Toward an Understanding of 1,5-Asymmetric Induction during Nucleophilic Addition to (Arene)chromium Tricarbonyl Complexes: Conformational Preference of the Chromium Tricarbonyl Tripod for Transmission of Chirality

Asymmetric induction during enolate nucleophile addition to chromium tricarbonyl complexes of 4-alkoxy-1-(trimethylsilyl)benzene derivatives, in which the alkoxy group (R*O) is derived from an optically pure alcohol, is analyzed in terms of the effect of the chiral auxiliary (R*) on the conformation of the tricarbonylchromium tripod. Crystal structure determination on the three complexes 3a (R* = 2-phenylisobornyl), 3b (R* = 2-methylisobornyl), and 3c (R* = 2-(3-[1,3]-dioxalanyl)isobornyl) reveals that the chiral group R* causes a rotation of the Cr(CO)3 group from the normally preferred orientation, the extent and direction of which are dependent on the auxiliary. NMR studies on seven complexes indicate the presence of a single conformation in solution for each, which is assumed to be the same as that shown in the solid state. There is a correlation between the observed asymmetric induction and the Cr(CO)3 tripod rotation, which is believed to be a result of the effect of this conformational distortion on the arene-centered FMO coefficients.