Cobalt-mediated radical reactions in organic synthesis. Oxidative cyclisations of aryl and alkyl halides leading to functionalised reduced heterocycles and butyrolactones
Abstract
Reactions between the CoI species derived from cobalt (III) salen (11) or cobalt(II) salophen (12) and (O-allyl) or (O-but-3-enyl) iodophenols lead to isolatable cobalt complexes, viz.(16) and (27), which can be converted into substituted benzofurans, i.e.(17) and (23), and benzopyrans, i.e.(28) and (34); similarly, interaction between compound (36) and CoI salen led in one step to 1,3-dimethyl-indole (37). Radical cyclisation of the acetal (38a) in the presence of CoI cobaloxime [from (10)] leads to the cis-ring-fused alkyl cobalt complex (42), which can be converted in a preparative manner into lactone (44) following 1,2-elimination [to (43)] and hydrolysis/oxidation, and into lactone (53) following insertion of molecular oxygen [to (50)], reduction [to (51)] and hydrolysis/oxidation.
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