Oxidation reaction aromatic side chain

Etard reagents (chromyl chloride and some derivatives) suffer from the problem that occasionally they can exhibit a lack of selectivity and low yields. They are useful in the selective oxidation of aromatic side-chains to a carbonyl group, aldehyde or ketone but in many instances, the formation of the initial complex is slow and yields are low because of difficulties in the work-up which lead to undesired over-reaction. Attempts have been made to solve the problems by the use of sonication [134]. A simple preparation of the liquid reagent was proposed and the Etard reaction itself together with the hydrolytic step were conducted under sonication, with some success (Scheme 3.25). 

Benzylic oxidation of aromatic side-chains is also a well established technology in the bulk chemicals arena, e. g. toluene to benzoic acid and p-xylene to ter-ephthalic acid [1,2]. These processes involve homogeneous catalysis by, e. g., cobalt compounds, however, and also fall outside the scope of this book. Ammoxi-dation of methyl-substituted aromatic and heteroaromatic compounds is performed over heterogeneous catalysts in the gas phase but this reaction is treated elsewhere (Section 9.5). Transition metal-substituted molecular sieves have been widely studied as heterogeneous catalysts for oxidation of aromatic side-chains in the liquid phase, but there are serious doubts about their heterogeneity [5,6]. Here again, a cursory examination of the literature reveals that supported palladium seems to be the only heterogeneous catalyst with synthetic utility [4]. 

An elegant example of a paired mediated reaction has been reported by Chaussard and Lahitte [69] EDF (Electricite de France), who use Cr(VI) generated at the anode of a divided cell to oxidize the methyl side chain of a nitro-aromatic and Ti(III) generated at the cathode to reduce the nitro group. The reduction step, due to the faster homogeneous rate can be performed within the cell, whereas the oxidation has to be performed in a stirred tank reactor. 

Reaction 1 has been postulated both in oxidations of alkanes in the vapor phase (29) and in the anti-Markovnikov addition of hydrogen bromide to olefins in the liquid phase (14). Reaction 2 involves the established mechanism for free-radical bromination of aromatic side chains (2). Reaction 4 as part of the propagation step, established in earlier work without bromine radicals (26), was not invoked by Ravens, because of the absence of [RCH3] in the rate equation. Equations 4 to 6, in which Reaction 6 was rate-determining, were replaced by Ravens by the reaction of peroxy radical with Co2+ ... 

The chemical reactions used to degrade these aromatic compounds are numerous and complex. As was mentioned in Chapter 16, some fungi initiate the attack on lignin with peroxidases and produce soluble compounds that can be attacked by bacteria. In other cases elimination reactions may be used to initiate degradation. For example, some bacteria release phenol from tyrosine by P elimination (Fig. 14-5). However, more often hydroxylation and oxidative degradation of side chains lead to derivatives of benzoic acid or of the various hydroxybenzoic... 

Reaction C. Oxidation of the Side Chain in Aromatic Compounds. (A., 122,184 133, 41 137, 308 141,144 147, 292 B., 7,1057 19, 705 Z. Ch., 4, 119) (Fittig).—When aromatic compounds containing aliphatic side chains attached to the nucleus are treated with certain oxidising agents (potassium permanganate, dilute nitric acid, and chromic acid), the side chain is oxidised until only a carboxylic group attached to the nucleus remains the end methyl group, if there are several carbon atoms... 

An attempt was made to reveal the mechanism for the formation of free radicals upon irradiation of titanium dioxide in the presence of benzene and toluene. Careful examination of the effects of oxygen and water showed that the presence of oxygen is essential for the reaction, and that under oxygen the oxidation of water contributes to the aromatic hydroxylation and the oxidation of toluene as a substrate leads to oxidation of its side chain (56). 

From Hydrocarbons, Friedel-Craft.—The aromatic hydrocarbons yield ring carboxy acids by other reactions than those effecting oxidation of a side chain. Carbon dioxide may be introduced directly into a benzene ring, thus converting a hydrogen into.carboxyl. This may be accomplished in the presence of aluminium chloride, Friedel-Craft reagent. 

REACTION OXIDATION OP THE SIDE-CHAIN OP AN AROMATIC COMPOUND... 

Partial catalytic oxidation of alkylaromatic hydrocarbons is interesting both from the industrial and the scientific point of view. The industrial interest is due to the availability of these substances from the petrochemical industry and to a number of applications for the possible oxidation products. Conventional gas phase oxidation concerns the side chain and leads mainly to benzoic acid or even to destruction of the aromatic ring. Oxidation under mild conditions could cease the reaction at earlier stages and reduce the number of the products formed. However, the appropriate catalyst for such partial oxidation has not been found yet. 

Organic acids can be prepared in many ways, four of which are described here (1) oxidation of primary alcohols or aldehydes, (2) oxidation of alkyl side chains on aromatic rings, (3) reaction of Grignard reagents with carbon dioxide, and (4) hydrolysis of alkyl cyanides (nitriles). 

When stirred in toluene under oxygen with solid potassium hydroxide and PEGMe, 4-nitrotoluene couples to the bibenzyl and styrene products. This coupling does not proceed by a direct reaction of the 4-nitrobenzyl radicals. Sonication increases the conversion rate and yield and more importantly, leads to a different chemical evolution with the formation of 4-nitrobenzoic acid. This sonochemical switching cannot be fully interpreted in the absence of accurate kinetic measurements. Oxidation of the side chain in alkyl aromatics was studied... 

Oxidation and Reduction Reactions on Aromatic Side Chains... 

A primary or secondary alkyl side chain on an aromatic ring is converted to a carboxyl group by reaction with a strong oxidizing agent such as potassium permanga nate or chromic acid... 

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