Varying the ratio of formic acid to triethylamine impacts on asymmetric transfer hydrogenation of ketones

Asymmetric transfer hydrogenation (ATH) is frequently carried out in the azeotropic mixture of formic acid (F) and triethylamine (T), where the F/T molar ratio is 2.5. This study shows that the F/T ratio affects both the reduction rate and enantioselectivity, with the optimum ratio being 0.2 in the ATH of ketones with the Ru-TsDPEN catalyst. Under such conditions, a range of substrates have been reduced, affording high yields and good to excellent enantioselectivities. In comparison with the common azeotropic F-T system, the reduction is faster. This protocol improves both the classic azeotropic and the aqueous-formate system when using water-insoluble ketones.

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► Asymmetric transfer hydrogenation with ruthenium catalyst.
► Formic acid-triethylamine mixture as reductant and solvent.
► The ratio of formic acid to triethylamine affects the hydrogenation.