Stereochemistry of Metalated Aldimines. 1. An ab Initio Study of the Potential Energy Surfaces of Isolated Acetaldimine Anions, their Monomeric Lithium and Sodium Ion Pairs, and Mechanistic Consequences

Equilibrium geometries and transition-state structures for syn/anti isomerization and CC rotation have been examined for the isomeric carbanions of acetaldimine and N-methylacetaldimine and for their ion pairs formed with Li and Na. Effects of the N-substituent and of the nature of the gegenion on the structures and topologies, relative stabilities, and activation energies are discussed and compared to oxyimine analogues. The π-conjugated syn- and αnti-configured azaallyl anions, best considered as amide anions rather than carbanions, are minima and their thermodynamic syn preference accounts for the regiochemistry of reactions of the free ions. Ion-pair formation reverses the relative isomer stabilities; a preference for the anti configuration is found for the unsolvated monomeric ion pairs and also for the di- and trisolvated lithioacetaldimines. The most stable structures of the Li and Na ion pairs of the syn-metaloaldimines all involve η1 Nσ coordination and in the anti isomers M+ coordinates to the azaallylic system either in a π-fashion or exclusively to N. Racemizations of enantiomeric π-complexes are fast processes. The activation barriers for syn/anti isomerization of the ion pairs are smaller than in the free anions and all isomeric ion pairs are readily available for aggregate formation. The relative stabilities and the geometries of the monomeric ion pairs suggest that metalated imines are aggregated in ether solutions and, moreover, that the monomeric ion pairs are not the reactive species in reactions with electrophiles. Ion-pair catalysis of the reaction of the syn-configured ion pair is proposed as a possible mechanism for the kinetically controlled formation of the syn product. The mechanism responsible for the regioselective formation of a new bond to the α-carbon in the syn position of a metalated alkylimine thus appears to be entirely different from the mechanisms thought to be operative in similar reactions of metalated oxy- and aminoimine. In the case of the metalated imines, cooperative effects appear necessary to allow for the regioselectivity of the electrophile addition and the ion-pair dimer is discussed as the smallest conceivable reactive species