Enol ether formation by disproportionation of the 1,5-biradical intermediate in the photocyclization of o-isopropoxybenzophenone

0040.4039/91 $3 00 Pergamon press plc

Teuahedron Letters, Vo1.32. No.7, pp 895-898, 1991 Rated III Great Britam

Enol Ether Intermediate

Formation by Disproportionation of the 1,5-Biradical of o-lsopropoxybenzophenone in the Photocyclization Peter

C h e mi st ry Department.

J

Wagner*

Michiaan

and Guy Laidig

State

tlmversrty.

East

Lansmg.

MI 48824

The 7,3-dloxane formed by uv irrad/ation of o-isopropoxybenzophenone IS produced by mtramolecular cyclization of a hydroxy enol ether, the dispropartlonation

product

of the

1,5-brrad[cal

formed

The enol ether IS stab/e /n the absence In

one

ketones, the for

of

It was

expected the

generate

have

enol to the

We

have

now

by gas separated

are

0.42

followed

and the

0.21 reaction

found

of

then

parent

by

that

forms

in benzene NMR

and

in agreement 0 71 and

(benzene-de)

established remamed

2

to

also

internal

a new

addmon

3 6

of

ionic

method the

to

hydroxy

and

with

R-OCH3 R=H

4 and

produces the

some

5 and earlier

characterized.sA

0 14 in methanol. we observed

895

an

only

catalysis.

M 4 In benzene

TLC

3 by

not

has

cyclizes

demonstrated

o-lsopropoxybenzophenone

preparative

was

formation

intramolecular

acid

o-alkoxyphenyl

No mechanism

1,5blradlcal

which

the

upon

of 0.02

abstraction.

1 gave

of product

suppositron,

that

only

3 1

sort

the

ether,

this

chromatography,

by

hydrogen

photocycllzatron

2: R = OCHB 5 R=H

the

products

this

that

R=OCH3 R=H

irradratlon

the

1,3-droxane day

enol

venfled

occurs

as determined were

to this

and

studied

UV

of

suggested2

ether

1 4

derivatives

2 but the

ethers, enol

studies

a hydroxy

to

We

state

o-rsopropoxy-p-methoxybenzophenone

and

recently

dlsproportionates

group

that

of 3;

We

addmon.

original

benzofuranol

formation

unique.

the noted

by triplet

of acid.

deuterated

6 in a 2’1 work.

The

Quantum When,

a 2 1 mixture

yield two

yields

however,

we

of 5 and

enol

ether 7.5

Addition

of one flake of naphthalenesulfonic

acid to the NMR tube caused

The lifetrme of 7 in “neutral” tubes 7 to convert completely to 6 within minutes. depended on how the tubes had been cleaned; when they had undergone a final ammonia

wash,

expected

acid-catalyzed

7 did not convert addition

to 6 after several of alcohol

to enol

weeks. ether,

Thus 7 undergoes In a rare

the

intramolecular

fashion. Scheme

1

d-d6

When proton

4-ds

signal

deuterium kinetic

was irradiated,

at

6 07

had shifted

concluded

to

isotope earlier

ketones,

which

at 5.75

The absence

and

to this

positron

from one of the isopropyl

by disproportronatron

kH/ko

Of 1.5.

bis-tertiary

ppm

of

There

are

in 6-d6

for

the

O-H

bond

breaking,

also

methyls.

now

examples

several

here

shows

a small

effects

transition

state

biradical

disproportionatlon

dlsproportronation

isotope

for dlsproportionation to

Involve an

at O-H

relative

1,6-hydrogen

unsaturated

presumably

alcohol

Indicate

to C-H.

transfers,

that the

primary

exothermic.

different

to

o-tert-

hydroxybrradlcals

enols, which involves breaking Both forms of disproportionatlon

very

support

of

effect,9 whereas drsproportionation to bond, shows larger isotope effects.2~1° The

7 IS

of such

It is interesting of

Since

competes

to isotope

of a C-H are quite

a much

It IS very so

a

8, with a

adds

rearrangement

that Thus

biradrcal

to 1 -(o-isobutenylphenyl)ethanol.7 relatively small. Disproportlonatron

involves

of a benzylrc

Indicated

intermediate

biradrcals, -6-s its occurrence

mechanism

suggested

butylacetophenone rsotope effect Is

ratio was 3.1.

in 7-ds

effect of

the 5/7

ppm

be formed

disproportlonatton our

6-ds

T-d6

earlier

both forms

of

unusual

that

well

with

897

drsproporhonation speculated that

that

the

rules

the

this

These

unlikely

findings

that

place.

The

protonated.

the double one

double

IS

favored

carbocation We

are

protecting studying Scheme

addition

and

that

occurs.

rapid

the kinetics

actually

rntramolecular

reactions

of an enol

in the absence

of added

acid strongly

alcohol

indicates

In these

that

make

proton

transfer

of

stable

that

nucleophlllc

this for

of Intramolecular

2

OH

that

free

to tell

by the hydroxyl

eventual addition

can site

must

lead the

still

hydroxy of

would

be

favored However,

whether

the

protonation

protonation, might

are

addition

Intermolecular

rotation

groups

is a single

to carbon.

ether

take

protonatlon

oxygen

enol

reaction

syn there

can

the

impossible

intramolecular

attack

whether

the

that

IS

or intramolecularly of

methyl

solvents

from it

intercase

ether

intramolecular

it clear

occur,

enol

the two

In that case a stereospecific models not

alcohols

to the

nonpolar

does

the

exploring

secondary

workers

from a methyl group and The deuterium labellrng

ether.1

rate-determining11

conformation In

at the of the

likely

is sufficiently

must be quite

look

of 6-de

protonated ant/

original

hydrogen enol

of 7 in benzene

intramolecular

stereochemistry.

to

spectrum

protonation

bond

a rare

possibility;‘2

for

specific

abstracted

It is very

bond then

conformation since the

allow stability

protonated

mechanistrc

The

ketone.

possibility.

t H-NMR

rs frrst

ketone

no uncatalyzed

equally group

starhng

disproportionated

The indefinite

suggests

to

triplet

1,6-biradical

out

ether.

back

to

of

random

intermediate

occur

before

what

group. be

specific

of alcohols

adapted

a

method

of

photooxidation

and

for

to enol

as

ethers.13

Acknowledgement.

This work

15052.

purchased

The

NMR was

was

generously

supported

wrth support

by NSF

by NSF grant

grant

No. CHE

No. CHE

88-

88-00770.

References

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(t, J = 7 4

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o-(1 -phenylethoxy)benzophenone

together

with

a mixture

of

also drastereomeric

benzfuranols. (9)

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