Accessing the Cloke-Wilson Rearrangement via Conjugate Addition of Phosphoranes to Michael Acceptors: A Route to Cyclopropanes and 5-Membered Ring Heterocycles Investigated by Density Functional and <i>Ab Initio</i> Theory

Conjugate addition of unstabilized Wittig-type phosphonium ylides to 1,1-diacceptor- and 1-acceptor-substituted alkenes is investigated by density functional theory and high-level <i>ab initio</i> (DLPNO-CCSD(T)) calculations. The results indicate that the initial conjugate addition step should be facile with barriers predicted to be between 0 and 21 kcal mol^-1. Potential intramolecular follow-up reactions include the formation of acceptor-substituted cyclopropanes as well as the formation of dihydrofuran derivatives via intramolecular S_N2-type transition state structures. The barriers calculated for these potentially valuable cyclization reactions are substantial with Gibbs free energies of activation between 19 and 40 kcal mol^-1. Competing reaction channels include Wittig olefination (for ketones and aldehydes), as well as Claisen condensation reactions. The reaction offers an alternative entry point to the nucleophile-catalyzed Cloke-Wilson rearrangement.