Competitive reactions in alkyne hydrogenation

Competitive hydrogenation reactions between alkynes have been studied over a palladium/carbon catalyst. The competitive reactions of I-pentyne, phenyl acetylene, 2-pentyne, and I-phenyl-l-propyne have been examined. The results show that in general, the competitive reaction results in a reduction of the hydrogenation rate for both alkynes. However, the 1-pentyne/2-pentyne couple revealed a rate enhancement for both alkynes. This is thought to be due to enhanced hydrogen transfer on the surface. Terminal alkynes reduced the rate of hydrogenation of a competing alkyne more effectively than internal alkynes. Phenyl acetylene was very sensitive to the presence of the second alkyne with its hydrogenation rate being reduced proportionately more than the aliphatic alkyne. This may be related to a disruption of its pi-pi stacking ability. Competitive hydrogenation increased selectivity to the respective alkenes but had no effect on the cis:trans ratio. (C) 2002 Elsevier Science B.V. All rights reserved.