The detailed mechanisms and origin of chemoselectivity of palladium-catalyzed alkoxycarbonylation of 1,3-diynes have been investigated by using DFT method. The computational reveal that the reaction preferably proceeds by the NH−Pd pathway, and the hydropalladation step is the chemoselectivity-determining step. The effectiveness of the NH−Pd catalytic system is attributed to the π-back-bonding, C−H⋯π interactions and d–pπ conjugation. The steric effects in hydropalladation transition states are mainly responsible for the origin of chemoselectivity.

Abstract

The palladium-catalyzed monoalkoxycarbonylation of 1,3-diynes provides a chemoselective method for the construction of synthetically useful conjugated enynes. Here, in silico unraveling the detailed mechanism of this reaction and the origin of chemoselectivity were conducted. It is shown that the alkoxycarbonylation reaction preferably proceeds by a NH−Pd pathway, which including three substeps: hydropalladation, CO migratory insertion and methanolysis. The effectiveness of the NH−Pd catalytic system is attributed to the alkynyl-palladium π-back-bonding interaction, C−H⋅⋅⋅π interaction in reactant moiety and d–pπ conjugation between the Pd center and alkenyl group. The hydropalladation step was identified as the rate- and chemoselectivity-determining step, and the first alkoxycarbonylation requires a much lower energy barrier in comparison with the second alkoxycarbonylation, in line with the experimental outcomes that the monoalkoxycarbonylation product was obtained in high yield. Distortion-interaction analysis indicates the more favorable monoalkoxycarbonylation (compared to double alkoxycarbonylation) is caused by steric effect.