Isomerization of Internal Alkynones to Vinylidenes in Tris(pyrazolyl)borate Ruthenium Complexes. Solution and Solid-State Kinetics

Internal alkynones of the type PhC CCOR (R Ph, Me) undergo thermal isomerization to disubstituted vinylidenes, Ru=C=C(COR)Ph in the coordination sphere of tris(pyrazolyl)borate ruthenium complexes. The treatment of [TpRuCl((PPr2Me)-Pr-i)(MeCN)] and [TpRuCI((Pr2PNHPy)-Pr-i)] with NaBAr4F (Ar-F = 3,5-bis(trifluoromethyl)phenyl) and the corresponding alkynone yields eta(1)-O-ketone complexes as kinetic products. These species are metastable and spontaneously isomerize to disubstituted vinylidenes both in solution and in the solid state. Kinetic studies in solution show that the isomerization rate is first-order in the concentration of the ketone complex, and thus the activation parameters have been obtained from Eyring plots. A concerted sigmatropic 1,2-acyl shift is proposed as the key step. The isomerization in the solid-state has been studied by IR spectroscopy and the results interpreted with the Avrami-Erofeev model. The X-ray structure of a disubstituted vinylidene complex has been determined for the complex [TpRu{=C=C(COPh)Ph}((PPr2Me)-Pr-i)(MeCN)][BAr4F].