Asymmetric transfer hydrogenation (ATH) is frequently carried out in the azeotropic mixture of formic acid (F) and triethylamine (T), where the FIT 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 ill 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. (C) 2012 Elsevier B.V. All rights reserved.
Lewis Base Adducts Derived from Transfer Hydrogenation Catalysts: Scope and Selectivity
作者:Zachariah M. Heiden、Bradford J. Gorecki、Thomas B. Rauchfuss
DOI:10.1021/om700996m
日期:2008.4.1
The coordination tendencies of the unsaturated 16e Lewis acid [Cp*Ir(TsDPEN)](+) ([1H](+)), where TsDPEN is H2NCHPhCHPhNTs-, are surveyed, together with parallel studies on analogous complexes such TsDACH (TsDACH = H2NC6H10NTs-) and Tsen (Tsen = H2NC2H4NTs-) derivatives as well as Rh analogues. Crystallographic analyses of the adducts of [Cp*IrL(TsDPEN)](+), where L = NCMe, NH3, PPh3, and CO, and [Cp*Ir(CO)(R,R-TsDACH)](+) are described. In the TsDPEN system, the Lewis base adducts contain an absolute configuration that is opposite that for the TsDPEN ligand and feature equatorial phenyl groups. In the case of [Cp*Ir(CO)(R,R-TsDACH)](+), both R and S metal centers cocrystallize. Isomerization of the R to the S metal center was first order in [Cp*(R-Ir)(CO)(R,R-TsDACH)](+) with minimal solvent effects. The pK(a) of the amine of the Lewis base adducts correlated linearly with the pK(a) of the free ligand in MeCN and the pK(a) of the amine (H2NCHPhCHPhNTs) of the Lewis base adduct in MeCN. Amines with pK(a) < 16 (MeCN scale), in the absence of additional hydrogen bonding to the TsDPEN ligand set, do not to bind to [1H](+), whereas bulky bases with pK(a) > 20 deprotonated the iridium amine.