The mechanism of [1,2] methyl Wittig migration in the simple free and lithiated anionic H2COCH3 model system is investigated at the CAS-MCSCF and CCSD(T) levels of theory, providing, on one hand, a picture of the processes taking place in the gas phase and, on the other hand, two extreme descriptions of ionic association relevant to the condensed phase (absent or tight anion-cation interaction). (A) The bonding situation of the free carbanion correlates with the homolytic dissociation limit, but the heterolytic case is significantly lower in energy, and an avoided crossing takes place: dissociation becomes consequently heterolytic in character. However, the heterolytic limit is not necessarily attained, because nucleophilic attack on the carbonyl carbon by methide occurs before an oxygen-carbon distance of 3 Angstrom is reached. (B) In contrast, in the lithiated system, the O-C bond is homolytically cleaved, because the lithium counterion stays firmly bound to oxygen, thus stabilizing the incipient radical anion of formaldehyde, and is unable to assist the O-C bond cleavage by setting an interaction with the detaching methyl carbon. The electrostatic complex between formaldehyde and methyllithium, although rather stable, does not appear to be immediately reachable, even if methyl radical can reassociate to formaldehyde radical anion not only from the same face from which it detached but also from the opposite face, attained by pivoting rather closely around lithium. The lithiated oxyanion is then readily obtained, in a radical coupling reassociation step. How solvation could modulate the cation-anion interaction is explored by allowing lithium to interact with three dimethyl ether molecules. Also this model confirms the preference for O-C homolytic cleavage. In summary, the study of this simple system clearly suggests a mechanistic dichotomy: the preferred heterolytic mechanism of the gas-phase reaction contrasts the homolytic cleavage indicated for the condensed phase reaction (although some dependence of the mechanism on the degree of cation-anion interaction and substitution is to be expected).
Mechanism of the anionic Wittig rearrangement. An ab initio theoretical study
ANTONIOTTI, Paola;TONACHINI, Glauco
1998-01-01
Abstract
The mechanism of [1,2] methyl Wittig migration in the simple free and lithiated anionic H2COCH3 model system is investigated at the CAS-MCSCF and CCSD(T) levels of theory, providing, on one hand, a picture of the processes taking place in the gas phase and, on the other hand, two extreme descriptions of ionic association relevant to the condensed phase (absent or tight anion-cation interaction). (A) The bonding situation of the free carbanion correlates with the homolytic dissociation limit, but the heterolytic case is significantly lower in energy, and an avoided crossing takes place: dissociation becomes consequently heterolytic in character. However, the heterolytic limit is not necessarily attained, because nucleophilic attack on the carbonyl carbon by methide occurs before an oxygen-carbon distance of 3 Angstrom is reached. (B) In contrast, in the lithiated system, the O-C bond is homolytically cleaved, because the lithium counterion stays firmly bound to oxygen, thus stabilizing the incipient radical anion of formaldehyde, and is unable to assist the O-C bond cleavage by setting an interaction with the detaching methyl carbon. The electrostatic complex between formaldehyde and methyllithium, although rather stable, does not appear to be immediately reachable, even if methyl radical can reassociate to formaldehyde radical anion not only from the same face from which it detached but also from the opposite face, attained by pivoting rather closely around lithium. The lithiated oxyanion is then readily obtained, in a radical coupling reassociation step. How solvation could modulate the cation-anion interaction is explored by allowing lithium to interact with three dimethyl ether molecules. Also this model confirms the preference for O-C homolytic cleavage. In summary, the study of this simple system clearly suggests a mechanistic dichotomy: the preferred heterolytic mechanism of the gas-phase reaction contrasts the homolytic cleavage indicated for the condensed phase reaction (although some dependence of the mechanism on the degree of cation-anion interaction and substitution is to be expected).
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