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.