 studied the effect of formation of second hydrogen bond in adjacent two-ring resonance-assisted hydrogen bonds in 1,5-dihydroxy-1,4-diene-3-pentanone and found that Gilli’s Q parameter correlates linearly with H-bond energy for four substituted species. Karabiyik et al.128 studied aromaticity balance, p-electron cooperativity and H-bonding properties in tautomerism of salicylideneaniline and found that delocalization in chelate chain estimated by PDI129 correlates with H-bond strength. The reported geometry based characteristic of p-electron delocalization belongs to a wide group of geometry based indexes of p-electron delocalization.101 6. Do the magnetic properties describe properly p-electron delocalization in quasi-aromatic rings?

Fig. 15. Scatter plot in the (dO/O, Q) space. Full and open circles indicate acacH fragments forming intra- and intermolecular hydrogen bonds, respectively.81

NMR spectroscopy of the magnetically active nuclei has long been very important tool for molecular structure determination of chemical compounds.130e132 Of various nuclei the most popular, particularly in organic chemistry, is 1H NMR spectroscopy, where the resonating nuclei belong to hydrogen atoms attached in various sites in a molecule. Quantitative measures in 1H NMR spectroscopy are chemical shifts, d/ppm, which are readily accessible both experimentally and theoretically. For characterization aromaticity it is important to note that due to the ring current effect in aromatic molecules, the aromatic protons resonate at d w7 ppm or higher

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whereas other protons bound to sp2 carbon atoms e.g., in olefins resonate at d w5 ppm or lower. To make clear application of 1H NMR spectroscopy for quasi-aromatic rings, computed values of 1H chemical shifts (at B3LYP/6-311þþg(d,p) and B3LYP/6-31g(d) level of theory) for the central CH in malondialdehyde (Fig. 16) and for 1,3-butadiene and benzene are presented in Table 2.

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the NICS concept are assumed to be not influenced by the framework of s bonds structure of the system studied.116 A plethora of successful applications have been presented in the last two decades.30,64,140e142 It should be added that application of NICS to a model system with a closed and open quasi-aromatic rings via OeH/N H-bond

Fig. 16. Analyzed malonaldehyde ‘closed’ and ‘open’ forms. Central CH groups related with data of Table 2 are given in blue. Table 2 Computed and experimental values of 1H chemical shifts (ppm) for central CH group Compound

d Ha

dHb

d Hc

Benzene 1,3-Butadiene Anthracene

7.575 6.724 8.583 8.306 7.896 9.037 8.027 7.730 7.864 7.847 5.620 5.167 5.065 6.018 5.927

7.232 6.284 8.168 7.866 7.543 8.549 7.669 7.526 7.611 7.517 5.905 4.675 4.670 5.406 5.355

7.339 6.267 8.397 7.980 7.440 8.697 7.894 7.743 7.658 7.604 8 < 5:30d ð5:63e Þ : 5:04f

Phenanthrene

malondialdehyde malondialdehyde malondialdehyde malondialdehyde malondialdehyde

(closed) (open, a) (open, b) (open, c) (open, d)

a dH computed at b3lyp/6-311þþg(d,p) level with respect to TMS (for b3lyp/6311þþG(d,p) geometries). b dH computed at b3lyp/6-31g(d) level with respect to TMS (for b3lyp/6311þþG(d,p) geometries). c Experimental values of dH from Spectral Database for Organic Compounds, SDBS.133 d,e Experimental values of dH at pH 7.4 (d) and average values (e) reported by Bertz and Dabbagh.134 f Experimental values of dH reported by Bothner-By and Harris.135

As we see, the 1H chemical shifts for malonaldehyde for both open and closed form do not differ significantly, are far away from the values of aromatic protons and resemble the values for olefinic systems. Other known magnetic properties like e.g., magnetic susceptibilities, anisotropy of magnetic susceptibility59 and magnetic susceptibility exaltation,60 describe the properties of a whole molecule.136 and have not been used for systems with quasiaromatic rings. It is well known that aromaticity of particular rings in polycyclic p-electron molecule may differ between themselves, which is also shown by 1H NMR chemical shifts for various positions of molecules. Qualitatively this phenomenon is well described by Clar rules.107,137 Typically, two outer rings of phenanthrene are highly aromatic whereas the central one is weakly aromatic.46,114,138 These properties of individual rings are well quantified by NICS index, a very effective magnetic characteristic of aromaticity introduced by Schleyer et al.56 By definition, the nucleus-independent chemical shift (NICS) is a computational method that calculates the absolute magnetic shieldings at the center of the ring taken with reversed sign. In the last two decades this concept has been developed by computing this property 1  A above the center (NICS(1)) or by considering the perpendicular component of the shielding tensor NICS(1)zz.139 The latter two representations of

