Reaction Calorimetry as a Tool for Understanding Reaction Mechanisms: Application to Pd-Catalyzed Reactions

Abstract

Reaction calorimetry can be employed for kinetic studies of complex catalytic reactions. In the calorimeter, the enthalpy balance around the reactor is continuously monitored and gives a profile of reaction heat vs time, which can be easily expressed in terms of reaction rate, conversion, and concentration of reactants/products. The data can be further analyzed using a methodology called reaction progress kinetic analysis, where the rate is expressed in terms of the concentration of substrates and a graphical expression is used to analyze the order in each substrate and in the catalyst. When studying a catalytic cycle, we can obtain useful information on catalyst activation/deactivation, substrate or product activation/deactivation, and the rate-limiting step. Thus, a detailed mechanistic picture can be obtained. In the present work we show an example of the role of reaction calorimetry in both fingerprinting and detailed study of some Pd-catalyzed aminations of aryl halides. These reactions are considered to be a very important route in making substituted aromatic amines. We observe how the overall kinetics of the reaction changes when changing the amine; the explanation takes into account the mechanism and proposes a change in the rate-limiting step.