Reduction of Phosphine Oxide by Using Chlorination Reagents and Dihydrogen: DFT Mechanistic Insights

Extensive DFT calculations provide detailed mechanistic insights into the metal-free reduction of phosphine oxide Ph₃P=O by using chlorination reagents O=CClX (X=COCl, Cl, OCCl₃ and Ph) and H₂. Fast electrophilic attack to the P=O group oxygen atom is favored by exergonic CO₂ release to form phosphonium Ph₃PCl⁺ and chloride Cl⁻, which may slowly cleave H₂ by an unstable HPh₃PCl complex yielding Ph₃PH⁺ and Cl⁻ ions in solution. Moderate heating is required to accelerate the slow H₂-activation step and to eliminate HCl to form phosphine Ph₃P instead of Ph₃PH⁺Cl⁻ salt as the desired product. Though partially quenched by Ph₃P (and reactant Ph₃P=O if present), borane B(2,6-F₂C₆H₃)₃ can be still combined with Cl⁻ and Ph₃P as reactive frustrated Lewis pair (FLP) catalysts.