Pericyclic Reactions by Transition Metals

The ring opening of the codimer (221) of cyclobutadiene and benzocyclo-butadiene rearranges on heating to benzocyclooctatetraene (224). This reaction inevitably involves, as indicated, the successive disrotatory opening of two cyclobutene rings via antiaromatic transition states. The reaction is 

Pettit and his collaborators found that the rearrangement is very strongly catalyzed by salts of silver or platinum. Thus addition of a solution of silver nitrate to a solution of (222) in alcohol at room temperature leads to complete conversion to (224) in less than 10 sec. This startling observation led to extensive studies of the effects of transition metals on other forbidden pericyclic reactions and many of them have indeed been found to undergo similar accelerations. 

The mechanisms of these reactions are still a matter of controversy and some at least proceed via organometallic intermediates formed by replacement of hydrogen atoms in the reactant by metals. However it seems very likely that the original examples [equation (5.325)] represent a true catalysis by a transition metal of a pericyclic reaction involving an antiaromatic transition state. 

Transition metals, particularly those at the end of each transition series, form n complexes to olefins in which the olefin acts as a donor and in which d electrons of the metal are simultaneously used for back-coordination (p. 294). Similar complexes are also formed by conjugated hydrocarbons and by aromatic hydrocarbons such as benzene. 

The formation of such a n complex is indicated in Fig. 5.41. The dative bond formed by the n electrons of the hydrocarbon arises mainly from an interaction between the HOMO of the hydrocarbon and an empty valence shell AO of the metal, while the reverse dative bond involves an analogous interaction between a filled d AO of the metal and the LUMO of the hydrocarbon. The nearer together these pairs of orbitals are, the stronger is the interaction and the more stable is the n complex. 

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