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Comparative Ab initio Treatment (Hartree-Fock, Density-Functional Theory, MP2 and Quadratic Configuration Interactions) of the Cycloaddition of Phosphorus Ylides with Formaldehyde

Published

Author(s)

A A. Restrepo-Cossio, Carlos A. Gonzalez, F Mari

Abstract

In this study we describe the results of the application of several ab initio methods to the reactions of four unstabilized ylides (H[d3]P=CH[d2], H[d2]MeP=CH[d2], HMe[d2]P=CH[d2] and Me[d3]P=CH[d2]) with formaldehyde to form their respective oxaphosphetanes. At the HF level, H[d3]P=CH[d2] and H[d2]MeP=CH[d2] proceeded to the formation of oxaphosphetane. However, at the B3LYP, MP2 and QCISD levels, these ylides react by a nucleophilic attack of the ylidic carbon on the carbonyl group, concomitant with the proton abstraction from the phosphorus atom to form 2-phosphino-ethanol. This unusual dependence of the reaction path with the level of theory utilized indicates that the reaction of H[d3]P=CH[d2], to date the most utilized ylide to model the Wittig reaction, is unsuitable. Al all levels of theory (HF, B3LYP and MP2), the reactions of Hme[d2]P=CH[d2] and Me[d3]P=CH[d2] with formaldehyde proceed in a cycloaddition-like fashion to yield oxaphosphetanes. The calculated barriers, for these processes varied considerably depending upon the level of correlation and the basis set employed. The geometries of reactants, intermediates, transition states and products did not change significantly with the level of theory or basis set employed. The use of B3LYP or MP2 calculations with g-31G* basis set is a reasonable compromise between computational expense and level of rigor to describe the Wittig reaction. Our results suggest that for alkyl substituted ylides, HF geometries at all points of the reaction profile resemble the ones obtained with B3YLP and MP2 methods. Therefore, the Wittig reaction can be properly described at the ab initio level by using B3YLP or MP2 single point energies on the respective HF geometries.
Citation
Journal of Physical Chemistry

Keywords

ab initio, density-functional theory, Hartree-Fock, MP2, QCI, reaction barriers, Wittig Olefination

Citation

Restrepo-Cossio, A. , Gonzalez, C. and Mari, F. (2021), Comparative Ab initio Treatment (Hartree-Fock, Density-Functional Theory, MP2 and Quadratic Configuration Interactions) of the Cycloaddition of Phosphorus Ylides with Formaldehyde, Journal of Physical Chemistry (Accessed December 6, 2022)
Created October 12, 2021