Conformational Analyses of Chlorosilanes and Alpha-Chlorosilanes: Development of Molecular Mechanics (MM2) Parameters and Application to the Reaction Mechanisms.
Molecular mechanics (MM2) parameters for chlorosilanes and $\alpha$-chlorosilanes have been developed based on available experimental data and ab initio molecular orbital (MO) calculations. The molecular properties have been fitted accurately using "electronegativity correction" terms. Conformational analyses of cyclic and acyclic chloro- and $\alpha$-chlorosilane derivatives have been performed using the new MM2 parameter set. The rotational potential functions of basic building blocks for chloro- and $\alpha$-chlorosilanes have been examined by using both MO and MM2 methods. The "gauche effect" in C-C-Si-Cl and Cl-C-Si-C torsional frameworks has been studied from MM2 results. EtSiH$\sb2$Cl and MeSiH$\sb2$CH$\sb2$Cl show the "gauche effect", which stabilizes the gauche conformation compared with the anti by 0.20 and 0.32 kcal/mol, respectively, while ClCH$\sb2$SiH$\sb2$Cl is more stable in anti conformation than the gauche by 1.2 kcal/mol. 1-Chlorosilacyclohexane is more stable with axial-Cl conformation than equatorial-Cl conformation by 0.64 kcal/mol. 1-Chlorosilacyclopentane is stable in the half-chair conformation by 0.64 kcal/mol. 1-Chlorosilacyclopentane is stable in the half-chair conformation. From analysis of the MM2-calculated steric energies, it is concluded that the "gauche effect" in EtSiH$\sb2$Cl and MeSiH$\sb2$CH$\sb2$Cl is due to the attractive electrostatic interaction between Me and Cl without contribution from steric crowdedness. On the other hand, repulsion between vicinal Cl atoms makes the gauche conformer in ClCH$\sb2$SiH$\sb2$Cl less favorable. Investigations of the rearrangement mechanism of $\alpha$-chlorosilanes to chlorosilanes have been carried out by using semi-empirical (AM1 and MNDO) methods with the help of ab initio calculations in the key steps. Dyotropic rearrangement is predicted to be the most favorable path in this rearrangement under gas phase thermal conditions. The calculation predicts a transition state structure in which both migrating R and Cl are half-way transferred with an antiperiplanar relation. A ring enlargement rearrangement reaction employing 1-(chloromethyl)-1,2-dimethylsilacyclopentane has been carried out using the Lewis-acid, AlCl$\sb3$, as catalyst. Ring bond migration is favored over exocylic Me migration. The stereochemistry and reaction mechanism of the ring enlargement reactions are explained by using both the transition structure calculated by MO methods and the conformations of the reactant calculated by MM2.