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

Lithium dimethylamide (monomer and dimer) and several carbonyl complex es proposed as intermediates in its CO insertion reaction have been in vestigated 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 paramete rs of the dimer are very close to those from solid-state determination s with stable lithium dialkylamides. The dimerization energy is predic ted 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 lith ium in the first intermediate derived from lithium dimethylamide monom ers and CO and a rather long C-O bond (1.30 Angstrom). This indicates a; significant alkoxycarbene character, rather than the classical carb amoyl structure. A second intermediate arising from a second CO insert ion exhibits a planar geometry with relatively short O-Li bonds and th e lithium atom coordinated to both oxygens. The calculations predict t he 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 r esults. A third intermediate, formally produced by the coupling of the two just described, shows two eta(2)-coordinated lithiums and a plana r 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 level s, respectively), explains the high yields of substituted hydroxymalon amides obtained under special reaction conditions. The properties of t he tetramethylurea dianion proposed as the precursor for dimethylforma mides were also calculated: the high electron density (-1.113) found f or the central carbon atom predicts the facility of this intermediate for producing tetramethylureas.