ISOTOPIC EXCHANGE IN THE PLATINUM-CATALYZED REDUCTIONS OF OLEFINS IN PROTIC SOLVENTS

This paper reports the isotopic compositions of alkanes obtained by the catalytic reductions of olefins using H-2 or D2 and platinum black in alcoholic and mixed organic/aqueous solvents. The isotope found in the product alkanes was predominantly that present in the solvent (ROH or ROD) regardless of the isotope present in the gas (H-2 or D2). The rate of exchange of isotopes of hydrogen between the protic solvent and the surface of the catalyst was fast relative to the rate of production of alkanes; consequently, the content of excess deuterium in the alkanes provided a measure of the rate of activation of the C-H bonds of the intermediate surface alkyls relative to the rate of their reductive elimination as alkanes. In deuterated alcohols (ROD), under conditions in which the rates of reduction were limited by mass transport of H-2 or D2 to the catalyst surface (the mass-transport-limited regime, MTL; T = 40-degrees-C, P(H-2) = 0.17 atm), the alkanes contained deuterium atoms in excess of that required for stoichiometric reduction of the double bond; for example, the average isotopic content of cyclooctane from reduction of cis-cyclooctene in ethyl alcohol-d was C8H4D12. Under conditions in which the rates of reduction were limited by the rate of a reaction occurring on the surface of the catalyst (the reaction-rate-limited regime, RRL; T = 20-degrees-C, P(H-2) = 2.4 atm), the extent of incorporation of excess deuterium was less than that observed under MTL conditions. A comparison of several deuterated alcohols ROD (R = methyl, ethyl, n-propyl, isopropyl, and tert-butyl) as deuterium donors in the reduction of cis-cyclooctene showed similar incorporation of deuterium into the cyclooctanes within each kinetic regime. The influence of acidity/basicity of the solvent on these reactions was small. The reductions of cis-cyclooctene under MTL conditions in D2O/THF (1:1 v/v) where the pD of the D2O was varied from 1 to 10 gave cyclooctanes with slightly more deuterium than analogous reductions in the deuterated alcohols; at pD 13, this trend was markedly reversed. Under MTL and RRL conditions, the rate of isotopic exchange of hydrogen between H2O (D2O) and the surface of the catalyst was slower at pH(D) 13 than in the range pH(D) 1-10.