Selective Electrochemical Mono- and Polysilylation of Halothiophenes

Electrochemical trimethylsilylation of mono- and polyhalothiophenes (hal = Cl, Br) has been examined. In order to predict the selectivity of the reaction in accordance with the halogen position on the ring, the reduction potentials of the commercially available halothiophenes have been determined by cyclic voltammetry. Thus, as expected, the bromo derivatives are more easily reduced than the chloro analogs, and the 2-position is more reactive than the 3-position. We obtained 2- and 3-(trimethylsilyl)thiophenes, 2-(trimethylsilyl)-3 (or 5)-halothiophenes, 2-(trimethylsilyl)-3,4,5-trihalothiophenes, 2,5-bis(trimethylsilyl)thiophene, 2,5-bis(trimethylsilyl)-3,4-dihalothiophenes, and 2,3,5-tris(trimethylsilyl)-4-halothiophenes with an excellent selectivity. In contrast, the silylation of 2-(trimethylsilyl)-3-halo- and 2,3,5-tris(trimethylsilyl)-4-halothiophenes occurred with ring opening and afforded 1,1,4,4-tetrakis(trimethylsilyl)-1,2-butadiene and hexakis(trimethylsilyl)-2-butyne, respectively. A coherent interpretation of the synthetic results is proposed in correlation with the measured potential values.