Reduction of Aromatic Aldehydes

Nickel, Raney nickel and copper chromite are other catalysts suitable for hydrogenation of aldehydes to alcohols with little if any further hydrogenolysis. Benzaldehyde was hydrogenated to benzyl alcohol over nickel [43], Raney nickel [45] and copper chromite [50] in excellent yields. In the last- 

Chemical reduction of aromatic aldehydes to alcohols was accomplished with lithium aluminum hydride [5i], alane [770], lithium borohydride [750], sodium borohydride [757], sodium trimethoxyborohydride [99], tetrabutylam-monium borohydride [777], tetrabutylammonium cyanoborohydride [757], B-3-pinanyl-9-borabicyclo[3.3.1]nonane [709], tributylstannane [756], diphenylstan-nane [114], sodium dithionite [262], isopropyl alcohol [755], formaldehyde (crossed Cannizzaro reaction) [i7i] and others. 

The chiral reagent prepared in situ from ( + )-a-pinene and 9-borabicyclo-[3.3.1]nonane reduced benzaldehyde-7- 7 to benzyl-/- 7 alcohol in 81.6% yield and 90% enantiomeric excess [709]. 

Reduction of the carhonyl group in aromatic aldehydes to the methyl group 

Triethylsilane in the presence of boron trifluoride [772] or trifluoroacetic acid [777] also reduced the aldehyde group to a methyl group. 

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This reaction is favored by moderate temperatures (100—150°C), low pressures, and acidic solvents. High activity catalysts such as 5—10 wt % palladium on activated carbon or barium sulfate, high activity Raney nickel, or copper chromite (nonpromoted or promoted with barium) can be used. Palladium catalysts are recommended for the reduction of aromatic aldehydes, such as that of benzaldehyde to toluene. 

The effect of cryptands on the reduction of ketones and aldehydes by metal hydrides has also been studied by Loupy et al. (1976). Their results showed that, whereas cryptating the lithium cation in LiAlH4 completely inhibited the reduction of isobutyraldehyde, it merely reduced the rate of reduction of aromatic aldehydes and ketones. The authors rationalized the difference between the results obtained with aliphatic and aromatic compounds in terms of frontier orbital theory, which gave the following reactivity sequence Li+-co-ordinated aliphatic C=0 x Li+-co-ordinated aromatic C=0 > non-co-ordinated aromatic C=0 > non-co-ordinated aliphatic C=0. By increasing the reaction time, Loupy and Seyden-Penne (1978) showed that cyclohexenone [197] was reduced by LiAlH4 and LiBH4, even in the presence of [2.1.1]-cryptand, albeit much more slowly. In diethyl ether in the absence of... 

In a water/chlorobenzene biphasic system, reduction of aromatic aldehydes by hydrogen transfer from aqueous sodium formate catalyzed by [ RuCl2(TPPMS)2 2] provided unsaturated alcohols exclusively (Scheme 10.7). Addition of 3-CD shghtly inhibited the reaction [13]. It was speculated that this inhibition was probably due to complexation of the catalyst by inclusion of one of the non-sulfonated phenyl rings of the TPPMS ligand, however, no evidence was offered. 

The reduction of the preformed tosylhydrazones with sodium borohydride may be effected in aprotic solvents, such as tetrahydrofuran or dioxane. The use of lithium aluminium hydride in nonhydroxylic solvents permits the reduction of aromatic aldehydes and ketones. 

Reduction of aromatic aldehydes to pinacols using sodium amalgam is quite rare. Equally rare is conversion of aromatic aldehydes to alkenes formed by deoxygenation and coupling and accomplished by treatment of the aldehyde with a reagent obtained by reduction of titanium trichloride with lithium in dimethoxyethane. Benzaldehyde thus afforded /ra/is-stilbene in 97% yield [206, 209]. 

Oxidation of hydrazines 9-33 Dimerization of arenes 9-37 Reduction of aromatic aldehydes 9-43 Reduction of aromatic acids... 

The steady state and stopped-flow kinetic studies on the horse liver enzyme are now considered classic experiments. They have shown that the oxidation of alcohols is an ordered mechanism, with the coenzyme binding first and the dissociation of the enzyme-NADH complex being rate-determining.15,26,27 Both the transient state and steady state methods have detected that the initially formed enzyme-NAD+ complex isomerizes to a second complex 27,28 In the reverse reaction, the reduction of aromatic aldehydes involves rate-determining dissociation of the enzyme-alcohol complex,27,29 whereas the reduction of acetaldehyde is... 

The reaction should be compared with the simultaneous oxidation and reduction of aromatic aldehydes under the influence of concentrated caustic alkali (Reaction LXIII.). 

The so-called crossed Cannizzaro reaction is synthetically more useful than the Cannizzaro reaction itself, as it can be applied for the preparation of alcohols in high yields, without loss of 50% of the product in the formation of the corresponding carboxylic acid. Typically, paraformaldehyde is used as a sacrificial reducing agent, together with the carbonyl compound which is to be transformed into the alcohol. The reaction thus serves as an alternative method to the use of complex hydrides for the reduction of aromatic aldehydes. 

