Catalysis of Organic..

Jackson and Spence 24127. Selectivity Control in 1-Phenyl-1-PropyneHydrogenation: Effect of ModifiersS. David Jackson and Ron R. SpenceWestCHEM, Department of Chemistry, The University, Glasgow, G12 8QQ, ScotlandAbstractsdj@chem.gla.ac.ukThe use of modifiers in controlling the selectivity in liquid phase alkynehydrogenation has been studied. Modifiers that are more strongly bound than theintermediate alkene inhibit hydrogenation and isomerization giving high stereo- andchemo-selectivity. One modifier increased the rate of alkyne hydrogenationIntroductionControl of selectivity in alkyne hydrogenation is a constant industrial problem. Inlarge scale acetylene hydrogenation plants carbon monoxide is often used as amodifier to enhance the selectivity to ethylene (1, 2, 3). This option is not wellsuited to liquid phase hydrogenations. However the basic concept where a morestrongly bonding agent is added to the feedstream is one that can be developed forliquid phase systems. Ideally the system should achieve 100 % conversion of thealkyne with no conversion of the alkene. Typically alkene selectivity is high untilmost of the alkyne has reacted. This is usually considered to be due to the strongeradsorption of the alkyne inhibiting re-adsorption of the alkene, so that only whenthere is insufficient alkyne to maintain high coverage does alkene hydrogenationbegin in earnest.In this paper we will report on using a series of modifiers to enhance selectivityduring 1-phenyl-1-propyne over a Pd/alumina catalyst. The modifiers, transcinnamaldehyde,trans-cinnamonitrile, 3-phenylpropionitrile, and 3-phenylpropylamine, were chosen to have a functionality that potentially could adsorbmore strongly than an alkene and to be unreactive under the reaction conditions.Results and DiscussionThe reaction profile of 1-phenyl-1-propyne over Pd/alumina is shown in Figure 1. 1-Phenyl-1-propyne was hydrogenated to cis-β-methylstyrene and phenylpropane.Only trace levels of trans-β-methylstyrene were detected and further hydrogenation