28336-57-4Relevant articles and documents
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
Grealis, John P.,Mueller-Bunz, Helge,Ortin, Yannick,Condell, Mark,Casey, Michael,McGlinchey, Michael J.
experimental part, p. 1552 - 1560 (2009/04/06)
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 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′-bi-cyclohexylidene.
