Electrophilic aromatic substitution: Electron transfer routes in side chain substitution

Tetrahedron Lettera

lie. 1, pp 41 - 44, 1975.

Per-

by Jay K Department

of Chemistry,

Koch1

Indiana

Unlverslty,

(Reoeived in US511 19 September 1974; received In a continuing For

series

the side chain

example,

iety

of Interesting

substltutlon

hexamethylbenzene

tic acid m the dark of electrophlllc

and thlocyanatlon

z The

reports,

of these

lncludmg

which proceed

electrophlllc and arene ment (es Scheme

etc

generally

to pentamethylbenzyl (chlorine,

Cl,

-

1974)

and coworkers of electrophlllc

chloride

substltutlon

+

a var-

nitration

electron-rich

HOAc

in ace-

comprise

and lodme),

and other

have decondltlons’

by chlorme

reactlons

bromme

on polyalkylbenzenes

m Scheme

include

In eq 2, followed 3),

a variety

47401

lVomaber

aroma

HCl

-

(1)

3-4

mechanism

conditions

under

halogenatlon,

Indiana

26

, Illummatl

Baclocchl

non-conventlonal

+ (1~s such as thlophenes,

Bloommgton,

in UK for pnblloatioa

of alkyl-aromatlcs 1s converted

scope

reactions

The heterolytlc

Printed in Great Britain.

ELECTROPHILIC AROMATIC SUBSTITUTION a TRANSFER ROUTES IN SIDE CHAIN SUBSTITUTION

ELECTRON

scribed

Prem.

first,

by either

I which

they propose

the formation a stepwlse

for

of a a-adduct

or synchronous

side

chain

I between

a-proton

substltutlon

under

the electrophlle

loss

and rearrange-

e_ g ,’

I

Jx+ Cl,

0

-

0

I

cl-

(4

ti CIH:, ‘cl

(3)

TI IL

L__ IIda

__..__*1._ rcc.erruy

side chain posltlons.

I____ “CZC‘.

_L_..._ II‘VWLI,

&we*er

may proceed

,

Al__& CUdL

^____L__ ar”llldLlC

by electron-transfer

41

_..l__r....r.__ IU”ILILUC1”‘I,

processes

^.LL__

e~t.ntzs

ill

the

5 The electron

iaiiClear transfer

Or

steps

42

IO.

in Scheme

II have been effected

manganese(III), Scheme

cerlum(IV)

either electrochemlcally

capable

of effecting

or by metal oxidants

one-electron

changes

1

such as cobalt(III),

5* 6

The formatron

of

II

(4)

(5) the aromatic substltutlon

cation-radical follows

portant m facllltatlng

Illuminati

et al

also capable ily undergo transfer

transfer

in a number

In particular,

of performmg one-electron

chlorine

reduction

may proceed

shown m Figure

of arenes

’

Slrmlarly,

transfer

showed similar

esr spectrum

The facile

behavior

mechamsm

reported

Thus,

described

acceptors

acetic

formation

directly

described

or 8

by Illum-

acid solutions

Other arenes

are

m charge

to nitrogen dioxide

of hexamethylbenzene

in Scheme

by

and lodlne read-

ions involved

relative

po-

substitution

In the cavity of an electron

obtained at room temperature

with an electron-transfer

bromine

under condltlons

mechanism

were nuxed directly

reactions

mtroruum

are lm-

We feel that

for aromatic

electron

side chain

low oxldatlon

oxidant

chlorine,

potential

of hexamethylbenzene

by an electron

1 was readily

available

Thus,

high reduction

The intense and well-resolved

benzene and durene slstent

oxidants

since

Two factors

a relatively

of a one-electron

and are also highly effective

have a reasonable

and hexamethylbenzene

spectrometer

m Scheme II

of the side chain substltutlon

as one-electron

We wish to show that chlormaflon mat1 =t al

process

mechanism,

as shown In eq 5

many of the electrophlles

with a variety

m nitration

transfer

of an a-proton

compound and the avallablllty

are fulfilled

complexes

indirectly

from the loss

the electron

tential for the aromatic both condltlons

11 1s the key to the electron

directly

of

spm resonance cation-radical

such as pentamethyl-

of the radical-catlon

1s con-

III

Scheme III

J# + -II

-

_H+

-.&

T

HZ.

Cl,

“-lk;

e

y$( + -&

+ cl-

vu /y\

clz-

+ Cl.

Cl.

etc.

(6)

(?)

no. 1

43

/

+I

7

Figure

1

The electron avoids

Esr spectrum of the hexamethylbenzene radical-cation (central obtained during the reaction of hexamethylbenzene and chlorine acid at 25 ‘C. Proton nmr field markers are m kHz.

transfer

the anstulatlnn r-----------

heterolytlc

and electron

since electrophlllc qulrements available

mechamsm

In Scheme

III Involves

readily

accessible

of cvclnhexadlenes _, ___.____________ and their raarrnnoement _ _----__ D-------transfer

pathways m Schemes

addition (eq 2) and electrotl

5b In some

electrophlllc

for an electron

transfer

transfer

substltutlons.

process

I and III,

intermediates

and

A ~~rrnrnrln e-- --- dIstInctton between

respectively.

1s difficult

to make

(eq 7) have much the same electronic

such as proton exchange,)

to occur,

portion) m acetlc

and the reaction

acceptors

must perforce

re-

are un-

take a heter-

olytlc pathway References E

Rz.t.,n.-ch, __ _ _ _ _ _ . ._ ,

E

Bacciocchi,

p

P.oo,nnr - -ac)-----

A Clana,

zinc-4 G ----

Tlll,W%,“.+, C------&.......)

G. Illumlnatl

and C

C.nv -..--

,?h,m Teal - . . . . . _ *..._

Paslnl,

J Amer

(a) (b) (c)

G Illuminate, G Illumlnatl, E Bacclocchl

(a) (b) (c) (d)

L J A n d rews and R M Keefer, J Amer Chem Sot K Nakamura, Bull Chem Sot , Japan, 44, 133 (1971) H Suzuki, _Ibld , 43, 879 (1970) R Astolfl, E Bacclocchl and G Illumlnatl, La Chum

(a) (b)

E I Helba, R M Dessau and W J Koehl. J J K Kochl, R T Tang and T Bernath, Ibid L Eberson, _Ibld , 2, 4669 (1967).

(c)

,

In? A”.,,

1111 ASA_

Chem

Sot

11072\ \‘I.+,

, E,

3953 (1965)

L Mandollnl and A Patara. Tetrahedron Letters, 4161 (1972) L Mandolml, E M Arnett and R Smoyer, J Chem Sot ,B. 2206 (1971) and G Illummatl, Tetrahedron Letters, 637 (1962)

Amer

, 86, 4158 (1964) el’Ind

Chem

Sot

, 95, 7114 (1973).

, 53, 1153 (1971) . 91. 6830 11969) L

44

Ho. 1

6

It is also possible with apparent Z-electron J K Kochl, J Amer Chem Sot , E,

7

L J Andrews and R N Keefer, Day, San Francisco, Calif , 1964

8

F A Cotton and G Wllkmson Pub , 1972, p 357

9

(a)

;119$)

(b)

C

Shubm,

Eaborn

A

and G

oxidants such 5061 (1973)]

“Molecular

“Advanced

Inorganic

A

Tabatskaya

and V

J

Wright,

Chem

J

Complexes

A , Sot

as Tl(III)

[I

In Orgamc

Chemistry”.

Chemistry”,

Koptyug, , B,

Third

Zhur 2262

org

(1971)

Ed

H

Elson

and

Holden

, Intersclence

Khlm

, -6, 2081