Alternatives to platinum group metals for catalysis

Abstract

Transition metal-catalysed reactions are ubiquitous in chemical processes. Their applications in small-molecule activation are observed across a range of industries however there are issues associated with their use. Platinum-group catalysts dominate homogeneous catalysis despite issues associated with their scarcity. First-row transition metals have received much attention for their reactivity, and their increased abundance provides an advantage over second and third-row metals. Stringent regulations on the presence of trace metals in pharmaceutical products, however, means that any metal-catalysed process has an associated cost with removing trace metals completely. Main-group elements offer potentially transition metal-free catalytic systems, avoiding the need to remove residual metals. Two-electron processes, highly developed for d-block catalysis, are difficult to apply to p-block elements due to their lack of d-orbital electrons. As such, for efficient metal-free catalysis to be developed, a different approach is necessary.

There are reports on the use of borane reagents as catalysts for hydroboration and borylation, proceeding through a transborylation mechanism. Transborylation is the redistribution of groups around boron atoms, akin to transmetalation. This mode of reactivity is an alternative turnover-enabling step to traditional oxidative addition, transmetalation and reductive elimination associated with transition metal-catalysts.

The generation of boronic esters by alkene and alkyne hydroboration has been reported using H-BR2 catalysts and H-B(OR)2 turnover reagents such as pinacolborane. Mechanistic investigations propose that these systems proceed through an initial hydroboration of the substrate, by the catalyst, before a transborylation step where a C-BR2/H-B(OR)2 exchange occurs to regenerate the catalyst. Reported systems rely on harnessing established stoichiometric activity for catalysis. For borane catalysis to become widespread, however, transborylation must be utilised in a range of reactions.

An investigation into enantioselective hydroboration, catalysed by a borane species, has been reported in this work. By harnessing the established reactivity of stoichiometric borane species for catalysis, a method for the enantioselective hydroboration of ketones has been developed.

This system demonstrated transborylation for O-BR2/H-B(OR)2 exchange. Further, an enantioselective borane catalyst for alkene hydroboration has been identified and investigated.

Initial examples of transborylation as a method for enantioselective main-group catalysis have been described, providing evidence that main-group catalysis has the potential to complement the reactivity of established platinum-group catalysts. The focus of this work is metal-free catalysis however abundant first-row transition metals also offer advantages over platinum-group metals, due to the reduced environmental impact associated with their use. Here, the development of iron-catalysed systems for small-molecule catalysis and materials applications is also described. A mechanistic investigation into a Heck type alkenylation catalysed by a simple iron salt is discussed and the initial development of an iron-catalysed cross-coupling system for poly-3-hexylthiophene synthesis is reported. These projects have investigated the use of iron as a sustainable alternative to third-row transition metals in catalysis, further demonstrating the catalytic potential of elements across the periodic table, with an aim toward improving the sustainability of chemical industries.