Sustainable Chemistry and Pharmacy 9 (2018) 46–50
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Liqueﬁed dimethyl ether (DME) as a green solvent in chemical reactions: Synthesis of O-alkyl trichloroacetimidates Afraz Subratti, Lorale J. Lalgee, Nigel K. Jalsa
The University of the West Indies, St Augustine Campus, Trinidad and Tobago
A R T I C LE I N FO
A B S T R A C T
Keywords: Solvent Dimethyl ether Trichloroacetimidates Alcohols
The trichloroacetimidate group is one of the most versatile leaving groups in organic synthesis. This is evident by its involvement in quite a number of total syntheses. Typical methods of preparation generally involve the use of toxic and hazardous solvents, speciﬁcally dichloromethane and chloroform. Liquiﬁed Dimethyl ether (DME), was investigated as an alternative solvent in the synthesis of commonly utilized trichloroacetimidates. A range of alcohol substrates were employed and moderate to excellent yields of the corresponding imidates were obtained. The low boiling point (−24 °C) of DME allows for rapid evaporation at atmospheric pressure, and its facile recovery.
1. Introduction Trichloroacetimidates serve as valuable intermediates in organic synthesis (Greene and Wuts, 1999; Schmidt and Zhu, 2009). They are often used as alcohol alkylating agents which are known to possess higher reactivity than the corresponding halides (Schmidt and Zhu, 2009). Synthetic approaches to trichloroacetimidates of benzyl (Eckenberg et al., 1993), propargyl (Li and Wang, 2007), Fmoc (Ali et al., 2003), diphenylmethyl (Ali et al., 2003; Howard et al., 2016), phthalidomethyl (Ali et al., 2004), tert-butyl (Armstrong et al., 1988), and allyl (Wessel and Bundle, 1985) functionalities have been reported. Conventional procedures for preparing these reagents utilize the base catalysed addition using Cl3CCN (Schaefer and Peters, 1961). Some alkyl trichloroacetimidate reagents involved in complex syntheses include, allyl: fused-ring alkaloid securinine (Honda et al., 2004); propargyl: 1,3-Diarylpropynes (Li and Wang, 2007); benzyl: funiculosin (Williams et al., 2000); methyl: bibenzimidazole oligomers (Yin and Elsenbaumer, 2005); and tert-butyl: phospho-amino acid building blocks for solid-phase peptide synthesis (Rothman et al., 2003). Furthermore, the Overman rearrangement of allylic trichloroacetimidates (Swift and Sutherland, 2007), as well as a similiar transformation of the benzylic counterparts (Adhikari et al., 2017), are synthetically important. Schmidt and co-workers pioneered the development of trichloroacetimidates as glycosyl donors (Schmidt and Michel, 1980). Even with an arsenal of protocols established within recent times, this method to date, still serves as one of the most powerful techniques in contemporary carbohydrate chemistry (Schmidt, 1986). The additional ⁎
Corresponding author. E-mail address: [email protected]
https://doi.org/10.1016/j.scp.2018.06.001 Received 21 April 2018; Received in revised form 2 June 2018; Accepted 3 June 2018 2352-5541/ © 2018 Elsevier B.V. All rights reserved.
publications every year on new methodologies involving trichloroacetimidates authenticates their value (Das and Mukhopadhyay, 2016). Examples of targets utilizing Schmidt glycosylation include: the tumor-associated globo H antigen (Lassaletta and Schmidt, 1996); Amphotericin B (Nicolaou et al., 1988); Streptococcus pneumoniae type 1 capsular polysaccharide antigen (Schmidt et al., 2010), glycosphingolipids (Yunpeng et al., 2006) and glycoconjugate vaccines for Candida albicans (Wu and Bundle, 2005). A well exploited feature of the glycosyl trichloroacetimidates is the ability to thermodynamically control anomeric selectivity upon introduction depending on the type of base used. This, by extension inﬂuences the stereochemical outcome of the resulting glycoside bond (Schmidt and Michel, 1984). Despite being an indispensable methodology, synthesis of trichloroacetimidates and their subsequent activation generally employ toxic organic solvents (MacMillan et al., 2013), mainly CH2Cl2 and CHCl3. Solvents play a critical role in deﬁning the environmental performance of a process. As such, signiﬁcant eﬀorts are made towards the minimization of their impact on the environment and human health (Palacios et al., 2006). Alternative solvents should be of low toxicity for humans and the environment, derived from a renewable resource, high solvating capacity and easily recoverable (Noyori, 1999; García et al., 2014), Substitutes such as 2-methyl-tetrahydrofuran (Alcantara et al., 2012), γ-valerolactone (Song et al., 2016), deep eutectic solvents (DES) (Ge et al., 2017), supercritical CO2 (Rayner, 2007) and water (Hailes, 2007) have emerged within recent times. Presently, there are no reports describing the synthesis of trichloroacetimidates in green solvents; however several exist for glycosylations involving such glycosyl donors in ionic liquids (Rencurosi et al., 2005).
