A DFT study of the mechanism of palladium-catalyzed alkoxycarbonylation and aminocarbonylation of alkynes: Hydride versus amine pathways

摘要

A DFT study on the palladium-bisphosphine catalyzed alkoxycarbonylation and aminocarbonylation of alkyne (propyne) is reported. The theoretical study explores the feasibility and the regioselectivity control of two independent mechanisms: the first is based on the active intermediate [Pd(II)(P _2)(H)]~+ (where P_2 = PH_2CH _2CH_2CH_2CH_2PH_2) for the alkoxycarbonylation reaction, and the second is based on the active species [Pd(II)(P_2)(NR_2)]~+ for the aminocarbonylation reaction. The study explains the role of solvent in increasing the yield and in controlling the selectivity of reaction to produce selectively the trans isomer in the alkoxycarbonylation reaction (hydride cycle) and the gem isomer in the aminocarbonylation reaction (amine cycle). In hydride cycle, the regioselectivity is mainly determined by the stability of the complex [Pd(II)(P_2)(COC_3H_5)(CH_3CN)] ~+; however, for the amine cycle, the regioselectivity is determined by the stability of the complex [Pd(II)(P_2)(C_3H _5CON(CH_3)_2)]~+. The calculations reveal that ligand simplification is not valid in addressing the regioselectivity behavior of alkoxycarbonylation and aminocarbonylation reactions. The kinetic data for the formation of the two key complexes show no difference between the gem and trans isomers which predict the regioselectivity to be a thermodynamically controlled process.