The temperature dependent photochemistry of phenyl azide in diethylamine

Tetrahedron Printed in

Letters,Vol.26,No.l8,pp Great Britain

2147-2150,1985

0040-4039/85 $3.00 + .OO 01985 Pergamon Press Ltd.

THE TEMPERATURE DEPENDENT PHOTOCHEMISTRY OF PHENYL AZIDE IN DIETHYLAMINE Elisa Leyva and Matthew Chemistry

Department,

S. Plats*'

The Ohio State University,

Columbus,

Summary: The photochemistry of phenylazide in diethylamine consistent with electrophilic singlet intermediates to products nitrenes as the temperature is lowered from 273 to 77K.

The photochemistry

of phenylazide

1 has been studied

and has been shown to be highly

complex.

photolysis

leads to the formation

of _1 in diethylamine

This chemistry

is best rationalized

cies such as 3 and/or with solvent

4.

by the intermediacy

The products

expected

8 and _9, azobenzene

[anilines

Low temperature

In 1966 Doering

spectroscopic

but a full understanding

Ohio

43210*

changes from products consistent with triplet

by chemical

and physical

methods

and Odum discovered

of 3H azepine

that the 2 5 in good yield.

of an electrophilic

for the reaction

of triplet

singlet

spe-

nitrene

2T _-

61 were formed in only trace amounts.

methods have been applied extensively

of azide photochemistry

is lacking.3

to aryl asides

In 1962 Wasserman

photo-

lyzed phenylazide at 77K in a variety of glasses and obtained the EPR spectrum of triplet 4 A few years later Reiser5 and coworkers followed the photolysis of phenylnitrene. phenylazide

at 77K by UV spectroscopy.

pentane-alcohol

glass

of a new UV spectrum. basis of Wasserman's at 10K by infrared assigned

The latter EPR study.

spectroscopy

spectrum

3.

given contradictory Reiser's

spectrum

'22 partly on the

In later work Dunkin

and Thompson7

from photolysis

for Reiser's

derived

the IR

of 1 and 2

and LeRoux it is not clear what species of phenylazide

IJV spectrum.

results and interpretations.8

observed

at 77K.

It is not clear

Flash photolysis

studies have

The most recent study prefers

to assign

as due to 3.

To shed further 1 at low temperature listed

to the nitrene

studied

and azirines

formed upon photolysis

is responsible

and the appearance

the photochemistry of 1 in argon -1 an intense band at 1895 cm which was

Based upon the IR work of Chapman

?T or 3) is predominantly what specie(s)

was assigned

and observed

to azacyclooctatetraene

of 1 with 313 nm light in an ether-

of the aside spectrum

In 1978 Chapman6

spectra of both azacylooctatetraenes naphthylazide.

Irradiation

led to the dissappearance

light on the identity(ies)

of the specie(s)

we have studied

chemistry

the matrix

in the Table, as usual the percent

lower in a polycrystalline

conversion

solid than in solution

2147

formed upon photolysis

of phenylazide.

of the starting

The results

material

(the m-p. of diethyl

is much

amine is 227K).

of are

2148

The data clearly

shows that there is a change in product

At 273 and 195K the only significant considerable

amounts

of azobenzene

to 77K the yield of azepine

are formed

drops to zero!

mine at 77K does not produce yields

product observed

(-73%) of triplet nitrene

derived

do not occur at 77K but at a higher

at 77K.

Triplet

phenylnitrene

(over many minutes)

measured

by EPR.

at 120K.

hours prior to thawing and chemical phenyl nitrene

amine.

It

versibly

dramatic similar

occurred

analysis.

cross to triplet

of phenylazide

dependence

This insured

reported

conditions

that all of the chemistry

below the melting

of

point of diethyl

species are formed upon photolysis

of 1 at 77K

Any 3 or 4 that is formed at 77K must irreprior to warming

temperature

of the product distribution

matrix

at 140K ('cl12 < 1 min) as

at 77K were kept frozen at 195K for 24

phenylnitrene

at the higher

to work with carbenes

low temperature

to very high

The reactions

very rapidly

at a temperature

inert in the matrix.

intersystem

photolysis

It decays

is possible that electrophilic

but are chemically

in frozen diethyla-

but leads instead

temperature

Samples which were photolyzed

triplet

of phenylazide

species

of triplet nitrene _2T as the EPR signal of 2T is stable "_ is stable in solid diethylamine even at lOOK, but decays

probably

slowly

relative

products.

with temperature.

