Delocalization does not always stabilize: A quantum chemical analysis of α-substituent effects on 54 alkyl and vinyl cations

Abstract

The effects of α-substituents on alkyl and vinyl cations are studied using high-level ab initio calculations. The geometries, stabilities, and electronic properties of 27 alkyl cations and 27 vinyl cations with α-substituents are computed at the B3LYP/6-3114-G(d,p), MP2/6-311+G(d,p), and CBS-Q levels. The substituents studied vary from strongly destabilizing (e.g., α-CN and α-CF3) to strongly stabilizing (e.g., α-OSi(CH3)3 and α-NH2). The calculations show that in the case of vinyl cations the stabilization provided by the α-substituents is larger by an average value of 4 kcal/mol than for the alkyl ones. This is the result of the intrinsically lower stability of vinyl cations (on average 17 kcal/mol). However, strong inductively donating or withdrawing α-substituents show different behavior. Because of the high amount of s character in the carbon-α-substituent bond in the vinyl cations (sp hybridized), more pronounced o effects are found than in the corresponding alkyl cations, leading to lower stabilization for inductively withdrawing α-substituents and higher stabilization for inductively donating ones. Thus, distinct effects of α-substituents on the stabilization of the cations are observed. However, no correlation is found between NBO-computed charge increases or bond-order increases at either the carbocationic center or at the α-substituent of the molecule and the stability provided by an α-substituent. This demonstrates the conceptual difference between stabilizing and electron-donating effects. Only for the Cβ-H hyperconjugative effect in the vinyl systems is a correlation with the computed reaction enthalpies observed. Finally, the effect of leaving-group variation is studied. Changing the leaving group from H to Cl yields geminal effects ranging from 7 kcal/mol destabilization to 9 kcal/mol stabilization of the neutral precursor.