Acyl radicals, sources

Acyl radical sources, other than aldehydes, are also available. The group transfer addition of an acyl radical has been reported by Zard and co-workers, where S-acyl xanthates serve as acyl radical sources [44]. Crich and co-workers reported that an acyl radical, generated from an aromatic acyl telluride by photolysis, adds to an allylic sulfide which contains an ethoxycarbonyl group to form the corre-.sponding y-unsaturated ketones [45]. The addition pathway involves Sh2 type reaction with extrusion of a /ert-butylthiyl radical. 

Reaction 21 is the decarbonylation of the intermediate acyl radical and is especially important at higher temperatures it is the source of much of the carbon monoxide produced in hydrocarbon oxidations. Reaction 22 is a bimolecular radical reaction analogous to reaction 13. In this case, acyloxy radicals are generated they are unstable and decarboxylate readily, providing much of the carbon dioxide produced in hydrocarbon oxidations. An in-depth article on aldehyde oxidation has been pubHshed (43). 

When applied to ketones, this is called Norrish Type / cleavage or often just Type I cleavage. In a secondary process, the acyl radical R —CO can then lose CO to give R radicals. Another example of a category 1 process is cleavage of CI2 to give two Cl atoms. Other bonds that are easily cleaved by photolysis are the 0—0 bonds of peroxy compounds and the C—N bonds of aliphatic azo compounds R—N=N—R. The latter is an important source of radicals R , since the other product is the very stable N2. 

Telluroesters (acyl tellurides) have been recognized as excellent sources of acyl radicals upon photolysis with a 250 W tungsten lamp, or thermal process (benzene at reflux) in the... 

With radical sources other than acyl peroxides, the rearomatization of the a-complex can take place by various, not always well characterized, reactions, such as oxidation by metal salts, hydrogen abstraction by intermediate radicals, disproportionation, and induced decomposition. 

Two sources of acyl radicals have proved to be useful for the homolytic acylation of protonated heteroaromatic bases the oxidation of aldehydes and the oxidative decarboxylation of a-keto acids. The oxidation... 

The possibility of using other sources of acyl radicals, such as tin hydrides and acyl chlorides, is complicated by the fact that homolytic acylation requires an oxidizing medium for the rearomatization of the... 

Figure 4.12. Second-order rate constants for reactions of hydrogen atom donors with various radical types at ambient temperature. Data sources group 14 (IV A) hydrides (15) aminyl radicals (69) amidyl radicals (70) alkyl radials with group 16 (VI A) hydrides (71) acyl radical with PhSeH (72).

Pyridine-2-thione-A-oxycarbonyl (PTOC) derivatives of carboxylic esters 53 were developed by Barton et al. and serve as a convenient source of acyloxyl radicals, which upon decarboxylation provide specific routes to free radicals (equation 82). This process can also proceed by a radical addition (equation 83). Acyl selenides (54) are a convenient source of acyl radicals, which can undergo decarbonylation also giving specific free radicals (equation 84). ° ... 

In 02-saturated solutions, a major portion of acyl radicals (60-80%) react with O2 as in Eq. (22), the rest undergoing decarbonylation and finally formation of tert-butylperoxyl radicals, Eqs. (20) and (23). The source of Craq02 +, which is clearly an intermediate on the basis of the effects of methanol and Mn2+, is reaction 24. As discussed in greater detail below, Craq02 + is produced by disproportionation of the initially generated Craq(V). 

Zard and colleagues have developed an interesting radical chain reaction based on the thionocarbonyl derivatives, where S-alkyl, S-acyl and S-alkoxyacyl xanthates are employed as new and very useful sources of alkyl and acyl radicals, as demonstrated in Scheme 53.168 The alternate undesirable pathways available to an alkyl or alkoxyacyl radical are not important in this reaction due to the reversible and degenerate (path A) nature of the adduct radicals.168 This reaction does not therefore complete with the expulsion of carbon dioxide (path B), in sharp contrast to previous processes based on stannane... 

Acyl radicals. Tributyltin hydride in combination with AIBN can generate acyl radicals from phenylselenol esters, which are a better source than acyl chlorides or phenylthiol esters for acyl radicals for intramolecular cyclization. 

Acyl radicals are very useful synthetic intermediates. Their preparation is not simple since the corresponding halides are highly electrophilic and cannot be used as radical precursors. Organocobalt compounds were proposed as suitable source of acyl radicals [44]. However, the use of acyl selenides proved to be more general [45, 46]. These radical precursors can be efficiently prepared from the corresponding carboxylic acids and esters [47]. Acyl phenyl selenides should be preferred, when possible, relative to acyl methyl selenides due to the consumption of two equivalents of tin hydride with this last system (Scheme 1) [4]. Acyl selenides have found many applications in tandem radical additions to alkenes. Examples of intermole-cular [Eq. (18)] [48,49] and intramolecular reactions [Eq. (19)] [50a] are reported. The enoyl selenide 68 give the unsaturated acyl radicals 69. This intermediate... 

Note that aldehydes can also be a source of acyl radicals ( 0=0) via reaction with transition metal salts such as Mn(III) acetate or Fe(II) compounds.Another useful variation employs imidoyl radicals as synthons for unstable aryl radicals. 

Delduc et al. have reported that the photolysis of S-alkyl and -acyl xanthates provides a useful source of free alkyl and acyl radicals. Photolysis of the imide (30) gives tetramethyl-cyclobutadiene (Kashima et al.). 

Irradiation of the cyclopentanecarboxylate derivatives (139) results in cycliza-tion to afford the spiro compounds (140). This process arises from Se-C bond cleavage, addition of the resultant radical to the pendant alkene group, and readdition of the PhSe radical. Several examples of the process are described as a synthetic path to novel bakkenolides. Irradiation of the stannane (141) in benzene brings about C-Sn bond cleavage and the formation of the tripropyltin radical. The irradiation of telluro derivatives such as (142) can be used as a source of a variety of alkyl and acyl radicals formed by the fission of the C-Te bond. ... 

A somewhat different approach to a polymer-supported radical source was described by Giacomelli and coworkers, who used an N-hydroxythiazole 2(3)-thione anchored to a Wang resin [95]. The reagent was prepared in solution and equipped with a pendant COOH group to facilitate attachment to the resin. The applicability of the reagent in radical-mediated reactions was then investigated via a Hunsdiecker reaction (Scheme 6.24). Treatment of the reagent with an acyl... 

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