J. Am. Chem. Soc., 133, xxx-xxx (2011).

© American Chemical Society

Silanones and Silanethiones from the Reactions of Transient Silylenes with Oxiranes and Thiiranes in Solution. The Direct Detection of Diphenylsilanethione

Svetlana S. Kostina and William J. Leigh*

Contribution from the Department of Chemistry & Chemical Biology, McMaster University, 1280 Main Street West, Hamilton, ON Canada L8S 4M1.

The transient silylenes SiMe₂ and SiPh₂ react with cyclohexene oxide (CHO), propylene oxide (PrO), and propylene sulfide (PrS) in hydrocarbon solvents to form products consistent with the formation of the corresponding transient silanones and silanethiones, respectively, via O- or S-atom abstraction from the three-membered heterocycles. Laser flash photolysis studies show that these reactions proceed via multi-step sequences involving the intermediacy of the corresponding silylene-oxirane or –thiirane complexes, which are formed with rate constants within a factor of two of the diffusion limit in all cases and exhibit UV-vis absorption spectra similar to those of the corresponding complexes with the “non-reactive” O- and S-donors, tetrahydrofuran and tetrahydrothiophene. The silylene-PrO complexes exhibit lifetimes of ca. 300 ns, nearly independent of the identity of the silyl substituents, and are longer-lived than the corresponding complexes with CHO, which are both in the range of 230-240 ns. On the other hand, the silylene-PrS complexes are considerably shorter-lived and vary with silyl substituent; the SiMe₂-PrS complex decays with the excitation laser pulse (i.e. τ ≤ 25 ns), while the SiPh₂-PrS complex exhibits τ ≈ 48 ± 3 ns. The decay of the SiPh₂-PrS complex affords a long-lived transient product exhibiting λ_max ≈ 275 nm, which has been assigned to diphenylsilanethione (Ph₂Si=S) on the basis of its second order decay kinetics and absolute rate constants for reaction with methanol, tert-butanol, acetic acid, and n-butyl amine, for which values in the range of 1.4 × 10⁸ – 3.2 × 109 M^-1s^-1 are reported. The experimental rate constants for decay of the SiMe₂-epoxide and –PrS complexes indicate free energy barriers (ΔG^‡) of ca. 8.5 and ≤7.1 kcal mol^-1 for the rate-determining steps leading to dimethylsilanone and -silanethione, respectively, which are compared to the results of DFT (B3LYP/6-311+G(d,p)) calculations of the reactions of SiH₂ and SiMe₂ with oxirane and thiirane. The calculations predict a stepwise C-O cleavage mechanism involving singlet biradical intermediates for the silylene-oxirane complexes, and a concerted mechanism for silanethione formation from the silylene-thiirane complexes, in agreement with earlier ab initio studies of the SiH₂-oxirane and –thiirane systems.

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