Enones transfer hydrogenation

In total, over the past six years, the chelating P,N-ligands have shown considerable promise in a variety of enantioselective processes, including transfer-hydrogenation and hydrosilylation of ketones, hydroboration of alkenes, conjugate addition to enones and Lewis-acid catalysed Diels-Alder reactions, in addition to those described above.128,341 It is anticipated that this list will continue to grow, and... 

More recently, using the cyclometallated iridium C,(7-benzoate derived from allyl acetate, 4-methoxy-3-nitrobenzoic acid and BIPHEP, catalytic carbonyl crotylation employing 1,3-butadiene from the aldehyde, or alcohol oxidation was achieved under transfer hydrogenation conditions [274]. Carbonyl addition occurs with roughly equal facility from the alcohol or aldehyde oxidation level. However, products are obtained as diastereomeric mixtures. Stereoselective variants of these processes are under development. It should be noted that under the conditions of ruthenium-catalyzed transfer hydrogenation, conjugated dienes, including butadiene, couple to alcohols or aldehydes to provide either products of carbonyl crotylation or p,y-enones (Scheme 16) [275, 276]. 

Scheme 27. Asymmetric counteranion-directed catalysis transfer hydrogenation of enone...

V,7V-Dialkyl derivatives of 1 have been successfully applied to the asymmetric addition of dialkylzinc reagents to aldehydes, giving products of moderate enantiomeric excess.In addition, ruthenium(II) complexes of 1 have been demonstrated to be excellent catalysts for the control of the enantioselective transfer hydrogenation of ketones to alcohols at catalyst loadings as low as 1 mol The ruthenium/1 complex has been applied to a range of ketone substrates, including cyclic enones and a-amino and alkoxy substituted derivatives. 

In order to facilitate recycling of the multiple TsDPEN-functionalized dendrimer catalysts, the same group recently reported the synthesis of a novel form of hybrid dendrimer ligands by coupling polyether dendrons with peripherally TsDPEN-functionahzed Newkome-type poly(ether-amide) dendrimer (Figure 4.28) [90]. The solubility of these hybrid dendrimers was found to be affected by the generation of the polyether dendron. The ruthenium complexes produced were applied in the asymmetric transfer hydrogenation of ketones, enones, imines and activated... 

Nucleophilic oxidation of electron-deficient alkenes is another route to epoxides. For example, reaction of enones with hydrogen peroxide and sodium hydroxide provides epoxides in good yield. The first attempt to turn this into an asymmetric transformation utilised the benzylchloride salt of quinine as a chiral phase transfer catalyst but only moderate enantioselectivity was obtained (55% with... 

Hydrogenation. Transfer hydrogenation (Pd/C, HCOONH4) affects alkenylfiirans and benzyloxy substituents. Enones are reduced to saturated alcohols. ... 

Cyclohex-2-en-l-one was converted initially to cyclohexanone but this was subsequently reduced at a rate comparable to the rate of appearance of the cyclohexanol e.g. after 30 min there was 40% reduction of the enone to equal amoimts of the saturated ketone and alcohol, while after 48 h there was 73% conversion of enone to essentially just cyclohexanol - no cyclohexen-l-ol was detected (10). Nitrobenzene was reduced selectively to aniline, but the conversion was only 20% after 48 h, while 4-nitrobenzaldehyde gave a mixture of mainly nitrobenzyl alcohol and smaller amounts of aminobenzyl alcohol and the aminobenzaldehyde. Heptan-l-al was also reduced to heptan-l-ol, while benzaldehyde under such basic conditions imderwent the Cannizzaro reaction to give the alcohol and benzoate. The findings summarized in this paragraph are all reproducible, while a notable transfer hydrogenation of oct-l-ene to octane noted in the earlier commimication has not been duplicated this will be commented on later. 

Scheme 3.26 Enantioselective transfer hydrogenation of ot,P-unsaturated enones.

Scheme 3.27 Enantioselective transfer hydrogenation (x,P-unsaturated aldehydes and enones by applying the asymmetric counterion directed catalysis concept.

Scheme 2.5 Enantioselective transfer hydrogenation of cyclic enones catalyzed by 11...

The organocatalytic transfer hydrogenation of cyclic enones has been also successfully achieved employing the imidazolidinone 12 [24], catalyst that also promotes enantioselective Diels-Alder reactions with cyclic enones [25]. The structure of the dihydropyridine reagent seems to have an important effect on the selectivity of the process since better enantioselectivities are observed when increasing the steric hindrance of the ester moiety 13 (Scheme 2.6). The reduction, which is performed with substoichiometric amounts of imidazolidinone 12 in ether at 0°C,... 

The asymmetric epoxidation of acyclic )S,)3-disubstituted o, )3-enones in acetonitrile, by peracetic acid and catalysed by an iron complex in which Fe(OTf)2 was coordinated by two 2-[l-(l-naphthyl)-2-naphthyl]-l,10-phenanthroline ligands (35) (R = m-xylyl), to the corresponding Q ,j8-epoxyketones with yield up to 88% and up to 92% ee was achieved. The epoxy ketone was further converted to functionalized )8-keto-aldehydes with an all-carbon quaternary centre." The transfer hydrogenation of acetophenone to 1-phenylethanol in isopropanol in the absence of added base was catalysed by a five-coordinated Fe(II) complex (36) and certain analogues. ... 

Later, the first enantioselective organocatalytic transfer hydrogenation involving cyclic enones was reported by MacMillan and co-workers following an operationally simple and rapid protocol that allowed access to chiral / -substituted... 

Scheme 9 Transfer hydrogenative couplings of 2-butyne to form allylic alcohols and conjugated enones. Yields are of material isolated by flash chromatography on silica gel...

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