This PhD thesis investigates aspects of the Mizoroki-Heck reaction which can be broadly divided into two parts: (1) employing a flow reactor to distinguish between surface and solution catalysed reactions of heterogenous catalysts whilst assessing their leaching behaviour under various reaction conditions; (2) investigating the catalytic behaviour of ligandless palladium at ultra-low catalyst loadings through reaction kinetics and kinetic modelling.
A flow reactor was constructed in two operational configurations: configuration 1 was used to study the surface and solution contributions of heterogenous catalysts which revealed that the majority of conversion is actually attributed to turnover in the homogenous phase; configuration 2 probed the influence of individual reactants on the leaching process showing that both the aryl halide and base have the largest impact on leaching.
To date, little is known about the mechanism and speciation of Pd(OAc)2 at ultra-low catalyst loadings. Kinetic profiling of the Heck reaction catalysed by “homeopathic” loadings of Pd(OAc)2 have been studied to gain an insight into the nature of the catalyst and its speciation. Orders in reagents and catalyst were extracted from relevant reaction profiles by utilising variable time normalisation analysis and reaction progress kinetic analysis. The reactions were monitored in situ by IR accompanied with offline HPLC quantification. Comparison of the results for reactions under varying reaction conditions are made to probe their impact on the homeopathic palladium catalysed Heck reaction.
The study reveals two distinctive kinetic regimes: between [Pd] of 0.003-0.01 mol% (30-100 ppm), catalyst deactivation was found to be minimal, and the observed rates showed a 1.7 order of dependence on [Pd]. In contrast, when [Pd] > 0.02 mol% (200 ppm), catalyst deactivation becomes competitive, whereupon the order of dependence on [Pd] decreased to 0.9. A simplified microkinetics model revealed the presence of at least two kinetically competent catalysts, represented by a monomeric (Pd1) and dimeric (Pd2) species in a 1:3 ratio. Surprisingly, Pd2 was found to be nearly 90 times more active than Pd1. This work provides direct kinetic evidence that a higher-order Pd species can be more active than a monomeric species, and the key role played by catalyst deactivation at higher catalyst loadings.