Group 13 exchange for catalytic turnover

Abstract

Transition metals are able to undergo redox-active processes, such as oxidative addition and reductive elimination with carbon-element bonds. These two fundamental steps have allowed for the majority of metal-catalysed processes to take place by providing access to multiple oxidation states. Precious metals, such as palladium, platinum, iridium, rhodium, and ruthenium, dominate transition metal catalysis due to their wide applicability and highly tuneable ligands. However, these metals can be toxic and have low abundance in the Earth’s crust, leading to high costs and difficult purification.

Group 13 elements are an attractive alternative to transition metals in catalysis due to their lower toxicity and cost. Group 13 elements are electron-deficient, giving them unique reactivity within the main-group; however, their synthetic utility has been limited to stoichiometric processes. Group 13 elements do not have access to empty d-orbitals, so cannot undergo the same redox-active processes that transition metals can, therefore their applications in catalysis are limited to redox-neutral processes. Therefore, new methods of catalytic turnover are required for the application of group 13 elements in catalysis. Group 13 exchange has been developed as a redox-neutral process for catalytic turnover, whereby classically stoichiometric reactions have been rendered catalytic. Group 13 exchange has been applied to the aluminium-catalysed C‒H borylation of alkynes with HBpin using a tethered Al/N Lewis pair, which shut down the competitive alkyne hydroboration pathway.

Transborylation, group 13 exchange between two boron atoms, has provided access to metal-free catalytic systems by extending traditional stoichiometric borane-mediated reactions. Transborylation has been applied to borane-catalysed hydroboration and reduction, classic stoichiometric reactions that have been rendered catalytic. This work has successfully extended the catalogue of transborylation-mediated catalysis: borane-catalysed C‒F arylation and esterification of alkyl fluorides demonstrated excellent functional group tolerance across a wide range of examples. The borane-catalysed asymmetric hydroboration of alkenes showed promise, with moderate yields and enantioselectivities, and improved the synthesis of chiral borane reagents. The borane-catalysed intramolecular 1,1-carboboration of silylalkynes gave access to a variety of silolenyl boronic esters and was the first example of a borane-catalysed carboboration reaction.