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Structural Isomers of Aryl-Substituted η3-Propargyl Complexes:  η2-1-Metalla(methylene)cyclopropene and η3-Benzyl Complexes

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journal contribution
posted on 10.10.2002 by Charles P. Casey, Timothy M. Boller, Stefan Kraft, Ilia A. Guzei
Hydride abstraction from C5Me5(CO)2Re(η2-PhC⋮CCH2Ph) (1) gave a 3:1 mixture of η3-propargyl complex [C5Me5(CO)2Re(η3-PhCHC⋮CPh)][BF4] (5) and η2-1-metalla(methylene)cyclopropene complex [C5Me5(CO)2Re(η2-PhCCCHPh)][BF4] (6). Observation of the η2-isomer requires 1,3-diaryl substitution and is favored by electron-donating substituents on the C3-aryl ring. Interconversion of η3-propargyl and η2-1-metalla(methylene)cyclopropene complexes is very rapid and results in coalescence of Cp* 1H NMR resonances at about −50 °C. Protonation of the alkynyl carbene complex C5Me5(CO)2ReC(Ph)C⋮CPh (22) gave a third isomer, the η3-benzyl complex {C5Me5(CO)2Re[η3(α,1,2)-endo,syn-C6H5CH(C⋮CC6H5)]}[BF4] (23) along with small amounts of the isomeric complexes 5 and 6. While 5 and 6 are in rapid equilibrium, there is no equilibration of the η3-benzyl isomer 23 with 5 and 6.