Hydrogenolysis and Homologation of 3,3-Dimethyl-1-butene on Ru/SiO 2 Catalyst: Implications for the Mechanism of Carbon-Carbon Bond Formation and Cleavage on Metal Surfaces

Abstract

The reactions of 3,3-dimethyl-1-butene (neohexene) with hydrogen in the presence of zero-valent ruthenium metal particles supported on silica are reported. The predominant reaction is the hydrogenation of neohexene to neohexane. Simultaneous but slower homologation and hydrogenolysis reactions are reported. The homologation and hydrogenolysis reactions run at approximately equal rates, suggesting a mechanistic link between the two processes. The relative quantities of C 1-C 5 and C 7 products and the variation of these quantities with respect to varying temperature, neohexene/hydrogen ratio, and contact time are reported. The distribution of the primary products, neopentane, isobutene, and methane for hydrogenolysis and 2,2-dimethyl pentane, 2,2,3-trimethyl butane, and 4,4-dimethyl-1-pentene for homologation is discussed in terms of current thought on the mechanism of homologation and hydrogenolysis of alkanes. The most likely and strongly indicated mechanism for the hydrogenolysis of neohexene involves the deinsertion of a methylidene fragment from a ruthenium-neohexyl intermediate which is also an intermediate in the hydrogenation of neohexene. The distribution of C 7 homologation products does not allow one to distinguish between a simple insertion mechanism and a mechanism passing through a metallacyclic intermediate.