Palladium-N-heterocyclic carbene (NHC) catalyzed C-N bond formation in a continuous flow microreactor. Effect of process parameters and comparison with batch operation

In this work, the Buchwald-Hartwig aryl amination is reported in a home-made continuous plug flow tube microreactor, using a homogeneous well-defined palladium-N-heterocyclic carbene (NHC) complex. The microreactor enabled a 100% conversion of the reagents within 30 min, even at very low catalyst concentrations. A direct comparison between batch and continuous flow reactions is described and shows that the Buchwald-Hartwig reaction is faster in the microreactor than in the batch case. An investigation of the influence of the operating parameters of the microreactor on the reaction was carried out. Increasing temperature allowed a faster conversion of the reagents; moreover, no effect on microreactor performance was found by changing tube diameter. The dependence of reaction kinetics on reagents and [Pd(NHC)] pre-catalyst concentrations was investigated based on initial reaction rates. The resulting expression for the rate of reaction showed some similarities with the one reported for palladium-phosphine catalyst, but also some important differences.