Towards cleaner PolarClean: efficient synthesis and extended applications of the polar aprotic solvent methyl 5-(dimethylamino)-2-methyl-5-oxopentanoate

Abstract

As a result of recent efforts in green solvent selection, methyl 5-(dimethylamino)-2-methyl-5-oxopentanoate, sold under the brand name Rhodiasolv PolarClean, has received considerable scientific and industrial attention as a possible non-toxic replacement for common polar aprotic solvents. However, the multicomponent nature and multi-step synthesis of this solvent remains an obstacle for its more widespread use and niche applications. In this work, a retrosynthetic approach was taken to identify novel shorter synthetic routes in alignment with green chemistry principles. High purity methyl 5-(dimethylamino)-2-methyl-5-oxopentanoate was obtained in novel single-step reactions via two different base-catalysed Michael additions from readily available building blocks. The more advanced synthetic route shows great potential owing to the swift (30 min), solvent-free reaction and catalytic amounts of base (<6.5 mol%). Green metrics analysis, including Atom Economy, Complete E factor, Carbon Intensity and hazard analysis found the new synthesis to be more sustainable than the patented routes. Application of this green solvent was demonstrated for the first time for O- and N-arylation in S_NAr reaction with solvent recovery with similar or superior yields compared to other green solvents. Moreover, broad opportunities for this green solvent in membrane science were identified, where the use of conventional, toxic polar aprotic solvents in large quantities is unavoidable. Important practical solvent properties and parameters such as dielectric constant, solubility parameters, solvent miscibility and NMR residual shifts have been determined to facilitate the uptake of methyl 5-(dimethylamino)-2-methyl-5-oxopentanoate as a green solvent.