Table I

Comparison of Asymmetric Transfer Hydrogenation of Acetophenone Using Ruthenium(II) Catalysts in Watera
Entry Catalyst Hydrogen sourceb S:Cc Time, h Conversion, % ee, % Reference
1 Ru-1 HCOONa 100 1 99 92 (75)
2 Ru-1 HCOONa 1,000 12 >99 89 (75)
3 Ru-2 HCOONa 100 1 >99 95 (65, 74)
4 Ru-2 Azeotroped 100 12 98 97 (73, 74)
5 Ru-2 FA-T/H2O 100 1.5 >99 97 (73)
6 Ru-2 FA-T/H2O 1,000 9 >99 96 (73)
7 Ru-2 FA-T/H2O 5,000 57 98 96 (73)
8 Ru-2 FA-T/H2O 10,000 110 98 94 (73)
9 Ru-3a HCOONa 100 2 99 85 (72)
10 Ru-3b HCOONa 100 2.5 >99 81 (68, 72)
11 Ru-4 HCOONa 100 2 99 97 (71)
12 Ru-4 HCOONa 1,000 20 95 96 (71)
13 Ru-5 HCOONa 100 12 84 71 (70)
14 Ru-6 HCOONa 100 10 95 50 (70)
15 Ru-7 HCOONa 100 5 97 60 (70)
16 Ru-8 HCOONa 100 3.5 >99 73 (70)

a The reaction was carried out at 40°C in 2 ml of water or a mixture of water and formic acid-triethylamine (FA-T in a 1.2:1.0 molar ratio) under inert gas protection

b Hydrogen source: 5 equiv. was used unless otherwise specified

c S:C is substrate to catalyst molar ratio

d Azeotrope refers to an azeotropic HCOOH-NEt3 mixture (2.5:1 molar ratio)