Conformational control of photochemical behavior. Cyclohexyl phenyl ketone

Tetrahedron Letter6

No. 27, pp 2557 - 2560, 1973.

Pergamon Press.

Printed in Great Britain.

CONFORMATIONAL CONTROL OF PHOTOCHEMICAL BEHAVIOR. CYCLOHEXYL PHENYL KETONE’ Frederick D. Lewis and Rmhard W. Johnson

(Received

in USA 9 April

Department of Chemistry Northwestern University Evanston, Illmom 60201 1973; reoeived in UKfor publication 29 Msy 1973)

Ground state molecular conformations can influence photochemical behavior m cases where excited state reactions are more rapti than conformational momerlzatlon. 2 Our mterest m the role of molecular conformation 111photochemistry led us to reexamine the photochemistry of cyclohexyl phenyl ketone @) and its -CIS- and

-4-i-butyl derivatives (2 and 2).

Previous

mvestlgatlons of J and 2 have revealed mefflclent formation of acetophenone, s presumably via consecutive type II elrminatlons (eq 1,2).

The antmlpated prnnary photoproducts 4 and/or 2

4 have not been ldentlfred.

The relative stablllty

of _j and 2

has been attributed

to the macces-

slbllrty of the v-hydrogen to the carbonyl group m the more stable conformers of _j (equatorial) and 2 (dlequatorml twist boat).)b

The mefflclency of product formation from 1 seemed

surprising for the followmg reasons.

(a) at least several percent of the ground state molecules

2557

No. 27

255s should be m conformer

la and (b) chair-chair

compete with radz&onless

interconversion m the excited state should

decay of conformer le.

Even more surprlsmg are the reported

results for 2 which we would expect to exist m a highly reactive chair conformatlon.4 The conformational preference of the benzoyl group was determined by base catalyzed equilibration of 2 and 3 (eq 3) to be 1.79 kcal/mole

&J&L

at 25’ C.

Tlus value 1s only slightly larger

(3)

mph 3

2 than those for the acetyl (1.52 kcal/molePa

and methyl groups ( 1.7 kcal/mole).5b

Thus

there IS no reason to expect -2 to have a most stable twist boat conformation.4 Irradlatlon of 2 m degassed benzene solution through Pyrex results m the formation of 6

(@= 0.024) as the only observed primary photoproduct (eq 4).6 The low quantum yield could

&Y+-

&pLphA,f$

(1)

2 result from inefficient biradrcal formatlon and/or reversion of the biradical mtermedlate to ground state ketone (k,).

Since alcohol solvents are known to suppress biradlcal reverslon.7

the effect of added 1-propanol on the quantum yield was mvestigated.

The quantum yield

increases with 1-propanol concentration to a value of 0.098 in 8.9 M 1-propanol-benzene. 8 Thus the inefhcrency of product formatlon from 2 1s due at least m part to blradical reversion. as IS the case for several bicycloalkyl phenyl ketones with similar ryld geometrles.s

The

excited state lifetune (T) of 2 was estimated by the usual Stern-Volmer method usmg 1,3pentadlene as the trlplet quencher m 4.5 M 1-propanol-benzene solution.

The value for I/ T =

6.8 x 109 (Table I) is in good agreement with that for endo-2-benzoylnorbornane

(7).s

Irradmtlon of 3 does not lead to the formatron of 6: however the quantum yield for disappearance of 3 (a - 0.1) 1s surprlsmgly high. have not been isolated.

The products are thermally unstable and

Stern-Volmer quenching of the disappearance of 3 m degassed benzene

gives a value of */T = 2.1 x 107 set-i . Thus the triplet llfetune of 3 is 300 times that of 2. Irradiation of I in degassed benzene solution results m the mefflcrent formatlon of 4 and an lsomerlc alcohol, tentatively ldentifled as 5 (# = 0.005). 1o Even at moderate converslons (
CpaUtltles

upon irradiation of 1 m benzene.

Addltlon of I-propanol again

No.

2559

27

Table I.

