Acid-Catalyzed Isomerization of Rhenium Alkyne Complexes to Rhenium Allene Complexes via 1-Metallacyclopropene Intermediates

The alkyne complexes C5Me5(CO)2Re(η2-MeC⋮CMe) (1) and C5H5(CO)2Re(η2-MeC⋮CMe) (6) underwent acid-catalyzed isomerization by way of 1-metallacyclopropene intermediates to form the allene complexes C5Me5(CO)2Re(η2-2,3-MeHCCCH2) (5) and C5H5(CO)2Re(η2-2,3-MeHCCCH2) (7). Stoichiometric reaction of 1 with CF3CO2H initially produced the kinetic addition product C5Me5(CO)2Re[η2-(Z)-MeHCCMeO2CCF3] (8-Z), which slowly isomerized to the thermodynamically more stable E isomer 8-E. The reaction of 6 with CF3CO2H at −73 °C produced only C5H5(CO)2Re[η2-(E)-MeHCCMeO2CCF3] (9-E), which isomerized at −60 °C to a 80:20 equilibrium mixture of 9-E and 9-Z. Treatment of 9-E and 9-Z with base led to formation of allene complex 7. The rate of this elimination was independent of base concentration. Labeling studies showed that the 1-metallacyclopropene intermediate C5H5(CO)2Re(η2-CMeCHMe)+CF3CO2 - (12-CF 3 CO 2 ) undergoes a number of important reactions which include, in order of decreasing relative rates: (1) addition of trifluoroacetate to give enol trifluoroacetate complexes, (2) deprotonation to give complexed allenes, (3) degenerate 1,2-hydride migrations, (4) hydride migrations to give η3-allyl complexes, and (5) deprotonation to give complexed alkynes.