Silanones and Silanethiones from the Reactions of Transient Silylenes with Oxiranes and Thiiranes in Solution. The Direct Detection of Diphenylsilanethione.
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The transient silylenes SiMe(2) and SiPh(2) 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. Laser flash photolysis studies show that these reactions proceed via multistep sequences involving the intermediacy of the corresponding silylene-oxirane or -thiirane complexes, which are formed with rate constants close to the diffusion limit in all cases and exhibit UV absorption spectra similar to those of the corresponding complexes with the nonreactive O- and S-donors, tetrahydrofuran and tetrahydrothiophene. The SiMe(2)-PrO and SiPh(2)-PrO complexes both exhibit lifetimes of ca. 300 ns, 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 SiMe(2)-PrS complex decays with the excitation laser pulse (i.e., τ ≤ 25 ns), while the SiPh(2)-PrS complex exhibits τ = 48 ± 3 ns. The decay of the SiPh(2)-PrS complex affords a long-lived transient product exhibiting λ(max) ≈ 275 nm, which has been assigned to diphenylsilanethione (Ph(2)Si═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 × 10(8) to 3.2 × 10(9) M(-1) s(-1) are reported. The experimental rate constants for decay of the SiMe(2)-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 SiH(2) and SiMe(2) 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 SiH(2)-oxirane and -thiirane systems.
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