[L-ASTERISK-RH(NBD)CL] (L-ASTERISK = CHIRAL CYCLIC MONOPHOSPHONITE) -A NOVEL CLASS OF RHODIUM(I) COMPLEXES AND THEIR EVALUATION IN THE ASYMMETRIC HYDROSILYLATION OF KETONES - INVESTIGATIONS OF THE EFFECTS OF TEMPERATURE AND LIGAND BACKBONE

A series of new rhodium(I) complexes[LRh(NBD)Cl] (L* = chiral cyclic phosphonite with a fused 1,4-dioxane or cyclobutane ring in the backbo ne) was synthesized via the corresponding borane-phosphonite adducts. They turned out to be highly active catalysts in the asymmetric hydros ilylation of ketones in a broad temperature range. Evaluation of the t emperature dependent measurements according to the Eyring formalism di sclosed a nonlinear relationship between ln P (P = ratio of the enanti omeric product alcohols) and the reciprocal of temperature marked by t he occurrence of points of inversion in the middle temperature region between -5 and 30 degrees C. These findings indicate a complexly compo sed selection mechanism that is controlled by at least two relevant pa rtial steps. Furthermore, by comparison of different catalysts based o n a 1,4-dioxane, a cyclobutane, or a 1,3-dioxolane ring in the backbon e of L, the conformational properties of the ligand backbone were rev ealed to be a crucial feature determining not only the extent but also the direction of chirality transfer, thus providing a versatile tool for the construction of stereocomplementary ligands. Accordingly, by p roper choice of the ligand backbone and the temperature, fairly good e nantioselectivities in the hydrosilylation of rather different ketones like acetophenone (82% ee) and pivalophenone (86% ee) can be achieved .