Remarkable Substituent Effects on the Activation Energy of Silylene Insertion into Silicon–Chlorine Bonds

Abstract

Insertion reactions of dimethylsilylene (Me(2)Si:) into the silicon-chlorine bond of various substituted chlorosilanes have been computationally studied by using DFT calculations with a 6-31++G(d,p) basis set. All of the insertions investigated herein were exothermic (more than 40 kcal mol(-1)) and proceeded via three-membered cyclic transition states (TS) with one substituent of a chlorosilane in the ring plane and two other substituents out of the plane. Among the two possible concerted insertion pathways, the pathway via a TS with a silylene lone-pair orbital approaching a chlorosilane silicon atom was preferred. Important nucleophilic interactions of silylene lone-pair orbitals with anti-bonding sigma orbitals of Si-Y (in plane) were supported by carrying out a natural bond orbital (NBO) analysis of the reaction. A radical pathway through the initial abstraction of chlorine from chlorosilane by the silylene cannot compete with the preferred concerted pathway. Systematic calculations for SiMe(2) insertion into various chlorosilanes (YR(2)SiCl; Y (in-plane substituent) = H, Me, NH(2), OH, F, SiH(3), PH(2), SH and Cl; R (out-of-plane substituent) = H, Me, iPr and tBu) have revealed remarkable substituent effects on activation free energies (DeltaG(double dagger)). In-plane (Y) substituent effects were mostly electronic. Electron-withdrawing substituents accelerate the insertion through enhanced nucleophilic interaction between silylene lone-pair orbitals with the sigma*(Si-Y) orbital at the TS. The DeltaG(double dagger) values correlate with the sigma(I) constants as a scale of the inductive effect of Y. Out-of-plane (R) substituent effects are mainly steric, and bulky substituents increase the activation free energy . Correlation of the DeltaG(double dagger) values with Taft's steric substituent constants (E(s)) was observed. There was no significant difference in the out-of-plane substituent effects between the electron-withdrawing Cl and the electron-donating Me groups.

Oxidative Addition and Reductive Elimination at Main-Group Element Centers

“…354 The effect of the substituents at silicon on the activation energy of insertion was also investigated. 355 Unlike the related reaction of silylenes with the C−Cl bonds (section 2.9), a radical pathway is much less favorable than direct insertion (ΔG ‡ = 44.1 kcal mol −1 for the radical pathway vs ΔG ‡ = 15.3 kcal mol −1 for the direct insertion). Calculations suggest that electron-withdrawing substituents Y in the chlorosilane facilitate the insertion pathway due to the increased interaction between the silylene lone pair and the σ* (Si−Y) orbital while the substituent R mainly exerts a steric effect.…”

“…Shortly after, Kira et al showed that the isolable silylene 183 oxidatively cleaved the Si–Cl bonds of dichlorodimethylsilane, tetrachlorosilane, and dichlorosilane to give the corresponding 1,2-dichlorodisilanes 465 (Scheme ), whereas reaction with dimethylchlorosilane afforded the Si–H insertion product 466 exclusively. , DFT calculations of the reaction of SiMe 2 with chlorosilanes ClSiYR 2 (R = Me or H; Y = H, Cl, F, Me, or SiH 3 ) revealed stereoelectronic substituent effects, of which the most important is the nucleophilic interaction of the silylene lone pair with the σ* (Si–Y) orbital . The effect of the substituents at silicon on the activation energy of insertion was also investigated . Unlike the related reaction of silylenes with the C–Cl bonds (section ), a radical pathway is much less favorable than direct insertion (Δ G ‡ = 44.1 kcal mol –1 for the radical pathway vs Δ G ‡ = 15.3 kcal mol –1 for the direct insertion).…”

Reactions of an Isolable Dialkylsilylene with Aroyl Chlorides. A New Route to Aroylsilanes

“…The insertion reactions of silylene 1 into the Si-Cl bonds of chlorosilanes have been found to occur cleanly [49,50]; hence, the concerted mechanism via three-membered cyclic transition states has been proposed. The mechanism has been supported by the detailed DFT calculations [55][56][57]. Scheme 1.…”

Carbenes and Nitrenes

“…142 The reaction has been shown to evolve through a transition state in which the silylene lone pair approaches the silicon atom of the chlorosilane.…”