The hydrogenation of cyclic molecules

The yields of cis-dialkylcycloalkanes observed in the hydrogenation of various cyclo- 

Most of the studies to date have employed either palladium [222—229] or platinum [220,224,226,228—235], commonly as Adams reduced platinum oxide, although nickel [228,236,237], rhodium [238,239], ruthenium [239], iridium [239], iron [237] and tungsten [237] have also been used. Many of these studies have been concerned with the stereochemistry of the hydrogenation of disubstituted cycloalkenes. Table 32 shows some typical results for the platinum- and palladium-catalysed hydrogenation of disubstituted cyclohexenes. Table 33 shows comparative results for the hydrogenation of 1,4-dialkylcyclohexenes over palladium, platinum and rhodium catalysts. 

From these results it can be seen that in the hydrogenation of 1,2-dialkylcyclohexenes the expected cis-1,2-dialkylcyclohexane is not the sole product. Similarly, in the hydrogenation of the 1,4-dialky lcyclohexenes where both the cis- and trans-cyclohexanes are expected, the trans-isomer being the thermodynamically more stable, the stereoselectivity varies from metal to metal. Thus with palladium, the cis/trans ratio approaches the equilibrium composition, whereas with platinum and rhodium, the equilibrium composition is never approached. It is also instructive to note that in the palladium-catalysed reactions, hydrogenation is accompanied by extensive alkene isomerisation [220—223], whereas with rhodium and platinum, little or no isomerisation is observed [220, 

Yields of cis-isomer in the hydrogenation of 1,4-dialkylcyclohexenes over rhodium, palladium and platinum catalysts at atmospheric pressure and 25°C in acetic acid solution 

Nickel has been reported to show behaviour similar to platinum [236], Further, in the reactions of cycloalkenes with deuterium, the product cycloalkanes are much more extensively exchanged over palladium than over nickel or platinum [236]. Such behaviour is not unexpected by comparison with the results obtained in the hydrogenation of linear alkenes (Sect. 3, p. 25). 

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