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.