5879
J. Am. Chem. SOC. 1995,117, 5879-5880
Stereospecific Hydrogen Rearrangement of a 1,4 Cation Radical
Scheme 1 l
Thomas A. Zona and Joshua L. Goodman*
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Department of Chemistry University of Rochester Rochester, New York 14627
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Received February 13, 1995 We recently reportedlathat the photooxidative decomposition of meso- and dl-3,6-diethyl-3,6-dimethyl1,2-diazacyclohexene (1) in benzene yielded 1,2dethyl-1,2-dimethylcyclobutannes(9) stereospecifically (Scheme 1).2 The reaction involved the initial loss of nitrogen from the azo cation radical to form the 1,4 cation radical 2. Under the reaction conditions, 2 is reduced to the 1,4 biradical3, which then either ring closes or cleaves to yield 2-methylbutene (8). The stereospecificity of the reaction is due to the fast combined rates of reduction and ring closure relative to C-C bond rotation. However, if 1 is oxidized in polar solvents where ion pair separation occurs, the subsequent reduction of 2 is slow and a 1,5 hydrogen rearrangement occurs to afford the isomeric cation radicals of olefins 4 and 5. In an effort to determine the relative lifetime of 2 under these conditions, we now report mechanistic details of this reaction using stereochemically and isotopically labeled substrates. In this regard, we have found that the hydrogen transfer occurs stereospecifically and with high regioselectivity, which suggests that although the reaction is exceedingly fast, it is highly selective. Our initial stereochemical studies were designed to probe the relative rates of the 1,5 rearrangement of 2 and C-C bond rotation using enantiomerically enriched 1 (Scheme 2). If the rate of C-C bond rotation of 2 to form 2', k,, is competitive with rearrangement, kH, then the rearrangement olefin should be a mixture of enantiomers 4 and 4'. Optically active (+)-1 was obtained by the resolution of the precursor 3,6-dimethyl3,6-diaminooctane with L-tartaric a ~ i d . ~The . ~ absolute configuration of the diamine was determined to be 3(R),6(R)by X-ray crystallography of the diamine-D-tartrate ~ o m p l e x .Irradiation ~ (1 > 420 nm) of dicyanoanthracene (0.5 mM) in a degassed acetonitrile solution containing cosensitizer biphenyl (0.2 M) and (+)-1 (0.01 M) at 20 "C affords a mixture of trans- and cis-3,6-dimethyl-2-octene (4,79%), 2-ethyl-5-methyl-1-heptene (5, 5%), and 3,6-diethyl-2,3,6-trimethyl-l-dehydropiperidine (1) (a) Zona, T. A,; Goodman, J. L. J . Am. Chem. SOC. 1993,115,4925. (b) Zona, T. A.; Goodman, J. L. Tetrahedron Lett. 1992, 33, 6093 (2) For previous work on the oxidation of azoalkanes, see: (a) Engel, P. S.; Robertson, D. M.; Scholz, J. N.; Shine, H. J. J. Org. Chem. 1992, 57, 6178 and references therein. (b) Bae, D. H.; Engel, P. S.;Hoque, A,; Keys, D. E.; Lee, W.; Shaw, R. W.; Shine, H. J. J. Am. Chem. SOC. 1985, 107, 2561. (c) Engel, P. S.;Kitamura, A,; Keys, D. E. J. Org. Chem. 1987, 52,5015. (d) Mendicino, M. E.; Blackstock, S. C. J. Am. Chem. SOC. 1991, 113,713. (e) Adam, W.; Grabowski, S.;Miranda, M. A.; Rubenacker, M. J. Chem. SOC., Chem.Commun. 1988, 142. (f) Adam, W.; Dorr, M. J. Am. Chem. SOC. 1987,109, 1570. (g) Shine, H. J.; Bae, D. H.; Hoque, A. K. M. M.; Kajstura, A.; Lee, W. K.; Shaw, R. W.; Soroka, M.; Engel, P. S.;Keys, D. E. Phosphorus Sulfur 1985, 23, 111. (h) Adam, W.; Walter, H.; Chen, G.-F., Williams, F. J. Am. Chem. Soc. 1992, 114, 3007. (3) Optical Resolution Procedures for Chemical Compounds; Newman, P . , Ed.; Manhattan Press: Riverdale, NY, 1978; Vol. 1, p 542. (4) The di-Mosher amide derivative was 86% R,R; 9% R,S; and 5% S,S, as determined by 500 MHz IH NMR analysis of the amide protons at 6.56.6 ppm. Product olefins derived from this mixture could have a maximum enantiomeric excess of 81%. The yield of the amide was typically >90%. The d 1:meso ratio was checked for azo compound 1 by GC and NMR and found to be in good agreement,
