2-propanol, transfer hydrogenation with

In transfer hydrogenation with 2-propanol, the chloride ion in a Wilkinson-type catalyst (18) is rapidly replaced by an alkoxide (Scheme 20.9). / -Elimination then yields the reactive 16-electron metal monohydride species (20). The ketone substrate (10) substitutes one of the ligands and coordinates to the catalytic center to give complex 21 upon which an insertion into the metal hydride bond takes place. The formed metal alkoxide (22) can undergo a ligand exchange with the hydride donor present in the reaction mixture, liberating the product (15). 

The real catalytic species 42 and key reactive intermediate 43 in asymmetric transfer hydrogenation with a chiral ligand 13-Ru(II) complex were isolated and characterized (Scheme 35) [120]. Examination of the reactivities of the two complexes as well as the kinetic study fully revealed the reaction mechanism. When the purple complex 42 is treated with 2-propanol at room temperature in the absence of any base, rapid elimination of acetone took place to produce the yellow Ru hydride species 43. The treatment of this 18-electron species 43 with... 

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). 

The procedure is very easy to reproduce and the asymmetric transfer hydrogenation may be applied to a wide range of aromatic ketones. Table 9.3 gives different substrates that can be reduced with the Ru(II)-(2-azanorbornylmetha-nol) complex in Ao-propanol... 

Remarkably, complex 25 was also able to reduce CO2 by transfer hydrogenation in 2-propanol [28]. While there have been many reports using H2 to reduce CO2, the work of Peris and coworkers is the only example of a hydrogen transfer reaction to reduce CO2 with 2-propanol [29]. The reduction is run in the presence of KOH,... 

The treatment of [Cp MCl2]2 (M = Rh and Ir) with (S,S)-TsDPEN gave chiral Cp Rh and Cp Ir complexes (12a and 12b Scheme 5.9). An asymmetric transfer hydrogenation of aromatic ketones using complex 12 was carried out in 2-propanol in the presence of aqueous KOH (1 equiv.) the results obtained are summarized in Table 5.4. In all of the reactions, the (S)-alcohols were obtained with more than 80% enantiomeric excess (ee) and in moderate to excellent yields. The rhodium catalyst 12a was shown to be considerably more active than the iridium catalyst... 

The transfer hydrogenation of a-keto- S -unsaturated esters, catalyzed by Ru(p-cymene)(TsDPEN) (TsDPEN monotosylated l,2-diphenylethylene-l,2-dia-mine) with 2-propanol as the hydrogen source, has been developed as an efficient method for the preparation of a-hydroxy-)S, y-unsaturated esters or acids. 

The rate constants of electron transfer with amines are much larger than those of hydrogen atom transfer, e.g. in the case of benzophenone, by over three orders of magnitude between triethylamine and 2-propanol. However, hydrogen atom transfer leads in most cases to irreversible reactions, but electron transfer is often reversible through the recombination of the ions... 

Excellent enantioselectivity was achieved for the transfer hydrogenation of pinacolone by using (S)-25a as a catalyst with 2-propanol in the presence of (CH3)2CHONa to give the S alcohol in >99% ee (Scheme 28) [90], 2,2-Dimethyl-cyclohexanone was reduced with the same catalyst with 98% optical yield. Reduction of cyclohexyl methyl ketone with (S)-25b gave the S alcohol in 66% ee. 

Asymmetric transfer hydrogenation of benzaldehyde-l-d with (R,R)-28 and (CH3)3COK in 2-propanol gave (R)-benzyl-l-d alcohol quantitatively in 98% ee (Scheme 41) [114], Introduction of electron-donating and electron-accepting groups at the 4 position had little effect on the enantioselectivity. Catalytic deuteration of benzaldehydes was achieved by use of the same complex (R,R)-28 and a 1 1 mixture of formic acid-2-d and triethylamine to give the S deuter-io alcohols in up to 99% ee (Scheme 42) [114], The dt content in the product alcohol was >99%. Only a stoichiometric amount of deuterium source is required to complete the reaction. 

Diastereoselective transfer hydrogenation of the chiral ketone (S)-A with (R,/ )-43 in 2-propanol gives (3S.4S)-B in >97% yield (Scheme 1.87) [327). Reaction with (S,5)-43 affords the 3R.4S alcohol predominantly. The degree and sense of diastereoface differentiation are mostly controlled by the chirality of the Ru catalyst. 

Keto Esters Asymmetric transfer hydrogenation of functionalized ketones is rare. However, an excellent optical yield is obtainable inreduction of methyl benzoylformate by using 2-propanol and with a catalyst system consisting of [RhCl(C6H]0)]2, (S,S)-DMDPEN, and KOH (Scheme 1.89) [313],... 

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