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

The resolution of (1α,5α,6α)‐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 β‐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.