Theoretical studies on the insertions of unsymmetrical alkynes into the metal-carbon bond of phosphanickelacycles: Electronic factors

Density functional calculations have been employed to study the insertion reactions of alkynes (RC≡CR′: R = H, R′ = H, Me, CF₃, Ph; R = Me, R′ = Ph; R = CO₂H, R′ = H, Me, CF₃, Ph) with the model phosphanickelacycle [NiBr(CH=CHCH₂PH_2-kC,P)(PH₃)]. Calculations with HC≡CH indicate that associative processes with insertion via 5-coordinate intermediates are preferred kinetically over an alternative mechanism involving initial displacement of a PH₃ ligand. Two possible trigonal-bipyramidal 5-coordinate intermediates were located with either Br or PH₃ occupying an axial position trans to the Ni-vinyl bond. The preference for an associative process was confirmed with HO₂CC≡CH and HC≡CMe. Computed 5-coordinate transition state energies for unsymmetrical alkynes are generally consistent with the regioselectivities observed with experimental analogues. One exception is HC≡CCF₃, for which the wrong regioisomer is marginally favored, although the computed energy difference between the transition states leading to opposite regioisomers is negligible. Both the observed and calculated results are discussed in terms of a simple model for predicting the insertion regioselectivities based on the polarization of the alkyne π⊥ orbital. In all cases, this model accounts well for the experimental regioselectivities but analysis of the computational results shows the success of this approach depends on both the alkyne and the 5-coordinate intermediate from which the insertion occurs. In particular, when electron-withdrawing substituents are present, a swap in regioselectivity is often predicted, depending upon whether insertion proceeds from the isomer with Br axial or from that with PH₃ axial.