J. Am. Chem. Soc., 133, xxx-xxx (2011).
© American Chemical Society
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 λmax ≈ 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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