New seven-membered ring chelates with unexpected enantioselectivity induction in asymmetric hydrogenation - Hint for a constant relative enantioselectivity Q for pairs of substrates determined by the structure of the catalysts

A series of new chiral 2,3-bis(diphenylphosphanylmethyl)-[1,4]dioxane-rhodium(I) chelates as catalysts in the asymmetric hydrogenation of (Z)-2-N-acylamino-acrylate derivatives, induce in the opposite direction compared with similar well-known catalysts possessing the same configuration, and lead to the unexpected product enantiomers. A thorough investigation of some substrate pairs led to the result that the relative enantioselectivity Q = q(H)/q(Me), the quotient of the enantiomeric ratios q for acid substrates (q(H)) and methyl esters (q(Me)), seems to be constant and characteristic for a catalyst with values influenced distinctly by the structure of the ligand. A new reduction of bisacetals to diethers by the reagent couple triethylsilane-tin tetrachloride was developed in order to obtain ligand 5a. Copyright (C) 1996 Elsevier Science Ltd