the NICS values were 2.2 and 2.5, respectively, whereas the HOMA values were 0.63 and 0.22 (Fig. 17). Moreover the closed form should be more stable due to intramolecular H-bonding.143 Interestingly, when Hþ is replaced by Liþ p-electron delocalization in the chelate ring increases to the value of HOMA¼0.945 and NICS to 1.79. The latter value is still far away from a typically value of NICS for aromatics w-9.0.64 These data indicate that NICS does not work well for describing p-electron delocalization in quasi-aromatic rings, whereas HOMA worked in a proper way in line with an increased stability.

Fig. 17. HOMA and NICS values for two conformation of (1Z,3E)-3-iminoprop-1-en-1-ol.

NICS values around functional groups and aryl structures (spatial NICS)115,144 can be computed as Through-Space NMR Shieldings (TSNMRS)144 and can be visualized as Iso-Chemical-ShieldingSurfaces (ICSS).144 Application of TSNMRS and ICSS approaches to enol tautomers of 1,3-dihydroxy-naphthyl-2-aldehyde145,146 showed that despite of two rotamers with quasi-aromatic rings, there is no difference between the spatial magnetic properties of the four structures presented in Fig. 18.

Fig. 18. Schemes of structures of enol tautomers of 1,3-dihydroxy-naphthyl-2aldehyde.

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Important conclusion can be drawn from investigation of the currents induced by external magnetic field perpendicular to the molecular plane.61,147,148 In aromatic rings the diatropic (anticlockwise) ring currents are observed.149 In the case of cyclic forms of ortho-acylphenols and their lithium analogues it was proved that neither Liþ nor Hþ contribute to aromatic p-electron delocalisation. Moreover no ring current is observed in the quasi-aromatic chelating ring (see Fig. 19).28 The observed equalization of bond lengths (high HOMA values w0.8) may be attributed to the electrostatic effect of Liþ on CO bonds and in consequence on the quasi-aromatic ring. The lithium b-ketoenolate and b-ketoenol current density maps for p-electron system obtained by ipsocentric approach150e152 are very similar and show the diatropic ring current in benzene ring and around oxygen atoms of the chelate chain, but not in the quasi-aromatic ring.

Fig. 20. Metal complexes of acetylacetone.

Fig. 21. Proton- or lithium-stabilized acetylacetone complexes.

behaviour of such compounds. Among them, 2,3-dihydro-1,4diazepinium salts also unable to form a closed, cyclic conjugated system, were studied.16 It was found that over a broad pH range, the monocation constitutes a preferred form (Fig. 22).

Fig. 22. Aromatic-like stability of 2,3-dihydro-1,4-diazepinium salts. Fig. 19. b-ketoenol current-density map for the p system, showing the current induced by a perpendicular external magnetic field, plotted at a height of 1 Bohr above the molecular plane. Arrows indicate the in-plane projection of current and contours the modulus of the total current; diatropic circulations are anticlockwise. Black-filled circles indicate projections of C positions, white-filled circles O, dot-filled circles H.28

In a study of a large set of polycyclic aromatic hydrocarbons153 it has been shown that cyclic p-electron delocalization is a necessary though not always sufficient condition for a presence of diatropic ring current. A good example is an artificially planarized structure of 1,3,5,7-cyclooctatetraene53, which has relatively low bond length alternation (HOMA w0.5) and no traces of diatropic ring current. From the presented results it can be concluded that unfortunately none of the magnetism based properties cannot serve as a descriptor of a p-electron delocalization observed in the quasiaromatic rings, since no ring currents can be attributed to these kind of cyclic structures. 7. Chemical reactivity criteria In addition to the above discussed physical phenomena, one may expect some chemical consequences on the reactivity of quasiaromatic systems. According to a definition given by Lloyd and Marshall16 an acyclic conjugated p-electron system can be called quasi-aromatic when possesses a significant mesomeric stability and shows typical properties of aromatic compounds, especially electrophilic substitution with retention of type. It was observed that certain complexes of acetylacetone easily undergo electrophilic substitution at b-carbon atom16 but apparently the metal d-electrons take part in a cyclic conjugated system thus contributing to its stability (Fig. 20). However, in this article we concentrate only on these cases in which Me¼Hþ or Liþ and therefore the conjugated electron system is limited to five out of six atoms in the cycle (Fig. 21): Despite this limitation, there are several literature examples supporting aromatic-like stability influencing relevant chemical

