Photoinduced Rearrangement of Aryl-Substituted Acylcyclohexasilanes.

In the present study, we examined photochemical transformations of acylcyclohexasilanes (Me₃Si)₂Si₆Me₁₂(Me₃Si)COR (compounds 1) with aromatic substituents attached to the carbonyl C atom. When photolyzed at wavelengths above 300 nm in hydrocarbon solvents, 1a- c ( 1a: R = o-Tol; 1b: R = 2,3-Me₂C₆H₃; 1c: R = 2,4-Me₂C₆H₃) afforded the previously unknown eight-membered heterocyclopolysilanes 3a- c, which feature endocyclic Si=C double bonds with remarkable selectivity. If two ortho-alkyl groups are present at the aromatic ring (compounds 1d- f), the primary products of type 3 ( 3d: R = Mes; 3e: R = 1,6-Me₂C₆H₃; 3f: R = 2,4,6- iPr₃C₆H₂) are photochemically unstable and undergo intramolecular C-H-bond addition reactions of a ortho-CH₃ group to the Si=C double bond, which leads to the formation of spirobenzocyclobutenes. When photolysis was performed in the presence of methanol, the expected 1,2-addition products (compounds 4) of the Si=C double bond were obtained quantitatively in all cases. 4a (R = o-Tol) and 4b (R = Mes) were isolated by crystallization from acetone and fully characterized spectroscopically and by single-crystal X-ray diffraction analysis. Interestingly 4b was obtained as an isomeric mixture arising from syn and anti addition of methanol to the endocyclic Si=C bond, whereas the syn addition was observed exclusively in the case of 4a. [ABSTRACT FROM AUTHOR]