The Roles of Ligands on Silicon

Abstract

<p> This thesis presents a study of the roles of ligands on silicon in two parts.</p> <p> In Part A, the relationship between the β-effect - the ability of a silyl group to stabilize a carbocation β to silicon, and the electron-withdrawing ability of the ligands on silicon was examined. The bromination of a series of (E)-β-silylstyrenes was chosen as a way to quantify the β-stabilization. The degree of syn-addition of bromine to (E)-β-silylstyrene was used as a measure of the stabilizing ability of the silyl group.</p> <p> The bromination of a series of four (E)-β-silylstyrenes was investigated [ligands on silicon: (SiMe3O)3; (CH3COO)3; (CCl3COO)3; (CF3COO)3]. It was found that the degree of syn-addition decreases as the silyl group electronegativity increases. Combined with the results previously obtained in our lab, two linear relationships between the degree of syn-addition and the silyl group electronegativity were observed in two groups of (E)-β-silylstyrenes [Group A, ligands on silicon: Me3-nXn, n = 1-3, X= Cl, F and (MeO)3; (Me3SiO)3 and Group B, (CH3COO)3; (CCl3COO)3; (CF3COO)3; (p-MeOPhO)3; (PhO)3; (p-ClPhO)3]. The degree of syn-addition was found to be more sensitive towards the group electronegativity for compounds in Group B. It was also found that, as the group electronegativity increases, the α-SiCH 1H and 29Si NMR chemical shifts of compounds in Group A decrease. In contrast, the α-SiCH 1H and 29Si NMR chemical shifts of compounds in Group B tend to increase.</p> <p> In Part B, a series of hydrosilanes were studied and their reactivities in iodination were found to be in the order: CH3SiH(OEt)2 > PMHS > CH3SiHCl2.</p> <p> The behavior of these organosilicon compounds is discussed in terms of the structural features of the silyl groups - the ligands on silicon.</p>