π-Complex Intermediates in Homogeneous and Heterogeneous Catalytic Exchange Reactions of Hydrocarbons and Derivatives with Metals

Abstract

The exchange of unsaturated and aromatic hydrocarbons and their derivatives with isotopic hydrogen on group VIII transition metals has been reviewed. Both classical and π-complex heterogeneous mechanisms proposed to account for the observed behavior have been compared. Evidence to distinguish between the application of the heterogeneous associative and dissociative π-complex exchange mechanisms is discussed. The recently discovered homogeneous equivalent of the above heterogeneous reactions is described in detail. Homogeneous associative and dissociative π-complex mechanisms are proposed to explain aromatic hydrogen exchange. Mechanisms are also proposed to explain the pattern of exchange observed homogeneously with platinum in the alkyl groups of alkyl aromatics. Possible mechanistic relationships between heterogeneous and homogeneous systems are considered using as representative unsaturated and aromatic series, the polycyclic hydrocarbons, the monohalogenated benzenes, the alkyl benzenes (and relevant simple alkanes), steroids, and synthetic hormones. The role of π-complexes in relating the present heterogeneous and homogeneous exchange reactions to the chemistry of inorganic coordination complexes is also considered.