The effects of CO2 pressure and pH on the Suzuki coupling of basic nitrogen containing substrates
The Suzuki coupling reaction of basic nitrogen containing substrates (2-bromo- and 2-chloro-4-aminopyridine, and 2-bromo and 2-chloropyridine) with phenylboronic acid using Pd(TPP)2Cl2/K3PO4 in acetonitrile–water biphasic solvent systems under a CO2 or a N2 atmosphere is discussed. It was observed that 2-halo-4-aminopyridine produced quantitative yields of coupled products under a CO2 atmosphere while the yields for the 2-halopyridines were poor. In contrast, the yields of coupled products for the 2-halopyridines substrates were quantitative under a N2 atmosphere while only poor yields were realized for the 2-halo-4-aminopyridines under the same conditions. Evidence is presented which suggests that the presence of CO2 alters the pH of the aqueous phase of the reaction system and the accompanying efficiency of the coupling process. Using a series of buffers to adjust the pH of the aqueous phase, the pH dependence associated with the efficiency of the coupling process is illustrated.