Effect of molecular structure on the hydrogenation and isomerisation of propenylbenzene isomers

Abstract

The hydrogenation and isomerisation of allylbenzene (AB), trans-b-methyl styrene (TBMS) and cis-b-methyl styrene (CBMS), in the liquid phase, was investigated over a 2.5% Rh/silica catalyst. When reacted individually, the cis-isomer gave the fastest rate of hydrogenation followed by allylbenzene, with the trans-isomer having the slowest rate giving a ratio of rates of CBMS:AB:TBMS of 4.2 : 2.8 : 1. The isomerisation reaction followed thermodynamic control. When co-hydrogenated, allylbenzene inhibited the hydrogenation of both CBMS and TBMS but allowed isomerisation: the rate of allylbenzene hydrogenation was unaffected. This behaviour is in keeping with terminal alkenes hydrogenating at edge/corner sites while internal alkenes hydrogenate on terrace faces. The terminal alkene inhibits fast diffusion of hydrogen into the sub-surface and hence prevents hydrogenation of the CBMS and TBMS. The model of sub-surface hydrogen, observed in palladium catalysis, being key for hydrogenation but not isomerisation, was found to explain the results in rhodium hydrogenation catalysis.

Comparison of C═C Bond Hydrogenation in C-4 Unsaturated Nitriles over Pt/Alumina

“…This is in contrast to that found when cis-2pentenenitrile and trans-3-pentenenitrile were hydrogenated over Raney Nickel, 2 where the conjugated cis-2-pentenenitrile reacted faster than the nonconjugated trans-3-pentenenitrile to give the saturated nitrile. It is also in contrast with a study of conjugated and nonconjugated alkenyl benzenes 10 where the conjugated cis-isomer was the most reactive. Nevertheless, it is in agreement with the study by Delmon and co-workers 9 who found the nonconjugated allyl cyanide reacted faster than crotononitrile.…”

Copper-Catalyzed Regioselective Synthesis of (E)-β-Fluorovinyl Sulfones

“…This is somewhat different from what one would expect from this type of reaction. Generally speaking, the reaction rate of catalytic heterogeneous hydrogenation correlates directly to the stability of the olefin in question—which in turn usually means the less sterically hindered isomer should react faster [21]. In our case, this would suggest that the ( E )-β-fluorovinyl sulfones should react faster, which is not what we observed experimentally.…”

Isomerization of Allylbenzenes

“…Recently, Jackson and co-workers investigated the ability of a 2.5% rhodium on silica system, prepared from RhCl 3 ·H 2 O by way of the incipient-wetness method, to isomerize allylbenzene . In particular, their study focused on how the molecular structure of the catalyst affected hydrogenation and isomerization of the isomers allylbenzene, trans -β-methylstyrene and cis -β-methylstyrene, as well as mixtures of the three compounds.…”