Silanones and Silanethiones from the Reactions of Transient Silylenes with Oxiranes and Thiiranes in Solution. The Direct Detection of Diphenylsilanethione.
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- 作者：Svetlana S. Kostina ; William J. Leigh
- 刊名：Journal of the American Chemical Society
- 出版年：2011
- 出版时间：March 30, 2011
- 年：2011
- 卷：133
- 期：12
- 页码：4377-4388
- 全文大小：1087K
- 年卷期：v.133,no.12(March 30, 2011)
- ISSN：1520-5126
文摘
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. Laser flash photolysis studies show that these reactions proceed via multistep sequences involving the intermediacy of the corresponding silylene鈭抩xirane or 鈭抰hiirane 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 SiMe2-PrO and SiPh2-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鈭?40 ns. On the other hand, the silylene-PrS complexes are considerably shorter-lived and vary with silyl substituent; the SiMe2鈭扨rS complex decays with the excitation laser pulse (i.e., 蟿 鈮?25 ns), while the SiPh2鈭扨rS complex exhibits 蟿 = 48 卤 3 ns. The decay of the SiPh2鈭扨rS complex affords a long-lived transient product exhibiting 位max 鈮?275 nm, which has been assigned to diphenylsilanethione (Ph2Si鈺怱) 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 脳 108 to 3.2 脳 109 M鈭? s鈭? are reported. The experimental rate constants for decay of the SiMe2鈭抏poxide and 鈭扨rS complexes indicate free energy barriers (螖G) of ca. 8.5 and 鈮?.1 kcal mol鈭? 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鈭扥 cleavage mechanism involving singlet biradical intermediates for the silylene鈭抩xirane complexes, and a concerted mechanism for silanethione formation from the silylene鈭抰hiirane complexes, in agreement with earlier ab initio studies of the SiH2鈭抩xirane and 鈭抰hiirane systems.