In a more general approach, eight examples of the Wolff—Kishner reduction of aromatic aldehydes and ketones are described using 80% hydrazine hydrate in toluene64 (Scheme 4.37). The reaction times are longer than described in the previous paper because less reactive substrates were used. Still, both the formation of the hydrazone and the reduction step are considerably faster than under thermal conditions the reduction proceeds at ambient pressure and in the absence of a solvent. The microwave reduction is compatible with other reducible functional groups such as aromatic OMe, Me, Cl or COOMe, which can otherwise cause problems under conventional reaction conditions64. 

C Kinetic isotope effects (KIEs) of a xylose reductase-catalysed cinnamalde-hyde reduction have been determined by 13C NMR using competition reactions with reactants at natural 13C abundance. The primary KIEs indicated that the chemical reaction steps are only partly rate limiting during reduction of aromatic aldehydes and slow steps occur outside the catalytic sequence. The aldo-keto reductase-catalysed... 

Alcohols are converted to alkyl iodides by reaction with A A-diethyl-aniline/borane/iodine. Reductive iodination is observed when this system reacts with ketones or with carboxylic acids934,935. Direct reduction of aromatic aldehydes to benzyl bromide is reported by Le Corre and coworkers, who have brominated benzaldehydes and acetophenones in presence of a trimethylamine/borane complex and have obtained benzyl bromides and a-bromoethylbenzenes, respectively936. See also Reference937. 

Aromatic alcohols are obtained by the hydrolysis of benzyl halides and by the reduction of aromatic aldehydes and ketones. 

METAL HYDRIDE REDUCTIONS OF AROMATIC ALDEHYDES AND KETONES 313... 

Arylcyclopropanes by Zinc Reduction of Aromatic Aldehydes in the Presence of BF3 OEt and an Alkene General ftocedure ... 

The reagent formed by reaction of Znij with NaCNBH3 in CHjClj allows the reduction of aromatic aldehydes and ketones as well as benzylic, allylic, and tertiary alcohols to hydrocarbons, probably by a radical process [LDl] (Section 2.4). Some comparable reductions are carried out in ether media starting from tertiary, benzylic, or allylic halides (Section 2.1). 

For aliphatic aldehydes and ketones, reduction to the alcohol can be carried out under mild conditions over platinum or the more-active forms of Raney nickel. Ruthenium is also an excellent catalyst for reduction of aliphatic aldehydes and can be used to advantage with aqueous solutions. Palladium is not very active for hydrogenation of aliphatic carbonyl compounds, but is effective for the reduction of aromatic aldehydes and ketones excellent yields of the alcohols can be obtained if the reaction is interrupted after absorption of one mole of hydrogen. Prolonged reaction, particularly at elevated temperatures or in the presence of acid, leads to hydrogenolysis and can therefore be used as a method for the reduction of aromatic ketones to methylene compounds. 

Ligand transfer has been suggested as the initial step in the reduction of aromatic aldehydes by Cr in acid solution. ... 

Since few aldehydes lack a-hydrogen atoms, the Cannizzaro reaction is of limited use in modern synthetic sequences, although a variation called the crossed Cannizzaro is occasionally used for the reduction of aromatic aldehydes. In this reaction excess formaldehyde is mixed with the aromatic aldehyde and becomes preferentially activated by base since it is present in excess. Thus, the aldehyde present to the lesser extent is reduced to the primary alcohol in high yield. For example. 

Pierre and Handel have studied the effect of [2.1.1]-cryptate on the lithium aluminum hydride reduction of cyclohexanone in diglyme [16]. The [2.1.1]-cryptate strongly complexes lithium ion and if sufficient cryptate is used to sequester all of the lithium ion, no reduction occurs. Apparently, lithium ion is needed as an electrophilic catalyst for the reduction to occur (see Eq. 12.8). Consistent with this interpretation is the observation that even in the presence of cryptate, reduction will occur if an excess of lithium iodide is also present. The relatively low reactivity of tetrabutyl-ammonium borohydride in benzene solution may also reflect this property, at least in part [9]. Likewise, the jS-hydroxyethyl quaternary ammonium ions may be better catalysts than non-oxygenated quaternary ions because the hydroxyl may hydrogen bond to carbonyl and provide electrophilic catalysis [5]. Similar, though less dramatic results, have been observed in the reduction of aromatic aldehydes and ketones by lithium aluminum hydride in the presence of [2.1.1]-cryptate [17]. 

In catalytic reductions of aromatic aldehydes and ketones, excess tritium and prolonged reaction times should be avoided since they may result in tritiolysis of the carbon-oxygen bonds, producing labeled methyl or methylene groups. Examples of tritio-deoxygena-tions can be found in Section 4.2.2.2. 

A critical review of polarographic researches on semicarba zones was given in the recently published article by Fleet and Zuman [164]. They showed that the four-electron reduction of semicarba-zones is preceded by protonation. They examined also quantitatively the effect of polar substituents on the reduction of aromatic aldehyde derivatives and found that in this series of compounds the values are relatively little sensitive to the effects of substituents (p =0.10 V). 

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