Rhenium Allenylidenes and Their Reactivity toward Phosphines: ATheoretical Study

Density functional and localMP2 calculations have been performed to study the electronic structure of the rhenium(I) allenylidene [(triphos)(CO)2Re(dCdCdCRR0)]þ species [triphos=MeC(CH2PPh2)3;R, R0 =aryl group] and its reactivity toward tertiary phosphines. The calculated electronic structure shows a relatively electron-rich nature of the [(triphos)(CO)2Re]þ synthon in agreement with the experimental behavior of the corresponding allenylidene complex [(triphos)(CO)2Re(dCdCdCPh2)]þ (R=R0=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 Cγ, which leads, however, to products higher in energy than those of the attack to CR. The computed behavior agrees with the experimental evidence showing that the products of the attack toCγ are kinetically favored, while the products of the attack to CR are thermodynamically favored. Finally, we addressed the mechanism of phosphine migration fromCγ toCR, finding a low-energy path corresponding to an incomplete detachment of the phosphine moiety that then shifts from the Cγ to the CR atoms while remaining weakly bound to the allenylidene unit.