Quantum

Yield and Kmetic Data for Cyclohexyl @PrOHa

Ketone

2

0.008 0.024 0.10 0.10

2 ;c

Phenyl Ketones k b q

0 13 0.098 0:;s

‘/ , set-1

‘I652.9 0.74 2390.70

61 76xx 109 106 6 8x log 7.1 2.1 x 109 107

aQuantum yield 1118.9 M l-propanol-benzene. bSlope of Stern-Volmer plots where kq 1s the rate constant for triplet quenching by pentadlene, assumed to be 5 x 109 M-1 set-1 ‘Data from ref. 9.

results

m a marked

quantum

increase

equatorial

m competition

expernnents Volmer

less than the quantum

conformer

le* must undergo

with other processes

The formation

yield for 4a (Table I), however,

Since the ground state population

yield for 5 was observed.

la (4-50/o)*t IS significantly excited

mthe quantum

which deactivate

of 4 upon excitation

different

(Figure

slopes,

1).

mdlcatlve

The two linear of product analyslsl

formatlon

state

provides

for la and I/ = 6.6 x 106 set-1

of I/

are m reasonable the product

accord

results

for

excltatlon

the axial conformer

lsomerlzatlon

abstractlon. with

of cyclohexyl

allows

rmg mversion

yields

for product biradlcal

The short llfetnne

-hydrogen

abstraction.

to compete

formation

of the Stern-

from a short

2 of the quenching

2 and 3.

for le.

data These values

Approximately

80% of

of le m 4.5 M 1-propanol-benzene.

phenyl ketone (2) have very short trlplet

-hydrogen

efflclent

with those for the model compounds

from mltial

In conclusion, cyclohexyl

= 1.7 x 109 see-1

the

by quenchmg

portlons

lived (la*) and a long lived (!e’+) excited values

Kinetic

(s_ 0.13),

state

of both la and le was confirmed

m 4.5 M I-propanol-benzene

plot have markedly

formation

followed by biradical

the excited

111the

of the axial conformer

yield for blradical ring mverslon

no increase

llfetlmes

of la

formation

from

reversion

to ground state ketone.

due to their favorable

precludes

In contrast,

with radmtionless

phenyl ketone (la) and -cls-4-.-butyl-

competltlon

the longer

decay processes.

la and 2 in benzene

solution

trlplet

geometries

of conformatlonal llfetlme

of le’

The low quantum

are due at least in part to

It 1s mterestmg

to note that conformatlon-

ally non-mobile excited ketones such as &, _& and _Swhich have geometrically x’-hydrogens, undergo exceptionally rapti v-hydrogen abstraction.

accessible

Furthermore,

the same

non-mobile conformations which enhance blradlcal formatloon also favor biradlcal reversion to ground state ketone, resulting in low quantum yields for product formatlon. 13 11 0 Figure 1. Stern-Volmer

9

plots for quenching of the formation of 4 from .I using high (upper curve) and low (lower curve) concentrations of 1,3-

7

@O

i-

5 i:

: * I

I

X1:01 References

1

:: :02 [ 1,3-Pentadlenel

1 I

x: io3

t

pentad

0.4 0.004

1. The authors thank the donors of the Petroleum Research Fund, admmlstered by the American Chemical Society, and PPG Industries for support of this research 2. F. D. Lewis and R. W. Johnson, 2. Amer. Chem. Sot., !@, 8914 (1972). -3. (a) C. L. McIntosh, Can. J. Chem. ,- 45, 2267 (1967); (b) A Padwa and D. Eastman, J Amer. Chem. Sot., ‘JI.462 m. 67 J H. Stocker and D. H. Kern, -Chem. CommT, -6s.-4. E. L. Ellel, N. L. Allinger, S. J. Angyal, and G A. Morrison, “Conformational Analysis, ” John Wiley and Sons, Inc., New York, 1965, Ch. 2. 5. (a) E L. Ellel and M. C. Reese, J. Amer. Chem. Sot., 40, 1560 (1968). (b) F. A. L. Anet, C. H. Bradley, and C W. Buchanan, m -258 (1971). IR (film), 1680 cm-l, nmr (Ccl, ) b 0.83 (s, 9H), 6. Isolated by silica gel chromatography 1. l-l.9 (m, 4H), 2 l-2.3 (m, lH), 2.82 (t, 2H), 4.7-5.4 (m, 3H), 7.4, 7.9 (m, 5H). Chem. SOC., @, 7. P. J. Wa er, I. E. Kochevar, and A. E Kemppamen,_J. Amer. --7489 (1978 8 The quantum yield 1s still increasing with added 1-propanol at this concentration 9. F D. Lewis, R. W. Johnson, and R. A. Ruden, J Amer. Chem. Sot , 94, 4292 (1972). -10. Preparative photolysls of 1 m 1-propanol followedby silica gel chromatography gave 4 spectral properties identical to those of an authentic sample prepared by the Grlgnard reaction of benzonltrlle with 4-pentenylmagnesmm bromide. Photolysls of 1 m benzene followed by chromatography gave 5 IR (fdm), 3450 cm-i, nmr (Ccl, ) C 0.8-2.0 (m, 10H), 4.2 (s, lH), 7.2 (m, 5H). The spectral propertles of 3 were not the same as an authentic sample of cyclohexylphenylcarbmol