Application of boronate complexes in Earth-abundant metal catalysis
Item statusRestricted Access
Embargo end date28/11/2020
Catalysis by Earth-abundant transition metals represents one of the most promising avenues towards a sustainable, environmentally-benign bulk and fine chemical industry. Increasing effort has been applied to the development of synthetic methods using Earth-abundant metals, such as iron and cobalt, as catalysts. Nevertheless, the current state of the art is largely reliant on either the synthesis of an unstable low oxidation-state complex or the use of a pyrophoric external activator, such as NaBHEt3 or EtMgBr. It is therefore more challenging to use low oxidation-state iron and cobalt than established precious metal catalysts based on, for example, palladium and platinum. This work focuses on the development of a new, operationally simple activation method. Using commercially available tetrafluoroborate salts and bis(imino)pyridine ligands, it was possible to carry out catalytic hydroboration reactions of alkenes and alkynes, as well as nitro-group reductions, without the addition of any external activator, thus greatly simplifying synthetic procedures. Mechanistic studies suggest that this endogenous activation method is made possible by the dissociation of fluoride from the tetrafluoroborate pre-catalyst, which leads to the formation of a hydridic boron or silicon ‘ate’ complex. This subsequently transfers hydride to the pre-catalyst, leading to reductive elimination of dihydrogen to give a catalytically active low oxidation-state metal species. Increased understanding of the reaction mechanism from in situ NMR studies, independent synthesis and reactivity screening of the putative boronate complex and hydride trapping experiments, allowed the activation method to be applied to a number of other reactions, namely, [2+2]-cycloaddition, C–H borylation and C–C activation. Based on the concept of the activation of a boronic ester reagent as a source of hydride, preliminary investigations have also been carried out to examine the possibility of using less reactive (lower pKa) nucleophiles to activate boron reagents for the transfer of larger groups, such as an aryl group. Reactivity was initially uncovered using n-butyllithium as a nucleophilic activator for the boron reagents (Scheme A2, Nu), and studies demonstrated that the reaction proceeds though a boron-zinc exchange mechanism. Initial studies towards an organometallic-free procedure were also undertaken.