Octenes isomerisation

The hydrogenation activity of the isolated hydrides 3 and 6 towards cyclooctene or 1-octene was much lower than the Wilkinson s complex, [RhCKPPhj) ], under the same conditions [2] furthermore, isomerisation of the terminal to internal alkenes competed with the hydrogenation reaction. The reduced activity may be related to the high stability of the Rh(III) hydrides, while displacement of a coordinated NHC by alkene may lead to decomposition and Rh metal formation. 

Llewellyn, Green, and Cowley isolated the Co-H complex [CoH(CO)3(IMes)] 26, a relatively stable complex under inert conditions [31], The authors examined the hydroformylation activity of 1-octene with Co-hydride complex 26. With 8 atm of syngas (H /CO) at 50°C for 17 h and 1 mol% 26, the conversion to aldehyde products was 47% with a l b of 0.78. However, 83% of the product was the internal aldehyde 2-methyl-octanal, indicating isomerisation competed with hydroformylation and the rate of isomerisation occurred faster than hydroformylation. 

The method of catalyst immobilisation appeared to affect its performance in catalysis. Catalyst obtained by method II showed a low selectivity in the hydroformylation of 1-octene (l b aldehyde ratio was even lower than 2) at a very high rate and high yields of isomerised alkenes (Table 3.2, entry 2), whereas procedure IV resulted in a catalyst that was highly selective for the linear aldehyde (with a l b ratio of 37) (entry 5). In accordance with examples from literature it is likely that procedure II gave rise to the ionic bonding of ligand-free rhodium cations on the slightly acidic silica surface [29],... 

Preliminary mechanistic studies on the methoxycarbonylation of 1-octene showed that two pathways to methyl nonanoate occur, one involving the direct carbonylation of 1-octene to the linear ester, the other the alkene isomerisation in competition with the first one. Subsequently, the linear product forms by tandem isomerisation of the internal alkenes, with the terminal alkyl intermediate being trapped by migration to CO at a higher rate than any branched alkyl species. This has been confirmed by the analysis of products... 

Metathesis of 1-octene leads cleanly to ethene and 7-tetradecene, but as the reaction proceeds also 2-octene is formed and metathesis products derived from the isomerisation reaction. It was found that after prolonged reaction times decomposition of the ruthenium alkylidene catalyst occurs. At least eight different products were formed and several of them have been identified [37], Figure 16.22 shows the identified compounds derived from Grubbs 1st generation catalyst (the 2nd generation gives basically the same result [38]). 

A patented process for the production of green notes applying bakers yeast for in situ reduction of enzymatically produced aldehydes [67, 68] has been called into question regarding the effective production of (Z)-3-hexenol. According to Gatfield s report [69] the isomerisation of (Z)-3-hexenol to (E)-2-hexenal is a very fast process. The latter undergoes facile conversion to hexanol. Beside this, baker s yeast can add activated acetaldehyde to ( )-2-hexenal, forming 4-octen-2,3-diol. 

Scheme 13.6 Isomerisation of 1-octen-3-ol into 3-octanone by using copper supported on alumina.

Comparison with other molecular sieves (Fig. 11) shows that the yields obtained with FER are very high indeed. As elaborated upon elsewhere [6-8], we have proposed that the isomerisation involves a bi-molecular mechanism in which e.g. di-methylhexene isomers crack selectively to isobutene and n-butene (Fig. 12). The mono-molecular mechanism requires the energetically unfavourable primary carbenium ions. Molecular modelling [7] has provided support for this mechanism in that the branched octenes can be formed in the intra-crystalline voids of FER but their diffusion out of the pores is hindered. 

We must remember that branched alkenes may contain centres of optical activity an example is (—)3,7-dimethyl-l-octene, which when hydrogenated on platinum shows little racemlsation, but on palladium this happens extensively, to an extent depending upon the form of catalyst and reaction conditions. - Isomerisation of the double bond to the 2-position negates the optical activity, so that it may return to the terminal position in either the (-1-)- or (—) form. When tetra-substituted alkenes of the type RR C=CCRR are hydrogenated, two centres of optical activity are created the fi-form gives the meso product, while the Z-form gives a racemic mixture. ... 

Octadiene on a palladium catalyst was hydrogenated at 323 K to a mixture of octenes and isomerised to other octadienes, but 5tot was somewhat low. °... 

In accordance with this assumption, a variety of alkali-metal zeolites (Li, Na, K, Rb, Cs) are shown to be active and selective (50). Addition of alkali metals suppressed by-product formation, possibly by a more effective neutralization of residual Bronsted sites. The NiX merits, when it comes to dimerization, have also been observed in the case of butene where octene was the major product. Yet isomerisation of butenes appeared to be much more rapid than its dimerization (51). 

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