Metal salts, addition radical cyclization

Wunderlich and Knochel recently published the alkylation of diaryliron compounds by alkyl iodides or benzyl chloride 1 (entry 33) [77]. The reaction works well with 98% pure FeCl2 2LiCl but not with 99.998% pure metal salt. Addition of other transition metal salts showed that nickel contained in the FeCl2 of 98% purity is the likely catalyst. Alkylarenes 3 were obtained in 65-88% yield. The method tolerates ester, nitrile, fluoride, or chloride substituents. Although the use of 5-hexenyl iodide did not provide a cyclized product, the initial formation of radicals cannot be excluded safely. 

To achieve low radical concentrations, most radical reactions are traditionally performed as chain reactions. Atom or group transfer reactions are one of the two basic chain modes. In this process the atom or group X is the chain carrier. A metal complex can promote such chain reactions in two ways. On one hand, the catalyst acts only to initiate the chain process by generating the initial radical 29A from substrate 29 (Fig. 10). This intermediate undergoes the typical radical reactions, such as additions or cyclizations leading to radical 29B, which stabilizes to product 30 by abstracting the group X from 29. A typical example is the use of catalytic amounts of cobalt(II) salts in oxidative radical reactions catalyzed by /V-hydroxyphthalimide (NHPI), which is the chain carrier [102]. 

This catalytic sequence is known as Kharasch addition or atom transfer radical addition (ATRA) [4]. Various polyhalogenated compounds such as CCI4 and CCI3CO2R are used as the organic halides, and transition metal salts or complexes are used as the catalyst [3]. Intramolecular version of the Kharasch addition reaction (atom transfer radical cyclization, ATRC) has opened novel synthetic protocols to the synthesis of carbocyde or heterocyles catalyzed by transition metals [5-7], and this has become a very important field in free radical cydization in organic synthesis. Transition metal-catalyzed Kharasch reactions sometimes afford telomers or poly-... 

Thus it seems now established that the cyclized products obtained by photolysis of o-allylphenols do not result from intramolecular addition of aryloxyl radical. Furthermore, the results obtained on metallic salt oxidation lead to the conclusion that intramolecular addition of an aryloxyl radical, even in what is probably the best situation (Cy5/Cy6), is not a favored process. More work is needed before a definitive conclusion can be reached. Nonetheless, this possibility has been retained to account for the cyclized products (48.5% yield) obtained by treating the chalcone (66 Ar =/ -HOC6H4) with aqueous alkali - K3Fe(CN)g (Scheme 35).This must be considered a very favorable case, however, since the (Cy 5) radical is benzylic. 

The photochemical behavior of o-allylanilines such as 88 has been studied. Such compounds cyclize to give the (Cy5) compounds, indolines (89) (Scheme 45). This result is clearly reminiscent of the photochemical behavior of o-allylphenols (Section VIII.3), and although the selectivity is the one expected from aminyl radical intramolecular addition, it also seems best explained by photochemical excitation of the n double bond. This is an interesting conclusion since metal-salt-induced cyclization of compounds such as 88 generally yields a mixture of (Cy5) and (Cy6) products. 

The sonochemistry of the other alkali metals is less explored. The use of ultrasound to produce colloidal Na has early origins and was found to greatly facilitate the production of the radical anion salt of 5,6-benzo-quinoline (225) and to give higher yields with greater control in the synthesis of phenylsodium (226). In addition, the use of an ultrasonic cleaning bath to promote the formation of other aromatic radical anions from chunk Na in undried solvents has been reported (227). Luche has recently studied the ultrasonic dispersion of potassium in toluene or xylene and its use for the cyclization of a, o-difunctionalized alkanes and for other reactions (228). 

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