Arene-Metal Complex in Organic Synthesis: Directed Regioselective Lithiation of (?-Substituted benzene)chromium Tricarbonyl Complexes

Article abstract of DOI:10.1021/jo00162a010

(3-Methoxybenzyl alcohol)chromium tricarbonyl complex (8) and (2-substituted 7-methoxy-1-tetralol)chromium complexes 14-17 are selectively lithiated at the 4-position and 6-position, respectively, by treatment with n-BuLi/TMEDA.The regioselectivity of this lithiation is improved with increasing bulk of the butyllithium reagent employed.Since the direct lithiation of the corresponding chromium-free arenes normally proceeds at the 2- and 8-positions, complementarily substituted arenes can be prepared by using chromium tricarbonyl complexes.The different lithiation is explained by the relative configuration of the chromium tricarbonyl group in the (?-arene)Cr(CO)3 complexes and the electrostatic factor.This rationalization is supported, at least in part, by X-ray crystallography of the complex 16.On the other hand, the chromium complexes of arenes without a free hydroxyl group, such as benzyl methyl ether or ethylene acetals of benzaldehydes, are lithiated at the 2-position preferentially.