Pentenes isomerisation

Above we have mentioned several heterogeneous applications such as the OCT process and SHOP. Neohexene (3,3-dimethyl-1-butene), an important intermediate in the synthesis of fine chemicals, is produced from the dimer of isobutene, which consists of a mixture of 2,4,4-trimethyl-2-pentene and 2,4,4-trimethyl- 1-pentene. Cross-metathesis of the former with ethene yields the desired product. The catalyst is a mixture of W03/Si02 for metathesis and MgO for isomerisation at 370 °C and 30 bar. The isobutene is recycled to the isobutene dimerisation unit [48],... 

Since the addition of methylene to an olefin should be exothermic, with the evolution of about 90 kcal/mole, isomerisations of the initially formed cyclo-propanes are very likely, since they only need about 64 kcal/mole. RRKM-studies demonstrate that this isomerisation should be faster than the rearrangement of cyclopropanes 32, 33 to the pentenes 34, 35 Numerous studies of the photochemical generation in the gas phase provided conclusive evidence in favour of these findings uo.iii.iis). 

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. 

Fluoride-induced rearrangements of terminal to internal isomers of perfluoro-alkenes are well established [70-72], for example the rapid isomerisation of perfluoro-l-pentene to the thermodynamically more stable isomer occurs in the presence of cesium fluoride (Scheme 33) [73], Allylic displacement reactions occur readily and with a stereochemistry suggesting steric control (Scheme 34) [74],... 

The reactions of linear or branched alkenes containing more than four carbon atoms reveal no types of reaction not already met, but their lower volatility permits their study as liquids or in solution as well as in the vapour phase. Thus for example the relative isomerisation rate r,7r/, of liquid 1-pentene over Pd/C at 290 K was independent of the conditions of agitation, showing there were no mass-transport effects attributable to the hydrocarbons. The presence of solvents was also without effect. ... 

Double-bond migration of terminal alkenes forms more of the Z-isomer than corresponds to thermodynamic equilibrium. A study of the exchange and isomerisation of 1-pentene-1,2- 2 on various types of nickel catalyst has shown that this is due to crowding at the active centre, so that the pentyl radical on losing a... 

---