Hydrogenation of a,/?-unsaturated carbonyl

Selective hydrogenation of a -unsaturated carbonyl compounds can be carried out by reduction with iron pentacarbonyl and a small amount of base in moist solvents. The method is applicable to oc -unsaturated aldehydes, ketones, esters, and lactones with negligible over-reduction of the carbonyl group and is susceptible to the steric environment of the olefin. Spectroscopic evidence suggests that solutions of equimolecular amounts of iodine and thiocyanogen contain an appreciable concentration of iodine thiocyanate. Addition of alkenes results in tran.r-addition to yield jS-iodo-thiocyanates which in base suffer rapid hydrolysis of the thiocyanate followed by ring closure to the episulphide (516). As a synthetic procedure this does not appear to be applicable to acyclic olefins. ... 

The homogeneous transfer-hydrogenation of a/ -unsaturated carbonyl compounds using ruthenium catalysts has been described, and is shown in Scheme 88. 

The regioselective reduction of the C=C bond of a, -unsaturated carbonyl compounds, a very important organic reaction, is achieved by means of hydrogen telluride and phenyl tellurol, under appropriate experimental conditions. 

For our initial studies we chose to evaluate the hydrogenation of two unsaturated carbonyl model prochiral substrates with rhodium complexes of chiral ferrocene diphosphine and tetraphosphine ligands using a standard set of conditions. The substrates screened were methyl a-acetamido cinnamate (MAC) and dimethyl iticonate (DIMI). The substrates, catalysts, conditions, and experimental results are shown in Table 1. 

The oxidation of a, -unsaturated carbonyl compounds under the usual conditions in DMF using PdCl2/CuCl/02 is very slow. However, regioselective oxidation of a, -unsaturated esters to 3-keto esters (equation IS), and a,3-unsaturated ketones to 1,3-diketones (equation 16) proceeds with NazPdCU in solvents such as S0% acetic acid, isc ropyl alcohol, and NMP. r-Butyl hydroperoxide and hydrogen peroxide are used as the reoxidants of the reduced palladium. The reaction proceeds slowly at room temperature but smoothly between SO and 80 C. Some typical examples of this process are shown in Table 1. 

Selective hydrogenation ofx,p-nnsatwated carbonyl compounds. Japanese chemists have effected selective hydrogenation of ,/7-unsaturated carbonyl compounds by a wine-red hydridoiron carbonyl complex generated in situ from iron pentacarbonyl and a small amount of base in moist solvents. Three different procedures can be used ... 

Kelly and colleagues explored the use of bisphenylenediol 103 as a catalyst in Diels-Alder reactions of a, -unsaturated carbonyl compounds. Activation of the dienophile occurred through double hydrogen bonding of the two hydroxyl functions on 103 to the carbonyl group on the dienophile. The reaction of cyclopentadiene with methyl vinyl ketone (equation 31) at ambient temperature showed, after a reaction time of 10 minutes, 3% of product formation in the absence of 103 against 90% of product formation in the presence of 0.4 equivalents of 103. 

Other cobalt catalysts that have been used for enantioselective C—C double bond reduction of a,/ -unsaturated carbonyl compounds are vitamin B12 in combination with zinc/acetic acid (up to 33% ee)S7 and bis(dimcthylglyoximato)cobalt(II) (12) with chiral amines as cocatalysts88 and molecular hydrogen as reducing agent. The best results have been obtained with complex 12 and quinine or (2.S )-A -[(/ )-1 -phenethyl]-2-quinuclidinecarboxaniide (hydrogenation of 1,2-diphenyl-2-propen-l-one 49% ee A, /V -dimethyl-5-benzylidenehydantoin 79% ee). 

Carbonyl hydrogenation is generally less facile than olefin hydrogenation, making selective hydrogenation of a, -unsaturated aldehydes to the allyl alcohol a special challenge. Substitution of the carbon atom attached to the carbonyl (i. e. from the aldehyde to the ketone), substantially increases the steric hindrance to carbonyl adsorption, hence the lack of reports in the literature of selective unsaturated ke-... 

