1-Phenyl-1,2-cyclohexadiene: Astoundingly High Enantioselectivities on Generation in a Doering-Moore-Skattebol Reaction and Interception by Activated Olefins

The resolution of (1 alpha,5 alpha,6 alpha)-6-bromo-6-fluoro-1-phenylbicyclo-[3.1.0]hexane (rac-5) provided the enantiomerically pure precursors (-)-5 and (+)-5 of 1-phenyl-1,2-cyclohexadiene. On treatment of (-)-5 with methyllithium in the presence of 2,5-dimethylfuran, the pure (-)-enantiomer of the [4+2] cycloadduct of 2,5-dimethylfuran onto 1-phenyl-1,2-cyclohexadiene was obtained exclusively. From this result, it is concluded that pure (M)-1-phenyl-1,2-cyclohexadiene ((M)-7) emerged from (-)-5 and was enantiospecifically intercepted to give the product. In the case of indene as trap for (M)-7, the (-)- and the (+)enantiomer of the [2+2] cycloadduct were formed in the ratio of 95:5. Highly surprising, remarkable enantioselectivities were also observed, when (M)-7 was trapped with styrene to furnish two diastereomeric [2+2] cycloadducts. Hence, the achiral conformation of the diradical conceivable as intermediate cannot play a decisive part. The enantioselective generation of (M)- and (P)-7 by the beta-elimination route was tested as well. Accordingly, 1-bromo-2-phenylcyclohexene was exposed to the potassium salt of (-)menthol in the presence of 2,5-dimethylfuran, and the enantiomeric [4+2] cycloadducts of the latter onto (M)- and (P)-7 were produced in the ratio of 55:45.