Nucleophilic displacement versus electron transfer in the reactions of alkyl chlorosilanes with electrogenerated aromatic anion radicals

Anion radicals of a series of aromatic compounds (C6H5CN, C6H5COOEt, anthracene, 9,10-dimethyl-, 9,10-diphenyl- and 9-phenylanthracene, pyrene and naphthalene) react with trialkyl chlorosilanes R1R2R3SiCl (R1-3 = Me, Et; R1,2 = Me, R3 = t-Bu) in multiple ways, following classical bimolecular schemes. The ratio of one-electron transfer (ET) to a two-electron process (SN2-like nucleophilic attack of the reduced form of mediator on the chlorosilane, with k2 ≅ 102-108 M−1 s−1) is inversely related to the steric availability of Si for nucleophilic displacement reactions. The nucleophilic substitution pathway mainly results in mono- and disilylated aromatic products. Paralleling the electrochemical data with DFT calculations, the role of silicophilic solvent (DMF) in SN process was shown to be quite complex because of its involvement into coordination extension at silicon, dynamically modifying energetics of the process along the reaction coordinate. Although 2,2'-bipyridine also forms delocalized persistent anion radicals, they do not induce neither ET nor SN reactions in the same manner as aromatic mediators. Silicophilicity of 2,2'-bipyridine being superior to that of DMF, a R3SiCl·bipy complex of hypercoordinated silicon with electroactive ligand was formed instead, whose reduction requires about 1 V less negative potentials than bipyridine itself.