Quantum mechanical investigation of the mechanism of Ni(0)-catalyzed cycloaddition reaction of 2-cyclobutanone with alkyne

The mechanism of the Ni(0)-catalyzed cycloaddition reaction of 2-cyclobutanone with alkyne was studied using the function M06-2X with two basis sets. The computational results indicate that the reaction occurs through two independent pathways, one of which is related to the coordinating of C=O bond of oxidative addition of 2-cyclobutanone to the Ni(0) center (pathway 1), and the other involves coordination of C(sp(3))-H bond (pathway 2) to form an intermediate containing either a Ni-C(O) or Ni-C (sp(3)) bond, then insertion an alkyne into either Ni-C(O) or Ni-C(sp(3)) bond. In the last step, the products are formed and the catalyst is regenerated through the reductive elimination. Our computational results are consistent with previous computational studies for cycloaddition of 3-azetidinone to alkyne except that the mechanism takes place through two independent pathways, i.e. there are no transition states or intermediates involved in the two pathways, which is contrary to what it has been indicated in previous studies. In addition, our results show that the Ni-catalyzed cycloaddition reaction of the 3-azetidinone with the alkyne is more active compared to the 2-cyclobutanone by free energy of 10.4 and 9.3 kcal/mol according to pathway 1 (7.4 and 8.1 kcal/mol according to pathway 2) calculated by the M06-2X(I) and M06-2X(II) methods, respectively. An analysis of the electrophilic and nucleophilic indices of the reactants was carried out.