Reactive intermediates from the photolysis of methylpentaphenyl- and pentamethylphenyldisilane

Article abstract of DOI:10.1021/ja00030a011

The photochemistry of methylpentaphenyldisilane (1) and pentamethylphenyldisilane (7) has been investigated using nanosecond laser flash photolysis techniques. Flash photolysis of 1 with 248-nm light in acetonitrile or isooctane solution gives rise to a complex mixture of transient species. A strongly-absorbing transient with λ_max, = 490 nm and τ ≤ 1.7 μs (in acetonitrile) has been assigned a 1,3,5-(1-sila)triene structure on the basis of comparisons to the transient spectroscopic behavior of 7, which is known to rearrange to a silatriene intermediate in high yield upon photolysis, and on the basis of comparisons of the reactivity of these species toward water, a series of alcohols, acetone, acetic acid, ethyl acetate, oxygen, alkenes, and silanes. The extinction coefficient for the long-wavelength absorption band of the silatriene from 1 has been estimated. Silyl-centered radicals have also been identified in the transient mixture on the basis of their UV spectra and quenching with 2-bromopropane. Quenching of the silatrienes from 1 and 7 is linear in concentration for oxygen, acetone, cyclohexene, and 2,3-dimethylbutadiene. While quenching of the two silatrienes by 2,2,2-trifluoroethanol and acetic acid are also linear in concentration, water, methanol, ethanol, and tert-butyl alcohol give rise to curved quenching plots. These results are consistent with an addition mechanism which involves initial, reversible formation of a zwitterionic silene-alcohol complex, followed by competing ultra- and intermolecular proton transfer. Rate constants for the bi- and termolecular quenching processes have been determined by quadratic least-squares analysis of the quenching plots. A deuterium isotope effect of k_H/k_D ≈ 1.7 has been measured for the intracomplex proton-transfer process for the reaction of the two silatrienes with water. Similar, nonlinear behavior is observed for the quenching of the silatrienes by methoxytrimethylsilane.