More sustainable biocatalytic imine reductions to chiral amines with hydrogen-driven NADPH recycling operated in batch and continuous flow

This project is in the area of industrial biotechnology for cleaner chemical manufacturing, and is relevant to the pharmaceutical and fine chemicals sectors. Biocatalytic imine reductions are a valuable component in the synthetic tool-box for synthesis of chiral amines for pharmaceuticals or other fine chemicals. Imine reductase enzymes depend on nicotinamide cofactors, usually NADPH, for hydride transfer to an enzyme-bound imine substrate, and conventional approaches for regenerating NADPH during these reactions usually rely upon super-stoichiometric glucose as the reductant. Use of the six-carbon sugar, glucose, for a single hydride-transfer makes the overall process atom-inefficient in contrast to metal-catalysed hydrogenations where hydrogen gas (H2) is added across the C=N double bond of an imine, giving an amine product with no by-products. This project exploits approaches that fall neatly between these alternatives, offering the chiral and functional-group selectivity of isolated-enzyme biocatalysis, alongside the atom-efficiency of H2-driven hydrogenation reactions under mild temperature and pressure. We achieve this by exploiting an innovative strategy for NADPH recycling which uses biocatalysed H2 activation to drive the biocatalytic reduction of NADP+ to NADPH, as well as handling the enzymes on a heterogeneous support. The H2-driven cofactor recycling has the added advantage of simplifying purification of the chemical product. As well as replacing glucose by H2 as atom-efficient reductant, our approach also avoids a pH change associated with the cofactor recycling step, thereby simplifying the process chemistry. The project is conjunction with industrial partner, AstraZeneca. Overall, the project aims to deliver a robust platform for more sustainable, H2-driven NADPH recycling to support application of imine reductases for generation of chiral amines. This may be implemented in batch or continuous flow. We aim to demonstrate this for a series of imine reductions relevant to the synthesis of pharmaceuticals and other fine chemicals, which can be implemented readily within standard industrially-used hydrogenation reactors. The approach would be suitable for late-stage functionalisation, as well as synthesis of chiral building blocks. Results will be presented at conferences spanning organic synthesis, biotechnology and chemical manufacturing topics. This project falls within the EPSRC 'Manufacturing the Future' research area, and is also relevant to 'Energy and Decarbonisation' since it decarbonises industrial biotechnology for chemical manufacturing. As such, it is relevant to the EPSRC strategic priority area of 'Engineering Net Zero'.