The importance of negative (anionic) hyperconjugation

Abstract

The consequences of negative (anionic) hyperconjugation (interactions of orbitals of π-symmetry of saturated groups with filled p orbitals) upon geometries, energies, and charge distributions are calculated for anions and related neutral systems using the ab initio split valence basis sets, 3−21+G and 4−31+G, which are augmented by a set of diffuse s and p functions on the non-hydrogen atoms. The β-fluorethyl anion, which has often been used for analysis, is not a minimum on the potential energy surface. Essentially complete transfer of negative charge to fluorine leads to rupture of the C-F bond and formation of a hydrogen bonded F − ethylene complex, 10, during geometry optimisation. The effects of fluorine negative hyperconjugation, assessed by using the trifluorethyl anion, are in accord with the implications of the “no-bond” formalism for negative hyperconjugation: the C-C bond is shortened by 0.10 Å and the C-F bond antiparallel to the carbanion lone pair is elongated by 0.13 Å compared to trifluoroethane. Similarly large geometrical differences are found when β-amino-and β-hydroxyethyl anion are compared with ethylamine and ethanol. The total anion stabilisation energies are quite large. The inductive contributors, evaluated using conformations in which the carbanion lone pair cannot interact hyperconjugationally with the C-X bond, are 11.3, 10.3 and 5.3 kcal mol -1 for F, OH and NH 2 in β-ethyl anions. The hyperconjugative contributions to the total effect are of similar magnitude. The potential rotational energy surfaces of fluoromethyl amine and aminomethanol also demonstrate the importance of negative hyperconjugation in neutral systems (anomeric effect). Unsaturated olefinic systems, e.g. the 2-fluorovinyl and 2,2-difluorovinyl anions, also show the expected C-F bond lengthening and CC bond shortening. The planar inversion barrier of the 2,2-difluorovinyl anion (11.9 kcal mol -1; 3−21+G//3−21+G) is lowered from that of the vinyl anion due to hyperconjugative stabilisation of the transition state. Fluorine negative hyperconjugation is also illustrated in the stabilisation of the rotational transition states of species like F 2BCH 2 - and F 2AlCH 2 -. Negative (anionic) hyperconjugation and the anomeric effect are the same phenomena from a qualitative molecular orbital viewpoint. Both are well established, important, and uncontroversial. We conclude that negative (anionic) hyperconjugation is of great significance in organic chemistry. Energetically, hyperconjugation is generally as important as the inductive effects of β-electronegative substituents. Rather large hyperconjugative stabilisations, on the order of 10kcalmol -1 for an optimally aligned substituent, are indicated. In such alignments, charge transfer from a carbanion lone pair to an electronegative group can play a significant role; polarisation may be operative in addition to or in place of charge transfer. There is a concerted set of geometrical changes which correspond to expectations based on “no bond resonance” formulations ( 2). Anomeric effects in neutral molecules, e.g. NH 2CH 2F, are similar: energetic stabilisation, conformational dependence, geometrical differences, and charge transfer. Negative hyperconjugation is not controversial; it is firmly established. We underscore Bingham's conclusion, 31 “anionic hyperconjugation is, indeed, a very real, general, and important concept”.