Vinyl anions geometry

Carbanions have been less studied, apart from CH3, 167,168 but included in a more recent set of calculations169 on several carbanions (CH3-, C2H5-, and ethynyl anion) are calculations on C2H3-. For reliable calculations on this type of molecule, diffuse orbitals must be added to the basis set. Several different basis sets were used, but the geometry of the neutral parent molecule was used in some of the calculations. The main aim of this paper was to investigate the electron density and difference densities, electron affinities, and proton affinities. The inversion barrier in the vinyl anion was ca. 142 kJ mol-1 which was in good agreement with that found by Lehn et a/.148 in an earlier calculation. 

Stereoelectronic features of vinyl anions are associated with their bent geometries and usual configurational stabilities in solution. Theory predicts a sizable inversion barrier for the parent vinyl anion (28.7kcal/ mol at the MP2/6-31+G(d)//HF/6-31 -nG(d) level). The two conformers of 1-propenyl anion can be formed stereospecifically in the gas phase (via desilylation of E- and Z-l-trimethylsilylpropene) and show different reactivity patterns. ... 

The sodium donates an electron to the LUMO of the triple bond (one of the two orthogonal 7t orbitals). The resulting radical anion can pick up a proton from the ammonia solution to give a vinyl radical. A second electron, supplied again by the sodium, gives an anion that adopts the more stable trans geometry. A final proton quench by a second molecule of ammonia or by an added proton source (t-butanol is often used, as in the Birch reduction) forms the E-alkene. 

The chemistry of phospholes and related phospholide anion complexes remains a very active area, which also continues to attract the attentions of the theoretical community. Among recent theoretical contributions are a consideration of the stability, structure and bonding in lithium- and beryl-lium-pentaphospholide systems, the aromaticity of the pentaphospholide anion (and its arsenic analogue) as probed by ring currents,the remarkable influence of fluorine-substitution (either at phosphorus or at a ring carbon) on the electronic and thermochemical properties of phospholes,and the effects of methyl and vinyl substitution at various positions on the geometries, relative stabilities and Diels-Alder reactivities of phospholes. An ab initio approach has been used to reinterprete some spectral and thermochemical properties of IH-phospholes. The synthesis and reactivity of phospholes of reduced... 

In contrast, reductions using Al/Hg seem to be stereoselective at high reaction temperatures to afford exclusively the more stable E-alkene product. The equilibration of the anionic intermediates under the reaction conditions is responsible for this isomerization. The temperature is a crucial factor in controlling the stereoselectivity of the process, since the geometry of the vinylic sulfone can be maintained when the reduction is carried out at low temperature. Partial scrambling of configuration is observed upon increasing the reaction temperature (Eq. 23).41... 

These studies demonstrate that the geometry of the carbanion site can be an important factor in the methylene stereochemistry of anionic oligomerizations and polymerizations of vinyl monomers. 

---