Acyl radicals stabilization

The same is true for decarbonylation of acyl radicals. The rates of decarbonylation have been measured over a very wide range of structural types. There is a very strong dependence of the rate on the stability of the radical that results from decarbonylation. For example, rates for decarbonylations giving tertiary benzylic radicals are on the order of 10 s whereas the benzoyl radical decarbonylates to phenyl radical with a rate on the order of 1 s . ... 

Other functional groups provide sufficient stabilization of radicals to permit successful chain additions to alkenes. Acyl radicals are formed by abstraction of the formyl hydrogen from aldehydes. As indicated in Table 12.7, the resulting acyl radicals are... 

Acyl radicals can fragment with toss of carbon monoxide. Decarbonylation is slower than decarboxylation, but the rate also depends on the stability of the radical that is formed. For example, when reaction of isobutyraldehyde with carbon tetrachloride is initiated by t-butyl peroxide, both isopropyl chloride and isobutyroyl chloride are formed. Decarbonylation is competitive with the chlorine-atom abstraction. 

Acyl radicals undergo dccarbonylation. For aliphatic acyl radicals the rate constant for decarbonylation appears to be correlated with the stability of the alkyl radical formed. Values of the dccarbonylation rate constant range from 4 s for CH3C 0) to 1.5x10s s l [lor (CH.,)2C(Ph)C( )0] at 298 °C.3M The loss of carbon monoxide from phenacyl radicals is endothermic and the rate constant is extremely low (ca 10 8 s 1 at 298 nC).388 Consequently, the reaction is not observed during polymerization experiments. 

Carbonyl group of the aldehyde decreases the BDE of the adjacent C—H bond. This is due to the stabilization of the formed acyl radical, resulting from the interaction of the formed free valence with Tr-electrons of the carbonyl group. For example, DC—H = 422kJmol 1 in ethane and D( n 373.8 kJ mol 1 in acetaldehyde. The values of Dc H in aldehydes of different structures are presented in Table 8.1. In addition, the values of the enthalpies of acylperoxyl radical reactions with aldehydes were calculated (D0 H= 387.1 kJ mol-1 in RC(0)00 H). 

Sometimes acylium ions lose carbon monoxide to generate an ordinary carbonium ion. It will be recalled that free acyl radicals exhibit similar behavior at high temperatures. Whether or not the loss of carbon monoxide takes place seems to depend on the stability of the resulting carbonium ion and on the speed with which the acylium ion is removed by competing reactions. Thus no decarbonylation is observed in Friedel-Crafts reactions of benzoyl chloride, the phenyl cation being rather unstable. But attempts to make pivaloyl benzene by the Friedel-Crafts reaction produce tert-butyl benzene instead. With compound XLIV cyclization competes with decarbonylation, but this competition is not successful in the case of compound XLV in which the ring is deactivated.263... 

In the preceding eqnation, the primary anion-radical gives the l-chloro-2,2,2-trifluoroethyl radical. In vivo, this radical was detected by the spin-trapping method (Poyer et al. 1981). Ahr et al. (1982) had presented additional evidence for the formation of the radical as an intermediate in halo-thane metabolism and identified l-chloro-2,2-difluoroethene as a product of radical stabilization. Metabolytic transformations of l-chloro-2,2-difluoroethene lead to acyl halides, which are relevant to halothane biotoxicity (Guengerich and Macdonald 1993). 

Thus acyl radicals are relatively insensitive, both as regards stability and reactivity, to substitution. The polar character and the consequent... 

Also, the results of the substituent effects in homolytic acylation of protonated heteroaromatic bases must be connected, as for homolytic alkylation, with the polar characteristics of the acyl radicals and the aromatic substrates, but not with the stabilization of the intermediate a-complexes. 

Because the addition steps are generally fast and consequently exothermic chain steps, their transition states should occur early on the reaction coordinate and therefore resemble the starting alkene. This was recently confirmed by ab initio calculations for the attack at ethylene by methyl radicals and fluorene atoms. The relative stability of the adduct radicals therefore should have little influence on reacti-vity 2 ). The analysis of reactivity and regioselectivity for radical addition reactions, however, is even more complex, because polar effects seem to have an important influence. It has been known for some time that electronegative radicals X-prefer to react with ordinary alkenes while nucleophilic alkyl or acyl radicals rather attack electron deficient olefins e.g., cyano or carbonyl substituted olefins The best known example for this behavior is copolymerization This view was supported by different MO-calculation procedures and in particular by the successful FMO-treatment of the regioselectivity and relative reactivity of additions of radicals to a series of alkenes An excellent review of most of the more recent experimental data and their interpretation was published recently by Tedder and... 

The Norrish type 1 cleavage of cyclic ketones has been found to be strongly influenced by the presence of a silyl group ji to the carbonyl group, where the acyl radical intermediate was believed to be stabilized by the fi effect184. For example, as shown in equation 41 with the ketone 349, Ci— C2 cleavage led to the diradical 350, and thence to the products 351... 

Radical reactions with acyl radicals sometimes involve decarbonylation as side-reactions, especially when stabilized secondary or tertiary radicals can be formed. These side-reactions can be suppressed using low-temperature reaction conditions together with different reducing agents such as tris(trimethylsilyl)silane.254... 

Some mechanistic aspects of the above cascade reaction deserve comment. Thus, after the intermolecular addition of the nucleophilic acyl radical to the alkene, the electrophilic radical adduct A, instead of undergoing reduction, reacts intramolecularly at the indole 3-position (formally a 5-endo cyclization) to give a new stabilized captodative radical B, which is oxidized to the fully aromatic system. (For a discussion of this oxidative step, see Section 1.5.)... 

Given the greater stability of a benzylic radical than an acyl radical, the rates of combination of benzylic/aryloxy pairs should be slower than those of phenylacyl/aryloxy pairs. 

From Figure 7.12b it is seen that in conjugated ketones the ( r, r ) state can be stabilized to such an extent that it will be energetically below the (n, r ) states. Then, all the way along the reaction path leading to the bent acyl radical, the T, surface remains below the T and surfaces, which for the initial geometry correspond to an excitation. In this case, the re-... 

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