Styrene compounds alkyne hydrogenation

Independent studies of the reduction of C=C and C=C bonds indicate that the latter is kinetically favored. Thus, in the absence of phenylacetylene, the rate of hydrogenation of styrene to ethylbenzene is about one order of magnitude faster than those for C=C bond reduction, indicating that the origin of the selectivity cannot be kinetic. The styryl compound represents a thermodynamic sink that causes virtually all the osmium present in solution to be tied up in this form, and therefore the kinetically unfavorable pathway becomes essentially the only one available in the presence of alkyne.31... 

Ene addition products have been isolated from reactions with various alkenes containing allylic hydrogen atoms compounds 49 and 50 are shown here as examples. Analogously, the reaction with a 1-alkyne furnishes the adduct 47 while styrene, in contrast, reacts to afford the [2 + 2] cycloaddition product 51. The latter mode of reaction, however, is no longer considered to be unusual since the tetraalkyldisilene 41 also forms [2 + 2] cycloadducts with various C=C double bond systems71-73. On the other hand, until very recently [2 + 2] cycloadditions of the tetraaryldisilene 9 were unknown. It has now been shown that 970, as well as 4171, can undergo cycloadditions with the C=C double bonds of styrene and 2-methylstyrene. [4 + 2] Cycloaddition reactions of disilenes with... 

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