Experimental Observation of β‑Carbon Elimination from Alkenylrhodium Complexes through Exchange Reactions of the Alkenyl Unit

We have investigated β-carbon elimination of an alkenylmetal complex to give the corresponding alkyne, a rare type of β-carbon elimination, through the study of the exchange reaction of the alkene unit in an unstrained metallacyclic alkenylrhodium complex. When alkenylrhodium complex 1 derived from 1-naphthylphosphine and diphenylacetylene was treated with another internal alkyne, an equilibrium mixture of 1 and alkenyl-group-exchanged complex 2 was formed even at 0 °C. An in situ trapping experiment using PPh3 revealed that the exchange reaction was mediated by β-carbon elimination of 1 to afford a diphenylacetylene complex as an intermediate. A theoretical study also supported the involvement of the β-carbon elimination. The effects of alkyne substituents are also described.