Intramolecular arylations, radical-based

Johnston and coworkers reported a base-free aryl amination method based on radical additions to azomethines through nonconventional addition pathways (equation 6)42,43. By this route, the aryl radical adds to nitrogen, rather than to the carbon of the ketimines. For example, the ketimine prepared from o-bromophenethylamine and acetophenone was subjected to tributylstannane and the radical initiator AIBN to give the corresponding indoline in 87% yield. The only side product observed was the directly reduced compound. The fact that only the intramolecular radical addition can afford the high yield limited its application in synthesis of other arylamines. 

Assoanine via Vasconine. A similar intramolecular addition of an aryl a radical generated in a more conventional manner to an aryl ring constituted an alternative approach [6] to the synthesis of 164 (Scheme 25). Thus, o-bromobenzylaniline 173 obtained by sodium borohydride reduction of the Schiffs base derived from 6-bromoveratraldehyde and the... 

These results provide the first detailed calibration for a series of intramolecular radical cation probes based on cycloaddition chemistry. The cyclization rate constants cover several orders of magnitude in timescale, an ideal case for using 1—3 as probes for radical cations of different lifetimes. However, the time-resolved experiments demonstrate that the application of radical cation probes, at least those based on aryl alkene cycloaddition chemistry, may be considerably less straightforward than similar experiments with free radical probes or clocks. Some of the problems that need to be addressed include the variation of products with the reaction conditions and method of radical cation generation, and the possibility of reversibility of the initial adduct formation. Furthermore, at least some radical cation reactions are quite sensitive to solvent and this may mean that calibrations for radical cation cycloadditions will have to be done in a variety of solvents. 

A copper-catalyzed redox condensation reaction between benzothia-zoles 101 and aldehydes (RCHO) or benzylic alcohols (RCH2OH) provides 2-aryl- and 2-alkyl-substituted benzothiazoles 102 (14TL1806).The reactions are carried out under mild conditions using environmentally benign TBHP as the oxidant. The proposed mechanism involves the ring opening of benzothiazoles initiated by the attack of an acyl radical on the thiazole ring followed by intramolecular condensation. This mechanism is based on the isolation of an anilide disulfide intermediate. 

The catalytic system proved not only applicable to alkyl hahdes, but also allowed for the intramolecular conversion of aryl halides. Interestingly, the corresponding Mizoroki-Heck-type cyclization products were formed selectively, without traces of reduced side-products (Scheme 10.27) [55]. Therefore, a radical reaction via a single electron-transfer process was generally disregarded for cobalt-catalysed Mizoroki-Heck-type reactions of aromatic hahdes. Instead, a mechanism based on oxidative addition to yield an aryl-cobalt complex was suggested [51]. 

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