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Dimethyl ether (DME) has received signiﬁcant attention within recent times as a clean alternative fuel; possessing similar physicochemical properties to traditional Liqueﬁed Petroleum Gas (LPG) (Arcoumanis et al., 2008). Due to the lack of a C-C bond, decomposition is achieved readily in the atmosphere (Naito et al., 2005). It is non-toxic (Varlet et al., 2014), environmentally safe and benign, readily available and cost eﬀective (Good et al., 1999). Unlike typical ethers, DME does not form peroxides (Naito et al., 2005). Apart from the growing interest as a fuel, DME is most commonly employed as an aerosol propellant (Aguayo et al., 2007), spot coolants (Schaidle et al., 2015) and as a synthetic building block for valuable chemicals (Wang et al., 2015). Several protocols have been reported which involve the use of DME as a low temperature extraction solvent. Its ability to solvate proteins and pigments derived from vegetable tissues (Furukawa et al., 2016); and essential oils from citrus peels has been demonstrated (Hoshino et al., 2014). A key advantage of these procedures is the simultaneous dewatering from the extracts due to the low solubility of water in DME (7–8 wt% at room temperature). This property was exploited for removing water from brown coal at −20 °C (Kanda and Makino, 2014) as well as from blue-green microalgae (Kanda et al., 2013); both procedures resulting in eﬃciencies of > 90%. In addition, it has also been used as a co-solvent in the transesteriﬁcation of triolein (Kuramochi et al., 2008; Maeda et al., 2017); as well as in electrochemical studies (Messaggi et al., 2017). To the best of our knowledge, there are currently no reports utilizing liqueﬁed DME as a reaction solvent in any widespread/mainstream synthetic organic reaction. Herein, we describe its utility in the synthesis of O-alkyl trichloroacetimidates. The reaction involved trichloroacetonitrile in the presence of a base, which installed the imidate via an addition pathway. Alcohols of various classes and reactivities were utilized for this evaluation. The set-up employed standard, relatively inexpensive laboratory equipment, that should be accessible by the majority of labs. This would also allow the incorporation of this technique in undergraduate exercises, without requiring high-pressure vessels.
Table 1 Evaluating the performance of bases. K2CO3
Yield: < 30% 3 equivalents Reaction time: > 3 h Heterogeneous reaction
Yield: > 60% 1.5 equivalents Reaction time: < 1 h Reaction solidiﬁes
Yield: > 60% 0.2 equivalents Reaction time: 1 h Homogenous reaction
We next sought to optimize the quantities of Cl3CCN and DBU used. Lower equivalents of Cl3CCN reduced the yield signiﬁcantly (> 20% reduction). Typically, higher amounts of Cl3CCN are used due to the reactivity of the resulting imidate (Schmidt and Michel, 1980). We found that 3 eq. of Cl3CCN gave the best results in reasonable time (1 h). Reduced quantities of DBU required longer reaction times whereas higher eq. did not impact the yield appreciably (< 3%). We then subjected the range of alcohols to the established conditions (Table 2). 2.2. General procedure Alcohols (1 g) were dissolved in the liqueﬁed DME (12 mL) and Cl3CCN (3 eq.) followed by DBU (0.2 eq.) was added. The resulting solution was stoppered under argon (introduced via a balloon), placed in a dry ice/acetone bath and allowed to stir for 1 h. After this, recovery of the DME was allowed and the resulting crude dissolved in 20 mL EtOAc. The solution was then washed with water (3 × 50 mL). The organic layer was dried using Na2SO4, ﬁltered and concentrated in vacuo to yield the trichloroacetimidated products. 2.3. Reaction setup See Fig. 1. 3. Results/discussion
3.1. Yields of imidates (products)
Overall, good yields of the imidates were obtained. Products 1, 2, 3, 4, 7, 8 and 13 were produced in > 80%. These yields were calculated using the mass of products obtained and are accurate within ( ± 2%) after 3 repetitions. This is likely due to the fact that these alcohols are all primary and hence sterically available to take part in the addition reaction. The isopropyl derivative 5 was obtained in 86% yield. Even though it is a secondary alcohol, the inductive eﬀect imparts a higher reactivity. Slightly lower yields were observed for 6 and 10 from propargyl alcohol and cyclohexanol respectively; the former is perhaps due to competitive reaction sites and the latter as a result of conformational sterics. For the reactions involving 1,3 propane diol and triethanolamine 3 eq. of Cl3CCN were added per hydroxyl group. Yields of the respective per-O-trichoroacetimidated alcohols, 11 and 12, were acceptable (> 65%). Partially trichoroacetimidated compounds for these substrates would have partitioned in the aqueous phase during work up. The glycosyl donor 14 was obtained in 80% yield exclusively in the α conﬁguration. This is a common substrate utilized in evaluating glycosylation methodologies as well as assembling oligosaccharides. The galactose derived imidate 15 was obtained in 75% yield. We were unsuccessful in synthesizing trichloroacetimidates of the alcohols shown in Fig. 2. Several plausible explanations were developed to account for these observations. Due to the high electron withdrawing activity taking place in 17, 18 and 20, the reduced nucleophilicity of those alcohols are more pronounced at −40 °C thereby rendering the substrates unreactive. Similarly, the reduced reactivity associated with the alcohols in the ortho position relative to the methoxy substituent on 19 and 21 are greater at low temperatures. We postulate that the high reactivity of the epoxide ring in 16 is responsible for the lack of
