1-Phenyl-1,2-cyclohexadiene: Astoundingly High Enantioselectivities on Generation in a Doering–Moore–Skattebøl Reaction and Interception by Activated Olefins
✍ Scribed by Manfred Christl; Hartmut Fischer; Mario Arnone; Bernd Engels
- Publisher
- John Wiley and Sons
- Year
- 2009
- Tongue
- English
- Weight
- 334 KB
- Volume
- 15
- Category
- Article
- ISSN
- 0947-6539
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✦ Synopsis
Abstract
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.