Reduction of Aliphatic Aldehydes

Several reagents reduce aldehydes preferentially to ketones in mixtures of both. Very high selectivity was found in reductions using dehydrated aluminum oxide soaked with isopropyl alcohol and especially diisopropylcarbinol [755], or silica gel and tributylstamane [756]. The best selectivity was achieved with lithium trialkoxyalumimm hydrides at —78°. In the system hexanal/ cyclohexanone the ratio of primary to secondary alcohol was 87 13 at 0° and 91.5 8.5 at —78° with lithium tris(/er/-butoxy)aluminum hydride [752], and 93.6 6.4 at 0° and 99.6 0.4 at —78° with lithium tris(3-ethyl-3-pentyl-oxy)aluminum hydride [752], 

Reduction of saturated aliphatic aldehydes to alkanes was carried out by refluxing with amalgamated zinc and hydrochloric acid (Clemmensen reduction) [760, 758] (p. 28) or by heating with hydrazine and potassium hydroxide (Wolff-Kizhner reduction) [280, 759] (p. 34). Heptaldehyde gave heptane in 72% yield by the first and in 54% yield by the second method. 

Other possibilities of converting the aldehyde group to a methyl group are desulfurization of the mercaptal (p. 104) and reduction of azines, hydrazones and tosylhydrazones (p. 106). 

An interesting reduction of aldehydes takes place on treatment with a reagent prepared from titanium trichloride and potassium [206] or magnesium [207] in tetrahydrofuran propionaldehyde gave a 60% yield of a mixture of 30% cis- and 30% /ronj-3-hexene [207]. 

A method for the conversion of unsaturated aliphatic aldehydes to saturated aldehydes is a gentle catalytic hydrogenation. Palladium is more selective than nickel. Hydrogenation over sodium borohydride-reduced palladium in methanol at room temperature and 2 atm reduced crotonaldehyde to butyralde-hyde but did not hydrogenate butyraldehyde [57]. Nickel prepared by reduction with sodium borohydride was less selective it effected reduction of crotonaldehyde to butyraldehyde but also reduction of butyraldehyde to butyl alcohol, though at a slower rate [57]. Hydrogenation of 2,2,dimethyl- 

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The simplest type of dehydration reaction is involved in the reduction of aliphatic aldehydes (76) of the type R—CHO, where R is an alkyl group (but not hydrogen). Dehydration follows the scheme (29), where B is a base ... 

Tin Hydrides. Tributyltin hydride reduces aldehydes to primary alcohols by simply heating in methanol. °° A mixture of BusSnH and phenylboronic acid (p. 815) reduces aldehydes in dichloromethane. °° Reduction of ketones was achieved with Bu2SnH2 and a palladium catalyst. ° Using triaryltin hydrides with Bp3 OEt2, where aryl is 2,6-diphenylbenzyl, selective reduction of aliphatic aldehydes in the presence of a conjugated aldehyde was achieved. 

Some examples of the effect of the electrode material on the branching of reactions are the reduction of benzyltriethylammonium nitrate in dimenthylformamide (DMF) to bibenzyl or toluene at aluminum or platinum electrodes [115] and the reduction of aliphatic aldehydes [116] or alkyl arylketones [117] at different materials. 

Copper chromite catalyzes reduction of aliphatic aldehydes to the alcohol, and Ru-on-carbon appears especially effective in aqueous medium. 

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. 

Reduction of aromatic and related aldehydes to alcohols Reduction of aliphatic aldehydes to alcohols... 

The ternary system consisting of a metallic catalyst, a chlorosilane, and a stoichiometric co-reductant has been reported by us for the first time to achieve the catalytic pinacol coupling. The homo coupling of aliphatic aldehydes is catalyzed by CpV(CO)4, Cp2VCl2, or Cp2V in the presence of a chlorosilane and Zn in DME to give the 1,3-dioxolanes 1 via the coupling and acetalization (Scheme 3) [18,19]. 

