Silicon α-Effect: A Systematic Experimental and Computational Study of the Hydrolysis of C_α- and C_γ-Functionalized Alkoxytriorganylsilanes of the Formula Type ROSiMe₂(CH₂)_nX (R = Me, Et;n= 1, 3; X = Functional Group)

Abstract

To understand the silicon α-effect in terms of an enhanced reactivity of the Si−OC bond of α-silanes of the formula type ROSiMe 2 CH 2 X compared to analogous γ-silanes ROSiMe 2 (CH 2 ) 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 CD 3 CN/D 2 O under basic and acidic conditions, using 1 H 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 α-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 α-effect" should not be used furthermore to explain the hydrolysis reactivity at the silicon atom of alkoxyorganylsilanes with functional groups in αor γ-position of the organyl groups (so-called αor γ-silanes).

Pentafluoroethyl-substituted α-silanes: model compounds for new insights

“…[1][2][3][4] The early picture of the so-called α-effect is a classical dative bond between a silicon atom and a geminal donor centre. 7, 8 Tacke and co-workers come to the conclusion that the term "silicon α-effect" should not be used furthermore to explain the hydrolysis reactivity at the silicon atom of alkoxyorganylsilanes with functional groups in αor γ-position. 6 Industrial use of this effect is made in the accelerated hydrolysis in the class of so-called α-silanes as surface mediators, cross-linking agents and other useful industrial products.…”

“…It was introduced by Kostyanovskii 5 in order to rationalize the reactivity and properties of such geminal systems. 8 However, fundamental interaction types have to be clearly distinguished from the complex interplay of reactants involving solvation, pre-aggregation and many more polarity or dipole-driven effects in solution. 7 Driven by these important applications, hydrolysis of α-silanes has intensely investigated by kinetic and theoretical methods revealing that the whole process is much more complicated than the original picture of the α-effect.…”

Nucleophilic substitution on silica surfaces: Comparison of the reactivity of α- versus γ-chlorosubstituted silanes in the reaction with sodium azide

“…Ethylene glycol (EG, Aldrich) was purified by drying with Na 2 SO 4 and distillation from Mg. Pluronic P123 (M av = 5800), EO 20 PO 70 EO 20 (BASF), N,N-dimethylformamide (DMF, VWR) and NaN 3 (Merck) were applied without purification.…”

Synthesis and Hydrolysis of Alkoxy(aminoalkyl)diorganylsilanes of the Formula Type R2(RO)Si(CH2)nNH2 (R = Alkyl, n = 1–3): A Systematic Experimental and Computational Study

“…The model compounds of the α‐, β‐, and γ‐aminosilane systems shown in Scheme allowed a systematic analysis of the electronic and steric impact of the different silicon‐bound substituents on the hydrolysis kinetics. With this combined experimental and theoretical study, we have extended a quite recently published study on the hydrolysis of C α ‐ and C γ ‐functionalized alkoxytriorganylsilanes of the formula type Me 2 (RO)Si(CH 2 ) n X (R = Me, Et; n = 1, 3; X = functional group) 4. The focus of the present study was the comparison of the hydrolysis kinetics of analogous α‐, β‐, and γ‐aminosilanes of the formula type R 2 (RO)Si(CH 2 ) n NH 2 (R = organyl; n = 1–3).…”