Photosubstitution of Dicyanobenzenes by Allylic Silanes, Germanes, and Stannanes via Photoinduced Electron Transfer

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Abstract
Photoreactions of 1,2- and 1,4-dicyanobenzenes (o- and p-DCB) with allylic silanes, germanes, and stannanes in acetonitrile were studied under various conditions. In these photoreactions, one of cyano groups of o- and p-DCB was replaced by allylic groups. With trimethyl(3-methyl-2-butenyl)silane, -germane, and -stannane, the photosubstitution occurred at both α- and γ-positions of the allylic moiety in a constant ratio, regardless of the kinds of metals. 1,3-Dicyanobenzene (m-DCB) was less reactive to the photosubstitution reaction. The reactivity of group 14 organometallic compounds increased in the order: Si- < Ge- < Sn-compounds. The photoreactions were sensitized by aromatic hydrocarbons such as phenanthrene and triphenylene, and also accelerated by adding Mg (ClO4)2 and alkylbenzenes. Quantum yields for the formation of the substitution products increased with decreasing oxidation potentials of group 14 organometallic compounds. A key step for the photoreactions is the reaction of radical anions of dicyanobenzenes with allylic radicals that are generated by cleavage of radical cations of group 14 organometallic compounds. The mechanistic features of these photoreactions are discussed.

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