Isomerization valence, strained hydrocarbons

To date there are no examples of the very highly strained cyclopropadiene (288), 1,2-cyclobutadiene (289), or 1,2-cyclopentadiene (290), either free or complexed to transition metals. Complexes of a valence isomeric form of cyclopropadiene (291) have been prepared,110 but as with the free hydrocarbon,111112 there is no evidence for any converting to the allene form [Eq. (46)]. 

A number of complexes, notably of Rh1, Pd°, and Ni° will induce valence isomerization of strained hydrocarbons. A typical example is the interconversion of norborna-diene and quadricyclane. 

Treatment of highly strained hydrocarbons with Pd complexes has been shown to induce valence bond isomerization. Some representative examples are shown in Scheme 4. The mechanisms of these reactions are not clear in most cases. However, a mechanism involving oxidative complexation, allylic rearrangement, and reductive decomplexation may be proposed for the Dewar benzene-to-benzene rearrangement. 

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