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.

Photolysis of a series of E or Z stereoisomeric α-R-substituted bromostyrenes (R = CH 3 , F or CN) in methanol yields E and/ or Z stereoisomeric styrenes, which stem from the corresponding vinyl radicals. The results show that the α-Me vinyl radical is a rapidly equilibrating, bent structure, while the α-F vinyl radical is a stable bent species, in agreement with earlier thermal results. The α-CN vinyl radical is assigned as a rapidly inverting bent and not a linear species from the

The photochemical reactions in methanol of the vinylic halides 1-4, halostyrenes with a methyl or a trifluoromethyl substituent at the alpha- or beta-position, have been investigated quantitatively. Next to E/Z isomerization, the reactions are formation of vinyl radicals, leading to reductive dehalogenation products, and formation of vinyl cations, leading to elimination, nucleophilic substitution, and rearrangement products. The vinyl cations are parts of tight ion pairs with halide as the counterion. The elimination products are the result of beta-proton loss from the primarily generated alpha-CH(3) and alpha-CF(3) vinyl cations, or from the alpha-CH(3) vinyl cation formed from the beta-CH(3) vinyl cation via a 1,2-phenyl shift. The beta-CF(3) vinyl cation reacts with methanol yielding nucleophilic substitution products, no migration of the phenyl ring producing the alpha-CF(3) vinyl cation occurs. The alpha-CF(3) vinyl cation, which is the most destabilized vinyl cation generated thus far, gives a 1,2-fluorine shift in competition with proton loss. The experimentally derived order of stabilization of the vinyl cations photogenerated in this study, alpha-CF(3) < beta-CF(3) < beta-CH(3) < alpha-CH(3), is corroborated by quantum chemical calculations, provided the effect of solvent is taken into account.

Alkyl cations of the form CH3C(+)HR have been investigated using high-level quantum chemical methods to study the influence of α-substituents R (R = H, CH3, CHCH2, C⋮CH, F, and Cl) on cation geometries and relative energies with respect to the neutral precursors CH3CH2R. The results of density-functional B3LYP computations with a variety of basis sets were compared with MP2, MP4, QCISD(T), and CBS-Q model chemistry results and benchmarked against experimental data. The results show that geometrical features are already accurately described using B3LYP/6-311G(d,p) or MP2/6-311G(d,p). For a systematic study of the energetics of α-substitution on alkyl cations, B3LYP/6-311+G(d,p) and MP2/6-311+G(d,p) computations form a useful compromise between accuracy (average deviation within 1 kcal/mol of the experimental error) and computational efficiency. The electronic structures of these species and their precursors CH3CH2R were studied using both natural bond orbital (NBO) and Atoms-in-molecules (AIM) analyses. These analyses clearly show that the electron-donating power of α-substituents at (partially) positively charged carbon atoms does not correlate well with the thermochemical stabilization of cations provided by such substituents.

No abstract