Catalysts unsaturated aldehydes

The simplest a 3 unsaturated aldehyde acrolein is prepared by heating glycerol with an acid catalyst Suggest a mechanism for this reaction... 

The direct oxidation of ethylene is used to produce acetaldehyde (qv) ia the Wacker-Hoechst process. The catalyst system is an aqueous solution of palladium chloride and cupric chloride. Under appropriate conditions an olefin can be oxidized to form an unsaturated aldehyde such as the production of acroleia [107-02-8] from propjiene (see Acrolein and derivatives). 

These reversible reactions are cataly2ed by bases or acids, such as 2iac chloride and aluminum isopropoxide, or by anion-exchange resias. Ultrasonic vibrations improve the reaction rate and yield. Reaction of aromatic aldehydes or ketones with nitroparaffins yields either the nitro alcohol or the nitro olefin, depending on the catalyst. Conjugated unsaturated aldehydes or ketones and nitroparaffins (Michael addition) yield nitro-substituted carbonyl compounds rather than nitro alcohols. Condensation with keto esters gives the substituted nitro alcohols (37) keto aldehydes react preferentially at the aldehyde function. 

Isopropyl alcohol can be oxidized by reaction of an a,P-unsaturated aldehyde or ketone at high temperature over metal oxide catalysts (28). In one Shell process for the manufacture of aHyl alcohol, a vapor mixture of isopropyl alcohol and acrolein, which contains two to three moles of alcohol per mole of aldehyde, is passed over a bed of uncalcined magnesium oxide [1309-48-4] and zinc oxide [1314-13-2] at 400°C. The process yields about 77% aHyl alcohol based on acrolein. 

The formation of pyridine derivatives from a, P-unsaturated aldehydes and ammonia involves formation of three bonds during the ring synthesis. For example, with an a, P-unsaturated aldehyde, both 2,5-substituted as well as 3,4-substituted pyridines can be obtained, depending on whether a 1,2- (eq. 17) or 1,4-addition (eq. 18) occurs with ammonia. Reactions are performed in the vapor phase with catalysts. 

Dijbner-von Miller Synthesis. A much less violent synthetic pathway, the Dn bner-von Miller, uses hydrochloric acid or 2inc chloride as the catalyst (43). As in the modified Skraup, a,P-unsaturated aldehydes and ketones make the dehydration of glycerol uimecessary, and allow a wider variety of substitution patterns. No added oxidant is required. With excess aniline the reaction proceeds as follows ... 

Chiral aluminum catalyst 2, prepared from Et2AlCl and a Vaulted biaryl ligand, is reported to be an effective Lewis acid catalyst of the Diels-AIder reaction between methacrolein and cyclopentadiene, affording the adduct in 97.7% ee [4] (Scheme 1.2). Although the Diels-AIder reaction with other a,/ -unsaturated aldehydes has not been described, that only 0.5 mol% loading is sufficient to promote the reaction is a great advantage of this catalyst. 

In 1989 Yamamoto et al. reported that the chiral (acyloxy)borane (CAB) complex 3 is effective in catalyzing the Diels-AIder reaction of a number of a,/ -unsaturated aldehydes [5]. The catalyst was prepared from monoacylated tartaric acid and bo-... 

To overcome these problems with the first generation Brmsted acid-assisted chiral Lewis acid 7, Yamamoto and coworkers developed in 1996 a second-generation catalyst 8 containing the 3,5-bis-(trifluoromethyl)phenylboronic acid moiety [10b,d] (Scheme 1.15, 1.16, Table 1.4, 1.5). The catalyst was prepared from a chiral triol containing a chiral binaphthol moiety and 3,5-bis-(trifluoromethyl)phenylboronic acid, with removal of water. This is a practical Diels-Alder catalyst, effective in catalyzing the reaction not only of a-substituted a,/ -unsaturated aldehydes, but also of a-unsubstituted a,/ -unsaturated aldehydes. In each reaction, the adducts were formed in high yields and with excellent enantioselectivity. It also promotes the reaction with less reactive dienophiles such as crotonaldehyde. Less reactive dienes such as isoprene and cyclohexadiene can, moreover, also be successfully employed in reactions with bromoacrolein, methacrolein, and acrolein dienophiles. The chiral ligand was readily recovered (>90%). 

C, 92% ee at -20 °C, 88% ee at 0°C in the reaction of acrolein and cyclopen-tadiene). This is unusual for metal-catalyzed asymmetric reactions, with only few similar examples. The titanium catalyst 10 acts as a suitable chiral template for the conformational fixing of a,/ -unsaturated aldehydes, thereby enabling efficient enantioface recognition, irrespective of temperature. 

