Stereochemistry and the use of H2/D2 mixtures as probes into the mechanism of hydrogenations catalyzed by cationic rhodium (DIPHOS) complexes

The stereochemistry of the hydrogenation of 4-tert-butylmethylenecyclohexane (1) and the use of D₂ or D₂/H₂ mixtures in place of H₂ furnishes evidence that hydrogenations using the catalyst precursor [Rh(DIPHOS)(COD)]⁺BF ₄ ⁻ proceed via more than one mechanism. This evidence includes the effect of changes in pressure and added triethylamine upon the kinetics and isomerization of 1, as well as the distribution of the added deuterium in the products of the reaction of 1 or norbornene with either D₂ or H₂/D₂ mixtures. That an alkene necessarily causes the equilibration of H₂/D₂ mixtures, although it need not involve any of the alkenes’s hydrogen atoms (e.g., norbornene), provides a clue to the process by which the mononuclear mechanism proposed by Halpern, which is dominant near one atmosphere of H₂, merges into another with increasing pressure. It has been proposed that the cationic complex [Rh(DIPHOS)S₂]⁺ is transformed in the presence of an alkene and hydrogen into a binuclear hydrido complex, such as those described by Sivak and Muetterties (1979) and Fryzuk (1982), which represent a far more active catalyst than its mononuclear precursor. Such an intermediate should readily catalyze the H₂-D₂ equilibration and the isomerization of an alkene in the presence of D₂ without necessarily introducing deuterium into the product. Published in Russian in Kinetika i Kataliz, 2006, Vol. 47, No. 1, pp. 88–97. Deceased. The text was submitted by the authors in English. This work was conducted at the University of Arkansas.