Cycloaddition/rearrangement cubane

Within the cubane synthesis the initially produced cyclobutadiene moiety (see p. 329) is only stable as an iron(O) complex (M. Avram, 1964 G.F. Emerson, 1965 M.P. Cava, 1967). When this complex is destroyed by oxidation with cerium(lV) in the presence of a dienophilic quinone derivative, the cycloaddition takes place immediately. Irradiation leads to a further cyclobutane ring closure. The cubane synthesis also exemplifies another general approach to cyclobutane derivatives. This starts with cyclopentanone or cyclohexane-dione derivatives which are brominated and treated with strong base. A Favorskii rearrangement then leads to ring contraction (J.C. Barborak, 1966). 

Intermediate metallacyclopentanes are also implicated in transition metal-catalyzed alkene cycloadditions to form cyclobutanes and the corresponding cycloreversions, e.g. dimerization of norbomadiene (73JA597) and rearrangements of cubane and other cyclo-butanoid hydrocarbons (78JA2573). 

Unstable P N derivatives undergo dimerisation (13.304), which is similar to Equation 13.281, but examples of further cycloaddition to produce cubane forms do not yet appear to have been established. Rearrangement of the type (13.305) is known (7.549) but further rearrangement to the cyclic ring (13.305c) does not (according to some investigations) occur. Compounds of types (13.306) may polymerise in alternative ways. 

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