Kinetics of a dual nickel and iridium photocatalysed cross-coupling
Over the past decade, photoredox catalysis has developed into an important tool for synthetic chemists. Of particular importance has been the dual photoredox/nickel catalysis technique pioneered by the Doyle, MacMillan and Molander groups in 2014. While there has been much development of the synthetic applications of these techniques, in both industrial and academic settings, the mechanisms of this type of reaction remain poorly understood and few bulk-reaction kinetic studies have been conducted. This work takes the silane-mediated bromide-bromide sp2-sp3 cross-coupling developed by the MacMillan group in 2016 as a case study to probe the mechanism, both of this particular transformation and as representative of the class of reactions as a whole. In order to study this reaction, an in situ illumination 19F NMR spectroscopy (LED-NMR) apparatus was constructed in-house. Systematic variation of the reaction conditions allowed for the impact of each of the reaction components on the kinetics of the system to be observed. Four components (light, aryl bromide, nickel, and iridium photocatalyst) were found to control the rate of aryl bromide consumption, but not the product selectivity, while two components (silane and alkyl bromide), control the product selectivity, but not the rate. One particularly important outcome of this monitoring was the direct observation of a key aryl-Ni(II) intermediate that is the major resting state of the nickel catalyst throughout the cycle. Subsequent 13C isotope labelling studies demonstrated that this complex undergoes Ir-photocatalysed conversion to products in competition with degenerate release of aryl bromide. The experimental observations enabled development of a minimal kinetic model which allows simulation of the reaction evolution. This model provides useful insights for optimisation of these processes in the laboratory, as well as providing a framework for evaluating the validity of existing, and future, mechanistic proposals.