Initiation of radical cyclisation reactions using dimanganese decacarbonyl. A flexible approach to preparing 5-membered rings
作者:Bruce C. Gilbert、Wilhelm Kalz、Chris I. Lindsay、P. Terry McGrail、Andrew F. Parsons、David T. E. Whittaker
DOI:10.1039/b000835o
日期:——
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.
用可见光光解二锰十羰基化合物 [Mn2(CO)10] 会产生锰五羰基自由基 [·Mn(CO)5],该自由基与卤代烃反应生成以碳为中心的自由基。与烯丙基或苯甲基卤代物,或具有较弱碳-卤素键的多卤代前驱体,发生高效的卤素原子抽取反应。立体相互作用也很重要,初级卤代物通常比次级或三级卤代物与 ·Mn(CO)5 反应更快。以碳为中心的自由基可以进行高效的二聚反应,或者在存在接受体双键的情况下,环化形成5员环。末端烯烃的环化反应会导致生成初级自由基,随后这些自由基可以通过碘或溴原子转移反应,或者在加入异丙醇的情况下进行氢原子转移反应。当环化反应在TEMPO存在的情况下进行时,还可以形成羟胺。这些高产率的环化-捕捉反应在温和的反应条件下发起,锰卤化物副产物 [类型为 XMn(CO)5] 可以通过简单的DBU后处理程序轻松与产物分离。