6533b861fe1ef96bd12c4679

RESEARCH PRODUCT

A theoretical study of the addition of CH3MgCl to chiral α-alkoxy carbonyl compounds

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Abstract A theoretical characterisation of the addition of CH 3 MgCl to the carbonyl group of 2-hydroxypropanal and 3,4-di- O -methyl-1- O -trimethylsilyl- l -erythrulose has been carried out. The PM3 semiempirical procedure and 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 ∗ // MP2/6–31G ∗ and MP4/6–31G ∗ //MP2/6–31G ∗ single point calculations. The geometry, harmonic vibrational frequencies, transition vector and electronic structure of the transition structures have been obtained. The dependence of the results upon computing method is analysed, discussed and 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 stereochemistry of the global process, which is retained throughout the reaction pathway. The second step is associated with the CC bond formation via 1,3-migration of the nucleophilic methyl group from the organomagnesium compound to the carbonyl carbon and the corresponding transition structure can be described as a four membered ring. The presence of a pathway on potential energy surface connecting the anti and syn chelate complexes via a conformational change allows us to discuss the possibility of a Curtin-Hammett/Winstein-Holness kinetics-type scheme. The results obtained show that the correct anti/syn product ratio is obtained when this kinetic analysis is taken into account in the calculations.