The nature of the intermediate formed in the methoxymercuration of p-substituted styrenes

The nature of the intermediate formed in the methoxymercuration of p-substituted styrenes

Tetrahedron Letters lo. 38, pp.3627 @PerPress Ltd. 1979. Printed 004&4039/ - 3630. in Great 79/ 091F3627$02.00/ 0 Britain. THE NATURE OF THE INT...

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Tetrahedron Letters lo. 38, pp.3627 @PerPress Ltd. 1979. Printed

004&4039/

- 3630. in Great

79/ 091F3627$02.00/

0

Britain.

THE NATURE OF THE INTERMEDIATE FORMED IN THE METHOXYMERCURATION OF P-SUBSTITUTED STYRENES Adolphus Lewis* and Jerome Azoro Department of Chemistry, Howard University Washington, D.C. 20059, U.S.A. ABSTRACT - The logarithms of the rates of methoxymercuration of p-substituted styrenes give excellent correlations with, both, (Jand 0'. Based on the values of p and p+ obtained, an unsymmetric bridged ion is proposed as the intermediate formed in these reactions.

The nature of the intermediate subject

of numerous

theoretical

studies.

_ currently,

have been proposed

formed during electrophilic

Based on the evidence

available,

by various

in the field

Ic’

metric

an open a-substituted

bridged

or cyclic

In recent years during

electrophilic the nature

atom,

should

On the other hand, nonbonding

(ii)

should

that symmetric

reaction

atom,

nonbonding

and the reaction

proceed

E, I,

spectroscopic,

and

for these intermediates

I - III,

and III

II,

represent

the

\‘

III

through

bridged

or cyclic

ion, and an unsym-

hence, capable

that the nature of the intermediate on various

and (iii) the structure

should

factors:

proceed

of the alkene

then bridging

through

a cyclic

atom is highly electronegative of the electrophilic

an open intermediate

concerning of forming

ions, II,

ion, I.

(1,2).

In regards to

of the electrophilic intermediate

formed

the nature of the

ion, II

and posatom

or III.

and/or does not possess

any

atom should not be important

and

Solvent

effect

studies

are more likely to be formed

the structure

of the alkene

a resonance-stabilized

3627

(i)

that if it is not highly electronegative

p- or d-electrons,

intermediate

evidence

evident

is dependent

then bridging

or cyclic

The available

kinetic,

has been the

)C6th-gE

a symmetric

it appears

if the electrophilic

bridged

kene is conjugated,

increasingly

the solvent,

p- nor d-electrons,

the reaction

solvents.

addition

polarizable

be important

(l), where

reactions

ion, respectively.

of the electrophilic

sesses easily

carbocation,

it has become

an electrophilic

structures,

\f+>/ ‘II’

>c+-celectrophile,

- stereochemical,

three different

workers

addition

suggests

allylic

indicate

in nonpolar

that if the al-

or benzylic

type of

3628

Bo. 38

cation, action

then the need for stabilization should

unconjugated

proceed

by bridging

of the electrophilic ion, I.

by way of an open intermediate

or the formation

of a resonance-stabilized

of the ion by bridging

of the electrophilic

ceed through

or cyclic

a bridged

ion, II

intermediate

On the other hand,

cation

atom should

is not possible,

be important

of a series of p-substituted

In these reactions,

anol at 25OC, eq 1.

p-XC8H4-CH=CH2

A.

X = CH3;

+

B.

X = H;

C.

d-electrons.

Further,

ly changes

the structure

of the alkene.

this reaction

the alkene

being present

to show a change

were carried

in excess.

in the absorbances

of IVF, which sorption

0.

for the styrenes,

The

out under pseudo

changing

of IV at their longest

on which

290 nm.

All reactions

wavelength

factor

because

meth-

of its

HOAc

(I)

X = NO2

on the ring effective-

is dominant,

one might

formed as the structure

first order

of the data.

exhibited

of

band.

acetate the de-

With the exception

have several

weak ab-

behavior,

as

time, over more than four half-lives.

coefficients

analyses

of Kezdy-Jaz-Bruylants

of 0.998 or better.

The

plots

(3)

The data are sum-

I.

Plots of log kX vs. u and 8 to the least squares

Both of the correlations

are shown in Figures

correlation

lines.

intermediates

alkene were important

The lines drawn

The correlation

changes

formed during these reactions in determining

have been curved,

1 and 2.

in these Figures

data are summarized

in Table

are excellent.

Since the plots show no signs of curvature,

stabilization

absorption

by monitoring

clean first order kinetic

by least squares

All such plots had correlations

in Table

with the mercuric

band, all of these compounds

rates were determined

should

F.

the substituent

spectrophotometrically

pseudo

hence,

+

X = CN;

of the intermediate

linear plots of log (At- A_) versus

II.

atom is possible

E.

on

IVA - F, in anhydrous

first order conditions,

rates were determined

has only one intense absorption

bands around

correspond

styrenes,

X = Br;

shown by excellent

marized

pro-

of substituents

p-XCSH4-FHCH2HgOAc

Thus, depending

in the nature

should

is changed.

