Stereochemistry and the use of H-2/D-2 mixtures as probes into the mechanism of hydrogenations catalyzed by cationic rhodium(DIPHOS) complexes

by Lin, S. T.; Siegel, S.

The stereochemistry of the hydrogenation of 4-tertbutylmethylenecyclohexane (1) and the use of D-2 or D-2/H-2 mixtures in place of H-2 furnishes evidence that hydrogenations using the catalyst precursor [Rh(DIPHOS)(COD)]+BF4- 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 D-2 or H-2/D-2 mixtures. That an alkene necessarily causes the equilibration of H-2/D-2 mixtures, although it need not involve any of the alkene'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 H-2, merges into another with increasing pressure sure. It has been proposed that the cationic complex [Rh(DIPHOS)S-2](+) 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 H-2-D-2 equilibration and the isomerization of an alkene in the presence of D-2 without necessarily introducing deuterium into the product.

Kinetics and Catalysis
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1608-3210; 0023-1584