Birch Reduction of Hexaphenyl- and Pentaphenylbenzene and an X-ray Crystallography and NMR Spectroscopy Study ofcis- andepi-1,2,3,4,5,6-Hexaphenylcyclohexane and of 2,3,5,6-Tetraphenyl-1,1′-bicyclohexylidene: Cannizzaro's Conundrum Revisited
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The Birch reduction of hexaphenylbenzene yields two isomers of 1,2,3,4,5,6-hexaphenylcyclohexane. The X-ray crystal structure of the all-cis isomer, 1, reveals that the severe steric crowding among the three axial phenyls is alleviated by a marked splaying out of those three aryl substituents relative to the positioning in a conventional chair structure. A second product, 2, was identified crystallographically and by NMR spectroscopy as the 1,3-diaxial-2,4,5,6-tetraequatorial (epi) isomer of hexaphenylcyclohexane, in which only five of the six additional hydrogen atoms are positioned on the same face of the C(6)Ph(6) precursor. A variable-temperature NMR study of the all-cis isomer 1 yielded a chair-to-chair inversion barrier of approximately 19 kcal mol(-1), which is somewhat higher than the previously reported values for all-cis-1,2,3,4,5,6-C(6)H(6)R(6) in which R=Me or CO(2)Me. The possible relevance to Cannizzaro's 1854 report of a product with the formula (C(7)H(6))(n) is discussed. By contrast, Birch reduction of pentaphenylbenzene led to the formation of 2,3,5,6-tetraphenyl-1,1'-bicyclohexylidene.
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