Rhenium Allenylidenes and Their Reactivity toward Phosphines: A Theoretical Study

Density functional and local MP2 calculations have been performed to study the electronic structure of the rhenium(I) allenylidene [(triphos)(CO)(2)Re(=C=C=CRR')(+) species [triphos = MeC(CH2PPh2)(3); R, R' = aryl group] and its reactivity toward tertiary phosphines. The calculated electronic structure shows a relatively electron-rich nature of the [(triphos)(CO)(2)Re](+) synthon in agreement with the experimental behavior of the corresponding allenylidene complex [(triphos)(CO)(2)Re(=C=C=CPh2)](+) (R = R' = Ph). Both the kinetics and the thermodynamics of the nucleophilic addition of tertiary phosphines PMe3-xPhx, (x = 0, 1, 2, and 3) have been considered. The results indicate lower activation energies for the phosphine attack to C-gamma, which leads, however, to products higher in energy than those of the attack to C-alpha. The computed behavior agrees with the experimental evidence showing that the products of the attack to C-gamma are kinetically favored, while the products of the attack to C-alpha are thermodynamically favored. Finally, we addressed the mechanism of phosphine migration from C-gamma to C-alpha, finding a low-energy path corresponding to an incomplete detachment of the phosphine moiety that then shifts from the C-gamma to the C-alpha atoms while remaining weakly bound to the allenylidene unit.