Hydrogen atoms unsaturated aldehydes

Reduction of unsaturated carbonyl compounds to the saturated carbonyl is achieved readily and in high yield. Over palladium the reduction will come to a near halt except under vigorous conditions (73). If an aryl carbonyl compound, or a vinylogous aryl carbonyl, such as in cinnamaldehyde is employed, some reduction of the carbonyl may occur as well. Carbonyl reduction can be diminished or stopped completely by addition of small amounts of potassium acetate (i5) to palladium catalysts. Other effective inhibitors are ferrous salts, such asferroussulfate, at a level of about one atom of iron per atom of palladium. The ferrous salt can be simply added to the hydrogenation solution (94). Homogeneous catalysts are not very effective in hydrogenation of unsaturated aldehydes because of the tendencies of these catalysts to promote decarbonylation. 

The aim of this work was to develop and present a new method for the synthes selective platinum catalysts for the hydrogenation of unsaturated aldehydes to unsatui alcohols. The method of preparation presented above makes it possible to obtain cata possessing polar platinum surface, due to the presence of active centers near the intei metal - partially reduced transition metal oxide. This transition metal oxide adsorb carbonyl oxygen atom whereas adjacent platinum atom interacts with carbonyl ca atom. In such a situation, the C=C bond is quite far from the surface so its adsorption i favored. There are also some additional parameters which should be taken consideration. The first is the size of platinum crystallites located on the catalyst sur Large platinum crystallites cause planar adsorption of aldehyde molecule in such adsorption of both double bonds is highly probable. It is known that a more sur... 

Although excellent yields of the unsaturated amides and urethans could be obtained, hydrolysis of the urethans gave poor yields of the aldehyde. The application of the Curtius degradation resulted in excellent yields of the various intermediates and a fair yield of the aldehyde. It appears that the presence of the heterocyclic moiety renders these aldehydes less stable than the corresponding aldehydes in the benzene series. Possibly the electron rich thiophene ring bestows a higher reactivity on the hydrogen atoms of the methylene carbon. 

After optimization of reaction conditions with a special focus on in situ catalyst generation, the pH value of the catalyst phase and the ratio of ligand to metal in the hydrogenation of prenal, the transferabihty of the catalyst system to other Q ,/f-unsaturated aldehydes was checked. The influence of steric hindrance at the C3-atom and the water solubiUty of the substrates on the reaction rate and selectivity to the unsaturated alcohol were analysed (Table 2). The initial concentration of the aldehyde in the organic phase was always 0.5 M. Apart from acrolein, which is not mentioned in the table, generally all kinds of Q ,/f-unsaturated aldehydes can be selectively hydrogenated with... 

Ru(II)-TPPTS to the corresponding unsaturated alcohols in biphasic mode. If one compares the reaction times until full conversion, it becomes clear that the reaction rate correlates with the solubility of the substrate in the aqueous phase, as expected. The latter decreases with increasing chain length or branching of the chain at the C3-atom. In contrast to heterogeneously catalysed hydrogenations of o , d-unsaturated aldehydes, the steric hindrance of substituents at the C3-atom only plays a minor role in the coordination mode of the substrate at the metal centre, since selectivity differences from croton-aldehyde to citral are marginal. 

An example of a structural substituent that is often metabolized (bioactivated) to an electrophile is the allyl alcohol substituent (C=C—C—OH). Allyl alcohol moieties are found in many commercial chemical substances, either as the free alcohol or as an ester or ether. As illustrated in Scheme 4.1, allyl alcohols (and also as their esters or ethers) that contain at least one hydrogen atom on the alcoholic carbon can be oxidized in the liver by alcohol dehydrogenase (ALDH) to the corresponding a, 3-unsaturated carbonyl metabolite, which is toxic in many cases [29-31]. The hepatotoxicity of allyl alcohol (1), for example, is due to its oxidation by ALDH to acrolein (2), an a,(3-unsaturated aldehyde, which undergoes Michael addition with cellular nucleophiles in the liver [29] (Scheme 4.1). Cyclic allyl alcohols (Scheme 4.1) are expected to undergo similar enzymatic oxidation to yield a,(3-unsaturatcd carbonyl metabolites and are also likely to be toxic. 

The acyl-alkv biradical obtained by ring-opening of a cyclic ketone is able lo undergo intramolecular disproportionation in one of two ways. A hydrogen atom may be transferred to the acyl radical from the position adjacent to the alkyl group, and this produces an unsaturated aldehyde (4.21). Alternatively, a hydrogen may be transferred to the alkyl radical from the position adjacent to the acyl group, and this results in the formation of a ketene (4.22). Many ketenes are labile, and the use of a nucleophilic solvent or addend. 

Reaction xxklll. (d) Condensation of Aldehydes with the Sodium Salts of certain Adds in the presence of Add Anhydrides (Perkin). (A., 100,126 227, 48 B., 10,68 14,1826 J. C. S., 21, 53 J 1877, 789.) —This is a reaction of very wide application, and one much used in the preparation of unsaturated aromatic carboxylic acids. It consists in heating together—usually to 180°—an aldehyde, the sodium salt of a fatty acid with at least one oc-hydrogen atom, and an acid anhydride. The following reactions then occur —... 

All the steps of this reaction are reversible but the position of the equilibrium is significantly in favour of the aldol, which generally may be obtained when the reaction is carried out at room temperature or below, followed by extraction and careful distillation under reduced pressure. When the required product is the unsaturated aldehyde the reaction is carried out at a higher temperature, and dehydration of the aldol occurs readily (e.g. 2-ethylhex-2-enal, Expt 5.212). In the case of aldehydes with only one a-hydrogen atom, aldol formation occurs but the resulting / -hydroxyaldehyde cannot undergo the dehydration step. 

Optically active aldehydes can be obtained by asymmetric hydroformylation of olefinic substrates when at least one asymmetric carbon atom is formed either by addition of a formyl group or of a hydrogen atom to an unsaturated carbon atom (Scheme 1, reactions (1) and (2)). In the case of trisubstituted olefins, two new asymmetric carbon atoms can form due to the cis stereochemistry of the reaction10), in the absence of isomerization, the formation of only one epimer is expected. 

Pentacarbonyl[methyl(methoxy)carbene]tungsten and other carbene complexes containing a hydrogen atom a to the carbene carbon atom react with butyllithium (but also with OMe" ) at low T to generate carbene anions. The moderate reactivity of these carbene anions toward carbon nucleophiles, including epoxides, aldehydes, a-bromoesters and a, -unsaturated carbonyl compounds can be used to prepare carbene complexes inaccessible via other synthetic routes (see refs. 7-9). The anion generated by treatment of (CO)5Cr[C(OMe)Me] with BuLi in THF at — 78°C is isolated as the air-stable bis(triphenylphosphane)iminium salt . 

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