Asymmetric bio-amination of ketones in organic solvents
ω-Transaminases, employed as a lyophilised crude cell-free extract, were successfully employed in organic solvent for the asymmetric amination of ketones without the need for immobilisation. Best activity was found for methyl tert-butyl ether (MTBE) at a water activity of 0.6. The ω-transaminases (9 different enzymes) accepted efficiently 2-propylamine as amine donor when used in the solvent, which is not the case when they are used in aqueous solution. The bio-amination in organic solvent showed several advantages such as higher reaction rates (up to 17-fold), general acceptance of 2-propylamine as amine donor, simple work-up procedure (i.e., no basification and extraction required), easy recycling of the catalyst and lack of substrate inhibition. The biocatalysts maintained their excellent stereoselectivity in MTBE allowing the preparation of optically pure amines (ee >99%) with up to >99% conversion.