Molybdenum-Mediated Vinylidene Rearrangement of Internal Acylalkynes and Sulfonylalkynes

Vinylidene rearrangement of internal acylalkynes and sulfonylalkynes at a d6 molybdenum center is presented. Treatment of [(η7-C7H7)MoBr(dppe)] with various acylalkynes R–C≡C–COR′ in the presence of NaBArF4 afforded the corresponding vinylidene complexes [(η7-C7H7)Mo[═C═CR(COR′)](dppe)][BArF4], which exist as mixtures of two rotamers, with respect to the Mo═Cα bond at room temperature, as evidenced by NMR studies. This provides the first example of vinylidene rearrangement of internal alkynes at a Mo center. X-ray diffraction studies revealed that the vinylidene ligand :C═CR(COR′) coordinates almost perpendicularly toward the C7H7 plane. A 13C-labeling experiment unveiled selective migration of the acyl group, which is in agreement with a nucleophilic 1,2-migration mechanism. When sulfonylakynes R–C≡C–SO2R′ were used as substrates, three types of products—η2-alkyne, vinylidene, and unexpected η2-phosphonioalkyne complexes—have been isolated, depending on the R group. The η2-phosphonioalkyne complex is con...