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

Abstract

The resolution of (1alpha,5alpha,6alpha)-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.

Cycloadditions of Oxacyclic Allenes and a Catalytic Asymmetric Entryway to Enantioenriched Cyclic Allenes

“…As noted earlier, one of the most exciting opportunities regarding cyclic allene chemistry is the possibility of intercepting enantioenriched allenes for the synthesis of enantioenriched cycloadducts. In a seminal study, Christl and co‐workers generated 1‐phenyl‐1,2‐cyclohexadiene in enantioenriched form, as judged by the formation of an enantioenriched cycloadduct, albeit in low yield, likely owing to the necessary use of organolithium reagents . Our laboratory recently disclosed a mild, alternative strategy whereby silyl triflate precursors to the desired cyclic allenes could be employed in enantioenriched form .…”

Efficient Trapping of 1,2‐Cyclohexadienes with 1,3‐Dipoles

“…Concerted reaction is also consistent with the high degree of stereochemical fidelity reported by Christl and Engels in the furan trapping of substituted 1,2-cyclohexadienes derived from enantioenriched precursors. [24] In analogy, we posit a concerted dipolar cycloaddition in these cases, with the observed diastereoselectivity arising from an endo-like transitions state (Figure 1, TS 2). Reaction of 1,2-cyclohexadienes with stable and readily accessible azide 1,3-dipolar partners was also of interest.…”

Recent advances in the syntheses, transformations and applications of 1,1-dihalocyclopropanes

“…35,92 In the absence of a trapping agent, 1-phenyl-1,2-cyclohexadiene was converted to a dimer or a trimer depending on the mode of generation of the diene. 35,92 In the absence of a trapping agent, 1-phenyl-1,2-cyclohexadiene was converted to a dimer or a trimer depending on the mode of generation of the diene.…”