Benzaldehydes radical reactions

In a direct comparison of the reactivity of 1-alkyl- and 2-alkylbenzotriazoles, compound 393 was lithiated in the presence of benzophenone with 1 equiv of LDA to give a mixture of alcohol 394 and dimer 395 (Equation 12) <1996LA745>. No reaction was detected at the carbon adjacent to the benzotriazol-l-yl moiety. When benzaldehyde was used instead of benzophenone, only dimer 395 was obtained. This suggests that a-benzotriazol-2-yl carbon radical reactions are much faster than those of a-benzotriazol-l-yl) carbanions. 

An estimate of the pK value for benzaldehyde radical-anion has also been obtained from fast cyclic voltammetry experiments over a range of pH values [14], Interpretation the results obtained in this case requires first deduction of an overall reaction scheme followed by numerical solution of the corresponding set of differential equations allowing simulation of the cyclic voltammogram. Reaction constants are then adjusted to give good simulations over a range of experimental conditions. The pKj can then be extracted from these reaction constants. 

The oxidation is initiated (a) by Fe to yield the benzoyl radical (99) which adds on a molecule of oxygen to form the perbenzoate radical (1(X)), this reacts with benzaldehyde (97) to yield perbenzoic acid (101) and another benzoyl radical (99)—these two steps constituting the chain reaction (b). The actual end-product is not perbenzoic acid (101), however, as this undergoes a rapid acid-catalysed, non-radical reaction (c) with more benzaldehyde (97) to yield benzoic acid (98). This latter reaction (c), being acid-catalysed, speeds up as the concentration of product benzoic acid (98) builds up, i.e. it is autocatalytic. That benzoyl radicals (99) are involved is borne out by the observation that carrying out the reaction at higher temperatures ( 100°), and at low oxygen concentrations, results in the formation of CO, i.e. by PhCO-> Ph- + CO. 

Radical intermediates were proposed at an early stage in the history of the Cannizzaro reaction, and this possibility has been resurrected to account for the detection of some 20% of a-D-benzyl alcohol in the products of the Cannizzaro reaction of a-D-benzaldehyde in aqueous alkaline dioxan. This has been rationalized in terms of formation of the benzaldehyde radical anion, which abstracts a H-atom from the solvent (Chung, 1982). Epr spectroscopy of reacting solutions of p-Cl-,/ -NO 2 and / -CF3-benzaldehyde as well as benzaldehyde itself in THF HMPA (9 1) yielded spectra of the aldehyde radical anions identical to those produced by the action of metallic... 

The relative ease of pinacolization is primarily determined by the reduction potential of the carbonyl group involved. Many reductants are therefore selective for aromatic and other electronically activate systems. Moreover, as a result of this ready reduction, pinacolization of such carbonyls can be effected by either anionic or radical routes. For example, treatment of aromatic aldehydes or ketones with Mg/TMSCl in HMPA promotes pinacolization via formation of an a-silyloxy carbanion - and nucleophilic attack on a second carbonyl group (equation 2). Furthermore, with benzaldehyde the reaction is stereodirecting with a preference for /it-coupling. Whilst an alternate coupling metht using the milder... 

Benzyl Chloride. Benzyl chloride is manufactured by high temperature free-radical chlorination of toluene. The yield of benzyl chloride is maximized by use of excess toluene in the feed. More than half of the benzyl chloride produced is converted by butyl benzyl phthalate by reaction with monosodium butyl phthalate. The remainder is hydrolyzed to benzyl alcohol, which is converted to ahphatic esters for use in soaps, perfume, and davors. Benzyl salicylate is used as a sunscreen in lotions and creams. By-product benzal chloride can be converted to benzaldehyde, which is also produced directiy by oxidation of toluene and as a by-product during formation of benzoic acid. By-product ben zotrichl oride is not hydrolyzed to make benzoic acid but is allowed to react with benzoic acid to yield benzoyl chloride. 

Much more complicated is the course of the reaction if the oxazirane is derived instead of from benzaldehyde from an aliphatic ketone. Here the possibility of an H-transfer does not occur. Further complications are found if the A -alkyl group can be attacked by the radicals. 

In the anodic decarboxylation of phenylacetic acid benzaldehyde is the major product (80%) at low current density (< 3.2mA/cm ). Its formation is supposed to occur by reaction of the intermediate benzyl radical with oxygen, which is possibly simultaneously generated at the anode [31]. 

For some halides, it is advantageous to use finely powdered lithium and a catalytic amount of an aromatic hydrocarbon, usually naphthalene or 4,4 -di- -bu(ylbiphcnyl (DTBB).28 These reaction conditions involve either radical anions or dianions generated by reduction of the aromatic ring (see Section 5.6.1.2), which then convert the halide to a radical anion. Several useful functionalized lithium reagents have been prepared by this method. In the third example below, the reagent is trapped in situ by reaction with benzaldehyde. 

Aldehydes, and particularly aromatic ones, are highly susceptible to autoxidation thus benzaldehyde (97) is rapidly converted into benzoic acid (98) in air at room temperature. This reaction is catalysed by light and the usual radical initiators, but is also highly susceptible to the presence of traces of metal ions that can act as one-electron oxidising agents (cf. p. 306), e.g. Fe3 , Co3 , etc ... 

When reactions with oxygen-containing acceptors were performed [3] in the 300-400°C region, the formation of adducts occurred with both Tetralin and mesitylene. This reaction was observed when benzyl radicals were generated from dibenzyl ether, dibenzyl sulfide, benzyl alcohol, and benzaldehyde. 

Perhaps due to oxidizing quinoid type electronic structure of benzotriazol-2-yl derivatives, some of their properties are completely different from those of isomeric benzotriazol-l-yl derivatives. Thus, anions derived from 2-alkylben-zotriazoles 388 are rapidly converted to appropriate radicals that undergo coupling to form dimers as mixtures of racemic 289 and meso 390 forms <1996LA745>. When the reaction mixture is kept for an extended period of time at —78 °C, (Z)- 391 and (E)- 392 alkenes are formed. When benzophenone is added to the reaction mixture, alcohols 387 are obtained in good yields however, benzaldehyde does not react under these conditions (Scheme 63). 

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