Tartrate-Derived Aryl Aldehyde Acetals in the Asymmetric Directed Metalation of Chromium Tricarbonyl Arene Complexes

A series of three chromium tricarbonyl benzaldehyde acetal complexes were prepared from benzaldehyde diethyl acetal and (+)-diethyl tartrate. These compounds were ortho-lithiated by a number of bases, and the diastereoselectivity of lithiation was determined by quenching of the resulting anions with Me₃SiCl. Complexes 1 and 2 were found to give moderate levels of diastereoselection, while the combination of complex 3 and 2.4 equiv of n-BuLi under equilibrating conditions gave an 86% de of silylated products. Other electrophiles gave slightly higher de's (88% to ≥94%), and all products derived from 3 were separable by column chromatography. Hydrolysis of the ortho-methylated product to the corresponding benzaldehyde complex (−)-21 as well as X-ray structural analysis of the trimethylsilylated complex demonstrated that preferential pro-R deprotonation had occurred.