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 subs titution increases the activation enthalpy for this process by approxi mately 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 th e diolate ring. Support for this view is seen in conformational analys is of the ring, performed by examining H-1 NMR coupling constants. The data suggest the extrusion is inhibited if the ring is flat and promo ted if it is highly puckered. This agrees with a mechanism involving m igration of carbon from oxygen to rhenium, followed by loss of alkene.