2.4.4- Trimethyl-2-pentene, metathesis

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... 

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... 

In the presence of Re207/Al203 that has been pretreated with Cu(N03)2 to suppress side reactions, the cross-metathesis of 2,4,4-trimethyl-2-pentene with 4-vi-nylcyclohexene (Eq. 10) proceeds with >15% selectivity [29]. The product iso-butenyl cyclohexene can be converted to isobutylbenzene, which is the starting material for the synthesis of the over-the-counter analgesic, ibuprofen. 

---