Aroyl radicals

Entry 5 is an example of the use of fra-(trimethylsilyl)silane as the chain carrier. Entries 6 to 11 show additions of radicals from organomercury reagents to substituted alkenes. In general, the stereochemistry of these reactions is determined by reactant conformation and steric approach control. In Entry 9, for example, addition is from the exo face of the norbornyl ring. Entry 12 is an example of addition of an acyl radical from a selenide. These reactions are subject to competition from decarbonylation, but the relatively slow decarbonylation of aroyl radicals (see Part A, Table 11.3) favors addition in this case. 

However, these latter phosphines Id - k also suffer photofragmentations. a-Cleavage occurs under formation of either aldehydes 2d - h, via subsequent H-abstraction of aroyl radicals from solvent (Scheme 3), or hetero- and carbocycle 7 and 8, via cyclization of the corresponding aroyl radicals under neighbouring group participation (Schemes 4 and 5). 

It is well known that benzophenone generates a biradical through n-ir electronic transition under irradiation ( 340 nm). Irradiation of a mixture of 1,4-benzoquinone (34) and aromatic aldehydes in the presence of benzophenone generates 2-aroyl-l,4-dihydroxybenzene (35) [47-49]. This reaction comprises of the abstraction of a formyl hydrogen atom of an aromatic aldehyde by the oxygen-centered radical of the benzophenone biradical to form an aroyl radical and a 1,1-diphenylhydroxymethyl radical, and addition of the nucleophilic aroyl radical to 1,4-benzoquinone (34) to form a phenoxyl radical derivative, which finally abstracts a hydrogen atom from an aromatic... 

The cathodic reduction of aroyl chlorides as seen earlier (equation 46) gives olefinic diethers involving reduction to aroyl radicals, combination of the radicals to give... 

Aromatic aldehydes have been reported to undergo 1,1-addition to the ketene dithioacetal S,S-dioxides (46 R = alkyl) in either the presence or absence of benzophenone to give adducts (47). The transformation is initiated by H abstraction from the aromatic aldehyde by the n,K excited state of the sensitizer or substrate aldehyde to form the aroyl radical (48) as intermediate. These adducts may be useful synthetic precursors of inda-nones. 

The reachon with aryl, acyl, or aroyl radicals, generated from the respective peroxides by thermal cleavage, leads to an arylation or acylation of the diamond surface (Figure 6.40). Substitutions performed on the aromatic rings then ahow for further modifications of the surface and for consecutive reactions. 

Interestingly, the acyl radicals formed show different chemical behaviors depending on their structures. Aroyl radicals from 42 were highly prone to performing an unprecedented intramolecular five- and six-membered cyclization via attack onto the azido moiety (Scheme 9), whereas acyl redicals from 43 undergo rapid decarbonylation. 

Diacyl peroxides are sources of alkyl radicals because the carboxyl radicals that are intitially formed lose CO2 very rapidly. In the case of aroyl peroxides, products may be derived from the carboxyl radical or the radical formed by decarboxylation. The decomposition of peroxides can also be accomplished by photochemical excitation. 

With 4,4-diacyl triafulvenes two principal fragmentation pathways have been observed5 s In 4-aroyl-4-acetyl triafulvenes 241 the molecular ion is followed by a fragment ion of probable structure 242 arising from primary loss of (CvI R), which surprisingly has incorporated a CH2 unit from the acetyl group and the exo-cyclic aryl residue. It is not unlikely that the (C7H6R)-residue corresponds to a substituted tropyl radical due to its well-known formation from electron-impact of benzylic precursors. 

The usual sources used for the homolytic aromatic arylation have been utilized also in the heterocyclic series. They are essentially azo- and diazocompounds, aroyl peroxides, and sometimes pyrolysis and photolysis of a variety of aryl derivatives. Most of these radical sources have been described in the previous review concerning this subject, and in other reviews concerning the general aspects of homolytic aromatic arylation. A new source of aryl radicals is the silver-catalyzed decarboxylation of carboxylic acids by peroxydisulfate, which allows to work in aqueous solution of protonated heteroaromatic bases, as for the alkyl radicals. 

Tertiary amines with an a-hydrogen are among the most effective electron donors other electron donors include alcohols, amides, amino acids, and ethers. A third process, direct hydrogen atom transfer from RH to the ketone, is not common hut does occur with some photoinitiators. The overall result is the same as the electron-transfer process. Although two radicals are produced by photolysis of the photoinitiator, only one of the radicals is typically active in initiation—the aroyl and amine radicals in Eqs. 3-48 and 3-49, respectively. The other radical may or may not initiate polymerization, hut is active in termination. The decrease in photoinitiator concentration during polymerization is referred to as photo-bleaching. 

Mattice37 observed the photorearrangement of 3,4-benzocumarin (78) to 4-hydroxyfluorenone (79) with an exceptionally low yield of 0.08% of the initial 78. Here, structural features determine the migration of the aroyl group to the meta-position with regard to the phenolic oxygen atom. For the eventual recombination, the meta-position of a phenoxy radical is extremely deactivated (Table III). 

As shown in [111], introduction of any substituent with both a donor and an acceptor nature in the phenyl radical at position 2 of heterocycle 115 impedes the reaction. Besides the desired aroylation products 117, N,Nf-diaroyl-1,2-phenylenediamines 118 are isolated. 

The related reagent combination, PhI(OAc)2/NaN3, has been employed for direct conversions of aromatic aldehydes to aroyl azides, ArCON3 [31]. These reactions were conducted in dichloromethane at room temperature and are thought to occur by a radical mechanism. 

The most common synthetic approaches were reported in CHEC-I. Subsequently, some new strategies have been described. A synthesis of pyridazino[4,5- ]pyridazine derivatives (25) consists of radical aroylation of pyridazine (23) and the cyclization of the resulting diketones (24) with semicarbazide (Scheme 4) <74M648>. 

IR spectra there is absorption in the region 1540-1620 cm-1, characteristic of the chelated cis-enol bands characteristic of a free carbonyl group are absent. The effect of substituents on the enolization of /3-di-ketones (in acetone) was studied by NMR spectroscopy. In the cu-acyl-2-acetoselenophene (20) and a> -acyl-3-acetoselenophene(21) series, the acyl groups COR increase the enolization in the following order of R methyl < a-furyl < a-thienyl < trifluoromethyl < phenyl < a-pyridyl. Thus, /S-diketones containing aroyl or heteroaroyl radicals together with selenienyl exist completely as the cis-enols.121... 

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