Silicon alpha-effect: A systematic experimental and computational study of the hydrolysis of c-alpha- and c-gamma-functionalized alkoxytriorganylsilanes of the formula type rosime2(ch2)(n)x (r = me, et; n=1, 3; x = functional group)

To understand the silicon alpha-effect in terms of an enhanced reactivity of the Si-OC bond of alpha-silanes of the formula type ROSiMe2CH2X compared to analogous gamma-silanes ROSiMe2(CH2)(3)X (R = Me, Et; X = functional group), a systematic experimental and computational study of the kinetics and mechanisms of hydrolysis of such compounds was performed. For this purpose, a series of suitable model compounds was synthesized and studied for their hydrolysis kinetics in CD3CN/D2O under basic and acidic conditions, using H-1 NMR spectroscopy as the analytical tool. To get more information about the reaction mechanisms, the experimental investigations were complemented by computational studies. These investigations demonstrated that the silicon alpha-effect cannot be rationalized in terms of a special single effect. The reactivities observed rather result from a summation of different components, such as electronic and steric effects, pD dependence, and hydrogen bonds between the functional group (or even protonated functional group) and the alkoxy leaving group. Therefore, the term "silicon alpha-effect" should not be used furthermore to explain the hydrolysis reactivity at the silicon atom of alkoxyorganylsilanes with functional groups in alpha- or gamma-position of the organyl groups (so-called alpha- or gamma-silanes).