1,2-Aryl radical rearrangements

IV-Nitrosqanilides are an alternative source of aryl radicals. There is a close mechanistie relationship to the decomposition of azo compounds. The JV-nitrosoanilides rearrange to intermediates that have a nitrogen-nitrogen double bond. The intermediate then decomposes to generate aryl radieals. ... 

The aryl radical thus formed attacks the substrate to give the intermediate 1 (p. 898), from which the radical 26 abstracts hydrogen to give the product. N-Nitroso amides probably rearrange to N-acyloxy compounds, which cleave to give aryl radicals ... 

In the classical procedure, base is added to a two-phase mixture of the aqueous diazonium salt and an excess of the aromatic that is to be substituted. Improved yields can be obtained by using polyethers or phase transfer catalysts with solid aryl diazonium tetrafluoroborate salts in an excess of the aromatic reactant.177 Another source of aryl radicals is A-nitrosoacetanilides, which rearrange to diazonium acetates and give rise to aryl radicals via diazo oxides.178... 

Thermolysis of 164 bearing a pendent olefin at 150 °C also allows the aryl radical in 167 to be captured in a 5-exo fashion, leading to 165 as a 1 1 mixture of diastereo-mers (Scheme 20.33). The corresponding Lewis acid-catalyzed [3,3]-sigmatropic rearrangement promoted by AgBF4 was facile at room temperature, making it possible for 166 to be isolated. Thermolysis of 166 at 75 °C afforded 165 in 80% yield. 

The flash vacuum pyrolysis of alkynes, arynes, and aryl radicals has been reviewed. A discussion of secondary reactions and rearrangements is included. The pyrolysis of cyclopentadienes has also been examined. The rates for the initial C—H bond fission and the decomposition of C-C5H5 have been calculated. A single-pulse shock study on the thermal decomposition of 1-pentyl radicals found alkene products that are formed by radical isomerization through 1,4- and 1,3-hydrogen migration to form 2- and 3-pentyl radicals. The pyrrolysis of f-butylbenzene in supercritical water was the subject of a report. ... 

Reaction (7.63) shows an example of C—S bond formation [73,74]. In fact, the aryl radical formed by iodine abstraction by (TMS)3Si radical rearranged by substitution to the sulfur atom, with expulsion of the acyl radical and concomitant formation of dihydrobenzothiophene (60). This procedure... 

When a radical is formed in a chain four or five carbon atoms removed from an aryl group, rearrangements that amount to intramolecular aromatic substitutions can occur. The initial radical attack will yield a cyclohexadienyl radical intermediate with an attached five- or six-membered ring, as in Equation 9.109. 

Scheme 22 presents some selected aryl radical cyclizations. The first example shows that isomerization of the intermediate radicals is again a concern. This isomerization, which is usually called the neo-phyl rearrangement,101 is promoted by the activating aldehyde group in the case at hand.102-103 The other examples illustrate that aryl radical cyclizations provide practical routes to benzo-fused heterocycles.104-105... 

In 2005, the group of Alcaide has reported the regiocontrolled preparation of biaryl-2-azetidinones (I and II, Fig. 17), via aryl-aryl radical cyclization and/or rearrangement of (3-lactam-tethered haloarenes [286]. 

Although the two competing intermediates, the hypothetical ketyl-aryl radical pair (4) and the oxaspirooctadienyllithium (3), are not the rate-determining transition states, they should lie at almost the same energetic level. The rearrangement is in accord with the intramolecular nucleophilic addition/elimination mechanism rather than with homolytic cleavage/recombination. 

Bose and colleagues described the construction of tetracyclic isoquinolines and quinazolines via aryl radical cyclization (equation 72)510. Sometimes, the radical formed as a result of 5-exo cyclization can undergo further rearrangement as was discovered by Engman and coworkers, who report the preparation of antioxidant 2,3-dihydrobenzothiophenes (96, 97) by radical ring closure (Scheme 10)516. 

An unusual 4-exo-trig cyclization accompanied by a later rearrangement was observed with aryl enolether 13 (Scheme 6) [48], The benzoxetane 14, which is formed in the initial cyclization step, undergoes ring-opening to form the phenoxy radical 15. 5-Endo cyclization of 15 and reduction of the resulting benzylic radical leads to the spirocycle 16. Diverse polycyclic products have also recently been obtained by intramolecular aryl radical cyclizations to tetrahydropyridines [49]. 

Aryloxymethylthiopyrano[3,2-c]pyran-5-ones, derived from a 4-mercaptopyran-2-one by thermal rearrangement of a propargyl ether, are suitable substrates for an aryl radical cyclisation. Only products arising from a 6-endo cyclisation are observed and in some instances one diastereomer of the annulated benzopyran is formed exclusively (Scheme 43) <06S2725>. 

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