Reactions of higher aliphatic olefins

The metal-catalysed hydrogenation of the higher olefins exhibit general features which are similar to those observed with the n-butenes. Thus, for example, the hydrogenation of hex-1-ene over Adams platinum catalyst [144] is accompanied by very low amounts of double-bond migration the relative rates of isomerisation and hydrogenation are in the ratio 0.03 1. Similarly, in the liquid phase hydrogenation of the n-pentenes over platinum—charcoal and iridium—charcoal [145], little or no isomerisation 

The rhodium—charcoal-catalysed hydrogenation of the n-pentenes at room temperature exhibits similar results to those found with platinum [145], Whilst at first sight this might appear to be unusual, the apparent lack of isomerisation can be readily accounted for by assuming that, as with the n-butenes [125], isomerisation only becomes the predominant reaction at relatively high temperatures, (E — Eh ) being positive. 

In the isomerisation of the tetra-substituted olefin 3,4-dimethylhex-3-ene over palladium—alumina [146], it has been shown that double bond migration is a necessary precursor to cis—trans isomerisation. This has been interpreted as showing that the mechanism involves a series of elementary steps, each of which is stereospecific, although no definite conclusions were drawn as to whether an addition—abstraction or an abstraction-addition mechanism was involved. 

Conclusive evidence for the participation of 7r-allylic intermediates in double bond migration has been obtained from a study of the nickel-catalysed hydrogenation of the isomeric olefinic esters methyl oleate and methyl elaidate using tritium as a tracer [147]. It was also concluded that in this system cis—trans isomerisation occurred by an addition—abstraction mechanism. 

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