Novel photooxidation of alkenes sensitized by p-dimethoxybenzene

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Novel Photooxidation

of Akaes

Uday T. Bhakrao* Organic

Division

II, Indian Institute

by p-Dime d

Wtixed

and Madabhushi Sridhar

of Chemical

Technology,

Abstract

: Photoexcited p-dimethoxybenzene catalyses to corresponding carbonyls in the presence of oxygen. Like

inorganic

also exhibit in

polar

photoinduced

solvents

semi-conductor

direction ( >280 This

in carrying

sensitizer

oxidative

conversions

benzene (DMB).

irradiated

is followed tography

procedure, at

to

not

promote

DMB

sensitization radical

of dioxetanes

and

with

column

reaction.

cations

(I mmol

medium

donating

generated

on irradiated

inorganic

to carbonyls

studies

in this

excited

states

by electron

of

studied

on the scope for

low lying

cations

possibility

groups

ejection3

extensively

We initiated

having

alkenes

as electron

are

carrying

transfer5. out

various

semiconductors.

Here

sensitized

by p-dimethoxy-

pressure

450W

in 500 ml of acetonitrile

Hanovia

atmosphere.

mercury

Progress

arc

lamp

of the reaction

were

isolated

by normal

solvents

alkenes

are not oxidized.

column

chrcma-

samples.

in

their

with

during; irradiation

each) are taken

and the products

polar

polar

Carbonyls

expect

reacting

the

replacing

of alkenes

and in deaerated

we

awaited.

l2h under an oxygen

efficiently

the

study

electron

but the studies

benzenes

conditions

and DMB6

temperature

proceed

holes

various

results are given in Yable I.

alkene

DMB

to

photooxidation

with authentic

of

to

conversions4, is still

of

as well

of amines to amine radical

us further

(> 280 nm) for

absence

could

tion

room

sleeve

and compared

reaction

by olefin

novel

having

semiconductors

methoxylated

500 007, India.

cleavage

oxygen2

(equivalent

arenes

homogeneous

by glc using SE-30

In the this

lead

under

the

oxidative

photooxidation

canpounds

phenomenon

rich

various

oxidative

to molecular cations

this

various

The representative

In a general

using a Pyrex

on

observation

we wish to report

and

Based

of electron

M-I) which initiate interesting

radical

identified

aromatic

transfer

arene

out

activity

and recently

several

electron

forming

surf ace).

as sensitizers similar

semiconductors,

Hyderabad

solvents are

the

formation

like

acetonitrile

only

from

oxidation

the

because

nonpolar

products

corresponding

super oxide anion (4’). possibly

but

readily

cleave

solvents

observed

dioxetanes

We could not identify

they

We find

under formed

the forma-

to carbonyls

in polar

solvents? To look which but

reacts

we could

for

the

possibility

efficiently

with

not observe

on6

singlet

formation hY

-flnB

OtlB+‘+ ~+oi’-

of

M

singlet

oxygen

oxygen of

as oxidant

forming

‘ene’

product

the ene product. *

_

_

we irradiated

Stilbene

POLAR SOLVLRT

BMB+’ Dt4B

[=I Scheme 4341

I

+

tetramethylethylene

as major

component

(4) is inert + m

towards

0;’

>co+o<

(85%)8, singlet

4342

9

oxygeii

but when irradiated

tentatively (Scheme

in presence

suggest the involvement I) as the major Table

I

of DMB

it cleaved

of a non-singlet

path for the observed oxidation

: p-Dimethoxybenzene K

Al krnr

to benzaldehyde.

oxygen mechanism,

Sensitized

These observations

possibly electron

transfer

of alkenes. Photooxidation

Alkenesa.

Products

Conversion

(chrtaical

60

yields1 b

Ph-CHO

Ph-

OO)C

+ph-m

PI-CnO

(bo)d

(W

zpm

30

(loo)

YeooC~m

30

-~ 6

a) Acetonitrile is used as solvent; irradiation time I2 h; b) Yields are based on the recovered starting material; c) Oligomeric products accounted for the remainer of consuned starting material; d) Yield is based on the expected formation of 2 moles of benzaldehyde from each mole of stilbene; e) Self-sensitizer. In conclusion, the first

observation

this

work

of reaction

ing the scope and mechanistic REFERENCES

1. 2. 3.

4. 5. 6. 7. 8. 9.

AND

offers

NOTES

a mild

method

under an electron

evaluation

for

oxidative

cleavage

rich arene sensitization.

of DMB sensitization

of

Further

alkenes

and

is

work extend-

is in progress.

:

IICT Communication No. 3155 R. Pottashnik, C.R. Coldshmit and M. Ottolenghi, Chem.Phys.Lett., 1971, 9, 424; Y. Taniguchi, A. Mioshi, N. Mataga, H. Shizuka and T. Morita, Chem.Phys.Lett., 1972, 15, 2223. 1.1. Grossweiner, C.W. Swenson and E.F. Zwicker, science, 1963, 141, 805; 3. Jortner, M. Ottolenghi and G. Stein, J.Am.Chem.!ioc., 1963, 58, 2712; G. Dobson and 1.1. Grossweiner, Trans.Faraday kc., 1965, 61, 708; H.1. Joschek and 1.1. Grossweiner, J.Am.Chem.Soc., 1966, 88, 3261; J. Zechner, G. Kohler, G. Grabner and N. Getoff, Chem.Phys.Lett., 1976, 27, 292; G. Grabner, W. Rauscher, J. Zechner and N. Getoff, J.Chem.Soc.,Chem.Commun. 1988, 222. Photoinduced Electron Transfer, Part D, M.A. Fox and M. Chanon Editors, Elsevier Science Publishers, The Netherlands, 1988, ~241-302. U.T. Bhalerao and M. Sridhar, J.Chem.Soc.,Chem.Canmun., 1993, 115. DMB has UV absorption above 280 nm $.,,., = 289 nm, E = 2600); Recovery of DMB in this reaction is > 80%. W.H. Richardson, F.C. Montgomery, M.B. Yelvington and H.E.0 ‘Neal, J.Am.Chem.Soc., 1974, 96, 7525. G.O. Schencl: and K. Schulte-Ute, Ann. 1958, 618, 185; C.S. Foote, Accts.Chem.Res., 1968, 1, 104; K. Gollnick, AdvamPhotochem., 1968, 6, 1. L.E. Manring, J. Erikson and C.S. Foote, J.Am.Chem.Soc., 1980, 102, 4275.

(Received in UK 13 May 1993)