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 H-2. 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 H-2 by an unstable HPh3PCl complex yielding Ph3PH+ and Cl- ions in solution. Moderate heating is required to accelerate the slow H-2-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 CI and Ph3P as reactive frustrated Lewis pair (FLP) catalysts.