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 an d 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 e stablished by determining the structures and relative energies of the putat ive intermediates involved. The energy barriers for the most important step s were also determined. The allyl and the alkyne moieties can be coupled by pi- or sigma-allyl complexes, although the pi-allyl reaction pathway is fo und to be less energetically demanding. The energy barrier for cyclopenteno ne formation is lower than for cyclohexenone, in agreement with the experim ental findings. Acetylene substituent have a noteworthy effect on the regio selectivity of the process, and this is manifested as a kinetic effect on t he computed energy barriers. The yield of five- or six-membered rings has a thermodynamic origin but also depends on the energy barrier when substitut ed allyls are used.