EXTRUSION OF ALKENES FROM RHENIUM(V) DIOLATES - THE EFFECT OF SUBSTITUTION AND CONFORMATION

The kinetics of alkene extrusion from several diolates of the type Cp*Re(O)(diolate) are measured. The data show that successive methyl substitution increases the activation enthalpy for this process by approximately 1 kcal/mol per methyl group. Reaction rates and Delta G(double dagger) also reflect substantial differences in Delta S-double dagger, which are attributed to differences in the average conformation of the diolate ring. Support for this view is seen in conformational analysis of the ring, performed by examining H-1 NMR coupling constants. The data suggest the extrusion is inhibited if the ring is flat and promoted if it is highly puckered. This agrees with a mechanism involving migration of carbon from oxygen to rhenium, followed by loss of alkene.