Liquefaction of the DME was carried out using a vacuum trap ﬁlled with dry ice/acetone. Two traps were placed in series to minimize wastage during optimization of the ﬂow pressure. A dispensing pressure of 8 psi was found to be most eﬃcient in condensing approximately 10 mL of DME per minute; which dispensed directly into a round bottom ﬂask immersed in a dry ice/acetone bath. Both methanol and benzyl alcohol were used for optimization of reaction conditions. We found that a volume of 12 mL per gram of substrate was ideal for complete solvation and increased handling time. After addition of the necessary reagents, the ﬂask was stoppered and stirred at −40 °C under argon which was delivered via a balloon (Fig. 1). From a safety perspective, the balloon also allows for controlled gaseous expansion if there is excessive pressure build up in this all-glass set up. After completion, the DME was left to evaporate at room temperature; the crude was then dissolved in EtOAc and washed with water. The performance of diﬀerent bases: K2CO3, NaH and DBU were evaluated using 2 eq. of Cl3CCN. Table 1 describes the results obtained. Since it is necessary to maintain a low temperature during reaction, K2CO3 proved to be the least reactive; and as such required higher equivalents and longer reaction times. NaH displayed higher reactivity as expected but since stoichiometric amounts are required, the reaction solidiﬁes signiﬁcantly due to the low solubility of the ions in DME. DBU was most eﬀective as it functioned catalytically without compromising reaction time/reactivity. This is especially advantageous as it fulﬁls another one of the 12 principles of green chemistry which encourage the use of catalysts (Phan et al., 2015). 47
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Table 2 Products obtained from reactions in DME.
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% Recovery of DME (3 replicates) 120 100 80 60 40 20 0 Inial Volume 1st Recovery 2nd Recovery Fig. 3. Graph showing percentage recovery of DME (3 replicates).
attribute the other 50% to losses during the reaction and recovery process; as well as small amounts that remained embedded in the resulting crude product. Despite the DME being highly volatile (BP: −24 °C), at least half of the initial solvent can be reused without any contamination. No column chromatography of the products was required since they proved to be spectroscopically pure. The ability of DME to solvate almost all of the substrates used looks very promising for its application in other organic transformations. This study also demonstrates the stability of this ether under strongly basic conditions. We are uncertain of its stability in strongly acidic solutions as preliminary studies of the reactions of glycosyl trichloroacetimidates using TMSOTf as a catalyst in DME resulted in a complex mixture of compounds. In adhering with recommendations for greener solvents, DME is a synthetic fuel generated from biomass, hence renewable; and is non-toxic to humans and the environment (Good et al., 1999). These ﬁndings are remarkable as the yields obtained were comparable to these reactions performed in CH2Cl2 and CHCl3, which are both listed on GSK solvent selection guide as having major issues (Byrne et al., 2016).
Fig. 1. Reaction in progress under argon gas using a balloon.
4. Conclusion In conclusion, we have successfully demonstrated the ﬁrst use of liqueﬁed DME as a reaction solvent. A range of alcohols were subjected to trichloroacetimidation in this ether. Limitations do exist in that only low temperature reactions can be performed; as well as the reduced solubility of some substrates at that operating temperature. Approximately 50% recovery of the solvent was possible. Simple methods for both liquefaction and recovery were developed and optimized. As part of our future work we plan to utilize these imidates by activating them in DME to build glycosides, oligosaccharides, surfactants, and polymers via nucleophilic displacement reactions. Investigations of other synthetic reactions in DME are also being undertaken.
Fig. 2. Unsuccessful alcohol substrates.
formation our desired product. This is evident in the complex NMR spectrum that resulted, indicating ring opening took place Compounds 22 and 23 were insoluble in DME at the prevailing temperature and as such no reaction occurred. It should be noted that no successful attempts at the trichloroacetimidation of these substrates have been reported with the traditional solvents. We also attempted to synthesize the known tert-butyl trichloroacetimidate but found the tert-butanol to be unstable under these strongly basic conditions, likely forming isobutylene. Performing the reaction using CH2Cl2 instead of DME yielded the same result. This is not surprising given that the synthesis of this trichloroacetimidate requires modiﬁcation to the typical conditions (Armstrong et al., 1988).
Acknowledgements The authors thank The University of the West Indies, St. Augustine Campus for funding this research; as well the Caribbean Gas Chemical Limited (CGCL) for providing the DME cylinders.
3.2. DME recovery
Conﬂicts of interest
As part of a green approach to using the DME, we embarked on recovering the solvent after reaction. This was facilitated with a DeanStark apparatus attached to the vacuum trap used for dispensing. The dry ice/acetone combination provided the cooling. Preliminary attempts to recovering the solvent were successful with approximately 50% eﬃciency (13 mL from 24 mL) (Fig. 3). A more specialized apparatus however may be able to further enhance the recovery process. We
The authors declare no conﬂicts of interest Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.scp.2018.06.001. 49
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