Predicting the direction of nucleophilic attack in vinyl halides: halogenophilic versus carbophilic reactivity of metal carbonyl anions

Abstract

Transition metal carbonyl anions ([M(CO)nL]‐, L=CO, Cp) were previously shown to react with activated vinyl halides (VinHal) via two distinct mechanistic pathways: ‘carbophilic’ addition–elimination substitution (AdNE) and halogenophilic mechanism. Such competition between attack at carbon and halogenophilic attack is relevant for many other reactions of soft nucleophiles with aryl, vinyl, and alkyl halides. As descriptors of the experimentally observed reaction pathway, various quantum‐chemical parameters (π* and σ*C‐Hal molecular orbital and reaction energies, Fukui coefficients ) and their combinations were examined. Correlation of the computational parameters with the kinetic data allowed to develop separate models for VinHal reactivity in AdNE and halogenophilic reactions, and to predict the direction of [M(CO)nL]‐ attack for the whole set of VinHal with reasonable accuracy. The lg(k) of ‘carbophilic’ AdNE reactions were found to roughly correlate with π* lowest unoccupied molecular orbital energies, but a three‐parameter model was required for a more accurate description. The lg(k) of halogenophilic reactions does not correlate with the energy of the corresponding σ*C–Hal molecular orbital, but is well described by a linear combination of density functional theory energies of Hal+ abstraction from VinHal and Hal+ affinity of [M(CO)nL]–. The developed approach can be also used to create a semiquantitative dual electrophilicity scale of vinyl halides (at carbon and at halogen) including the cases, when one of the reaction pathways is experimentally unobservable, because of the much higher rate of competing process. Copyright © 2012 John Wiley & Sons, Ltd.