Dihydrodiazepinium salts undergo also a variety of reactions characteristic for ‘typical’ benzenoid compounds, like electrophilic substitution taking place at C-6 carbon atom (Fig. 23).16,154

Fig. 23. Electrophilic substitution of diazepinium salts.

The kinetics of readily proceeding halogenations resembles that of activated benzene derivatives despite the fact that the attack of electrophile occurs on an already positively charged species. The salts may also be nitrated under standard procedure16 affording nitro-compounds that may be reduced to amino derivatives, which in turn resemble benzenoid amines in their reactivity. These amines, upon nitrous acid treatment, are capable of undergoing Sandmeyer reactionethe transformation typical of aromatic amines.16 Other compounds iso-p-electronic with dihydrodiazepinium salts also reveal a similar behaviour towards electrophiles. Among them, enols and enolate ions from 1,3-dicarbonyl compounds are of special importance as ambident nucleophiles. Many fundamental synthetic transformations including alkylation, acetylation, aldol reaction or Michael or Dieckmann condensation involve alkali enolates as intermediates. For ambident systems it is important to estimate the reactive site of the molecule under certain conditions and with given reagent. Consequently, many 1,3-dicarbonyl compounds have been extensively studied. Thus, malonamide can be readily brominated, iodinated and nitrosated (Fig. 24) to form C-substituted derivatives.155

Fig. 24. C-nitrosation of malonamide.

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The kinetics of C-nitrosation has been studied in detail and the data obtained strongly supports the mechanism of electrophilic substitution on the enol tautomer. Recently, several enols of 2-substituted malonamides were subjected to structural investigations revealing the presence of a strong hydrogen bond responsible for structure stabilization.156 The temperature dependence of the differences in OeH and O.H bond lengths as well as other bonds in conjugated systems has also been investigated.157 Similar mechanism involving the presence of quasi-aromatic moiety was postulated for halogenations of ethyl cyanoacetate (Fig. 25).158

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The quasi-aromatic behavior of enolisable 1,3-dicarbonyl compounds is not limited to halogenations or nitrosation reactions. Also, other electrophiles like ArSþ or ArSeþ species induce the SE reaction. Thus, benzene-selenyl bromide and chloride react smoothly with b-ketoesters to give a-phenylseleno product, which is a useful intermediate in a double bond formation in the molecule (Fig. 28).165 Similar reactivity of 1,3-dicarbonyl compounds has been recently observed. Arylbis(arylthio)sulfonium ions ArS(ArSSAr)þ, which were generated by electrolysis of diaryl disulfide (ArSSAr), were found to serve as ArSþ equivalent in electrophilic substitution

Fig. 25. Halogenation of cyanoacetate.

The reaction was found to occur by rate-limiting diffusioncontrolled halogenation of the enol tautomer. Also, a kinetic study allowed the first kinetic discrimination between carbon and oxygen reactivity of enols.159 The nitrosation reaction was chosen as the model transformation because of its importance in the synthesis of nitroso-ketones or keto-oximes (depending on whether the substituted group is a primary or secondary structure). In the case of C-nitrosation of acetyloacetone due to the observed catalysis by added nucleophile, the reaction is postulated to proceed through a rate-determining electrophilic attack at the olefinic carbon of the acetyloacetone enol form (Fig. 26).

reaction of both aromatic compounds and 1,3-dicarbonyl derivatives (Fig. 29).166

Fig. 29. Arylbis(arylthio)sulfonium ion reaction with acetoacetyl acid.

Fig. 26. Nitrosation of acetylacetone.

This mechanism has been supported by further study160,161 being in accordance with fundamental contribution from Williams’ group.162 Among a variety of similar investigations, it is noteworthy to mention the results obtained by Iglesias et al.163,164 Upon extending their nitrosation studies to other than acetylacetone 1,3-dicarbonyl compounds like 2-acetylcyclopentanone, the ‘unexpected’ reaction mechanism was found suggesting that the enol form of 1,3-diketone undergoes nitrosation of give a chelatenitrosyl complex X that subsequently rearranges to the stable Cnitroso compound in the rate-limiting step (Fig. 27).

