Resolution of sterically overcrowded ethylenes; a remarkable correlation between bond lengths and racemization barriers

Article abstract of DOI:10.1016/S0040-4039(00)78887-3

The synthesis and resolution of 2-methyl-9H-thioxanthene-9-(9H-thioxanthen-9-ylidene) 1 and related structures 2-5, being the first examples of thermally stable optically active sterically overcrowded ethylenes, are reported. Substitution of the sulfur atom in the thioxanthene part of 1 with an X group (2, X = C(CH₃₂; 3, NCH₃ 4, O) results in a change in racemization barrier (1, 27.4 kcal.mol_-1; 3, 21.3 kcal.mol¹ 4, < 20 kcal.mol^-1) depending upon the aryl-X bond lengths.