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 Ph₃ P=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 Ph₃ PCl⁺ and chloride Cl^- , which may slowly cleave H₂ by an unstable HPh₃ PCl complex yielding Ph₃ PH⁺ and Cl^- ions in solution. Moderate heating is required to accelerate the slow H₂ -activation step and to eliminate HCl to form phosphine Ph₃ P instead of Ph₃ PH⁺ Cl^- salt as the desired product. Though partially quenched by Ph₃ P (and reactant Ph₃ P=O if present), borane B(2,6-F₂ C₆ H₃ )₃ can be still combined with Cl^- and Ph₃ P as reactive frustrated Lewis pair (FLP) catalysts.