Catalyst-Free Nucleophilic Substitution of Deuterium in Pyridine- d 5 with Phosphorus Nucleophiles Triggered and Driven by Electrophilic Acetylenes: Mechanism Revisited

Catalyst- and solvent-free reaction of pyridine- d 5 with secondary phosphine oxides and sulfides and phosphinic acid esters in the presence of benzoylphenylacetylene (50–55 or 70–75 °C, 7–35 h) gives 4-phosphorylated pyridines- d 4 and - d 3 (the latter having hydrogen at position 2) in 43–73% total yield along with differently deuterated chalcones, the reduced benzoylphenylacetylene. The scrutinized analysis of deuterium distribution in the above substitution products with emphasis on protium insertion in the pyridine ring reveals that the substitution studied involves the isomerization of the primary 1,3(4)-dipole intermediate to carbene localized at the position 2. Quantum chemical calculations (B3LYP/6-311++G**) indicate that diphenylphosphine oxide or trace water likely mediates the carbene generation stage.