THEORETICAL-STUDIES OF CHEMICAL INTERACTIONS - AB-INITIO CALCULATIONS ON LITHIUM DIALKYLAMIDES AND THEIR CARBONYLATION REACTIONS

Lithium dimethylamide (monomer and dimer) and several carbonyl complexes proposed as intermediates in its CO insertion reaction have been investigated by means of ab initio calculations (6-31+G//6-31G and MP2/6-31+G*//6-31G). The calculated values of the main geometrical parameters of the dimer are very close to those from solid-state determinations with stable lithium dialkylamides. The dimerization energy is predicted to be -60.7 and -59.9 kcal/mol at 6-31+G//6-31G and MP2/6-31+G*//6-31G levels, respectively. Calculations show a eta(2)-coordinated lithium in the first intermediate derived from lithium dimethylamide monomers and CO and a rather long C-O bond (1.30 Angstrom). This indicates a; significant alkoxycarbene character, rather than the classical carbamoyl structure. A second intermediate arising from a second CO insertion exhibits a planar geometry with relatively short O-Li bonds and the lithium atom coordinated to both oxygens. The calculations predict the double carbonylation to be a thermodynamically favorable process (-37.6 and 27.7 kcal/mol at 6-31+G and MP2/6-31+G* levels, respectively), in contrast to previous reports but in agreement with experimental results. A third intermediate, formally produced by the coupling of the two just described, shows two eta(2)-coordinated lithiums and a planar arrangement of the main heavy atoms: the calculated high stability (-85.8 and -91.2 kcal/mol at 6-31+G//6-31G and MP2/6-31+G*//6-31G levels, respectively), explains the high yields of substituted hydroxymalonamides obtained under special reaction conditions. The properties of the tetramethylurea dianion proposed as the precursor for dimethylformamides were also calculated: the high electron density (-1.113) found for the central carbon atom predicts the facility of this intermediate for producing tetramethylureas.