Homolytic addition radical cations

Simple mechanistic considerations easily explain why heterolytic dissociation of the C — N bond in a diazonium ion is likely to occur, as a nitrogen molecule is already preformed in a diazonium ion. On the other hand, homolytic dissociation of the C —N bond is very unlikely from an energetic point of view. In heterolysis N2, a very stable product, is formed in addition to the aryl cation (8.1), which is a metastable intermediate, whereas in homolysis two metastable primary products, the aryl radical (8.2) and the dinitrogen radical cation (8.3) would be formed. This event is unlikely indeed, and as discussed in Section 8.6, homolytic dediazoniation does not proceed by simple homolysis of a diazonium ion. 

The gaseous dichlorocarbene radical cation reacted with alkyl halides via a fast electrophilic addition to form a covalently bonded intermediate (CI2C—X—R)+ in a Fourier transform ion cyclotron resonance mass spectrometer. This intermediate fragments either homolytically or heterolytically to produce net halogen atom or halogen ion transfer product. Addition of carbonyls to the carbene ion is followed by homolytic cleavage of the C-O bond to yield a new carbene radical cation. 

Triplet sensitization of sulfonium salts proceeds exclusively by the homolytic pathway, and that the only arene escape product is benzene, not biphenyl or acetanilide. However, it is difficult to differentiate between the homolytic or heterolytic pathways for the cage reaction, formation of the isomeric halobiaryls. Our recent studies on photoinduced electron transfer reactions between naphthalene and sulfonium salts, have shown that no meta- rearrangement product product is obtained from the reaction of phenyl radical with diphenylsulfinyl radical cation. Similarly, it is expected that the 2- and 4-halobiaryl should be the preferred products from the homolytic fragments, the arene radical-haloarene radical cation pair. The heterolytic pathway generates the arene cation-haloarene pair, which should react less selectively and form the 3-halobiaryl, in addition to the other two isomers. The increased selectivity of 2-halobiaryl over 3-halobiaryl formation from photolysis of the diaryliodonium salts versus the bromonium or chloronium salts, suggests that homolytic cleavage is more favored for iodonium salts than bromonium or chloronium salts. This is also consistent with the observation that more of the escape aryl fragment is radical derived for diaryliodonium salts than for the other diarylhalonium salts. 

While radical species can hardly abstract H atom from benzene rings, some of them, in particular HO radical, are easily trapped by aromatic molecules. Radical hydroxylation most often follows Fenton-type chemistry that involves generation of HO in the course of homolytic decomposition of HjOj induced by an iron(II) salt and addition of HO to an aromatic nuclear to give hydroxycyclo-hexadienyl radical I. This radical may dimerize, be oxidized to phenols (Cu(II) is one of the most effective oxidants for this), or undergo an acid-catalyzed collapse to radical cation [15, 28]. Scheme 14.3 shows the classical mechanism suggested by Walling [28]. 

Ionic disproportionation of NO can be promoted in non-polar media by the addition of Bronsted acids and Lewis acids [10]. The photochemical activation of the nitrosonium donor-acceptor complex via irradiation of the charge-transfer absorption band produces the aromatic radical cation. The most direct pathway to aromatic nitration proceeds via homolytic coupling of the aromatic radical cation with NO [16] because the intermediate subsequently undergoes very rapid deprotonation ... 

In this chapter, we discuss reactions that either add adjacent (vicinal) groups to a carbon-carbon double bond (addition) or remove two adjacent groups to form a new double bond (elimination). The discussion focuses on addition reactions that proceed by electrophilic polar (heterolytic) mechanisms. In subsequent chapters we discuss addition reactions that proceed by radical (homolytic), nucleophilic, and concerted mechanisms. The electrophiles discussed include protic acids, halogens, sulfenyl and selenenyl reagents, epoxidation reagents, and mercuric and related metal cations, as well as diborane and alkylboranes. We emphasize the relationship between the regio-and stereoselectivity of addition reactions and the reaction mechanism. 

The remaining two reactions, which will be described in this section, are related through the well-known formation of azo compounds 39 by the addition of nucleophiles to diazonium salts (40). The former compounds lose nitrogen by homolytic and the latter by heterolytic processes to give, respectively, aryl radicals and sometimes, as has been recently demonstrated, aryl cations. Because the equilibrium between 39 and 40 is so dependent on solvent, gegenion, and additives, both reactions [equations (8) and (9)] often occur simultaneously. ... 

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