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Understanding the mechanism of the addition of organomagnesium reagents to 2-hydroxypropanal: An ab initio molecular orbital analysis

✍ Scribed by Mónica Oliva; Vicent S. Safont; Juan Andrés; Raquel Castillo; Vicente Moliner

Publisher
John Wiley and Sons
Year
1997
Tongue
English
Weight
369 KB
Volume
65
Category
Article
ISSN
0020-7608

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✦ Synopsis

The molecular mechanism for the gas-phase addition of Ž . Ž . organomagnesium reagents: CH MgCl, 2CH MgCl, CH Mg, and CH Mg plus 3 3 3 2 3 2

Cl Mg, to 2-hydroxypropanal as a model of chiral ␣-alkoxy carbonyl compounds is 2 investigated at the 6-31G* basis-set level of calculation. An extensive exploration of the reactive potential energy surface was carried out in order to locate and characterize the stationary points. The geometry of stationary points and the harmonic vibrational frequencies, transition vectors, and electronic structure of the transition structures were obtained. The theoretical results are analyzed, discussed, and compared with previous theoretical and available experimental data. The first step corresponds to the exothermic formation of the chelate complexes without an 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. For the reactions of w Ž .

x one equivalent of an organomagnesium compound CH MgCl or CH Mg , the second 3 3 2 step for the intramolecular mechanism is associated to 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 anti attack being the most favorable pathway. CH MgCl is a 3 Ž . more powerful quelant agent than is the CH Mg system. Therefore, the reaction 3 2 pathway associated to the nucleophilic attack of CH MgCl q 2-hydroxypropanal presents 3 Ž . a larger barrier height than that of CH Mg q 2-hydroxypropanal addition. The 3 2 Ž . inclusion of a second equivalent corresponding to the 2CH MgCl and CH Mg q

3 3 2

Cl Mg systems yields an intermolecular mechanism, the barrier height decreases, and 2

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