Isobutenes metathesis

Because one might expect steric hindrance to be important, it is worth mentioning that the metathesis of alkenes branched at the double bond has been reported. Thus, isobutene gives (small) quantities of 2,3-dimethy 1-2-butene and ethene (16, 17) ... 

Calculated Equilibrium Distributions for the Metathesis of Isobutene at Different... 

Tebbe and co-workers reported the first olefin metatheses between titanocene-methyli-dene and simple terminal olefins [13]. They showed cross-metathesis between isotopically labeled isobutene and methylenecyclohexane to be catalyzed by titanocene-methylidene. This process is referred to as degenerate olefm metathesis as it does not yield any new olefin (Scheme 14.6). The intermediate titanacyclobutane has been isolated and characterized [14], and its stability [15] and mechanism of rearrangement [16] have been investigated. 

Although the reaction of a titanium carbene complex with an olefin generally affords the olefin metathesis product, in certain cases the intermediate titanacyclobutane may decompose through reductive elimination to give a cyclopropane. A small amount of the cyclopropane derivative is produced by the reaction of titanocene-methylidene with isobutene or ethene in the presence of triethylamine or THF [8], In order to accelerate the reductive elimination from titanacyclobutane to form the cyclopropane, oxidation with iodine is required (Scheme 14.21) [36], The stereochemistry obtained indicates that this reaction proceeds through the formation of y-iodoalkyltitanium species 46 and 47. A subsequent intramolecular SN2 reaction produces the cyclopropane. 

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],... 

The isomerization of light olefins is usually carried out to convert -butenes to isobutylene [12] with the most frequently studied zeolite for this operation being PER [30]. Lyondell s IsomPlus process uses a PER catalyst to convert -butenes to isobutylene or n-pentenes to isopentene [31]. Processes such as this were in larger demand to generate isobutene before the phaseout of MTBE as a gasoline additive. Since the phaseout, these processes often perform the reverse reaction to convert isobutene to n-butenes which are then used as a metathesis feed [32]. As doublebond isomerization is much easier than skeletal isomerization, most of the catalysts below are at equilibrium ratios of the n-olefins as the skeletal isomerization begins (Table 12.5). 

Despite the high selectivity of the reaction, the formation of by-products must be thoroughly monitored since severe specifications are usually imposed on the C4 raffinate (the C4 stream leaving the plant after the isobutene elimination) used later on as a feedstock to alkylation, metathesis or 1-butene extraction (where oxygenates act as poisons). 

The 2,4,4-trimethyl-l-pentene is not wasted because a dual catalyst can be used to ensure that it is converted into 2,4,4-trimethyl-2-pentene as this gets used up by ethenolysis. With a 1 3 catalyst mixture of a W03/Si02 metathesis catalyst and a MgO isomerization catalyst (at 370 °C and 30 bar ethene pressure), an average of 65-70 % conversion of the diisobutene can be achieved with approximately 85 % selectivity for neohexene. The coproduct isobutene can be recycled to an isobutene dimerization reactor. Neohexene is used to make the class of synthetic... 

It may be calculated from thermodynamic data that the metathesis of isobutene in a closed system at 25°C can yield only 1.4% each of ethene and 2,3-dimethylbut-2-ene reaction (22) (Mol 1975). At 200°C the estimated equilibrium conversion is still less than 6% of each product. It is clear that to drive reaction (22) from left to right it is necessary to remove the ethene as fast as it is formed. The reaction is catalyzed by Me4Sn-treated MoO t/Ti02 (x = 2.3-2.9) at 20°C (TOP =... 

Various industrial companies have developed a metathesis process for the production of 2-methylbut-2-ene (isoamylene), which can be used to make isoprene via oxidative dehydrogenation. It is produced via cross-metathesis of isobutene and but-2-ene reaction (8). Cross-metathesis can also be carried out with propene instead of but-2-ene, or a mixture of the two. 

Myrcene is also accessible from isoprene, which derives from the steamcracker s C5-cracking fraction by dehydrogenation. In contrast, construction of its carhon skeleton, for example from acetylene and acetone or hy metathesis of isobutene and hut-2-ene, are declining in importance. [125] The sodium-catalysed dimerisation in the presence of diisopropylamine leads to myrcene. 

At the beginning of the sixties, laboratory studies showed that diisobutene (2,4,4-trimethyl pent-2-ene) could be cleaved with ethylene over a classical metathesis catalyst to produce neohexene and isobutene [13 -15]. 

Various industrial companies have developed a process for the production of isoamylene (2-methyl but-2-ene), which is a precursor of isoprene (obtained by oxidative dehydrogenation). It can be produced by cross-metathesis of isobutene with but-2-ene or propene ... 

Cyclization of Linalool 97 to yield 2-methylcyclopenten-2-ol has also been achieved under solvent-free conditions (with GH2), yielding isobutene as a by-product (Scheme 2.40). Both metathesis products were then converted to useful fuel compounds. 

Hie neohexene process starts with the acid-catalyzed dimerization of isobutene, followed by metathesis with ediylene, to give neohexene, an intermediate in the manufacture of synthetic musk, and regenerate isobutene. 

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