A THEORETICAL-STUDY OF THE ADDITION OF CH3MGCL TO CHIRAL ALPHA-ALKOXYCARBONYL-COMPOUNDS

A theoretical characterisation of the addition of CH3MgCl to the carbo nyl group of 2-hydroxypropanal and 3,4-di-O-methyl-1-O-trimethylsilyl- L-erythrulose has been carried out. The PM3 semiempirical procedure an d the ab initio method at HF level of theory with the 3-21G and 6-31G basis sets have been applied to identify the stationary points on the potential energy surface. The correlation effects have been included by using the MP2/6-31G method, as well as by means of MP3/6-31G*// MP 2/6-31G and MP4/6-31G*//MP2/6-31G* single point calculations. The geo metry, harmonic vibrational frequencies, transition vector and electro nic structure of the transition structures have been obtained. The dep endence of the results upon computing method is analysed, discussed an d compared with previous theoretical and available experimental data. The first step corresponds to the exothermic formation of the anti or syn chelate complexes without energy barrier. These stationary points correspond to puckered five membered rings, determining the stereochem istry of the global process, which is retained throughout the reaction pathway. The second step is associated with the C-C bond formation vi a 1,3-migration of the nucleophilic methyl group from the organomagnes ium compound to the carbonyl carbon and the corresponding transition s tructure can be described as a four membered ring. The presence of a p athway on potential energy surface connecting the anti and syn chelate complexes via a conformational change allows us to discuss the possib ility of a Curtin-Hammett/Winstein-Holness kinetics-type scheme. The r esults obtained show that the correct anti/syn product ratio is obtain ed when this kinetic analysis is taken into account in the calculation s. (C) 1998 Elsevier Science B.V.