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

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 ≈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.