Hydrogenation and isomerization of propenylbenzene isomers over Pt/alumina

The hydrogenation and isomerization of the three isomeric propenylbenzenes, allylbenzene (AB), trans-β-methylstyrene (TBMS) and cis-β-methylstyrene (CBMS), in the liquid phase, was investigated over a 1% Pt/alumina catalyst at 313 K and 1 barg. When reacted individually, the cis-isomer gave the fastest rate of hydrogenation followed by AB, with the trans-isomer having the slowest rate giving a ratio of rates of CBMS:AB:TBMS of 57:19:1. All isomers gave high selectivity to phenylpropane, with isomerization controlled by thermodynamic constraints. Competitive hydrogenation revealed that AB adsorbed on a different site from CBMS and TBMS and that AB inhibited hydrogenation of the other two isomers. In contrast, isomerization was unaffected. These results indicate that AB adsorption limits the supply of hydrogen to the other isomers such that reductive elimination of the half-hydrogenated alkyl species to the alkane is depressed while β-elimination of the alkyl to give an olefin is not affected.

Hydrogenation of alkynyl substituted aromatics over rhodium/silica

The cascade reactions of phenylacetylene to ethylcyclohexane and 1-phenyl-1-propyne to propylcyclohexane were studied individually, under deuterium and competitively at 343 K and 3 barg pressure over a Rh/silica catalyst. Both systems gave similar activation energies for alkyne hydrogenation (56 ± 4 kJ mol−1 for phenylacetylene and 50 ± 4 kJ mol−1 for 1-phenyl-1-propyne). Over fresh catalyst the order of reactivity was styrene > phenylacetylene ≫ ethylbenzene. Whereas with the cascade hydrogenation starting with phenylacetylene, styrene hydrogenated much slower phenylacetylene even once all the phenylacetylene was hydrogenated. The activity of ethylbenzene was also reduced in the cascade reaction and after styrene hydrogenation. These reductions in rate were likely due to carbon laydown from phenylacetylene and styrene. Similar behavior was observed with the 1-phenyl-1-propyne cascade. Deuterium experiments revealed similar positive KIEs for phenylacetylene (2.6) and 1-phenyl-1-propyne (2.1). Ethylbenzene hydrogenation/deuteration gave a KIE of 1.6 obtained after styrene hydrogenation in contrast to the inverse KIE of 0.4 found with ethylbenzene hydrogenation/deuteration over a fresh catalyst, indicating a change in rate determining step. Competitive hydrogenation between phenylacetylene and styrene reduced the rate of phenylacetylene hydrogenation but increased selectivity to ethylbenzene suggesting a change in the flux of sub-surface hydrogen. In the competitive reaction between 1-phenyl-1-propyne and propylbenzene, the rate of hydrogenation of 1-phenyl-1-propyne was increased and the rate of alkene isomerization was decreased, likely due to an increase in the hydrogen flux for hydrogenation and a decrease in the hydrogen species active in methylstyrene isomerization.

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

Organofluorine compounds are finding increasing application in a variety of fields such as pharmaceutical, agrochemical, and material sciences. However, given the scarcity of fluorine-containing natural products, advancement in this area depends almost entirely on the development of new synthetic methodologies. In this article, we present the synthesis of a series of previously undescribed (E)-β-fluorovinyl sulfones via a simple copper-catalyzed addition of hydrogen fluoride to alkynyl sulfone starting materials in varying yields and E/Z selectivities. The hydrogenation of these products was also explored and compared with the hydrogenation of the related Z isomers. These new products may find interesting applications, given the versatility of vinyl sulfones in chemical synthesis and the unique properties of vinyl fluorides in biological settings.