Radicals arenes, heterocycles

The photolysis of donor-acceptor systems shows a reaction pattern of unique synthetic value. Direct irradiation of the donor-acceptor pairs, such as arene-amine, leads by intramolecular electron transfer, to amine radical cations and arene radical anions. The generated radical cation and radical anion intermediates undergo cyclization reactions providing efficient synthetic routes to N-heterocycles with a variety of ring sizes. 

Nitrogen heterocycles continue to be valuable reagents and provide new synthetic approaches such as NITRONES FOR INTRAMOLECULAR -1,3 - DIPOLAR CYCLOADDITIONS HEXAHYDRO-1,3,3,6-TETRAMETHYL-2,l-BENZISOX AZOLINE. Substituting on a pyrrolidine can be accomplished by using NUCLEOPHILIC a - sec - AM IN O ALKYL ATION 2-(DI-PHENYLHYDROXYMETHYL)PYRROLIDINE. Arene oxides have considerable importance for cancer studies, and the example ARENE OXIDE SYNTHESIS PHENANTHRENE 9,10-OXIDE has been included. An aromatic reaction illustrates RADICAL ANION ARYLATION DIETHYL PHENYLPHOSPHONATE. 

From a synthetic point of view, bond forming steps are the most important reactions of radical ions [202]. Several principle possibilities have been described in Section 8.1 and are summarized in Scheme 52. Many carbo- and heterocyclic ring systems can be constructed by (inter- and intramolecular) radical addition to alkenes, alkynes, or arenes. Coupling of carbonyl radical anions leads to pinacols either intra-or inter-molecular which can be further modified to give 1,2-diols, acyloins or alkenes. Radical combination reactions with alkyl radicals afford the opportunity to synthesize macrocyclic rings. These radical ion-radical pairs can be generated most efficiently by inter- or intramolecular photoinduced electron transfer. 

The use of Bu3SnH and AIBN has become commonplace in the synthesis of annulated arenes by the intramolecular aromatic substitution via aryl and heteroaryl radicals (see Chapter 13). Nevertheless, a simpler and more environmentally friendly protocol should be the direct generation of the o-aryl or heteroaryl radicals by UV-induced homo lytic cleavage of an aryl halide with subsequent intramolecular attack onto an arene in proximity. Accordingly, tricyclic [2,1-a] fused heterocycles were regioselectively formed in high yields from N-(2-arylethyl)-2-iodoimidazoles upon irradiation in acetonitrile (Scheme 14.18) [92]. 

Several efficient procedures have been developed that allow the cross-dehy-drogenative synthesis of heterocycles via intramolecular cyclization of acetanilide radicals. Fagnou and co-workers developed a palladium-catalyzed intramolecular coupling of arenes and alkanes in air. A new sp -sp C-C... 

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