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Rh III ‐ and Ir III ‐Catalyzed Asymmetric Transfer Hydrogenation of Ketones in Water
Author(s) -
Wu Xiaofeng,
Li Xiaohong,
ZanottiGerosa Antonio,
Pettman Allan,
Liu Jianke,
Mills Allan James,
Xiao Jianliang
Publication year - 2008
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200701258
Subject(s) - catalysis , chemistry , transfer hydrogenation , aqueous solution , formate , aqueous two phase system , medicinal chemistry , nuclear chemistry , organic chemistry , ruthenium
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*MCl 2 ] 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.