Transfer hydrogenation of unsaturated

In the presence of aqueous NaOH, palladium(II) chloride was effective for the transfer hydrogenation of unsaturated acids, azlactones and phenylpyruvic acid (Scheme 3.36) at 65 °C although in quite long reaction times (typically 16 h) [255], For these water-soluble substrates organic solvents were not required. No attempt was made to clarify the nature of the active catalytic species, which -under these conditions- may well be a fine colloid of Pd metal. 

P-33 - Transfer hydrogenation of unsaturated ketones catalyzed by Al-(sopropoxide dispersed on MCM-41... 

Sinou, D. Safi, M. Claver, C. Masdeu, A. (1991) Catalytic transfer hydrogenation of unsaturated substrates with formates in the presence of water soluble complexes of rhodium, J. Mol Catal, 68, L9-12. 

Ru/tppms catalysts exhibited excellent yields (98%) in the transfer hydrogenation of unsaturated aldehydes such as cinnamaldehyde or crotonaldehyde, to the corresponding unsaturated alcohols under mild reaction conditions (30-80°C), with HCOONa in an aqueous/organic two phase system. Similarly, ruthenium modified with the water soluble, air stable phosphine 100 (pta Table 5) is an effective catalyst for the chemoselective transfer hydrogenation of a,P-unsaturated aldehydes to unsaturated alcohols using formate in an aqueous/ organic two phase system. In contrast, Rh/pta afforded the cor-... 

Transfer hydrogenation of aldehydes with isopropanol without addition of external base has been achieved using the electronically and coordinatively unsaturated Os complex 43 as catalyst. High turnover frequencies have been observed with aldehyde substrates, however the catalyst was very poor for the hydrogenation of ketones. The stoichiometric conversion of 43 to the spectroscopically identifiable in solution ketone complex 45, via the non-isolable complex 44 (Scheme 2.4), provides evidence for two steps of the operating mechanism (alkoxide exchange, p-hydride elimination to form ketone hydride complex) of the transfer hydrogenation reaction [43]. 

The product, although sensitive to light and air, was an effective catalyst for the transfer hydrogenation of several ketones in propan-2-ol. Unsaturated ketones were used with a SCR of 500 1, and mostly gave high selectivity and modest yields (Table 15.8). 

Brunner, Leitner and others have reported the enantioselective transfer hydrogenation of alpha-, beta-unsaturated alkenes of the acrylate type [50]. The catalysts are usually rhodium phosphine-based and the reductant is formic acid or salts. The rates of reduction of alkenes using rhodium and iridium diamine complexes is modest [87]. An example of this reaction is shown in Figure 35.8. Williams has shown the transfer hydrogenation of alkenes such as indene and styrene using IPA [88]. 

Various a,P-unsaturated aldehydes were also selectively reduced to give allylic alcohols by this catalytic system (Scheme 5.19). The transfer hydrogenation of ali-... 

A possible mechanism for the P-alkylation of secondary alcohols with primary alcohols catalyzed by a 1/base system is illustrated in Scheme 5.28. The first step of the reaction involves oxidation of the primary and secondary alcohols to aldehydes and ketones, accompanied by the transitory generation of a hydrido iridium species. A base-mediated cross-aldol condensation then occurs to give an a,P-unsaturated ketone. Finally, successive transfer hydrogenation of the C=C and C=0 double bonds of the a,P-unsaturated ketone by the hydrido iridium species occurs to give the product. 

Subsequently, List reported that although the method described above was not applicable to the reduction of a,P-unsaturated ketones, use of a chiral amine in conjunction with a chiral anion provided an efficient and effective procedure for the reduction of these challenging substrates [210]. Transfer hydrogenation of a series of cyclic and acyclic a,P-unsaturated ketones with Hantzsch ester 119 could be achieved in the presence of 5 mol% of valine tert-butyl ester phosphonate salt 155 with outstanding levels of enantiomeric control (Scheme 64). A simple mechanistic model explains the sense of asymmetric induction within these transformations aUowing for reliable prediction of the reaction outcome. It should also be noted that matched chirality in the anion and amine is necessary to achieve high levels of asymmetric induction. 

Scheme 64 Enantioselective transfer hydrogenation of a, 3-unsaturated ketones...

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. 

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