Tetrahedron Letter6 No. 13, pp 1041 - IObk, 1973. Pergamon Press. Printed in Great Britain.

THE STEREOCHEMICAL FATE OF THE DIRADICAL INTERMEDIATE IN THE PIIOTOLYSIS

OF CYCLGDODECEN-2-ONE

A. IuIarchesi~&~~ G. Pagani.*’ and U. M. Pagnoni+

‘Istituto di Chimica Organica dell’universita, via Campi 183, Modena, Italy

*Northwestern University, Department of Chemistry, Evanston. Illinois 60101 U.S.A.

(ticeived in UK 8 PebruarJ 1973; accepted for publication 14 February 19’73)

cis-Cyclododecen-3-one QfJ) was reported3 to be the only photoprodrrct of -

dodecen-a-one (I). This result implies that: i) the diradical intermediate y) formed by v -

hydrogen abstraction of cis-cyclododecen-2-one (rr) evolves stereoselectively to the trans. cis- - --

cyclododeca-1.3-dienol &J), which in turn tautomeriaes to (BE); ii) the cl&cyclododecene-3- -

one QJJ) is unusually stable to U.V. irradiation. However, a, B-unsaturated ketones are known4

to suffer U.V. promoted v-hydrogen abstraction to give mixtures of the isomeric cis and trans - -

8. y-unsaturated ketones; in these compounds5 and in their y, R-analoguese the cis-trans --

interconversion occurs readily.

In the effort to reconcile Noxaki’s unprecedented results with the above cited reports,

we investigated more closely the photochemical behaviour of the -cyclododecen-a-one.

trans-Cyclododecen-2-one [(JU, 5% in cyclohexane, 50 ml1 was irradiated (internally,

Pyrex apparatus thermostatted at 20’ C) with a 125 watt high pressure Hg lamp: after 8 hr

both the cis- and the trans-cyclododecen-3-one @I> and @J were formed (yield >90%*) in a

84: 16 ra; by v.p.c.** (all glass analytical apparatus: silanixed column, 2 m, 3% NPGS on

*A high melting material, presumably a dimer, was also obtained in-l% yield. **

The analytical v.p.c. conditions under which the crude reaction mixture was reported3 to

give a single peak were inadequate to detect successfully the trans-isomer: in fact the

Properties reported for the ketone @J) are consistent with a mixture of (m) and a).

1041

1042 No. 13

silanixed Chromosorb W, 80-100 mesh, 150’). Four fractional crystallizations from n. pentane

afforded the cis-isomer in a pure state [(ra), m.p. 50, lit.3 39; I. R. (nujol): vC=C 1698 cm-i;

hEtCH 95% - max 289 (log E = 1.77); iHnmi (CDCl,): 4.26 T, dtt HP, 4.58 T, dtt HV, 6.80 T, dd

Ha. 7.59 m, Ha,, 7.85 7 m -CH, -C=, 8-9 7 (10H) CH, : SJBv 11, 9JaB = 3Jvb = 7.5, 4 Jga =

4J av = 1.251. Fractional crystallization of the residue from mother liquors of (UI) afforded in

low yield the trans-cyclododecen-3-one [@), m.p. 28-29; I. R. (film) vc=C 1705 cm-i, vczc

1660 cm-*, v( H\

CZC,~) 978 cm-‘; XEFxH g5w 294 (log E = 2.75); iH nmr (CDCl,): 4.36 T

dtt H6, 4.62 7 dtt Hy, 6.98TddHa, 7.547 mH,,, 7.94 T m -CH, -C=, 8-9 m (10H) CH, ;

3Ji% = 15.2, aJae = sJYb = 7, 4Jps = 4Jav = 0.7 Hz].

The observation that the cis-6, Y - -unsaturated

C 84% 16sH

11 rv

ketone m) isomerized (-80%) into the

trans-8, Y -ketone a) when injected in a v. p. c. apparatus working on preparative conditions

enabled us to obtain substantial amounts of mixtures rich in (Iy): from these mixtures trans-

cyclododecen-3-one a) could be obtained in moderate yield by several fractional crystal-

lixations.

trans-Cyclododecen-3-one a) underwent conversion into cis-cyclododecen-3-one (rs) -

upon irradiation (1% cyclohexane, 50 ml) in the same apparatus as before. After 6 hr a photo-

stationary mixture of (HI) to a) in the ratio 84: 16 was obtained (Fig. 1). Irradiation of pure

(I@ led to the same steady state mixture.

*At 100 MHz, first order analysis

No. 13 1043

I -I

I _I

I

I

- I I

- I

I

1

I - I

1

- I

I - \

\ ‘,_----

1 I I I I 0,2345676

1

hrs

2

hrs

progress of photochemical isomerization -L $J+ was monitored

periodic withdrawal aliquots and them by v.p.c. (Fig. without

elimination the solvent. low conversions concentration of -

(rI)3 to a value of (45 min): product undergoes conversion

into mixture of and (nr) is richer (UL) (30% 2 hrs) the photostationary

mixture (16% 8 hrs).

foregoing results that: i) the cis- trans-cyclododecen-3-ones @J) -

and @) are not photochemically stable but are capable of interconversion to reach a photo-

stationary state. This is sound on considering that intramolecular energy transfers in B,Y-

unsaturated carbonyl compounds are a common process ii) the -cyclododecen-3-one t@)

is formed directly from the cis-cyclododecen-2-one @I) via the diradical intermediate &); -

Scheme 2 ?

1044 No. I)

iii) the bis allylic diradical (V) is an intermediate common both to (I@) and to (IV); iv) the

stereochemical fate of the diradical intermediate &) is not greatly influenced by the conforma-

tional preferences’ of the medium ring. In the diradical (V) free rotation is expected both at

the 5 and the 5 bond. Accordingly the dienol @I) arises (Scheme 2) by counterclockwise

rotation of the C,, carbon p orbital while the dienol (VA) arises .through a clockwise rotation of

the C, carbon p orbital associated with the clockwise 180” rotation of the C, carbon p orbital:

judging from models the two rotations are probably simultaneous since the trans, trans-cyclo- --

dodeca-1,3-dienol has very little chance for formation.

Acknowledgment, One of us (G. P. ) is greatly indebted to Frederick D. Lewis for

suggestions and discussion.

1.

2.

3.

4.

5.

6.

7.

References

Authors to whom quieries should be addressed.

On leave from the University of Milan, Italy.

II. Nozaki, T. Mori. and R. Noyori. Tetrahedron, 22. 1207 (1966).

N. C. Yang and M. J. Jorgenson, Tetrahedron Letters, 1964, 1203; G. A. Carlson

and J. H. Bateman. $&, 1967, 4151.

H. Morrison, ibid., 1964, 3653; K. Takeda. J. Horibe. and H. Minato, Chem. -

Comm. , 1971, 87; D. 0. Cohen and A. A. Baum, 1. Amer. Chem. Sot., 5% 2153 ---

(1970).

H. Morrison, 2. Amer. Chem. Sot., 87, 932 (1965). ---

Conformational controls on the fate of the diradical intermediate have been discussed

in acyclic systems: see e.g., F. D. Lewis and R. W. Johnson,_J. Amer. Chem. So>. ,

94, 8914 (1972) and references cited therein.