EXTRUSION OF ALKENES FROM RHENIUM(V) DIOLATES - ENERGETICS AND MECHANISM

The kinetics of extrusion of ethylene, 2-butene, and bicyclo[2.2.2]oct-2-ene from Cp*Re(O)(OCH(R)CH(R)O) (Cp* = eta-C-5(CH3)(5)) are measured, as are the kinetics bf oxidation of norbornene, norbornadiene,and trans-cyclooctene by Cp*ReO3. Activation parameters calculated from the data show that strain in the double bond has a large effect on the enthalpy of activation for alkene oxidation but little or no effect on the enthalpy of activation for alkene extrusion. Entropies of activation for alkene extrusion are less than zero. A secondary deuterium isotope effect of 1.3 at 99.5 degrees C is measured for extrusion of ethylene-d(4). Taken together, this data is inconsistent with a concerted mechanism for interaction of alkenes with Cp*ReO3 but consistent with a stepwise mechanism with a metallaoxetane intermediate.