Hammett Studies on Alkene Extrusion from Rhenium(V) Diolates and an MO Description of Metal Alkoxide−Alkyl Metal Oxo Interconversion

Rates of cycloreversion of phenyl-substituted diolates of the type Cp*Re(O)(diolate), where diolate = phenylethane-1,2-diolate, exhibited a linear relationship with Hammett σ - constants, with ρ = 0.42. This suggests a slight buildup of electron density but little charge development in the transition state. The better fit with σ - constants suggests a resonance interaction between the phenyl ring and the breaking C−O bond. On the other hand, similar 1,2-diphenylethane-1,2-diolates showed curved Hammett behavior, with acceleration by electron-donating and electron-withdrawing groups. Such behavior offers more evidence for a nonsymmetric transition state and stepwise cleavage of the C−O bonds. Molecular orbital calculations suggest a rationale for bonding changes in these systems.