Among the many chiral Lewis acid catalysts described so far, not many practical catalysts meet these criteria. For a,/ -unsaturated aldehydes, Corey s tryptophan-derived borane catalyst 4, and Yamamoto s CBA and BLA catalysts 3, 7, and 8 are excellent. Narasaka s chiral titanium catalyst 31 and Evans s chiral copper catalyst 24 are outstanding chiral Lewis acid catalysts of the reaction of 3-alkenoyl-l,2-oxazolidin-2-one as dienophile. These chiral Lewis acid catalysts have wide scope and generality compared with the others, as shown in their application to natural product syntheses. They are, however, still not perfect catalysts. We need to continue the endeavor to seek better catalysts which are more reactive, more selective, and have wider applicability. 

Below is a table of asymmetric Diels-Alder reactions of a,/ -unsaturated aldehydes catalyzed by chiral Lewis acids 1-17 (Fig. 1.10, 1.11). The amount of catalyst, reaction conditions (temperature, time), chemical yield, endojexo selectivity, and optical purity are listed (Table 1.32). 

Cobalt boride has been used for reducing unsaturated aldehydes to unsaturated alcohols improved results are obtained by addition of ferric chloride or chromium chloride (6S). It is a low-activity catalyst. 

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. 

Another example of this preference is found in the enantiospecific syntheses of tritium-labeled leukotrienes(/i). Commercially available 3-nonyn-l-ol was converted to its phosphorane (16) and Wittig-coupled with the unsaturated aldehyde (17) to afford 18, which was reduced over Lindlar catalyst to give 19. 

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. 

Much like the oxidation of propylene, which produces acrolein and acrylic acid, the direct oxidation of isobutylene produces methacrolein and methacrylic acid. The catalyzed oxidation reaction occurs in two steps due to the different oxidation characteristics of isobutylene (an olefin) and methacrolein (an unsaturated aldehyde). In the first step, isobutylene is oxidized to methacrolein over a molybdenum oxide-based catalyst in a temperature range of 350-400°C. Pressures are a little above atmospheric ... 

Supported Lewis acids are an interesting class of catalysts because of their operational simplicity, filterability and reusability. The polymer-bound iron Lewis-acid 53 (Figure 3.8) has been found [52] to be active in the cycloadditions of a, S-unsaturated aldehydes with several dienes. It has been prepared from (ri -vinylcyclopentadienyl)dicarbonylmethyliron which was copolymerized with divinylbenzene and then treated with trimethylsilyltriflate followed by THF. Some results of the Diels-Alder reactions of acrolein and crotonaldehyde with isoprene (2) and 2,3-dimethylbutadiene (4) are summarized in Equation 3.13. 

Binaphthol-derived titanium complexes [64], prepared from chiral ligands 65 (Figure 3.13), also performed very well in the cycloadditions of conjugated aldehydes with cyclic and acyclic dienes. Judging from the absolute configurations of endo and exo adducts, this catalyst should cover the re-face of carbonyl on its u tz-coordination to s-trans a,/l-unsaturated aldehydes, and hence dienes should approach selectively from the si-face. 

When the iron-based catalyst 66 was used, a high level of enantiomeric excess in the cycloadditions between cyclopentadiene (18) and a,/i-unsaturated aldehydes [65] was observed. The cycloadditions were carried out in the presence of 2,6-di-t-butylpyridine (Scheme 3.15) which was added to scavenge residual acid impurities. 

When unsymmetrical ketones were used in this reaction (with BF3 as catalyst), the less highly substituted carbon preferentially migrated. The reaction can be made regioselective by applying this method to the a-halo ketone, in which case only the other carbon migrates. The ethyl diazoacetate procedure has also been applied to the acetals or ketals of a, P-unsaturated aldehydes and ketones. ... 

Rovis and co-workers have also shown that pre-catalyst 129 is competent with a wide range of Michael acceptors including oc,P-unsaturated aldehydes, amides, nitriles, esters, thioesters, vinylphosphonates and vinylphosphine oxides (Scheme 12.25) [58,60],... 

Nanostructured Pt(0) catalysts supported on cross-linked macromolecular matrices (Figure 5) have recently been evaluated in the hydrogenation of the a,P-unsaturated aldehyde, ( , Z)-3,7-dimethyl-2,6-octadienal (citral) (Scheme 10) [25]. 

We describe here (i) the selective hydroformylation of 1,3-dienes to p,y-unsaturated aldehydes promoted by catalyst A and (ii) the efficient hydrosilylation of aromatic nitriles to A,iV-disilylamines using catalysts B and C. 

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