On the deprotonation of η6-1,3-dimethoxybenzene-Cr(CO)3 derivatives: Influence of the reaction conditions on the regioselectivity

The regioselectivity of deprotonation/alkylation reactions of η6-1,3-dimethoxybenzene-Cr(CO)3 (5), η6-1,3-dimethoxy-5-methylbenzene-Cr(CO)3 (6) and 2-substituted derivatives of these compounds was investigated. It is shown that the regioselectivity highly depends on the reaction conditions. For instance, deprotonation of η6-1,3-dimethoxy-2-(trimethylsilyl)benzene-Cr(CO)3 (10) with n-BuLi followed by silylation or methylation affords 4-substituted products while the use of LiTMP at −78 °C cleanly gives rise to 5-substituted products. The regioselective alkylation of complexes of type 6 at the 5-methyl group is easily achieved. The usefulness of the latter possibility is demonstrated in a synthesis of olivetol dimethyl ether (33).