Stereoselectivity reversal of a photochemical reaction in the solid state

oo40-4039/85 $3.00 + .oo 01985 Pergamon Press Ltd.

Tetrahedron Letters,Vo1.26,No.48,nD 5903-5906,1985 Printed in Great Britain

STEREOSELECTIVITY

REVERSAL

OF A PHOTOCBEMICAL

REACTION

Stephen Evans, Nalamasu Omkaram, John R. Scheffer* Department of Chemistry, University of British Columbia,

IN THE SOLID STATE

and James Trotter* Vancouver, Canada V6T lY6

The stereoselectivity of Norrish type II cyclobutanol formation resulting from Abstract. photolysis of a-adamantyl-p-methoxyacetophenone is altered in favor of the more hindered cis isomer as the reaction medium is changed from isotropic liquid phases (benzene or acetonitrile) to Based on the reactant X-ray crystal structure, it is suggested that the interthe pure crystal. mediate 1,4-biradical in the solid state is born in, and restricted to, a conformation which is In the relatively unrestricted solution ideal for direct closure to the more hindered product. environment, however, conformational isomerism of the biradical is faster than closure, thus leading to a predominance of the less hindered trans cyclobutanol. Dedicated

to Professor

Of the several ways in which compared

to isotropic

of biradical between

intermediates.

The compound

have shown that the competition

type II reaction

of biradical

in favor of the less motion hindered

closure

pathway

is shifted

strongly

Similar

(cyclobutanol

formation)

by the solid state medium,

even

product.

studied was a-adamantyl-p-methoxyacetophenone

by Friedel-Crafts

(2,

Scheme I), mp 80-81

'C,

of anisole with adamantyl acetyl chloride. The choice of 5 lb was based on the work of Lewis, Johnson and Kory who showed that photolysis of

irradiation

acylation

(2)

with the output

and the more hindered The quantum yields

in solution

(obenzene

in benzene

Cyclobutanol (mp 110-112

for ketone

the presence

exclusively analogously,

the cyclobutanols affording

upon

laser (337 nm), the less hindered cyclobutanol 2b 3b in a ratio of 2.6 in benzene and 2.0 in acetonitrile. !? 2IL measured at 313 nm using valerophenone actinometry, were 0.08

assigned

was found to undergo

"C) upon attempted

than the alternative

l$, behaves

from a nitrogen

cyclobutanol

and its structure 2

= 0.04) affords

We find that ketone

lb m' and 0.12 in moist acetonitrile.

chromatography

indicated

reactivity

its effect on the behavior

by solid state immobilization.'

the stereoselectivity

and 3a3a(ratio 1.3 in benzene).

rather

process,

involves

from our laboratory

in the Norrish

gives the more sterically

a-adamantG:acetophenone 2

phase can alter chemical

from studies of the Norrish type II reaction in other organized 2 3 4 inclusion complexes, seolites and liquid crystals. In this communication

significantly

this pathway

compound

studies

and closure

the less motion

that, in addition,

can be altered

prepared

crystalline

of his 65th birthday.

have been reached

media such clathrate

though

on the occasion

one of the most intriguing

Previous

cleavage

in favor of cleavage,

we report

the organic

liquid media,

1,4-biradical

conclusions

Harry H. Wasserman

Photoproduct unambiguously

facile dehydration

chromatography.

regioisomer

of a molecular

2

plane of symmetry.

5903

by silica

gel column

spectroscopic

to the cyclobutene

The identity

was established

was separated

by conventional

derivative

methods. c

of the dehydration product as 4b I?/ir 13 by proton and C NMR spectroscopy, which 7

5905

In contrast afforded

This represents reasons ketone which

to the solution

the more hindered a complete

The results

to our previous

related

This distance

is consistent

geometry

is preferred It also differs

in the solid state.1

a The carbonyl oxygen to Ha distance for ketone E that the approximate

of the

is 2.66 1.

upper limit for hydrogen

is the sum of the van der Waals radii of the atoms involved, 2.72 d in the case of 10 The oxygen to Hb distance, at 2.96 & is well above this limit, and the

for ketone lb. abstraction

accessible

y-hydrogen

is not a prerequisite

conformation emerges.

as its ketonic

atom (Hc) also lies outside

atom Ha lies outside

This value corroborates

If we make the reasonable

our previous

for the success

assumption

precursor,

that the 1,4-biradical

essentially biradical

the 90,O geometry

2b, however,

requires

C(2)-C(3)

bonds.

C(l)-C(2)

bond.

of the Norrish

One such process

consists

This leads to conformer of cyclobutanol

is born in a conformation

stereoisomer

2b requires

medium."

%rlr The packing

neighbors

have parallel

bond rotation.12

diagram

However,

either by a preference

liquid

phase of n-butyl

crystalline

cyclobutanol

complex

Both Hrovat,

stereoisomer

increased

On the other hand, Goswami,

is retarded

or

the There-

production

of

by the crystal lattice Nearest

upon attempted

in solution,

lattice

C(l)-C(2)

and if, as seems

of photoproduct

B (less hindered)

z

in this

over conformer

A

of B or both. stereoselectivity

results with those obtained

and Casal, et. al. 2b (photolysis

found that the proportion in proceeding

et. al, 2a (Norrish

cyclobutanol

the C(l)-C(2)

the fact that its biradical

et. al.4 (photolysis of lo-nonadecanone

stearate)

of 5-nonanone)

of this

to form E.

whereas

the predominance

for conformer

our cyclobutanol

media.

urea inclusion

which

are absent

in solution,

ratios

of the aryl group around

that this is the case.

