Isomeric 2-octenes

Scheme 4.127 Regio- and stereoselective hydroformylation of isomeric 2-octenes (for the structure of Rh-cat. 1 compare Scheme 4.126).

Further novel observations are the hydroformylation of ethylene over graphite nanofiber-supported Rh catalysts,270 the transformation of a mixture of isomeric octenes to Cg-aldehydes,271 and the preparation of linear long-chain dialdehydes by the hydroformylation of linear a,co-dienes.272... 

TABLE 6. Reactivities of isomeric octenes relative to 1-octene in competitive ... 

Let us see if we can devise an acceptable mechanism for this dimerization. There are a great many isomeric octenes if our mechanism should lead us to just the two that are actually formed, this in itself would provide considerable support for the mechanism. 

The structural abbreviation Ts is used for p-toluenesulfonate Yield determined by gas chromatography Isomeric octenes identified as by-products... 

For simple alkenes there is a ca 4 kJ mol difference in the enthalpy of hydrogenation for their single internal double bond. See References 11 for the isomeric hexenes D. W. Rogers and K. Dejoongruang, J. Chem. Thermodyn., 20, 675 (1988) for isomeric heptenes and D. W. Rogers, K. Dejoongruang, S. D. Samuel, W. Fang and Y. Zhao, J. Chem. Thermodyn., 24, 561 (1992) for isomeric octenes. 

Water is not always split off in one direction only, and dehydration is often accompanied by isomerization, rearrangement, or even cleavage of the carbon chain. Thus heating 1-butanol with 85% phosphoric acid or 60% sulfuric acid leads to mixtures of 1- and 2-butene.1 In several cases the individual olefins can be isolated from the mixture of isomers by fractional distillation, as F. C. Whitmore and his colleagues found for the isomeric octenes.2 An isomerization that was actually desired occurs when cyclohexene or its homologs are heated over alumina at 470-480°, this affording alkylcyclopentenes.3... 

The hydroformylation of mixtures of Cg-olefins is a process with huge economic importance. A typical example is di-n-butene, consisting of isomeric -octenes, methylheptenes, and dimethylhexenes. The mixture is produced from Raffinate II, in which isomeric butenes are dimerized (e.g., by IFP Dimersol [47] or Octol process [48]). Hydroformylation of di- -butene produces linear and alkyl-branched Cg-aldehydes, which are converted to diisononyl phthalate (DINP), another additive for flexible PVC with immense industrial relevance. For this application, the use of terminal aldehydes is preferred. 

A process is disclosed for the preparation of a mixture of isomeric octenes by dimerizing w-butene in the SCF state with a NiO catalyst on a silica-alumina support. 

COT is prepared by the polymerization of ethyne at moderate temperature and pressure in the presence of nickel salts. The molecule is non-planar and behaves as a typical cyclic olefin, having no aromatic properties. It may be catalytically hydrogenated to cyclo-octene, but with Zn and dil. sulphuric acid gives 1,3,6-cyclooclairiene. It reacts with maleic anhydride to give an adduct, m.p. 166 C, derived from the isomeric structure bicyclo-4,2,0-octa-2,4,7-triene(I) ... 

In examination of various disubstituted cyclic olefins, the following decreasing isomerization order was adduced Pd Rh, Ru, Pi > Os > r 84). At 20% conversion of 1-octene to octane, the ratio of isomerization to hydrogenation in isopropanol for various unsupported metals was Pd (2.05), Rh (0.125), Ru (0.121. Pt (0.025), Ir (0.025), Os (0.009) 82). Palladium is used frequently when migration and isomerization are wanted platinum, when they are to be avoided (2J24). 

For this specific task, ionic liquids containing allcylaluminiums proved unsuitable, due to their strong isomerization activity [102]. Since, mechanistically, only the linkage of two 1-butene molecules can give rise to the formation of linear octenes, isomerization activity in the solvent inhibits the formation of the desired product. Therefore, slightly acidic chloroaluminate melts that would enable selective nickel catalysis without the addition of alkylaluminiums were developed [104]. It was found that an acidic chloroaluminate ionic liquid buffered with small amounts of weak organic bases provided a solvent that allowed a selective, biphasic reaction with [(H-COD)Ni(hfacac)]. 

Because of the difficulty in selling large amounts of C10 and Cl8+ olefins, these were isomerized to internal olefins followed by disproportionation. Here the important internal olefins for the synthesis of LAB are obtained (Fig. 5) [34]. Equations (4) and (5) illustrate the isomerization and disporportionation on 1-octene as an example. 

Hydrosilylation by Ziegler-type catalyst systems [e.g., Ni(acac)2/AlEt3] has been examined for the reaction of 1-octene with EtjSiH in benzene 178). Complications include competing isomerization and reduction to metal however, 1,3-dienes or terminal acetylenes are readily hydrosilylated withRC i CH, the major product is CH2 CR. CRiCHSiXj. 

A catalyst used for the u-regioselective hydroformylation of internal olefins has to combine a set of properties, which include high olefin isomerization activity, see reaction b in Scheme 1 outlined for 4-octene. Thus the olefin migratory insertion step into the rhodium hydride bond must be highly reversible, a feature which is undesired in the hydroformylation of 1-alkenes. Additionally, p-hydride elimination should be favoured over migratory insertion of carbon monoxide of the secondary alkyl rhodium, otherwise Ao-aldehydes are formed (reactions a, c). Then, the fast regioselective terminal hydroformylation of the 1-olefin present in a low equilibrium concentration only, will lead to enhanced formation of n-aldehyde (reaction d) as result of a dynamic kinetic control. 

Chart 1 Ligands used for isomerizing hydroformylation of internal octenes and 2-pentene. 

The ligands synthesized were also apphed to the isomerizing hydroformylation of more reactive 2-pentene. At 120 °C/ 20 bar quantitative conversion of olefin to aldehydes was achieved within 40 min. Trends similar to those described for internal octene hydroformylation were found. The regioselectivity obtained for the individual ligands tends to be 5% higher compared to that for the octenes. Thus, in the presence of 10 75% of n-hexanal were determined, compare Table 3. Obviously, 2-pentene is able to react more smoothly to the terminal isomer compared to olefins having the double bond in an more internal position. Illustrative for this effect are also literature results obtained for 2- and 4-octene.4,5... 

Disubstituted Alkenes. Simple 1,2-disubstituted alkenes such as 2-octene or cyclohexene, which produce only secondary aliphatic carbocation reaction intermediates, do not undergo reduction upon treatment with a Brpnsted acid and an organosilicon hydride. Even when extreme conditions are employed, only traces of reduction products are detected.192 203 207-210,214 An exception is the report that 4-methyl-2-pentene forms 2-methylpentane in 70% yield when heated to 50° for 20 hours with a mixture of Et3SiH/TFA containing a catalytic amount of sulfuric acid. It is believed that 4-methyl-2-pentene is isomerized to 2-methyl-2-pentene prior to reduction.203... 

Bicyclooctanes, isomerization paths of, 20 282 Bicycio (2.2.2)-2- octene, 20 269, 270 Bifimctional catalysts, ring closure, cyclization, 29 311-316 C5 cyclization, 29 311... 

Fig. 3. The thermal isomerization of 1,1-dicyclopropylethylene. (X) axis Time in minutes (Y) axis Percentage yield o 1,1-dicyclopropylethylene 0, 1-cyclopropyl-cyclopentene 0, bicyclo[3,3,0]octene-l.

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