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Remarkable Substituent Effects on the Activation Energy of Silylene Insertion into Silicon–Chlorine Bonds
Author(s) -
Xu Zheng,
Jin Juan,
Li Zhifang,
Qiu Huayu,
Jiang Jianxiong,
Lai Guoqiao,
Kira Mitsuo
Publication year - 2009
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200901285
Subject(s) - chlorosilane , substituent , chemistry , silylene , lone pair , natural bond orbital , crystallography , computational chemistry , photochemistry , stereochemistry , silicon , density functional theory , organic chemistry , molecule
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 σ orbitals of SiY (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, i Pr and t Bu) have revealed remarkable substituent effects on activation free energies (Δ G ≠ ). 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 σ*(SiY) orbital at the TS. The Δ G ≠ values correlate with the σ 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 Δ G ≠ 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.