Amide Coordination Effects in Organolithiums.

Organolithiums containing the amide group are examined by ab initio molecular orbital calculations with the 3-21G basis set. Amide coordination with the metal cation results in a large thermodynamic stabilization of the ion pair. Basis set superposition errors at 3-21G are estimated to favor the complex by 10-15 kcal mol{sup {minus}1}; nevertheless, qualitative trends at this level are believed to be reliable. The calculations stabilization energy due to the amide drops off depending upon whether lithiation occurs {alpha}, {beta}, or {gamma} to the amide - provided the cation is accessible to the amide oxygen. Without correction for basis set superposition error, stabilization energies at 3-21G (in kcal mol{sup {minus}1}) are 45 in acetamide, 40 in benzamide, and 38 in syn-bicyclo(1.1.1)-pentane-2-carboxamide. Amide coordination effects in lithiocubanes are also estimated and found to be large. Thus, thermodynamics plays an important role in amide-assisted metalations. In addition, formation of an acetamide-methyllithium complex is found to be 37.5 kcal mol{sup {minus}1} exothermic relative to separated molecules, suggesting that formation of this complex lies along the metalation reaction pathways. This complexation facilitates the reaction kinetically. Analysis of electron density distributions and electrostatic potentials shows that the carbanion-lithium and the amide-lithium interactions are primarily closed-shell ones, more » being essentially ionic bonds. « less