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Understanding the mechanism of the addition of organomagnesium reagents to 2‐hydroxypropanal: An ab initio molecular orbital analysis
Author(s) -
Oliva Mónica,
Safont Vicent S.,
Andrés Juan,
Castillo Raquel,
Moliner Vicente
Publication year - 1997
Publication title -
international journal of quantum chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.484
H-Index - 105
eISSN - 1097-461X
pISSN - 0020-7608
DOI - 10.1002/(sici)1097-461x(1997)65:5<719::aid-qua38>3.0.co;2-u
Subject(s) - chemistry , nucleophilic addition , transition state , computational chemistry , intramolecular force , potential energy surface , ab initio , nucleophile , exothermic reaction , reagent , molecular orbital , aldehyde , ring (chemistry) , molecular geometry , gaussian orbital , molecule , stereochemistry , organic chemistry , catalysis
The molecular mechanism for the gas‐phase addition of organomagnesium reagents: CH 3 MgCl, 2CH 3 MgCl, (CH 3 ) 2 Mg, and (CH 3 ) 2 Mg plus Cl 2 Mg, to 2‐hydroxypropanal as a model of chiral α‐alkoxy carbonyl compounds is 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 one equivalent of an organomagnesium compound [CH 3 MgCl or (CH 3 ) 2 Mg], the second step for the intramolecular mechanism is associated to the C(SINGLE BOND)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 3 MgCl is a more powerful quelant agent than is the (CH 3 ) 2 Mg system. Therefore, the reaction pathway associated to the nucleophilic attack of CH 3 MgCl+2‐hydroxypropanal presents a larger barrier height than that of (CH 3 ) 2 Mg+2‐hydroxypropanal addition. The inclusion of a second equivalent corresponding to the 2CH 3 MgCl and (CH 3 ) 2 Mg+Cl 2 Mg systems yields an intermolecular mechanism, the barrier height decreases, and the process can be considered as an assisted intermolecular mechanism: The first equivalent forms the chelate structure and the second one carries out the nucleophilic addition to the carbonyl group. The most favorable pathway corresponds to an intermolecular mechanism via an anti attack for the addition of 2CH 3 MgCl. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65 : 719–728, 1997