Carbonylative Cycloaddition of Allyl Halides and Acetylenes Promoted by Ni(CO)4. A DFT Study on the Reaction Mechanism

The [2 + 2 + 1] carbonylative cycloaddition of allyl halides and acetylenes promoted by Ni(CO) 4 is an elegant route to synthesize cyclopentenones and other cyclic and linear products. This paper reports the results of a DFT study on a proposed reaction mechanism, the energy profile for which was established by determining the structures and relative energies of the putative intermediates involved. The energy barriers for the most important steps were also determined. The allyl and the alkyne moieties can be coupled by π- or σ-allyl complexes, although the π-allyl reaction pathway is found to be less energetically demanding. The energy barrier for cyclopentenone formation is lower than for cyclohexenone, in agreement with the experimental findings. Acetylene substituents have a noteworthy effect on the regioselectivity of the process, and this is manifested as a kinetic effect on the computed energy barriers. The yield of five- or six-membered rings has a thermodynamic origin but also depends on the energy barrier when substituted allyls are used.