Distinct chemical reactivity of quasi-aromatic systems is not limited to electrophilic substitution at the carbon atom within the conjugated system but also manifests itself in a modification of the activity of other parts of the molecule. A relevant example comes from the chemistry of natural products and concerns alkylation reaction of magniferinda highly pharmacologically active xantone glucoside. Mangiferin 1 subjected to the treatment of a variety of alkyl or benzyl halides under reaction conditions used for a phenol ether preparation, undergoes O-alkylation at C-3, C-6 and C-7 phenolic groups. The C-1 OH group remains unaffected pointing out on a unique stability of apparent quasi-aromatic fragment of the molecule (Fig. 30).167 The above selected examples clearly indicate, in our opinion, the fact that the increased mesomeric stability of the quasi-aromatic systems affects their chemical behaviour in several ways among, which the tendency to undergo electrophilic reactions at the conjugated p-electron part of the molecule being the most profound manifestation. 8. General conclusions

Fig. 27. Mechanism of nitrosation of 1,3-dicarbonyl compounds.

Quasi-aromatic ring when fused to the benzenoid hydrocarbons cause changes in their properties as if the benzene ring was fused.

Fig. 28. Benzene-selenyl chloride reaction with b-ketoester.

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Fig. 30. Mangiferin 1 reaction with alkyl- or benzyl halides.

There is no diatropic ring current observed in the quasiaromatic rings. Therefore the magnetic properties do not work well as identifiers of the p-electron delocalization. There is no cyclic p-electron delocalization within the quasiaromatic ring, even if pi-conjugation is still responsible for the extra stabilization of such ring (e.g., due to more preferable partial charges distribution, more effective HOMO-LUMO overlapping, the smaller HOMO-LUMO gap, etc.) and if the decrease of the bond length alternation is observed. Electrostatic interactions with the chelated cation increases the p-electron delocalization in the quasi-aromatic rings. Closed forms of quasi-aromatic systems are significantly more stable than the open ones. Alike in aromatic systems, the electrophilic substitution take place in quasi-aromatic rings. References and notes 1. ISI Web of Science, retrieved in March 2013. 2. Aromaticity, Pseudoaromaticity, Antiaromaticity; Proceedings of an International Symposium, Jerusalem, 1970; Bergmann, E. D.; Pullman, B., Eds.; Israel Academy of Science and Humanities: Jerusalem, Israel, 1971. 3. Minkin, V. I.; Glukhovtsev, M. N.; Simkin, B. Ya Aromaticity and Antiaromaticity-Electronic and Structural Aspects; Wiley: New York, NY, 1994. 4. Issue dedicated to aromaticity and electron delocalization Chem. Rev. 2001, 101, 1115. 5. Issue dedicated to electron delocalization Chem. Rev. 2005, 105, 3433. 6. Pozharski, A. F. Khim. Geterocikl. Soedin. 1985, 867. 7. Williams, R. V. Chem. Rev. 2001, 101, 1185. 8. Dewar, M. J. S.; McKee, M. I. Pure Appl. Chem. 1980, 52, 1431. 9. Buhl, M.; Hirsch, A. Chem. Rev. 2001, 101, 1153. 10. Chen, Z.; King, R. B. Chem. Rev. 2005, 105, 3613. 11. Rzepa, H. Chem. Rev. 2005, 105, 3697. 12. Boldyrev, A. I. Lai-Sheng Wang, Chem. Rev. 2005, 105, 3716. 13. King, R. B. Chem. Rev. 2001, 101, 1119. 14. Calvin, M.; Wilson, K. W. J. Am. Chem. Soc. 1945, 67, 2003. 15. Mester, L. J. Am. Chem. Soc. 1955, 77, 4301. 16. Lloyd, D.; Marshall, D. R. In Aromaticity, Pseudo-aromaticity, Anti-aromaticity, Proceedings of an International Symposium; Jerusalem, 31 Marche3 April, 1970; Bergmann, E. D., Pullman, B., Eds.; Israel Academy of Science and Humanities: Jerusalem, Israel, 1971; p 85. 17. Lloyd, D.; Marshall, D. R. Chem. Ind. 1964, 1760. 18. Lloyd, D.; McDougall, R. H.; Marshall, D. R. J. Chem. Soc. C 1966, 780. 19. Lloyd, D.; Marshall, D. R. J. Chem. Soc. C 1958, 118. 20. Barnett, C.; Cleghorn, H. P.; Cross, G. E.; Lloyd, D.; Marshall, D. R. J. Chem. Soc. C 1966, 93. 21. Gorringe, A. M.; Lloyd, D.; Wasson, F. I.; Marshall, D. R.; Duffield, P. A. J. Chem. Soc. C 1969, 1449. 22. Barnett, C. Chem. Commun. 1967, 637. 23. Barnett, C. J. Chem. Soc. B 1967, 2436. 24. Gorringe, A. M.; Lloyd, D.; Marshall, D. R. J. Chem. Soc. C 1970, 617.  ski, M. K.; Czarnocki, Z.; Hafelinger, G. Katritzky A. R. 25. Krygowski, T. M.; Cyran Tetrahedron 2000, 56, 1783. 26. Masui, H. Coord. Chem. Rev. 2001, 219e221, 957. 27. Palusiak, M.; Simon, S.; Sol a, M. J. Org. Chem. 2006, 71, 5241.