interactions

are present

in other organized

about either

in turn closes

rings which would clash violently

these steric

to compare

Direct closure

suited for its formation,

isomerization

atom

has the same basic

of the less hindered

in the solid state reflects

or a faster rate of cyclization

It is interesting

A).

of a 180' rotation

(Scheme I) indicates

aromatic

can be explained

z

ideally

is 59.4'

angle for ketone lb is 83.3'. ?f% of hydrogen atom Ha will have

A by rotation

B (Scheme I), which

a conformational

these same conformers

(more hindered)

of confozzer

hydrogen

for the cyclobutanol

Formation

3b.

group,

dihedral

I (conformation

cyclobutanol

The

(3.17 i).

type II reaction. 1,4-hiradical

explanation

from abstraction

shown in Scheme

prior isomerization

fore, the predominance precursor

resulting

leads to the more hindered

that coplanar

conclusion'

that the intermediate

a plausible

the limit

the mean plane of the carbonyl

The X-ray data show that the O(l)-C(l)-C(2)-C(3)

This means

medium

This stands in

Dalton and Turro for the type II photoreaction

by Gagosian,

with our suggestion'

T, by which the hydrogen

likely,

(Scheme I) in

six atom arrange-

carbon atoms.

abstraction

of

and oxygen.

only other sterically angle

showed that a boatlike

structure

adopts a conformation

forms a chairlike

of a-cyclohexylacetophenones

cr-adamantylacetone.

atom abstraction hydrogen

assumed

W

of ketone lb V% ratio being 0.5.

To probe the

in the solid state.

oxygen atom and the intervening

studies which

type II reaction

from the conformation closely

carbonyl

samples

the c/z

the X-ray crystal and molecular

atom, H a, is that which

y-hydrogen

ment with the abstracting

for the Norrish

of stereoselectivity

(P21/n, R = 0.051) show that compound

the most accessible

contrast

reversal

of crystalline

isomer ,3b_as the major product,

change, we determined

for this dramatic lb.

laser irradiation

results,

cyclobutanol

of the crystalline

of the less hindered

from the less ordered

type II reaction

in the smectic

(trans)

to the more ordered

of the crystalline

complexes

phase. formed

5906

between

aralkyl

ketones

in the pure crystalline

compared

ketones

to the solution

Acknowledgement. for financial 1. 2.

and Dianin's

compound)

as well as Ariel, et. a1.l (irradiation

state) noted little difference

results.

Clearly,

not all organized

We thank the Natural

Sciences

in cyclobutanol media exert

and Engineering

of type II

stereoselectivity

the same effect.

Research

Council of Canada

support.

S. Ariel,

V. Ramamurthy,

(a) P.C. Goswami, -Can. J. N., Chem. =.,

105,

and J. Trotter, 2. -Am. Chem. z.,

P. de Mayo, N. Ramnath,

in press;

3.

N.J. Turro and P. Wan,

4.

D.A. Hrovat,

5.

F.D. Lewis,

6.

Similar

G. Bernard,

N. Omkaram,

a slight decrease

of type II systems Tetrahedron

trapped

E.,

_, 25

in the fragmentation

in urea inclusion

decreases

Kelso, A.E. Kemppainen,

in the stereoselectivity

to cyclization

complexes.

H.N. Schott

106,

7033 (1984).

96, 6100 (1974).

of cyclobutanol

have been noted previously

J.M. McGrath,

and Y-F. Wong,

3655 (1984).

R.W. Johnson and D.R. Kory, --J. Am. Chem. S&.,

in solvent polarity

6959 (1983).

and J.C. Scaiano, 2. -Am.

J.H. Liu, N.J. Turro and R.G. Weiss, 2. -Am. Chem. &.,

slight

105,

J.R. Scheffer

(b) H.L. Casal, P. de Mayo, J.F. Miranda

5155 (1983) observed

ratio upon photolysis

increases

J.R. Scheffer

formation

and discussed.

accompanying

P.J, Wagner,

and R.G. Zepp, 2. -Am. Chem. &.,

P.A.

96,

7506 (1972). 7.

The cyclobutanols

formed by photolysis

regioselectivity.

R.B. Gagosian,

of 1-adamsntylacetone

also dehydrate

with this

J.C. Dalton and N.J. Turro, 2. -Am. Chem. g.,

92, 4752

(1970). 8.

R.B. Gagosian,

J.C. Dalton and N.J. Turro, 2. -Am. Chem. %.,

assume that it is hydrogen chemistry

of o-adamantylacetone,

J. Am. Chem. e., ---

10.

A. Bondi, J. phys.

for more hindered

A.

12.

(cis) cyclobutanol

and C.K. Hu,

105,

5354 (1983).

thus resulting

formation from the singlet excited state a is nicely accommodated using

Dalton and Turro

of conformer

A in its singlet

in a large cis/trans

ratio from the longer-lived,

much lower

These authors

work on the photo-

J.A. Whittle

and L. Walsh, 2. -Am. Chem. G.,

noted by Gagosian,

The shorter lifetime

A to B isomerization, cis/trans

(1975).

Chem., 68, 441 (1964).

of 1-adamantylacetone conformation

97, 5189

For additional

see R.R. Sauers, M. Gorodetsky,

Z.Q. Jiang, J.R. Scheffer

W.K. Appel,

The preference

is abstracted.

93, 5520 (1971).

9.

11.

atom Hb which

conformationally

ratio

state precludes

(4.9 in benzene).

equilibrated

much The

triplet biradical

is

(1.0 in benzene).

Preliminary

studies from our laboratory

derivatives

which lack this type of packing arrangement

stereoselectivity

reversals

indicate

that a-adamantyl-p-substituted do not show significant

in their solid state photochemistry.

(Received in USA 14 June 1985)

acetophenone cyclobutanol