Stereochemical Analysis of 1,1,2,2-Tetraarylethanes. 1. Static Stereochemistry

Empirical force field calculations on 1,1,2,2-tetraphenylethane (TPE) and 1,1,2,2-tetrakis(2,6-xylyl)ethane (TXE) show that these molecules adopt a C₂ ground-state conformation in which the methine hydrogens are in anti positions. The dihedral angles of all four rings are equally signed; these systems can therefore be viewed as four-bladed molecular propellers. Comparison of the geometric parameters for TPE and TXE reveals that TXE is a highly strained molecule, as demonstrated by the unusual elongation of the central C-C bond and the unusual enlargement of the central C-C-C angles. The ¹H NMR spectra of tetramesitylethane (TME) and 1,2-dimesityl-1,2-bis(2,4,6-trimethoxyphenyl)ethane (2) at the slow exchange limit are also consistent with a C₂ conformation in solution. The racemic and meso forms of 2 have been separated and configurations have been assigned to the two stereoisomers on the basis of their NMR spectra. Treatment of the static stereochemistry of 1,1,2,2-tetraarylethanes in terms of a group theoretical approach leads to a description of the full permutation-inversion group, to the calculation of the number of isomers for any given substitution pattern in such systems, and to the recognition of systems which are stereochemically correspondent to 1,1,2,2-tetraarylethanes.