Unsymmetrical secondary aliphatic amines have been prepared by reaction of alkyl halides with benzylidene amines and subsequent hydrolysis 814 by reaction of alkyl halides with alkyl amines 5 by reduction of amine-aldehyde adducts 8-8 and by dealkylation of tertiary amines with dibenzoyl peroxide. ... 

RCOOH - RCHO (12,485). This borane (2 equiv.) reduces acids at room temperature to thexylboronic acid and aldehydes, which are best isolated as the sodium bisulfite adduct. Yields of aliphatic aldehydes are in the range 80-95%. Reduction of aromatic acids is slow, and yields are significantly lower. 

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... 

Appropriate activation of carboxyl groups enables reduction of aliphatic carboxylic acids to the corresponding aldehydes. The electroreduction of iminium salts prepared from aliphatic carboxyKc... 

Recently, the electrolysis of aliphatic carboxylic acids in an undivided cell and in the presence of triphenyl phosphine has been reported, which turned out to be one of the most reliable methods for the reduction of aliphatic carboxylic acids to the corresponding aldehydes (Scheme 26) [11, 52]. In this reaction. 

Scheme 26 Cathodic reduction of aliphatic carboxylic acids in the presence of triphenylphosphineto aldehydes R alkyl, aryl, yields 36 -100%.

The electrochemical reduction of aliphatic amides in dilute hydrochloric acid will tolerate other functional group.s active towards the alternative hydride reducing agents. With aliphatic amides the electrochemical step generally leads to the aldehyde because the intermediate imine is rapidly hydrolysed under the conditions... 

Reduction of aliphatic acid chlorides follows a roughly similar course78. That is, tetrameric products similar to 54 are the major products, together with small amounts of the corresponding aldehyde. The latter has been suggested to be formed from both acyl radical... 

Bis(triphenylphosphine)copper(i) tetrahydroborate [(Ph3P)2CuBH4] has found use as a reagent for the reduction of aliphatic and aromatic acid chlorides to the corresponding aldehyde, and is an alternative to the standard Rosenmund procedure. This is illustrated in Expt 6.120. The reagent may be prepared by either of the following two methods both preparations should be conducted in an efficient fume cupboard as hydrogen is evolved. 

The catalytic reduction of carboxylic acid chlorides by the Rosenmund procedure may be used for the preparation of aliphatic aldehydes but its application is mainly for the synthesis of aromatic aldehydes (e.g. Expt 6.120). Alternative procedures for the chemical reduction of acid chlorides include reduction with... 

This reaction occurs to a greater extent in benzene than in isopropyl alcohol.13 27a It appears that an excess of aluminum iso propoxide has the desired effect of speeding up the reduction reaction and thus minimizing side reactions in the case of aliphatic aldehydes.13 The excess, however, is disadvantageous with aromatic aldehydes (see p. 195). 

The use of a relatively soluble base such as CS2CO3 allows good product yield. No products are formed via carbopalladation. Therefore the reaction is considered to occur on a dienolate anion generated from the enal to give an aryl(7r-allyl)palladium intermediate. The regioselectivity seems to be determined in the reductive elimination of the product. Treatment of aliphatic aldehydes with aryl bromides brings about aldol condensation followed by y-aryla-tion to afford 2 1 coupling products (Eq. 27). Note that y-arylation products are also produced in the arylation of a tin-masked dienolate [65,66]. 

Bis(N-inethylpiperazinyl)aluniinum hydride (li. This hydride was originally prepared from aluminum hydride and N-methylpiperazine, and was used to reduce carboxylic acids directly to aldehydes. It can be prepared more conveniently from lithium aluminum hydride and the amine. It is useful for reduction of aliphatic and aromatic acids to aldehydes (80-95% yield). Significantly, it reduces a,p-unsaturated acids to aldehydes without reduction of the double bond (70-80% yield). ... 

Selective reduction of ketones. Luche and Gemal have reported selective reduction of ketones in the presence of aliphatic aldehydes using sodium boro-hydride in combination with cerium(III) chloride. This reagent also effects selective reduction of conjugated aldehydes in the presence of saturated ones. 

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