Selective hydrogenation of aj unsaturated carbonyl contpoandsJ Several substrates of this type have been selectively hydrogenated to a, -unsaturated alcohols with catalysis by a chromium-promoted Raney nickel catalyst. ... 

As a further example, a,j8-unsaturated carbonyl compounds possess two sites which can undergo reduction the carbonyl group and the adjacent C—C-double bond. While an abundant hterature is available on the selective reduction of the carbonyl group, often achieved with relative ease, the selective hydrogenation of the double bond suffers from the lack of simple methods using practical chemicals. Selective ionic hydrogenation of ,j8-unsaturated carbonyl compounds with cyclohexane was previously known to proceed only in superacidic conditions due to the necessity of dicationic, superelec-trophilic activation of the enones. H-form zeolites with acidity well below superacidity are however also able to induce the reduction of o j8-unsaturated carbonyl compounds with cyclohexane, in analogy, or sometimes even better than the parent , superacid mediated reactions. ... 

Lower valent tungsten halides are a new class of deoxygenation agents, e.g. for the conversion of carbonyl or epoxy compounds into olefins . A new reagent, generated in situ from iron pentacarbonyl and a small amount of base in moist solvents, selectively and efficiently hydrogenates the ethylenic portion of a,/ -unsaturated carbonyl compounds, such as ketones or lactones, under mild conditions. Aliphatic tert. amides can be easily reduced to alcohols by alkali metals in hexa-methylphosphoramide and a protic cosolvent such as tert-butanol. Aldehydes can be obtained from acids by catalytic reduction of intermediate carboxylic alkoxyformic anhydrides . Sec. nitro compds. are converted into ketones by the joint action of a nitrite ester and NaNOg under mild, non-acidic conditions . 

Manganese. Some years ago, a 7r-oxapropenyl complex was suggested as an intermediate in the hydrogenation of ajS-unsaturated carbonyl compounds in the presence of CoH(CO)4. A compound recently prepared... 

This method ensures the deposition of very reactive metal nanoparticles that require no activation steps before use. We shall review here the following examples of catalytic reactions that are of interest in line chemical synthesis (a) the hydrogenation of substituted arenes, (b) the selective hydrogenation of a, 3-unsaturated carbonyl compounds, (c) the arylation of alkenes with aryl halides (Heck reaction). The efficiency and selectivity of commercial catalysts and of differently prepared nanosized metal systems will be compared. 

Catalytic systems at very low metal loading 0.1% (w/w) obtained in this way can be conveniently used in the hydrogenation of a,P-unsaturated ketones to the corresponding saturated carbonyl compounds with very high efficiencies and selectivities. In Table 4 we report the results obtained in the selective hydrogenation of 4-(6-methoxy-2-naphthyl)-3-buten-2-one, 1, and 2-acetyl-5,8-dimethoxy-3,4-dihydronaphthalene, 2, to the corresponding saturated carbonyl products (I), which are important intermediates... 

The amide group shows a prominent directivity in the hydrogenation of cyclic unsaturated amides by a cationic iridium catalyst, and much higher diastereo-selectivity is realized than in the corresponding ester substrates (Table 21.7). In the case of / ,y-unsaturated bicyclic amide (entry 3), the stereoselectivity surpasses 1000 1 [41]. An increase of the distance between the amide carbonyl and olefmic bond causes little decrease in the selectivity (d, -unsaturated amide, entry 6) compared with the case of the less-basic ester functionality (Table 21.6, entry 5). 

The R,S-family 33, and of course its enantiomer, provide high enantioselectiv-ities and activities for the reductions of itaconic and dehydroamino acid derivatives as well as imines [141], The JosiPhos ligands have found industrial applications for reductions of the carbon-carbon unsaturation within a,/ -unsaturated carbonyl substrates [125, 127, 131, 143-149]. In contrast, the R,R-diastereoisomerof30 does not provide high stereoselection in enantioselective hydrogenations [125, 141]. 

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