A computational study on the mechanism for the GaCl3-catalyzed [4+1] cycloaddition of α,β-unsaturated ketone and 2,6-dimethylphenyl isocyanide

The reaction of the GaCl3-catalyzed [4+1] cycloaddition of α,β-unsaturated ketone with 2,6-dimethylphenyl isocyanide leading to unsaturated γ-lactone derivative has been investigated using the density functional theory with the B3LYP hybrid functional. According to our calculations we found that the reaction is stepwise and exothermic. The reaction proceeds via three steps. The first step is the coordination of GaCl3 to the oxygen atom in mesityloxide leading to a more electrophilic C3 atom. At the second step, 2,6-dimethylphenyl isocyanide attacks mesityloxide to form the C3–C6 bond with GaCl3 activator, which is the rate-limiting step. Finally, the C6–O bond is formed to give the five-member cycle product due to the attack of the C6 atom to the O atom. In addition, our calculations also suggest that GaCl3 activator can be easily detached from the product. The theoretical results are in good agreement with the recent experimental observations.