NUCLEOPHILIC RING-OPENING OF EPOXIDES BY ORGANOLITHIUM COMPOUNDS - AB-INITIO MECHANISMS

Ab initio calculations (MP4/6-31+G//6-31+G*+DELTAZPE) have been perfo rmed on the ring-opening reaction of ethylene oxide by monomeric and d imeric lithium hydride and by dimeric methyllithium, both with retenti on and with inversion of configuration at the carbon being attacked. S everal highly ionic transition structures were located. The large pref erence for reaction with inversion is due to the higher degree of rupt ure of the C-O bond in the retention transition structure. Cationic as sistance, i.e. Li+ coordination to the epoxide oxygen, significantly l owers the activation energies. An intermolecular push-pull mechanism, or an intramolecular mechanism in which the lithium aggregate is opene d to form a chain, is proposed for reaction in moderately polar solven ts.