The reactions

crease

X = F;

is

then stabilization

and the reaction

the effects

of the mercury fiSO%

Hg(OAc)2

nonbonding

expect

bridging

if the alkene

or III.

We have tested these ideas - (i) and (iii) - by determining the rates of methoxymercuration

atom is small and the re-

can be ruled out (4).

the nature of the intermediate

being concave

of the ion by bridging

in the structure

upward.

This is the expected

of the mercury

would

of the transition

If the structure

formed,

of the

the plot in Figure

behavior

have increased

states,

I

since the need for

as the ability

of the

Ilo. 38

3629

aryl group to stabilize

the ion decreased. TABLE

I

of IV by Hg(OAc)B

Ratesa of Methoxymercuration

in CH30H at 250C

IV

105Uv1,

Ratioc

104 kobs,Sd

A

178.

9.79

3280. f 61

B

164.

10.6

665. f 19

C

185.

9.38

584. 2 17

D

133.

13.1

162. ? 5

E

140.

12.5

5.93 + 0.33

F

8.02

217.

3.42 f 0.07

dye

aAll rates were determined a minimum of three times. bThe initial concentration of the Hg(OAc)2, as determined by titration of the stock solution with standard KSCN, was 0.0174 M. The same Hg(OAc)2 stock solution was u ed throughout this study. CRatio refers to Uig(0Ac)210/W3 . 8The average pseudo first order rates. eThe uncertainty is tie standard deviation.

LOU?.I-

?.I-

X6-

** w

Figure 1. Plot of log kX vs. 0. evalues were taken from ref. 5. The value of p suggests carbon

in the transition

The

for these reactions.

cyclic mercurinium

ion, can be ruled out.

Halpern

(7) for the hydroxymercuration

and Tinker

identities P’

is

too

of these p-values; low

to

be consistent

the excellent

w

charge

is developed

Thus, structure

Our value of p is identical

II,

with both

u

The

on the benzylic

the symmetric

bridged

or

with that (-3.3) found by

of a series of aliphatic

correlations

with structure

PI

LI

Figure 2. Plot of log kX vs. 0'. ot-values were taken from ref. 6.

that a large amount of positive

states

LO

olefins.

Based on the

and c?; and the fact that

I (8), the open a-mercuriocarbocation,

we propose

3630

190. 38

TABLE Correlations

II

of log kX with the Substituent

Subst. Const.

Constants

log kfltb

Pa

rc

u

-3.16 + 0.09

-1.08 + 0.04

0.998

o+

-2.76 + 0.11

-1.34

0.997 -

?; 0.05

aThe uncertainty is the standard deviation. bThe Y-intercept. The experimental value is -1.18 f 0.03. CThe correlation coefficient. that the intermediate cyclic mercurinium

formed

in these reactions

is of structure

ion.

IIIA,

the unsynmetric

bridged

or

a+

H'

'H

IIIA Further

studies

of these reactions

are in progress.

REFERENCES

1.

(a) F. Freeman, Chem. Rev., 75, 439 (1975) and (b) Loizos, Tetrahedron, 34, 224771978).

2.

(a) M.F. Ruasse, A. Argile, and J.E. Dubois, J. Am. Chem. Sot., 100, 7645 (1978); (b) ’ J.E, Dubois, J. Toullec, and G. Barbier, Tetrahedron Lett., 4485 (197Or (c) M.J.S. Dewar and R.C. Fahey, Angew. Chem. Int'l Ed., 3, 245 (1964); (d) W.B. Dolbier, J. Chem. Ed., 4&, 342 (1969); (e) R. Bolton, "ComprehensTve Chemical Kinetics," Vol. 9, C.H. Bamford and C.F.H. Dubois and M.F. Ruasse, J. Org. Tipper, Eds., Elsevier, New York, 1973, p. 1; and (f) J.E. e, 38, 493 (1973).

3.

F.J.

4.

a) H.H. Jaffe, I1971).

5.

L.P. Hammett,

6.

H.C. Brown and Y. Okamoto,

7.

J. Halpern

8.

The average value of p+ for the hydration of substituted styrenes at 250C is -3.4. K.M. Koshy, D. Roy, and T.T. Tidwell, J. Am. Chem. Sot., 101, 357 (1979) and references to Deno's Schubert's work therein.

K&zdy, J. Jaz, and A. Bruylants,

(Beoeived

Organic

and H.B. Tinker,

in U15a 29 May 1979)

Oubois and M.M. de Ficquelmont-

Bull. Sot. Chim. Belg., 67, 687 (1958).

Chem. Rev., 53, 191 (1953)

"Physical

J.E.

Chemistry,"

and (b)

Second

J.O. Schreck,

Ed., McGraw-Hill,

J. Am. Chem. Sot., 80, 4979

J. Chem. Ed., 48, 103

New York,

1970, p. 356.

(1958).

J. Am. Chem. Sot., 8!& 6427 (1967).