Osmium unsaturated aldehydes

Osmium makes a sluggish carbonyl hydrogenation catalyst but has the unusual property of reducing a, -unsaturated aldehydes to the unsaturated alcohol in good yield (85). The system has proved erratic high selectivity can only be obtained through prereduction of the catalyst just before use. 

Reduction of unsaturated aldehydes seems more influenced by the catalyst than is that of unsaturated ketones, probably because of the less hindered nature of the aldehydic function. A variety of special catalysts, such as unsupported (96), or supported (SJ) platinum-iron-zinc, plalinum-nickel-iron (47), platinum-cobalt (90), nickel-cobalt-iron (42-44), osmium (<55), rhenium heptoxide (74), or iridium-on-carbon (49), have been developed for selective hydrogenation of the carbonyl group in unsaturated aldehydes. None of these catalysts appears to reduce an a,/3-unsaturated ketonic carbonyl selectively. 

Iridium,204,205 together with osmium, has been not widely used in catalytic hydrogenation. Recently, however, iridium or iridium-based catalysts have been shown to be effective in various hydrogenations, such as in selective hydrogenation of a,P-unsaturated aldehydes to allylic alcohols (Section 5.2), of aromatic nitro compounds to the corresponding hydroxylamines (Section 9.3.6), of halonitrobenzenes to haloanilines without loss of halogen (Section 9.3.2), and in the stereoselective hydrogenation of carbon to carbon double bonds (see, e.g., eqs. 3.25-3.27 and Table... 

Osmium and iridium catalysts have been shown to be highly selective for the formation of unsaturated alcohols by hydrogenation of a,P-unsaturated aldehydes without any additive. Good yields of allyl alcohol (73%), crotyl alcohol (90%), and cinnamyl alcohol (95%) (eq. 5.26) were obtained by the hydrogenation of acrolein, crotonaldehyde, and cinnamaldehyde, respectively, over 5% Os-C catalyst both with and without solvent.63... 

Protected a,3-dihydroxy aldehydes have been prepared by oxidation of acetals of a,B-unsaturated aldehydes with osmium tetroxide in the presence of (23), and a remarkable level of enantioselection ee 2 90%) thereby achieved. Oxidation of chiral acetals of a,p-unsaturated aldehydes in which chirality resides in the noncarbonyl moiety with osmium tetroxide-t ydroquinine acetate (or dihydroquitudine acetate) may be regard as a process in which double stereoselection is at work and a high dia-stereoisomeric ratio of products may be obtained. 

Reduced osmium on carbon is an excellent catalyst for selective hydrogenation of aj3-unsaturated aldehydes to unsaturated alcohols.1 Cinnamaldehyde — cinnamyl alcohol (95% yield). Reduced rates are observed with alumina as the support. This selective reduction is not applicable to a,/3-unsaturated ketones thus hydrogenation of mesityl oxide afforded methyl isobutyl ketone. 

Treatment of a solution of 19 in acetone - water with N- methyl morpholine N- oxide and Osmium tetroxide will give the single cis diol which can be cleaved by reaction with lead tetraacetate in benzene to give the sensitive dialdehyde 20. Treatment of 20 with benzylammonium trifluoroacetate in benzene and refluxing will give the alpha,beta unsaturated aldehyde 21. 

Resolution of crystalline rac-29 (by means of brucine) gives 29, which, after conversion to 30, affords 15 Scheme 9) by Dieckmann condensation. The bicyclic hydroxy ketone 15 reacts with crotylmagnesium bromide to furnish 31a -I- 31b. On successive treatment with osmium tetroxide and lead tetraacetate the diastereoisomeric aldehydes arise which can be dehydrated to afford 16 -I- 32. The latter compound, obtainable by chromatography, on Wittig reaction leads to 33 which, on catalytic hydrogenation, gives 18 Scheme 9). Oxidation converts 18 to 19, and by successive Wittig reaction, followed by selective ozonolysis, unsaturated aldehyde 20 finally is formed. 

Reductions - The use of AIH3 selectively to reduce the carbonyl group in a series of A cyclopentenones has been described. Selective reduction of the carbonyl group in a,6 unsaturated aldehydes is reported, using 5% Osmium on Carbon as hydrogenation catalyst. ... 

Unsaturated aliphatic aldehydes were selectively reduced to unsaturated alcohols by specially controlled catalytic hydrogenation. Citral treated with hydrogen over platinum dioxide in the presence of ferrous chloride or sulfate and zinc acetate at room temperature and 3.5 atm was reduced only at the carbonyl group and gave geraniol (3,7-dimethyl-2,6-octadienol) [59], and crotonaldehyde on hydrogenation over 5% osmium on charcoal gave crotyl alcohol [763]. 

The physical properties, preparation and reactions of ruthenium tetroxide have been reviewed by Lee and van den Engh, Rylander," Haines and Hetuy and Lange. A more vigorous oxidant than osmium tetroxide, its reaction with double bonds produces only cleavage products. " Under neutral conditions aldehydes are formed from unsaturated secondary carbons while carboxylic acids are obtained under alkaline or acidic conditions. For example, Shalon and Elliott" found that ruthenium tetroxide reacted with compound (11) to give the corresponding aldehyde under neutral conditions, but that a carboxylic acid was formed in acidic or alkaline solvents (equation 23). 

Hydroxyketones are versatile intermediates in the synthesis of pharmaceutical intermediates and heterocyclic molecules. a-Aryl hydroxyketones have been prepared by reaction of aryl aldehydes with 1,4-dioxane followed by reduction with lithium aluminum hydride (LAH) and by the selective LAH reduction of a-silyloxy a,P-unsaturated esters." WissneC has shown that treatment of acid chlorides with tris(trimethylsilyloxy)ethylene affords alkyl and aryl hydroxymethyl ketones. 1-Hydroxy-3-phenyl-2-propanone (3) has been generated by the osmium-catalyzed oxidation of phenylpropene and by the palladium-catalyzed rearrangement of phenyl epoxy alcohoP both in 62% yield. 

Oxidative Cleavage of Alkenes. An alternative to the oxidative cleavage of alkenes using ozone or the Lemieux-Johnson protocol has been reported recently. Under the action of catalytic osmium tetroxide, with oxone as a reoxidant, a variety of substituted alkenes were cleaved efficiently to furnish carbonyl compounds (eq 37). Any of the aldehydes that are produced via this sequence are immediately oxidized in situ to give the corresponding acid clearly this does not happen for any ketones so produced. Even electron deficient alkenes such as o , -unsaturated carbonyl compounds could be conveniently oxidized, although the products then underwent a decarboxylation reaction to produce the corresponding diacid. 

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