Catalysis of Organic..

244 1-Phenyl-1-Propynealkene. When there is little or no re-adsorption, the production of phenylpropane isinhibited. At 55 % conversion, 1PP hydrogenation modified with TCA producesonly 13 % of the amount of phenyl-propane that is produced in the absence of themodifier. At 100 % conversion of 1PP a similar reduction is seen when TCN is themodifier. This inhibition of alkene re-adsorption is also observable in the cis:transratio of methylstyrene. The high cis:trans values confirm that cis-β-methylstyrene isthe primary product with only around 0.1 % of the alkene produced being trans-βmethyl-styrene.When re-adsorption and isomerisation occur the cis:trans valuedrops to 19, reflecting the thermodynamic drive to produce the trans isomer.Therefore the unsaturated aldehyde, the unsaturated nitrile, and the saturatedamine all bond more strongly than methylstyrene to the palladium surface. Thesaturated amine has adsorption energetics that are close to those of methylstyreneshowing only a limited effect. The saturated nitrile however enhances not just thealkyne hydrogenation but also the alkene hydrogenation. This suggests that thesaturated nitrile affects the rate determining hydrogen transfer aspect of thehydrogenation. Rate enhancement effects have been previously observed incompetitive reaction systems containing alkynes and alkenes (4, 5) and a similarconclusion drawn. The exact mode of operation is however unclear and requiresfurther investigation.Experimental SectionThe catalyst used throughout this study was a 1% w/w palladium on aluminasupplied by Johnson Matthey. The support consisted of θ-alumina trilobes (S.A.~100 m 2 g -1 ) and the catalyst was sized to 99 %) were used without further purification.No significant impurities were detected by GC. The gases (BOC, >99.99 %) wereused as received.The reaction was carried out in a 0.5 L Buchi stirred autoclave equipped with anoil jacket and a hydrogen-on-demand delivery system. 0.05 g of catalyst was addedto 278 mL of degassed solvent, propan-2-ol. Reduction of the catalyst wasperformed in situ by sparging the solution with H 2 (300 cm 3 min -1 ) for 30 minutes at343 K at a stirring speed of 300 rpm. After reduction, the autoclave was adjusted to313 K under a nitrogen atmosphere and 1 mL of modifier was added in 10 mL ofdegassed propan-2-ol. The system was pressurized to 3 bar hydrogen and the stirrerset to 1000 rpm. After 1 h, 1 mL of 1-phenyl-1-propyne was added to the reactor in10 mL of degassed solvent. Samples were taken at defined time intervals andanalyzed by GC. Specific reactions were repeated at different stirrer speeds andequivalent rates and selectivities were observed indicating and absence of masstransfer within the system.