Alexander J. Kos

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 A and the C-F bond antiparallel to the carbanion lone pair is elongated by 0.13 A 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 NH2 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 F2BCH2- and F2AlCH2-. 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. NH2CH2F, are similar: energetic stabilisation, conformational dependence, geometrical differences, and charge transfer. Negative hyperconjugation is not controversial; it is firmly established. We underscore Binghams conclusion,31 “anionic hyperconjugation is, indeed, a very real, general, and important concept”.

Regioselektive dimetallierung von aromaten. Bequemer zugang zu 2,2′-disubstituierten biphenylderivaten

Abstract Lithiation of biphenyl with 2.4 mol of n-butyllithium in the presence of TMEDA led directly to the 2,2′-dilithio derivative (I) in modest, but preparatively useful yields. I, in turn, was converted to a variety of products. The activation of the 2′-position of 2-lithiobiphenyl was shown directly by a separate experiment. MNDO calculations indicate stabilization in I by double bridging and in 2-lithiobiphenyl by intramolecular π interaction of Li with the o-phenyl group. Similar interactions in substitution transition states rationalize the specificity of the reactions observed.

C2Li6 structural isomers

Abstract Three C 2 Li 6 isomers characterized by triple (III), double (VII), and single (VI) CC bonds are revealed by minimal basis set ab initio calculations to be favorable minima on the singlet potential energy surface.

The structures of lithium and magnesium derivatives of acetic acid ‘dianions’

Ab initio and semi-empirical MNDO calculations indicate that monomeric dilithiated and bis-magnesyl derivatives of carboxylic acids (Ivanoff reagents) favour the 1,3-1,3′ doubly bridged structures generally found in metallated Y-conjugated dianion systems.

The relationship between the energies of carbanions, R–, and their lithiated counterparts, RLi. An ab initio study

While the methyl stabilisation energies of fifteen C1–C3 organolithium compounds, RLi, reflect (attentuation factor = 0·71) the methyl stabilisation energies of the corresponding anions, R–, deviations are found when the degree of charge localisation or delocalisation is changed significantly by association with lithium.

Stabilization of the alleged ‘bishomoromatic’ bicyclo[3.2.1]octa-2,6-dienyl anion by counterion interactions and by hyperconjugation

Hyperconjugation and inductive effects, rather than homoaromaticity, are responsible for the stabilization of the title anion in the gas phase; interaction of the double bond with the Li+ gegenion in the endo geometry contributes additionally in solution.

X-Ray crystal structure of [{o-C6H4(CHPh)2}{Li(tmeda)}2](tmedaMe2NCH2CH2NMe2). Unsymmetrical lithium bridging and (E,E)-conformation of the phenyl groups

The lithium atoms in the title compound are shown by X-ray analysis to prefer quite different locations, in contrast to the symmetrical double bridging exhibited by other 1,4-dilithium compounds, but in agreement with model MNDO calculations; furthermore, the (E,E) conformation of the two phenyl groups differs from the (E,Z) conformation reported in methyltetrahydrofuran solution.

Symmetrical double bridging in dilithiated propene,CH2CHCHLi2

Two symmetrically doubly lithium-bridged structures for dilithiated propene,CH2 CHCHLi2(3) and (4), are indicated to be comparably stable by ab initio(3-21G//3-21G) calculations.

The influence of carbanion orbital orientation and charge distribution on the structures of polylithium compounds

Symmetrical (C2v) double bridging, commonly exhibited by 1,4-dilithium compounds e.g. in (1) and (2), is not favoured in (4) and (5) since charge deloclization in the dianion, hybridization, and orbital orientation effects are more important than quadrupole-like electrostatic interactions (3).

β-Substituted cyclopropyl radicals

Ab initio and MNDO molecular orbital calculations support experimental evidence that 2-metallo-substituted cyclopropyl radicals prefer cis- while 2-halogenocyclopropyl radicals prefer trans-configurations.