Aryl Radical Cyclizations with Grignard Reagent

Cobalt-catalyzed Aryl Radical Cyclizations with Grignard Reagent 

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In a parallel study, it was found that chelating chiral diamines 208 or 209 are well suited as ligands to promote Kumada-type couplings of primary and secondary alkyl halides 202 with aryl Grignard reagents 203 (entry 4) [281]. This reaction was applicable to alkyl bromides and alkyl iodides, while alkyl chlorides gave only low yields. Acetal and ester functions are tolerated. A notable feature is the stereoretentive arylation of fra s-a-bromo acetals with excellent diastereo-selectivity. The involvement of radicals is supported by the stereoconvergent formation of cxo-phenvI norbornane from both endo- or exo-bromonorbomane (cf. Part 1, Fig. 9) and radical 5-exo cyclizations (see below). 

Co(I) species and radical 688. Cyclization of 688 would lead to the formation of radical 689 that then combines with the Co(I) species to afford the organometallic species 690. Reductive elimination produced 687 and regenerated the Co(0) species. Whereas a variety of aryl and heteroaryl Grignard reagents readily participated in this cascade, o-substituted aryl Grignards appeared to be too sterically encumbered to efficiently react. 

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