5. At 173 and 153K . to azepine 5. Upon further cooling

Photolysis

3 or 4 as trapable

distribution

is aaepine

with temperature

by Moss' and Tomioka. 9

enhance

triplet

to 195K as direct

gives almost entirely

carbene

reported

azepine herein

These workers

chemistry

relative

5.

The

is

found that to singlet

chemistry.

Table

1.

The distribution of products in diethylamine as a function

Temp (K)l'

Phenylazide

formed upon photolysis of temperature.

Aniline -_

1

9

Asepine PI

273a

5

of phenylazide

Aaobensene

6

80%

195b

5%

76%

l-32

173c

67%

9%

3%

153c

78%

3%

7%

77d+=

7%

35%

22%

21%

(b) 4 hours photolysis; (a) 2 hours photolysis at 3500#, rayonet reactor; (c) 3.5 hours photolysis; (d) 5 hours photolysis. Note the product ratios are insensitive to the length of the photolysis; (e) After 5 hours photolysis the sample was warmed to 195K where it was kept at these temperatures for 24 hours before thawing.

These results

can be interpreted

scheme has been proposed lysis of _' leads

earlier,

to singlet

(kl) and irreversible

in several ways in terms of Scheme

see references

phenylnitrene

intersystem

crossing

6 and 8 and references

22 which partitions to the triplet

between

2T which

1.

(Note this

therein.)

valance

Photo-

tautomerism

is lower in energy

than

2149

2S, 3 and 4. ..- -

It is possible

that 3 (or 4) is formed at 77K in the diethylamine

can not react with nucleophiles of the matrix

experiment

some temperature

between

observed

products.

as the temperature pheuyluitreue

(knuc Z 0).

matrix

but

On the long timescale

3 will eventually convert to the ground state triplet nitrene, at I. 77 and 195K. The triplet nitrene will then ultimately give the

increase singlet

under these conditions

Alternatively

is differeut

in IR aud UV spectroscopy.

the ratio of kisc/kl

is lowered.

iu diethylamine

Experiments

Scheme

1

I

RH 1

H,i

dependent

that the partitiouiug

than iu the less reactive

are currently

possibilities.

2T N

may be temperature

It is even possible

6 N

iu progress

and of

matrices

to distiuguish

these

used

2150

Ref ereuces

1. Alfred P. Sloan fellow aud Camille aud Heury Dreyfuss, teacher, scholar. 2.

W. Vou E. Doeritlgaud R. A. Odum, Tetrahedrou (1966) _22,81.

3.

M. Platz, "Asides aud Nitreues" E. V. Scriveu ed., Academic, Sau Diego, CA (1984).

4.

E. Wasserman, Prog. iu Phys. Org. Chem. (1971) 8, 319.

5. A. Reiser aud V. Frazer, Nature (Loudou) (1965) 208, II_ 682. 6.

0. L. Chapmau aud J. P. LeRoux, J. Am. Chem. Sot. (1978) __I 100, 282.

7.

I. Duukiu aud P. C. P. Thomson, Chem. Comm. (1980) 499.

106, 5228 aud refereuces a. A. K. Schrock aud G. B. Schuster, J. Am. Chem. Sot. (1984) _-thereiu. 9.

100, 4475 aud references see R. A. Moss aud M. A. Joyce, J. Am. Chem. Sot. (1978) _-?. thereiu.

10.

see H. Tomioka, N. Hayashl, Y. Izawa, V. P. Senthilnathau aud M. S. Plats, J. Am. Chem. Sot. (1983) 1,lI5, 5053 aud references therein.

11.

All aualyses were performed ou a Hewlett Packard 58308 GC usiug a 6' SE-30 columu. The products were identified by coiujectiou with autheutic samples aud GC-MS. The error is +_3%. (Received

in USA 8 January 1985)