Decarboxylative cross-coupling: bimetallic nanoparticles as catalysts and low-temperature optimisation

Decarboxylative cross-coupling chemistry is an emerging field of research that promises the synthesis of a variety of biaryl products without the need for expensive and challenging to handle organometallic reagents. Via interaction with a transition metal catalyst, organometallic species are generated in situ from carboxylic acid functionality, eliminating carbon dioxide in the process. However, inducing the decarboxylation of even highly activated carboxylic acids often requires forcing conditions: high reaction temperatures; long reaction times; and in some cases, stoichiometric quantities of transition metal mediator. Such conditions bring with them issues regarding functional group tolerance and side reaction prevalence, which limit the broader applicability of the method. The work detailed herein explores the applicability of both monometallic and bimetallic nanoparticles as potential solutions to these problems. A wide library of bimetallic catalysts have been synthesised containing group 10 (nickel or palladium) and group 11 (copper, silver or gold) transition metals, characterised via atomic absorption spectroscopy, transition electron microscopy and energy-dispersive x-ray spectroscopy and subsequently screened against a typical decarboxylative cross-coupling protocol utilising fluorinated potassium benzoate salts. Palladium/copper bimetallic nanoparticles were found to be excellent catalysts under both the literature conditions of 130 C and the lower temperature of 100 C. Palladium rich alloys performed best, and a synergistic effect was observed – the bimetallic nanoparticles displaying greatly enhanced reactivities over their monometallic analogues. Capping agent choice was also found to have a pronounced effect on catalyst activity, with the ethylene glycol-capped palladium/copper 1:1 bimetallic nanoparticle catalyst reaching reaction completion in 1 hour at 100 C, outperforming competing homogeneous catalyst systems. Substrate scope was also explored under these conditions, utilising small scale DFT calculations to make informed substrate choices. The result of this work was the elucidation of an interesting phenomenon, wherein certain substrate parings performed considerably better than others. Through follow up experiments evidence is presented that suggests this phenomenon is kinetic in nature, with substrate choice affecting the rates of the group 10 and group 11 catalytic cycles – parameters that are key to a reliable decarboxylative cross-coupling reaction. Finally, scope for the replacement of toxic and harmful solvents typically used in DCC protocols was also investigated. Success was found utilising poly(ethylene glycol) both as a direct analogue for problem solvents such as diglyme and in smaller quantities as an additive in desirable but underperforming solvent systems to boost reactivity.