Benzoyl radicals stability

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 . ... 

The benzoyl radical from initiator III is more reactive than the parasubstituted benzoyl radical from initiator II. The isopropyl substituent stabilizes the benzoyl radical and make it less reactive. 

All the acyl radicals have nucleophilicities between that of a primary and a secondary alkyl radical i.e, the acetyl radical is more nucleophilic than the ethyl radical, but the benzoyl radical is much less nucleophilic than the benzyl radical. The different polarizability of these last two radicals, due to their different configuration, was considered an important factor of this behavior. An incipient positive charge in a transition state similar to a charge-transfer complex (27) can be stabilized in the benzyl radical 32) by the aromatic orbitals, but not in the benzoyl radical, in which the unpaired electron occupies a hybrid orbital (33). 

An investigation of the substrate dependence on the rate of samarium-mediated reductive elimination of jS-acyloxysulfones has provided insights into the mechanism of this transformation and allowed for the development of a chemoselective elimination process." Electron transfer to the sulfonyl and/or benzoyl group is probably reversible and can occur into both acceptor groups. A difference in carbon radical stabilities... 

Phenyl radical, side by side with methyl radical, carbon dioxide and methyl benzoate, was also stabilized in an inert matrix as a product of UV photolysis of acetyl(benzoyl)peroxide [112] (Pacansky and Brown, 1983). Of nine IR bands of the radical C6H5, intense absorption at 710 cmwhich was shifted to 519 cm for the deuterium-labelled radical C Ds, has been assigned to out-of-plane CH deformation. The bands of the phenyl radical... 

Such nucleophilic displacements are likely to be addition-elimination reactions, whether or not radical anions are also interposed as intermediates. The addition of methoxide ion to 2-nitrofuran in methanol or dimethyl sulfoxide affords a deep red salt of the anion 69 PMR shows the 5-proton has the greatest upfield shift, the 3- and 4-protons remaining vinylic in type.18 7 The similar additions in the thiophene series are less complete, presumably because oxygen is relatively electronegative and the furan aromaticity relatively low. Additional electronegative substituents increase the rate of addition and a second nitro group makes it necessary to use stopped flow techniques of rate measurement.141 In contrast, one acyl group (benzoyl or carboxy) does not stabilize an addition product and seldom promotes nucleophilic substitution by weaker nucleophiles such as ammonia. Whereas... 

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... 

If the dibenzoyl ketyl ion is not included in the ion-pair complex with the alkali cation, it exists in the trans form. The potassium salt exists in the cis form, in which both charged oxygen atoms are in the vicinity of the metal cation (Bauld 1965). On benzoylation, the cis ion-radical pair gives rise mainly to cw-dibenzoyl stilbene. Meanwhile, the reduction of dibenzoyl under conditions, which do not stabilize the contact ion pair, is not stereospecific (Bauld 1965, Thiele 1899). Scheme 3.49 compares the results of these two different reactions of the same substrate. 

The higher stability of the benzoyl cation agrees with the higher sensitivity to polar effects of the corresponding radical. 

The monomer addition scheme, shown at the top, requires an initiator which is capable of removing a hydrogen atom from the allylic position of the butadiene, resonance stabilization of the radical from AIBN does not permit this initiator to effect this reaction while benzoyl peroxide is capable of reaction to remove a hydrogen atom and initiate the reaction. On the other hand the polymeric radical addition scheme requires that homopolymerization of the monomer be initiated and this macroradical then attack the polymer and lead to the formation of the graft copolymer. Huang and Sundberg explain that the reactivity of the monomer... 

The radical end (7) may be formed via homolytic decomposition of the vanadium-carbon bond in the presence of MMA and be stabilized by the vanadium ion. This stabilization is likely to cause a living radical polymerization of MMA. A similar conclusion has been reached by Lee et al. 113) who found that a mixture of chromium acetate with benzoyl peroxide initiated a living radical polymerization of MMA at low temperatures below 30 °C. 

In particular, crude polymerizates prepared in the presence of AIBN as initiator, which yield resonance stabilized radicals (17) that are unable to extract hydrogen from hydrocarbon supports (I, 18) show the same content of non-extractable rubber as that of the polymerizates prepared in the presence of active radicals in the hydrogen extraction from hydrocarbon polymers, such as those derived from the decomposition of benzoyl peroxide. 

Hi) Reduction of Glycosyl Halides with Radical Rearrangement. The reduction of acetylated or benzoylated halides or selenides with a low concentration of tributylstannane leads to 2-deoxy sugar derivatives (Scheme 11). The driving force of this radical reaction is the 1,2-cA-selective migration of an ester group because of the stabilization of the radical at C-2.46... 

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