Of carboxylic acids to aldehydes

D E L E P I N E Aldehyde Oxidation Mild oxidation of aldehydes to carboxylic acid using silver salts... 

The oxidation of aldehydes to carboxylic acids can proceed by a nucleophilic mechanism, but more often it does not. The reaction is considered in Chapter 14 (14-6). Basic cleavage of (3-keto esters and the haloform reaction could be considered at this point, but they are also electrophilic substitutions and are treated in Chapter 12 (12-41 and 12-42). 

Oxidation of Aldehydes to Carboxylic Acids - Addition of Oxygen... 

Oxidation of Aldehydes to Carboxylic Acids Investigated in Micro Reactors Cas/liquid reaction 26 [CL 26) Homc eneously catalyzed oxidation of butyraldehyde to butyric acid... 

The oxidation of aldehydes to carboxylic acids in aqueous solutions usually takes place with less powerful agents than those required to oxidize 1° alcohols to aldehydes => it is difficult to stop the oxidation at the aldehyde stage. 

On the basis of the obtained results and the literature[17-18], we assume that during the oxidative decomposition of p-coumaric acid 1, three kind of reaction can happen before the opening of the aromatic ring (Scheme 1) cleavage of the very reactive exocyclic double bond to give p-hydroxybenzaldehyde 2, hydroxylation of the aromatic ring to yield 3,4-dihydroxycinnamic acid 3 and oxidation of aldehydes to carboxylic acids such as oxidation of 2 to p-hydroxybenzoic acid 4. Compound 2 can be hydroxylated to yield 3,4- dihydroxybenzaldehyde 6. and Compound 4 can also be hydroxylated to yield 3,4-dihydroxybenzoic acid 5. 

The active site of GO contains a covalently modified tyrosyl radical coordinated to a Cu(II) ion constituting thereby a novel mononuclear active site in which both the metal ion and a ligand are redox active. Recently, evidence was provided for the same type of active site in glyoxal oxidase (82), which catalyzes the oxidation of aldehydes to carboxylic acids, Eq. (3). 

The silver oxide oxidation of aldehydes to carboxylic acids is aided by the addition of benzyltriethylammonium chloride the active agent is thought to be TEBA-Ag(OH) [16]. 

For the dehydrogenation of CH—XH structures, for example, of alcohols to ketones, of aldehydes to carboxylic acids, or of amines to nitriles, there is a wealth of anodic reactions available, such as the nickel hydroxide electrode [126], indirect electrolysis [127, 128] (Chapter 15) with I , NO, thioanisole [129, 130], or RUO2/CP [131]. Likewise, selective chemical oxidations (Cr(VI), Mn02, MnOJ, DMSO/AC2O, Ag20/Celite , and 02/Pt) [94] are available for that purpose. The advantages of the electrochemical conversion are a lower price, an easier scale-up, and reduced problems of pollution. 

Scheme 19 Anodic oxidation of aldehydes to carboxylic acids at the nickel oxide electrode.

Effective conditions for oxidation of aldehydes to carboxylic acids with KMn04 involve use of t-butanol and an aqueous NaH2PC>4 buffer as the reaction medium.173 Buffered sodium chlorite is also a convenient oxidant.174 175 An older reagent for carrying out the aldehyde — carboxylic acid oxidation is silver oxide. 

There are few reported oxidations of this type with TPAP in organic solvents, one of the advantages of the reagent being that the alcohol-to-aldehyde oxidation rarely proceeds further. One natural product which did involve such a step is antascomicin B using TPAP/NMO/PMS/CH Cl [85], In aqueous base however [RuO ] is a much more powerful oxidant than TPAP in organic media, perhaps because oxidation of aldehydes to carboxylic acids may proceed via an aldehyde hydrate, the formation of which is inhibited by the molecular sieves used in catalytic TPAP systems. 

For natural product syntheses involving alcohols to acids cf. 2.2 below for large-scale oxidations cf. 2.3.7 for oxidation of aldehydes to carboxylic acids cf alkanes, 4.1.1. 

Taylor and Flood could show that polystyrene-bound phenylselenic acid in the presence of TBHP can catalyze the oxidation of benzylic alcohols to ketones or aldehydes in a biphasic system (polymer-TBHP/alcohol in CCI4) in good yields (69-100%) (Scheme 117) °. No overoxidation of aldehydes to carboxylic acids was observed and unactivated allylic alcohols or aliphatic alcohols were unreactive under these conditions. In 1999, Berkessel and Sklorz presented a manganese-catalyzed method for the oxidation of primary and secondary alcohols to the corresponding carboxylic acids and ketones (Scheme 118). The authors employed the Mn-tmtacn complex (Mn/168a) in the presence of sodium ascorbate as very efficient cocatalyst and 30% H2O2 as oxidant to oxidize 1-butanol to butyric acid and 2-pentanol to 2-pentanone in yields of 90% and 97%, respectively. This catalytic system shows very good catalytic activity, as can be seen from the fact that for the oxidation of 2-pentanol as little as 0.03% of the catalyst is necessary to obtain the ketone in excellent yield. 

Oxidation Alcohols and Aldehydes. The oxidation of alcohols to aldehydes and of aldehydes to carboxylic acids is routine, and is catalyzed by alcohol dehydrogenase and aldehyde dehydrogenase, respectively. 

Aldehyde oxidase catalyzes the oxidation of aldehydes to carboxylic acids by dioxygen, but also catalyzes the hydroxylation of pyrimidines. Despite its rather broad specificity for substrates, it may well be that aldehyde oxidase should be regarded primarily as a pyrimidine hydroxylase. Thus, xanthine oxidase and aldehyde oxidase catalyze the hydroxylation of purines and pyrimidines respectively. The oxygen incorporated into the product comes from water, not 02. The dioxygen serves as the electron acceptor and other oxidizing agents may be used. 

The use of metal oxo-complexes for the oxidation of aldehydes to carboxylic acids is also well-known (Fig. 9-36), although, once again, the isolation of intermediate complexes is relatively rare. In particular, high oxidation state manganese or chromium complexes are commonly used for this process. 

Benzyl and allyl alcohols are oxidized with iodosylbenzene 18 in refluxing dioxane to aldehydes [67]. Further oxidation of aldehydes to carboxylic acids does not take place. Aliphatic primary alcohols are not oxidized under the conditions. Ligand exchange of 18 with alcohols produces alkoxy-A3-iodanes, which result in reductive /3-elimination to give aldehydes [Eq. (33)]. 

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