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.