A MNDO-Study of Solvent Free and Solvated Dimeric Lithium Ion Pairs of Acetaldoxime. Models for Dimeric Aggregates of Lithiated Oxime Ethers

Cyclic and stack-type structures of solvent-free and solvated dimeric lithium ion pairs of isomeric carbanions of acetaldoxime were investigated as models for lithiated oxime ethers. The adequacy of MNDO is evaluated critically with reference to ab initio results. Severe overestimation of the pyramidalization at the carbanionic center and insufficient carbon lone-pair-lithium core repulsion are found to cause the following artifacts: excessive charge localization, the breakdown of the pseudo-π conjugation within the ligands, and orbital orientation towards the cation; standard MNDO calculations yield entirely unacceptable results. A simple method is proposed to counteract some of these deficiencies and results thus obtained are in significantly better agreement with ab initio data. Solvated cyclic ( C _i symmetric) dimers were found to be the most important aggregated species; they are preferred over bis-solvated monomeric ion pairs at the MNDO level. The magnitudes of the relative energies are such that a definitive conclusion regarding the predominance of monomeric or dimeric aggregates cannot be made. Mechanistic implications with regard to the experimentally observed syn -axial entry of electrophiles into configurationally fixed lithiated oxime ethers are discussed.