Cycloadditions metallacyclopentane

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). 

Metallacycloalkanes are proven key intermediates in metal-catalyzed cycloadditions and cycloreversions of alkenes. The relationship of some iron metallacyclopentane derivatives with bis(olefin) complexes has been investigated theoretically. Scheme 1 shows a general route from bisalkene... 

The regiospecificity of the [2 + 2] cycloaddition is usually high, giving 1,3-dispirocyclobutanes, while in thermal reactions the 1,2-dispiro system is preferred (11 1). Steric constraints in the metallacyclopentane intermediate are obviously responsible for the observed regioselec-tivity. ... 

The involvement of metallacycles has been proposed for the [3+2] cycloaddition of meth-ylenecyclopropanes with alkenes to produce methylenecyclopentanes.l Oxidative cyclization of a methylenecyclopropane and an electron-deficient alkene produces a spi-rocyclic metallacyclopentane 86. Cyclopropane ring opening followed by reductive elimination affords the observed methylenecyclopentane products 87 and 88 (Scheme 65). Another report describes the novel use of nanostructured nickel clusters as catalysts. ]... 

---