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

Photolysis of dimanganese decacarbonyl [Mn2(CO)10] using visible light produces the manganese pentacarbonyl radical [·Mn(CO)5] which reacts with organohalides to form carbon-centred radicals. Efficient halogen-atom abstraction occurs with allylic or benzylic halides or polyhalogenated precursors bearing a weak carbon–halogen bond. Steric interactions are also important and primary halides generally react much faster with ·Mn(CO)5 than secondary or tertiary halides. The carbon-centred radicals can undergo efficient dimerisation or, in the presence of an acceptor double bond, cyclisation to form 5-membered rings. Cyclisation of terminal alkenes leads to primary radicals, which can then react by iodine- or bromine-atom transfer or, on addition of propan-2-ol, hydrogen-atom transfer. Hydroxylamines can also be formed when cyclisation reactions are carried out in the presence of TEMPO. These high-yielding cyclisation–trapping reactions are initiated under mild reaction conditions and the manganese halide by-products [of type XMn(CO)5] can be easily separated from products by a simple DBU work-up procedure.