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Biographical sketch

Tadeusz Marek Krygowski was born in Poznan, Poland (1937), and received his M.Sc. degree at the Adam Mickiewicz University (Poznan, 1961). He then moved to the Department of Chemistry of the Warsaw University where he obtained his Ph.D. (1969) and D.Sc. (1973) degrees. In 1983 he was promoted to the position of a Professor of Chemistry. In 2008 he retired but continued his association with the University as an emeritus professor. In 2010 he received the Foundation of Polish Science prize, which is considered the most prestigious scientific award in Poland. In 2012 he became  dz University. His main research interests involve the Doctor honoris causa of the q o studies of structural effects of intra- and intermolecular interactions, various phenomena associated with s and p-electron delocalization, definition of aromaticity, and long-distance consequences of H-bonding. He is married to Maria Krygowska (mathematician) and the father of two daughters, Kinga and Aleksandra. His hobbies are national music and hiking on not too high mountains.

dz, Poland (1974). After graduating in 2001 he received Marcin Palusiak was born in q o his M.Sc. Degree (in crystallography) at the Faculty of Physics and Chemistry of the  dz. He completed his Ph.D. in structural chemistry in 2005 at the UniUniversity of q o  d versity of q o z, working with professor S1awomir J. Grabowski. In 2009 he received habilitation in the field of physical organic chemistry. His main research areas are chemical bonding, noncovalent interactions in solid state and gas phase, aromaticity, and related p-electron effects.

Barbara Bankiewicz was born in Bia1ystok, Poland (1982). In 2006 received the M.Sc. degree (in organic chemistry) and in 2015 the Ph.D. degree (in computational and quantum chemistry), both at the Faculty of Biology and Chemistry of the University of Bia1ystok. Her main scientific interests involve noncovalent interactions, especially hydrogen and halogen bonds, molecular modelling, computational and quantum chemistry. Her hobbies are fire dancing and pyrotechnics.

Zbigniew Czarnocki was born in Warsaw, Poland, in 1954. He received B.Sc. and M.Sc. degrees in 1977 and in 1983 he obtained his Ph.D. degree from the University of Warsaw (Poland) working with Professor Jerzy T. Wrobel. In 1984 he moved to McMaster University (Hamilton, Canada) to perform his two-year post-doctoral study with Professor David B. MacLean. In 1987 and 1989 he visited McMaster University as research associate and visiting professor. After returning to University of Warsaw, he obtained his habilitation degree (summa cum laude) in 1993 and in 1996 he became a full professor at the Faculty of Chemistry (University of Warsaw). From 1996 till 2002 he served as a vice-Dean of the Faculty and from 1996 he is a leader of the Laboratory of Natural Products Chemistry. His research interest focuses on stereoselective synthesis of natural products, modern catalytic reactions and pharmacochemistry of various heterocyclic compounds. In 1987 he was recipient of the International Scientific Exchange Award (Government of Canada) and in 1993 he was awarded by the Polish Academy of Sciences. He is a member of the American Chemical Society and the International Society for Tryptophan Research. He graduated 16 Ph.D. students and supervised 54 M.Sc. theses. He has also authored over 110 publications, 5 review articles and 6 patents.