Alkyl and Acyl Radicals

Stabilization of benzoyl and acrylyl radicals could be formally considered in terms of resonance structures (27—31) 

Actually both thermochemical and ESR data suggest that the stabilization of benzoyl and acrylyl radicals is determined only by conjugation with the lone pair of the oxygen atom (27, 30), with no contribution whatsoever from structures 28, 29, 31. The nucleophilic character of the acyl radicals can be related in the ground state to resonance structures like 27 and 30 and in the transition state (21)  

The relative rates of acetylation and benzoylation of 2- and 4-substituted quinolines ire reported in Tables 26 and 27. The use of quinoline models with only one reactive position is more suitable than that of 4-substituted pyridines because 

To correlate the two series of substituted pyridines and quinolines, relative rates of alkylation were determined also with 2- and 4-substituted quinolines. The following sequence of nucleophilicity of alkyl and acyl radicals was obtained methyl primary alkyl acetyl i-chlorobenzoyl -chlorobenzoyl m-methoxybenzoyl benzoyl -methylbenzoyl -methoxybenzoyl sec. alkyl f-alkyl benzyl. 

A peculiar characteristic of this sequence is that the change of structure of the acyl radicals has a small effect on its polar character, while the effects are very strong in the alkyl radicals. That is further supported by the low value of p (—0.49) in Hammett correlation of the acylation rates of 4-cyanoquinoline relative to 4-chloroquinoline by m and -substituted benzoyl radicals 8). 

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Cyclization of both alkyl and acyl radicals generated by selenide abstraction have also been observed. 

R., 262 J. pr., [2], 51, 186.)—Cyanoacetic, CH CNICOOCjHj, has similar properties to malonic and acetoacetic esters, inasmuch as the methylene hydrogen atoms are successively replaceable by sodium and this latter by alkyl and acyl radicals. 

In the previous sections, the reactions of nucleophilic alkyl and acyl radicals with electron-deficient aromatics via SOMO-LUMO interaction have been described. At this point, we introduce the reactions of electrophilic alkyl radicals and electron-rich aromatics via SOMO-HOMO interaction, though the study is quite limited. Treatment of ethyl iodoacetate with triethylborane in the presence of electron-rich aromatics (36) such as pyrrole, thiophene, furan, etc. produces the corresponding ethyl arylacetates (37) [50-54]. 

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... 

Heating thiazole with phenylazotriphenylmethane at 75 °C for 24 hours affords 2-phenyl-5-triphenylmethyl thiazole (75 Scheme 43). The 1-adamantyl radical and other nucleophilic alkyl and acyl radicals react with 2-substituted benzothiazoles in a homolytic ipso substitution yielding the corresponding 2-alkyl or 2-acylbenzothiazole (Scheme 44) (77CC316). 

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.). 

In the first step, the carbon centered radical is generated. The second step involves the addition of this radical to the protonated ring. The third step consists of the rearomatization of the radical adduct by oxidation. The rates of addition of alkyl and acyl radicals to protonated heteroaromatic bases are much higher than those of possible competitive reactions, particularly those with solvents. Polar effects influence the rates of the radical additions to the heteroaromatic ring by decreasing the activation energy as the electron deficiency of the heterocyclic ring increases. 

More interest has been shown in the past year in the laser-induced processes involving organic molecules. One such study is the laser irradiation (193.3 nm) of the ketones (1-4). This study has shown that the Norrish Type I process is dominant resulting in a-fission and the formation of alkyl and acyl radicals. The ultimate products formed are alkanes and carbon monoxide. Norrish Type I reactivity is also observed in more complex molecules such as the carbohydrate derivatives (5) and (6). Irradiation of these in solution again brings about a-cleavage to give the isomeric radicals ]) ... 

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. ... 

The introduction of an acyl group activates the heteroaromatic ring towards further acylation, which however always takes place exclusively at the positions X and y to the heterocyclic nitrogen (the protonated nitrogen is by far the main activating factor, which determines the positional selectivity). Thus, if a heterocyclic compound has two reactive positions, it is easy to obtain diacyl derivatives, but only one isomer (for example 2,4-diacylderivatives in the case of quinoline), whereas the monoacylderivatives prevail only at very low conversions. Due to the nucleophilic character of alkyl and acyl radicals, the behavior of homolytic... 

In contrast to the oxidative generation of radicals described above, reductions of alkyl iodides using tris(trimethylsilyl)silane also produces alkyl radicals under conditions suitable for Minisci-type substitution." Carboxylic acids are also useful precursors for alkyl" and acyl" radicals via silver-catalysed peroxide oxidation, or from their l-hydroxypyridine-2-thione derivatives using Barton s method," the latter in non-aqueous conditions. 

The term "Norrish-Type-2 cleavage" is understood as the photolysis of aldehydes or ketones at 230-330 nm, generating alkyl and acyl radicals, which stabilise themselves, for example, by intramolecular hydrogen abstraction or fragmentation. [215]... 

Reactions with Radicals. Isoquinoline does react with radicals to give addition to the 1-position with subsequent loss of hydrogen. The Minisci reaction is a well-known example of this transformation. It is an effective reaction for the addition of hydroxymethyl, alkyl, and acyl radicals. Formyl and carbamoyl groups have also be added using this method. Acidic conditions help to promote the reaction. Alkylation is effective when carried out with alkyl iodides or alkyl xanthates. Hydroxymethyla-tion can also be carried out. Photochemical-induced free radical reactions have also been reported. Alkylation is possible using ethanol or propanoic acid but low yields are often obtained. Addition of a phenyl group can be achieved in 81% yield. ... 

Delduc P, Tadhan C, Zard SZ. A convenient source of alkyl and acyl radicals. J Chem... 

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