4346
Photochemistry of Bicyclo[3.2.0]hept-3-en-2-ones1" Robert L. Cargill,* lb A. Bradford Sears,lb Jeffrey Boehm,lb and M. Robert WillcottlC
Contribution from the Departments of Chemistry, University of South Carolina, Columbia, South Carolina 29208, and University of Houston, Houston, Texas 77004. Received November 17, 1972 Abstract: Irradiation of bicyclo[3.2.0lhept-3-en-2-onesyields 7-ketonorbornenes via a nonconcerted, diradical path which is superficially a [1,3]sigmatropic change. When the irradiation solvent is methanol an additional set of products, methyl 2-~yclohexenecarboxylates, are formed. These esters arise via a concerted, intramolecular [2 21 cycloaddition leading to a cyclopropanone and subsequent decomposition of the hemiketal of the cyclopropanone. Evidence upon which the mechanistic conclusions are drawn is presented.
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A
lthough cyclopentenones commonly undergo a variety of bimolecular cycloaddition reactions, the mechanisms of which have been studied extensively, the intramolecular cycloadditions of cyclopentenones have received much less attention. There are six pos21 cycloadditions (excluding sible intramolecular [2 hydrogen transfer processes) of y-substituted cyclopentenones such as 4-methyl-2-cyclopentenone. The olefinic ?r bond may in principle undergo cycloaddition in either of two directions with each of three allylic c bonds, C-1-C-5, C-4-C-5, and C-4-C-6. For example, 4,6) new bonds are formed between in path a (2,3 C-2 and C-6, and between C-3 and C-4, whereas in 6,4) new bonds are formed between C-2 path d (2,3 and C-4, and between C-3 and C-6. These processes are illustrated in Scheme I.
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Scheme I
(5 A
12.3 + 4.61 path a
12.3
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5,4l
path e
E
D
12.3
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6,41
/
5b2 4 12.3 + 4.51
path b
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B
[2.3
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5.11
C
Numerous examples of isomerization via path a are known in both cyclopentenones and cyclohexenones, the most famous example being the verbenone-chrysanthenone rearrangement.4 One isomerization of a (1) (a) Grateful acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research; (b) University of South Carolina; (c) University of Houston. (2) (a) E. J. Corey, J. D. Bass, R. LeMahiew, and R. B. Mitra, J . Amer. Chem. SOC.,86, 5570 (1964); (b) W. L. Dilling, Chem. Reu., 66, 373 (1966); ( c ) P. G. Bauslaugh, Synthesis, 2, 287 (1970); (d) P. de Mayo, Accounts Chem. Res., 4,41 (1971). (3) (a) W. L. Dilling, T. E. Tabor, F. P. Boer, and P. P. North, J . Amer. Chem. Soc., 92, 1399 (1970); (b) P. de Mayo, J. P. Pete, and M. F. Tchir, Can. J . Chem., 46, 2535 (1968); (c) P. E. Eaton, Accounts Chem. Res., 1, 50 (1968); (d) R. L. Cargill, A. C. Miller, D. M. Pond, P. de Mayo, M. F. Tchir, K. R. Neuberger, and J. Saltiel, Mol. Photochem.. 1, 301 (1969).
cyclopentenone via path b has been demonstrated by C h a ~ m a n and , ~ Agosta and Kende have found appropriately substituted cyclopentenones to rearrange via path c . ~Path d is followed by several substituted cyclopentenones,' but we are unaware of any cyclopentenones which undergo isomerization via paths e or f. We describe here our studies of the photochemistry of bicyclo[3.2.0]hept-3-en-2-ones which undergo intramolecular [2 21 cycloadditions via both paths a and b.8 We shall discuss first evidence which establishes the nonconcerted nature of path a, then experiments which indicate the concerted nature of path b. Irradiation of ketone l a in pentane, methylene chloride, or methanol gave the anti-head-to-tail dimerg and 7-ketonorbornene (3a) in 10-20% yield. The photQisomerization of 1 to 3 is a simple example of an allylic shift in which the migrating atom is carbon and which is restricted to being suprafaciaL4 If this transformation is concerted the conservation of orbital symmetry requires that the stereochemistry of the migrating carbon be retained.10 On the other hand, isomeriza-
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(4) (a) J. J. Hurst and G. W. Whitham, J . Chem. Soc., 2464 (1960); (b) W. F. Erman, J. Amer. Chem. Soc., 89, 3828 (1967); see also (c) H. E. Zimmerman and D. J. Sam, ibid., 88, 4905 (1966); (d) H. E. Zimmerman and R. L. Morse, ibid., 90,964 (1968); (e) R. F. C. Brown, R. C. Cookson, and J. Hudec, Chem. Commun., 823 (1967); (f) R. C. Cookson and D. C. Warrell, J . Chem. SOC.C, 1391 (1967); (9) R. C. Cookson, Quart. Reo., Chem. Soc., 22, 423 (1968); (h) L. A. Paquette, G. V. Meehan, and R. F. Eizember, Tetrahedron Lett., 995, 999 (1969); (i) H. Prinzbach, H. Hagemann, J. H. Hartenstein, and R. Kitzing, Chem. Ber., 98, 2201 (1965); (j) P. J. Kropp, J . Amer. Chem. Soc., 89, 1126 (1967). ( 5 ) L. L. Barber, 0. L. Chapman, and J. D. Lassila, J . Amer. Chem. SOC.,91, 3664 (1969). (6) (a) W. C. Agosta, A. B. Smith, A. S . Kende, R. G. Eilerman, and J. Benham, TetrahedronLett., 4517 (1969); (b) W. C. Agosta and A. B. Smith, J . Amer. Chem. SOC.,93, 5513 (1971). (7) (a) D. A. Plank and J. C. Floyd, Tetrahedron Lett., 4811 (1971); (b) T. Matsura and K. Ogura, Bull. Chem. SOC.Jap., 43, 3187 (1970); (c) T. Matsura and I
