Homolytic alkylation phenylation

It is estimated that thiophene reacts with phenyl radicals approximately three times as fast as benzene. Intramolecular radical attack on furan and thiophene rings occurs when oxime derivatives of type (112) are treated with persulfate (8UCS(Pt)984). It has been found that intramolecular homolytic alkylation occurs with equal facility at the 2- and 3-positions of the thiophene nucleus whereas intermolecular homolytic substitution occurs mainly at position 2. 

Coupling of phenoxy radicals to give dimeric quinone methides 1. Homolytic or heterolytic cleavage of side-chains (Ca-C/ , alkyl-phenyl) and aromatic rings... 

The corrinoid-mediated reduction of polyhaloethenes has been the subject of a recent study, which reports reaction via homolytic C-halogen bond fission. The elimination of a further halogen radical affords haloalkynes, which lead to acetylene itself.56 The electron transfer-induced reductive cleavage of alkyl phenyl ethers with lithium naphthalenide has been re-examined in a study which showed that it is possible to reverse regioselectivity of the cleavage (i.e. ArOR to ArH or ArOH) by introduction of a positive charge adjacent to the alkyl ether bond.57 A radical intermediate has been detected by ESR spectroscopy in the reduction of imines to amines with formic acid58 which infers reacts takes place via Lukasiewicz s mechanism.59... 

Although a number of such radical reactions are known, few promise much synthetic potential, Examples include the 2-phenylation of imidazole and benzimidazole by benzoyl peroxide, but both products are more readily obtained by other routes. Homolytic alkylations of imidazole and benzimidazole also occur at C-2, but usually give indifferent yields [10]. A potentially useful reaction is the synthesis of 2- and 4-trifluoromethylimidazoles from imidazoles and photochemically generated trifluoromethyl radicals. 1-Substituted imidazoles are largely substituted at C-5 in these reactions benzimidazole reacts initially at the 4-position [11-14]. 

Kim and coworkers introduced silyl radical mediated addition of alkyl radical to silyloxy enamine 76. The silyloxy enamine moiety is readily accessible from a variety of functionalities. The mechanistic concept is illustrated in the Scheme 12 and involves the addition of R radical to 76 to give the radical adduct 77 and the subsequent homolytic cleavage of N-O bond to yield the desired product 78 and a silyloxy radical 79. The latter undergoes 1,2-phenyl migration to give the silyl radical 80 that abstracts halogen from the alkyl halide to regenerate the R radical. 

The case of benzoin alkyl ethers illustrated in Figure 8.15 is a remarkable example of the effect of complexation with cyclodextrins. Such molecules normally undergo homolytic dissociation in solution (the Norrish type 1 process described in section 4.4) and there is practically no intramolecular hydrogen atom abstraction (Figure 8.15). When the benzoin alkyl ethers are complexed with a cyclodextrin to form a 1 1 association, it can be shown that one of the phenyl rings fits inside the cyclodextrin cavity in aqueous solution. When the solid complex is irradiated only the photoproducts resulting from hydrogen atom transfer are detected the opposite behaviour from irradiation of the crystal of benzoin alkyl ether as well as of solutions in benzene. 

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). 

This second mechanism is less satisfactory because cyclization of the carboxy radical 5-40 is unlikely. Alkyl carboxy radicals are highly unstable and they decarboxylate so rapidly that fragmentation would be expected to occur faster than cyclization. The previous mechanism has at least two other advantages. First, it proceeds through the radical 5-41, which is stabilized by resonance with two phenyl substituents, so that hydrogen abstraction to form 5-41 should compete effectively with homolytic scission of the acyl bromide to form 5-40. Second, it will occur rapidly because it is a radical chain process the bromine radical formed in the cyclization of 5-41 regenerates 5-41 from the acyl hypobromite. 

Kropp and coworkers have been concerned with systems in which homolysis is followed by rapid electron transfer, such as the photolysis of certain alkyl halides [55]. Photolysis of the norbomyl phenyl sulfoxide 110 was examined. The notion was that sulfide photolysis is usually homolytic, but the sulfinyl radical is more electronegative than the sulfenyl (thiyl) radical, which might assist in electron transfer reactions. Thus, it was thought, ionic reactivity might be observed. In addition to inversion of the sulfur center and deoxygenation (yide infra), norbomane 111 and norbomene 112, both presumed to... 

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