Ketones samarium catalysts

Since this initial finding there have been significant improvements in the degree of asymmetric induction observed in these reactions, most notably due to the use of chiral Lewis acids. In one example Evans and co-workers developed a chiral samarium catalyst (17) that was capable of reducing aryl ketones (18) in the presence of i-PrOH with high levels of enantioselectivity.19... 

Evans et al.106 report an asymmetric transfer hydrogenation of ketones using samarium(III) complex (108) as the catalyst at ambient temperature in 2-propanol. The products showed ee comparable with those obtained through enantioselective borane reduction (Scheme 6-48). 

Moreover, propargyl oxiranes 202 were found to react with samarium diiodide and ketones to form a,a -dihydroxyallenes 203 with moderate to high anti-diastereo-selectivities (Scheme 2.62). Aurrecoechea and co-workers [99] reported this reductive coupling to proceed smoothly in the absence of a palladium catalyst, i.e. a direct electron transfer from the samarium(II) to the substrate has to take place in order to generate an allenyl/propargyl samarium intermediate of type 184/185, which is then regioselectively trapped by the electrophile. 

An intriguing chiral samarium complex for the Meerwein-Pondorf-Verley (MPV) reduction of ketones has been reported by Evans.100 The soluble catalyst, prepared as indicated in Figure 46, promoted the asymmetric MPV reduction of aryl methyl ketones in up to 97% ee with as little as 5 mol % loading (Figure 47). 

Ni(0) catalyst. A radical 5-exo cyclization to the potentially zinc or nickel-complexed ketone provides an alkoxyl radical that combines with the co-produced Ni(I) species. A transmetalation to a zinc alkoxide regenerates the catalyst and forms the zinc cyclopentoxide, from which products 79 are liberated on hydrolysis. A bimetallic Cu(I)-Mn(II) system provided similar results (see Sect. 8.4). Analogous samarium diiodide-mediated reactions require in contrast stoichiometric amounts of the reducing agent and are less diastereoselective [26, 27],... 

Evans and co-workers demonstrated that lanthanide complexes containing a chiral tridentate amino dialkoxo ligand (Fig. 76) effectively catalyze the ATH of aryl methyl ketones (334). Terbium- and samarium-based catalysts gave the highest enantioselectivities (>97% ee). 

A related intramolecular reaction of this type is the so-called Evans-Tishchenko reaction [166]. Here, a P-hydroxy ketone is reduced in the presence of an aldehyde yielding 1,3-diol monoesters. Several metal catalysts such as samarium iodide [166, 167] and zirconocene complexes [168] are effective. The reaction is highly diastereo-... 

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