1,3-Cycloaddition of Pyrazole to the Allenylidene Ligand in [Re{C=CCPh2}(CO)2{MeC(CH2PPh2)3}]

The reaction of the allenylidene complex [Re{C=C=CPh2}(CO)2(triphos)]OTf (1) (triphos = MeC(CH2PPh2)3; OTf = -OSO2CF3) with pyrazole gave the 1,2,3-diheterocyclization product [Re{C=CHCPh2(N2C3H)3}(CO)2(triphos)]Y (Y = OTf, 2OTF; Y = BPh4, 2BPh4). The molecular structure of this complex was determined by a single-crystal X-ray analysis. Treatment of 2 with sodium methoxide gave the pyrazolyl-functionalized alkynyl derivative [Re{CºCCPh2(N2C3H3)}(CO)2(triphos)] (3) via selective deprotonation of the vinylic hydrogen, followed by back opening of the metal-bound heterocycle. Protonation of 3 with triflic acid in dichloromethane re-generated 1 and free pyrazole. The overall result of this deprotonation/protonation sequence allowed us to propose a reliable mechanism for the formation of the 1,2,3-diheterocyclization product 2OTf.