Rhodium catalyzed pH Dependent Asymmetric Transfer Hydrogenation of Imines in Water with Formic acid-Triethylamine as Hydrogen Donor

Rhodium catalyzed asymmetric transfer hydrogenation (ATH) of imines has been performed with variation in formic acid (F) and triethylamine (T) molar ratios in water. The F/T ratio is shown to affect both the reduction rate and enantioselectivity, with the optimum ratio being 1.1 in the ATH of imines with the Rh(1S,2S)-TsDPEN catalyst. Use of methanol as a cosolvent enhanced reduction activity. A variety of imine substrates have been reduced, affording high yields (94–98%) and good to excellent enantioselectivities (89– 98%) in 6-10 min reaction time. In comparison with the common azeotropic F–T system, the reduction with 1.1/1 F/T is faster.