Birch Reduction of Hexaphenyl‐ and Pentaphenylbenzene and an X‐ray Crystallography and NMR Spectroscopy Study of cis‐ and epi‐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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abstract
AbstractThe 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 C6Ph6 precursor. A variable‐temperature NMR study of the all‐cis isomer 1 yielded a chair‐to‐chair inversion barrier of ≈19 kcal mol−1, which is somewhat higher than the previously reported values for all‐cis‐1,2,3,4,5,6‐C6H6R6 in which R=Me or CO2Me. The possible relevance to Cannizzaro's 1854 report of a product with the formula (C7H6)n is discussed. By contrast, Birch reduction of pentaphenylbenzene led to the formation of 2,3,5,6‐tetraphenyl‐1,1′‐bicyclohexylidene.
