Department of Chemistry and Chemical Biology, McMaster University, 1280 Main Street West, Hamilton, ON, Canada, L8S 4M1.
J Am Chem Soc. 2011 Mar 30;133(12):4377-88. doi: 10.1021/ja107881p. Epub 2011 Mar 8.
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.
瞬态甲硅烷基 SiMe(2) 和 SiPh(2) 在烃溶剂中与环己烯氧化物 (CHO)、丙烯氧化物 (PrO) 和丙烯硫醚 (PrS) 反应，分别形成与相应瞬态硅酮和硅硫酮形成一致的产物。激光闪光光解研究表明，这些反应通过多步序列进行，涉及相应甲硅烷基-环氧化物或-硫代环氧化物配合物的中间体，这些配合物在所有情况下都以接近扩散极限的速率常数形成，并表现出与非反应性 O 和 S-供体四氢呋喃和四氢噻吩相似的紫外吸收光谱。SiMe(2)-PrO 和 SiPh(2)-PrO 配合物的寿命均约为 300 ns，比与 CHO 形成的相应配合物的寿命长，CHO 的寿命均在 230-240 ns 范围内。另一方面，甲硅烷基-丙烯硫醚配合物的寿命要短得多，并且随甲硅烷基取代基而变化；SiMe(2)-PrS 配合物随激发激光脉冲衰减（即 τ ≤ 25 ns），而 SiPh(2)-PrS 配合物则表现出 τ = 48 ± 3 ns。SiPh(2)-PrS 配合物的衰减提供了一种具有长寿命瞬态产物，其最大吸收波长 λ(max)≈275 nm，根据其二级衰减动力学和与甲醇、叔丁醇、乙酸和正丁胺的反应的绝对速率常数，将其分配给二苯基硅烷硫酮（Ph(2)Si═S），对于后者，报道的值在 1.4×10(8) 到 3.2×10(9) M(-1) s(-1) 范围内。SiMe(2)-环氧化物和 -PrS 配合物的实验衰减速率常数表明，导致二甲基硅酮和 -硅硫酮的速率决定步骤的自由能垒 (ΔG(‡)) 分别约为 8.5 和 ≤7.1 kcal mol(-1)，这与 DFT（B3LYP/6-311+G(d,p)）计算的 SiH(2) 和 SiMe(2) 与环氧化物和硫代环氧化物的反应结果进行了比较。该计算预测了涉及单重态双自由基中间体的逐步 C-O 断裂机制，以及来自硅烯-硫代环氧化物配合物的硅硫酮形成的协同机制，这与早期对 SiH(2)-环氧化物和 -硫代环氧化物系统的从头算研究一致。
