Palladium-Catalyzed Methoxycarbonylation Investigated by Design of Experiments

The synthesis of carboxylic esters is of fundamental importance in the chemical industry and the corresponding products find numerous applications for polymers, cosmetics, pharmaceuticals, agrochemicals, and other manufactured chemicals. In this context, palladium-catalyzed methoxycarbonylation of olefins using the cutting-edge ligand pytbpx represents an 100%-atom economical one-pot synthesis to a great variety of valuable esters. Despite intensive studies to understand this catalytic system, systematic investigations of the influence of the reaction conditions are still missing. In particular, studies of the quantitative interactions of the different reaction conditions are interesting. In this context, here we report the alkoxycarbonylation of di-iso-butylene, which is of interest for large-scale technical applications, using a design of experiment (DoE) approach. An excellent product yield of 95% after 3.5 h is achieved without formation of waste products in significant amount (<1%). The analysis of the developed reaction model allows the identification of the significant control parameters for this specific catalytic system. Furthermore, it was possible to identify the reasons of sensitivity related to the amount of acidic cocatalyst (H2SO4) used. While the literature-known inhibition of the reaction at small acid concentrations is confirmed, an unfavorable effect for increased amount of acid was observed. The assumption is that protonation of both nitrogen atoms of the ligand for high acid concentrations hinders the proton shuttle effect of the pyridyl unit, and therefore the rate-determining deprotonation step is decelerated. The optimization described here provides a rational basis for future process designs, which will possibly make this efficient synthetic route even more sustainable.