Rh III- and Ir III-catalyzed asymmetric transfer hydrogenation of ketones in water.
Asymmetric transfer hydrogenation (ATH) of ketones by formate in neat water is shown to be viable with Rh-TsDPEN and Ir-TsDPEN catalysts, derived in situ from [Cp*MCl2]2 (M=Rh, Ir) and TsDPEN. A variety of ketones were reduced, including nonfunctionalized aryl ketones, heteroaryl ketones, ketoesters, and unsaturated ketones. In comparison with Ir-TsDPEN and the related Ru II catalyst, the Rh III catalyst is most efficient in water, affording enantioselectivities of up to 99 % ee at substrate/catalyst (S/C) ratios of 100-1000 even without working under an inert atmosphere. The aqueous phase reduction is shown to be highly pH-dependent; the optimum pH windows for TOF greater than 50 mol mol(-1) h(-1) for Rh- and Ir-TsDPEN are 5.5-10.0 and 6.5-8.5, respectively. Outside the pH window, the reduction becomes slow or stagnant depending on the pH. However, the enantioselectivities erode only under acidic conditions. At a higher S/C ratio, the aqueous ATH by Rh-TsDPEN is shown to be product- as well as byproduct-inhibited; the product inhibition appears to stem at least partly from the reaction being reversible. The aqueous phase reduction is simple, efficient and environmentally benign, thus presenting a viable alternative for asymmetric reduction.
Wu
Xiaofeng
X
Liverpool Centre for Materials and Catalysis, Department of Chemistry, University of Liverpool, Liverpool L69 7ZD, UK.
Li
Xiaohong
X
Zanotti-Gerosa
Antonio
A
Pettman
Allan
A
Liu
Jianke
J
Mills
Allan James
AJ
Xiao
Jianliang
J
D000438:Alcohols / Q000138:chemical synthesis* / Q000737:chemistry; D002384:Catalysis; D006863:Hydrogen-Ion Concentration; D006865:Hydrogenation; D007495:Iridium / Q000737:chemistry*; D007659:Ketones / Q000737:chemistry*; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000737:chemistry*; D012238:Rhodium / Q000737:chemistry*; D013237:Stereoisomerism; D013997:Time Factors; D014867:Water / Q000737:chemistry
D000438:Alcohols; D007659:Ketones; D009942:Organometallic Compounds; D014867:Water; D007495:Iridium; D012238:Rhodium