Lewis acid-catalyzed Friedel-Crafts alkylation of ferrocene with allylchlorosilanes

Friedel-Crafts alkylations of ferrocene, 1, with allylchlorosilanes in the presence of Lewis acid catalysts under mild conditions gave regiospecific [1-methyl-2-(alkylchlorosilyl)ethyl]ferrocenes in fair to good yields depending upon the substituents on silicon, along with small amounts of dialkylated products. The alkylation of ferrocene with 1.2 equiv of(2-methylallyl)-dimethylchlorosilane, 2b, at 0 degrees C gave monoalkylated ferrocene 3b and an isomeric mixture of dialkylated products in 76% and 4% yields, respectively. The yield of the dialkylated ferrocene mixture increased to 21% when 2 equiv of alkylating agent 2b was used. The two components of the dialkylated product mixture were identified as 1,1'-dialkylated ferrocene 4b (17%), and 1,3-dialkylated ferrocene 4b' (4%). A higher yield of the 1,1'-diadduct compared to the 1,3-diadduct indicates that the electron-donating alkylsilyl group does not greatly enhance a second alkylation on the same ring. The reactivities of allylchlorosilanes in the ferrocene alkylations decrease in the following order: allyldialkylchlorosilane > allyl(alkyl)dichlorosilane much greater than allyldichlorosilane approximate to allyltrichlorosilane. The catalytic efficiencies of Lewis acids for the alkylations decrease in the following order: HfCl4 > ZrCl4 > AlCl3 > AlBr3 > TiCl4.