The mechanism of nucleophilic substitution at silicon

Abstract

Using multinuclear n.m.r. spectroscopy (1H, 13C, 29Si and 170 ), the interactions of diastereotopic (PhCHMeSiMe2X; X=CL, Br, 0S02CF3) and chiral (PhMeHSiX; X=CL, 0S02CF3) silanes with a diverse range of nucleophile (Nu = amides, ureas, pyridines, phosphine oxide and amines) are examined in detail. The dominant species in the diastereotopic silylating solutions are four coordinate 1:1 ionic [PhCHMeSiMe2-Nu]+X- adducts. These complexes undergo exchange with nucleophiles and their parent silanes at a rate limited by the initial attack of nucleophile or counterion at the silicon atom of the adduct. The halide counterions of stable silane-nucleophile adducts can also induce isomerization of uncomplexed silanes. The kinetic and thermodynamic results are consistent with the operation of two different mechanisms for the racemizations of PhCHMeSiMe2X (X=CL, Br). Racemization involving Less stable silane-nucleophile complexes may proceed via rate determining attack by a second molecule of nucleophile at the adduct, formed in the initial, rapid, pre-equilibrium step. Two molecules of nucleophile are utilized resulting in an order of 2 or 1.5 in nucleophile, depending upon the degree of ion-pair dissociation. An alternative pathway is preferred with increasing stability of the silane-nucleophile salt. Adduct formation is followed by halide exchange leading to racemization of silane. One molecule of nucleophile is required in this case corresponding to a first or a half order in nucleophile. The kinetic results are correlated with the rate of alcoholysis reported by Frye, the relative equilibrium constant for the formation of [Me3Si-Nu]+X- complexes and the donor strength of nucleophile as measured by Taft's Beta values. The good correlation with the Beta values provides evidence that the rate of racemization is dependent upon the nucleophilicity of the donor species. The excellent linear correlation with Frye's ln k1 values further verifies the mechanism first postulated by Chojnowski. Rapid pre-equilibrium formation of [R3Si-Nu]+X- adducts is followed by slow attack of a second molecule of nucleophile. With strong nucleophiles, four coordinate ionic [PhMeHSi-Nu]+Cl- salts undergo second nucleophilic attacks yielding stable pentacoordinate [PhMeHSi-(Nu)2]+CL- ionic adducts. The collapse of the N-methyl resonances of DMF observed in the interactions of PhMeRSiCl (R=H, Me) with DMF may serve as an indication of N-silylation of DMF.