Under these conditions, ortho-, metaand para-alkyl substituted alkynes gave full conversion to E-stilbenes with excellent selectivity (Table , 1b-5b). In the case of 1-methyl-2-(phenylethynyl)benzene (3a)and 1-ethyl-4-((4hexylphenyl)ethynyl)benzene (4a) the catalyst loading had to be increased to 2 mol% to afford the desired Ealkenes in high isolated yields of 99 and 89%, respectively (Table , 3b and 4b). At 60 o C with a catalyst loading of 1 mol %, the yields of 3b and 4b were only 17 and 51 %, respectively. Moreover, F-, Cl-, and Br-substituted substrates were tolerated yielding corresponding E-products in high yields (Table , 6b-9b). Notably, the ester functionality remained unaltered under the given reaction conditions yielding desired product with 95% yield (Table ). The ketone substituted substrate 10a yielded E-1-(4-styrylphenyl)ethane-1-ol (10b) with 88% yield (Table ). Substrates bearing strong electron-withdrawing groups such as CF3 and COOMe underwent up to 5% over hydrogenation (Table , 9b and 11b). More challenging substrates containing electron-donating groups such as OEt and NMe2 were also successfully reduced to corresponding E-alkenes with 98 and 99% yields and excellent E-selectivity (Table , 12b and 13b). Furthermore, unprotected phenol-and aniline-substituted alkynes (14a, 15a) were obtained in 89 and 99% isolated yield (Table , 14b and 15b). In the case of 2-(phenylethynyl)pyridine (16a) a mixture E/Z isomers in a 34:66 ratio was detected. Prolonging the reaction time from 20 to 25 h resulted in an increased E/Z ratio of 78:22 and 16b was isolated in 69% yield (Table ). Other heterocycles containing sulphur or oxygen (17a and 18a) where readily converted into the corresponding Ealkenes in 96 and 99 % isolated yield (Table , 17b and 18b). Only traces of product were obtained in the presence of a NO2 group (Table , 19b). The practical applicability of the system was demonstrated upon up-scaling giving E-stilbene in >99% yield on a gram scale (Table ). In the next step, aryl-alkyl substituted alkynes were investigated. These substrates tend to be more challenging in selective semihydrogenation due to over reduction and isomerization. Under the given reaction conditions nonactivated aryl-alkyl alkynes bearing several alkyl group substituents afforded corresponding E-styrene derivatives in good to excellent yields (Table , 20b-24b). For instance, 20a gave the desired alkene with a E/Z ratio of 86:14 in 78% yield together with only 3 % over-hydrogenated product (Table ). In the case of the sterically hindered 1-tbutyl-2-phenylacetylene (25a) as well as the conjugated diyne 26a only traces of product were detected (Table , 25b, 26b). It has to be mentioned that the removal of alkyne impurities in the presence of olefins via alkyne semihydrogenation is of interest for industrial purposes since it benefits the subsequent polymerizations of some light hydrocarbon fractions from steam cracking. Due to the high selectivity of the introduced protocol, we investigated the potential applicability of this system for the purification of olefins. Accordingly, treating a mixture of 1a and 1c