Collection of Czechoslovak Chemical Communications - digital archive 

Abstract

The nature of the intramolecular coordination in dimethyl 2-lithio-2,4,4-trimethylglutarate (I) as the model of living dimer of methyl methacrylate was studied by comparison with the specific solvation of methyl 2-lithioisobutyrate, using ab initio SCF calculations with the 3-21G basis set. In both cases, electron-density donation from the incident oxygen leads to a weak coordination bond (20 to 30% of full covalency) accompanied by polarization and weakening of the original O-Li bond. The directionality of any O-Li or C-Li bond in all the systems under study is poor and the position of Li results from a complex interplay of covalent and electrostatic interactions with its molecular environment. The covalent part of the bond system can be characterized as a system of polycentric electron-defficient bonds. Possible conformational geometries of I were examined by ab initio calculations and the results were compared to those of the semiempirical MNDO method. The inclination of MNDO to create artificial local energy minima in previously unsuspected cases was detected and shown to be avoidable if the valence sphere of Li is saturated by electron donors. The formation of aggregation dimers of I was studied by MNDO and the most stable conformers were found. The study was paralleled for the di-tert-butyl ester analogue of I. The tendency of I to intramolecular coordination and self-aggregation is discussed in view of the known kinetics of the first steps of anionic propagation.