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/6-311+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 σ 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.

Calculated stabilities and structures for carbocations and singlet carbenes bearing electron‐withdrawing groups

“…Tables 3 and 4 give the corresponding data, including enthalpies of hydrogenation in Table 4, for EWG-CH singlet carbenes and their rotational transition states, all calculated at the G3MP2 level. Our results agree well with published computational results [14][15][16][17][18][19][20][21][22][23][24] where comparisons are possible.…”

“…More recent computational work by van Alem, Lodder and Zuilhof has also been reported. [24] In this study, we report high level computational results on a variety of EWG-CH 2 + cations and their conformational isomers. Electronic substituent effects on the conformers are compared, and similarities between EWG-CH 2 + species and their conjugate base carbenes, EWG-CH, are discussed.…”

Characterization of CF Bonds with Multiple‐Bond Character: Bond Lengths, Stretching Force Constants, and Bond Dissociation Energies

“…[47,48] Ions R 2 CF + contain a CF + bond, which is isoelectronic with the carbonyl bond and which by pp donation from the F atom can obtain double-bond character. This is reflected by an increased gas-phase acidity of the carbenium ions and their enhanced reactivity towards electrophilic attack.…”

Selective removal of 1,2‐propanediol and 1,2‐butanediol from bio‐ethylene glycol by catalytic reaction

“…The reason for the higher conversions of PDO and BDO may be ascribed to their molecular structures. The electrondonating effect of methyl 59,60 and ethyl 61 in PDO and BDO molecules can stabilize the carbonium ions in the course of diol dehydration (pinacol-rearrangement mechanism). Moreover, because the electron-donating capacity of ethyl is larger than methyl, the reactivity of BDO should be higher than PDO.…”