Metathesis neohexene

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],... 

Further important industrial applications of olefin metathesis include the synthesis of 3,3-dimethyl-l-butene ( neohexene , intermediate for the production of musk perfume) from ethene and 2,4,4-trimethyl-2-pentene, the manufacture of a,co-dienes from ethene and cycloalkenes (reversed RCM), and the ROMP of cyclooctene and norbomene to Vestenamer and Norsorex , respectively. 

In the Phillips neohexene process147 2,4,4-trimethyl-2-pentene (8) is converted by cleavage with ethylene to neohexene (9) used in the production of a perfume musk. The starting material is commercial diisobutylene. Since it is a mixture of positional isomers (2,4,4-trimethyl-2-pentene and 2,4,4-trimethyl-l-pentene) and the latter (7) participates in degenerative metathesis, effective utilization of the process requires the isomerization of 7 into 8. A bifunctional catalyst system consisting of an isomerization catalyst (MgO) and a heterogeneous metathesis catalyst is employed 131... 

Dimethylbut-l-ene (neohexene), which is inactive to self-metathesis, undergoes cross-metathesis with internal alkenes to high conversion when catalysed by WCl6/Et20/Bu4Sn153. 

Cross metathesis of ethylene with internal alkenes provides a facile route to terminal alkenes. A number of processes have been described that use this transformation however, the only products, besides neohexene,that appear to be important are the a,o>-dienes that result from metathesis of cyclic alkenes with an excess of eAylene. This family of compounds should find a wide variety of applications. 

A metathesis similar to OCT came on stream about the same time as the SHOP process. Ethenolysis43 of a mixture of 2,4,4-trimethyl-2-pentene and 2,4,4-trimethyl-1-pentene yields 3,3-dimethyl-l-butene (commonly called neohexene) plus isobutylene (equation 11.15). 

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... 

Metathesis of 3,3-dimethylbut-l-ene (neohexene) has not been reported (see Warwel 1983a), although degenerate cross-metathesis with 3-methylbut-l-ene has been observed (Howard 1980a). 

In the commercial manipulation of hydrocarbon feedstocks, the metathesis reaction clearly provides a valuable unit process, which may be combined with other processes either in the same reactor using successive layers of the requisite catalysts, or in consecutive reactors. The neohexene and SHOP processes have already been discussed and three other examples are shown in Scheme 17.1. 

The elucidation of the role of double-bond isomerization activity in metathesis process is an example of the helpfulness of the four-center mechanism. As the scheme predicted, in certain applications the elimination, of double-bond isomerization activity (acidic isomerization sites were destroyed by various mild caustic treatments) prevented secondary metathesis reaction resulting in very high selectivity to specific products ( 5). In contrast, in other applications (e.g., linear olefin and neohexene processes) to obtain a high level of productive metathesis, the mechanistic scheme indicated a need for enhanced isomerization activity this was accomplished by addition of a very selective double-bond isomerization catalyst to the scheme ( ),... 

The latest (1980) commercial application of olefin metathesis is Phillips Neohexene Process ( ). Neohexene, an intermediate in the synthesis of a perfume musk, is produced by cross-metathesis of diisobutylene with ethylene (i,e., ethylene cleavage) over a bifunctional (double-bond isomerization/metathesis) catalyst system (Figure 7) ... 

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]. 

From a practical point of view, commercial diisobutene is a mixture of 2,4,4-trimethyl pent-2-ene, used in the cross-metathesis reaction, and of 2,4,4-trimethyl pent-l-ene (not modified by the metathesis reaction), in a relatively large amount (20 to 25%). In order to valorize the process, it is necessary to use commercial diisobutene and so to convert the terminal non useful olefin into the internal useful one. This can be done by mixing the metathesis catalyst (W03/Si02) with an isomerization catalyst (typically magnesia) together in the reactor. Typically, with a 1 3 mixture of W03/Si02 and MgO, at 370°C, under a 30 bar pressure and with a ethylene to diisobutene ratio of 2. A conversion of 65% of diisobutene is achieved with a neohexene selectivity around 85%. 

It is still unclear how the initiation step in alkene metathesis occurs and how the initial carbene forms. Commercial applications of metathesis include the triolefin process, in which propylene is converted to ethylene and butene, the neohexene process, in which the dimer of isobutylene, Me3CCH=CMe2, is metathesized with ethylene to give Me3CCH=CH2, an intermediate in the manufacture of synthetic musk, and a 1,5-hexadiene synthesis from 1,5-cy-clooctadiene and ethylene. Two other applications, SHOP and ROMP (Shell higher olefins process and ring-opening metathesis polymerization), are discussed in the next section. 

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. 

W03/Si02 has a high potential for practical applications in metathesis. The high-operation temperatures make it less susceptible to trace quantities of catalyst poisons such as air and water and to coke formation. Recently, this catalyst has extensively been studied in relation to its industrial use for the metathesis of long-chain olefins, such as oct-l-ene and industrial hept-l-ene [70], The optimum reaction temperature of an 8 wt% W03/Si02 catalyst was found to be 733 K, The optimum temperature for regeneration in a flow of air is 823 K, A W03/Si02 catalyst is presently used in the OCT process and also in Phillips s neohexene process [2],... 

Neohexene musk scent for detergent perfume industry is synthesized by metathesis from ethylene and, , -trimethyl- entene. 

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