L*Rh(NBD)Cl] (L* = 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

Abstract

A series of new rhodium(I) complexes [L*Rh(NBD)Cl] (L* = chiral cyclic phosphonite with a fused 1,4-dioxane or cyclobutane ring in the backbone) was synthesized via the corresponding borane−phosphonite adducts. They turned out to be highly active catalysts in the asymmetric hydrosilylation of ketones in a broad temperature range. Evaluation of the temperature dependent measurements according to the Eyring formalism disclosed a nonlinear relationship between ln P (P = ratio of the enantiomeric product alcohols) and the reciprocal of temperature marked by the occurrence of points of inversion in the middle temperature region between −5 and 30 °C. These findings indicate a complexly composed selection mechanism that is controlled by at least two relevant partial steps. Furthermore, by comparison of different catalysts based on a 1,4-dioxane, a cyclobutane, or a 1,3-dioxolane ring in the backbone of L*, the conformational properties of the ligand backbone were revealed 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 proper choice of the ligand backbone and the temperature, fairly good enantioselectivities in the hydrosilylation of rather different ketones like acetophenone (82% ee) and pivalophenone (86% ee) can be achieved.