Next Generation Enzymatic and Integrated Catalytic Approaches for Amide Synthesis

New routes to pharmaceuticals and other advanced materials are urgently required for a sustainable future. In this project we aim to develop novel, more efficient and sustainable methods for constructing amide bonds, which are common in many leading pharmaceuticals, agrochemicals, polymers and other valuable materials. Typically, amides are constructed synthetically from carboxylic acids and amines using well established coupling reagents. Although this traditional approach is widely used, it is extremely wasteful, lacks selectivity and uses toxic reagents. Coupling of carboxylic acids and amines typically requires one equivalent or more of coupling reagents, creating considerable waste as well as problems in reaction purification. Protecting groups are often necessary to block other reactive functionality in the precursors, so multiple steps (protect-couple-deprotect) are usually required to generate a single amide bond, consuming further expensive and deleterious reagents. Racemization/epimerization is also a common problem when coupling chiral precursors. This loss of stereochemistry is problematic in the synthesis of drugs which need to be produced as single stereoisomers. Finally, traditional amide coupling reactions typically employ dipolar aprotic solvents or chlorinated solvents, which present further safety issues and increased costs associated with their disposal.

In this project we aim to use a biotechnology-based approach to deliver amides in a more efficient and environmentally sustainable manner. To achieve this, we will explore two complementary methods for producing amides. First, we aim to engineer natures catalysts (enzymes) to create new enzyme variants (mutants) that can couple a wide range of acid and amine substrates. In addition, we plan to combine enzymes with transition metal catalyst to create new integrated catalytic approaches to amides. By combining the best of enzymatic and chemocatalysis, we aim to open new transformations and routes to valuable amides that would be inaccessible using existing methods. Nature has created a number of ways to couple acids and amines to make amide bonds with the most common methods relying on a molecule called ATP to activate the carboxylic acid group facilitating attack of the amine substrate. Such enzymes are called amide ligases and they possess binding sites for both the carboxylic acid and amine substrates. Normally the amide ligases nature provides have narrow substrate scope. We propose engineering both binding sites of the ligase enzymes to create new mutant enzymes that can couple a much wider range of substrates. The new enzymes will work in water, require no additional expensive or toxic reagents and can therefore be utilised for the more environmentally and cost-effective synthesis of valuable amides required for production of pharmaceuticals and other important molecules. In addition to amide ligases enzymes, we will also explore the utility of a different class of enzyme, the nitrile hydratases (NHase), for amide synthesis. NHase add water to nitriles (molecules with -CN groups) producing to primary amides (-CONH2). To broaden the scope of NHase we aim to combine these enzymes with a transition metal catalyst that can install a functional group on the primary amide to create more diverse secondary amides (-CONHR) which are typically found in pharmaceuticals etc. Normally combining enzymes with metal catalysts is problematic as the two catalysts are incompatible. For example, metals can bind to enzymes and deactivate the catalysts. To overcome this problem, we have devised a range of methods for compartmentalising enzymes and metal catalysts, in such a way that the two can be combine in a single (one-pot) reaction. This can also provide more direct routes to amides from alternative feedstocks (precursors).