UNDERSTANDING THE MECHANISM OF THE ADDITION OF ORGANOMAGNESIUM REAGENTS TO 2-HYDROXYPROPANAL - AN AB-INITIO MOLECULAR-ORBITAL ANALYSIS

The molecular mechanism for the gas-phase addition of organomagnesium reagents: CH3MgCl, 2CH(3)MgCl, (CH3)(2)Mg, and (CH3)(2)Mg plus Cl2Mg, to 2-hydroxypropanal as a model of chiral alpha-alkoxy carbonyl compou nds is investigated at the 6-31G basis-set level of calculation. An e xtensive exploration of the reactive potential energy surface was carr ied out in order to locate and characterize the stationary points. The geometry of stationary points and the harmonic vibrational frequencie s, transition vectors, and electronic structure of the transition stru ctures were obtained. The theoretical results are analyzed, discussed, and compared with previous theoretical and available experimental dat a. The first step corresponds to the exothermic formation of the chela te complexes without an energy barrier. These stationary points corres pond to puckered five-membered rings, determining the stereochemistry of the global process, which is retained throughout the reaction pathw ay. For the reactions of one equivalent of an organomagnesium compound [CH3MgCl or (CH3)(2)Mg], the second step for the intramolecular mecha nism is associated to the C-C bond formation via 1,3-migration of the nucleophilic methyl group from the organomagnesium compound to the car bonyl carbon and the corresponding transition structure can be describ ed as a four-membered ring, the anti attack being the most favorable p athway. CH3MgCl is a more powerful quelant agent than is the (CH3)(2)M g system. Therefore, the reaction pathway associated to the nucleophil ic attack of CH3MgCl + 2-hydroxypropanal presents a larger barrier hei ght than that of (CH3)(2)Mg + 2-hydroxypropanal addition. The inclusio n of a second equivalent corresponding to the 2CH(3)MgCl and (CH3)(2)M g + Cl2Mg systems yields an intermolecular mechanism, the barrier heig ht decreases, and the process can be considered as an assisted intermo lecular mechanism: The first equivalent forms the chelate structure an d the second one carries out the nucleophilic addition to the carbonyl group. The most favorable pathway corresponds to an intermolecular me chanism via an anti attack for the addition of 2CH(3)MgCl. (C) 1997 Jo hn Wiley & Sons, Inc.