Initiation of radical cyclisation reactions using dimanganese decacarbonyl. A flexible approach to preparing 5-membered rings

Photolysis of dimanganese decacarbonyl [Mn-2(CO)(10)] using visible light p roduces the manganese pentacarbonyl radical [Mn-.(CO)(5)] which reacts with organohalides to form carbon-centred radicals. Efficient halogen-atom abst raction occurs with allylic or benzylic halides or polyhalogenated precurso rs bearing a weak carbon-halogen bond. Steric interactions are also importa nt and primary halides generally react much faster with Mn-.(CO)(5) than se condary or tertiary halides. The carbon-centred radicals can undergo effici ent dimerisation or, in the presence of an acceptor double bond, cyclisatio n to form 5-membered rings. Cyclisation of terminal alkenes leads to primar y radicals, which can then react by iodine- or bromine-atom transfer or, on addition of propan-2-ol, hydrogen-atom transfer. Hydroxylamines can also b e formed when cyclisation reactions are carried out in the presence of TEMP O. These high-yielding cyclisation-trapping reactions are initiated under m ild reaction conditions and the manganese halide by-products [of type XMn(C O)(5)] can be easily separated from products by a simple DBU work-up proced ure.