Catalytic Hydrofunctionalization of Alkynes through P-H Bond Addition: The Unique Role of Orientation and Properties of the Phosphorus Group in the Insertion Step

The puzzling question of alkyne insertion into Pd-P and Pd-H bonds leading to the formation of new Pd-C, C-P, and C-H bonds was explored by theoretical calculations at the CCSD(T) and B3LYP levels of theory. The key factors responsible for selectivity of catalytic hydrofunctionalization of alkynes were resolved and studied in details for the models of hydrophosphorylation, hydrophosphinylation, and hydrophospination reactions. In contrast with the generally accepted mechanistic picture, the calculations have shown that several pathways are possible depending on the nature and geometrical arrangement of the phosphorus group. It was found that the product of alkyne insertion into the metal-hydrogen bond should be easily formed under kinetic-control conditions, while the product of alkyne insertion into the metal-phosphorus bond may be formed in certain cases under thermodynamic control. For the first time, the calculations have revealed the role of the oxygen atom in the reactivity of P=P(O)R-2 groups and the role of the interactions involving the lone pair of the P=PR2 group in the reagent. The fundamental properties of the Pd-P, C-P, and P-H bonds were reported, and the larger bond strength upon increasing the number of oxygen atoms bound to phosphorus (P=PR2, P(O)R-2, and P(O)(OR)(2)) have been shown. The relationship between bond energy, acidity, and reactivity of the studied phosphorus compounds has been determined.