Pentene, -2, metathesis

In this process, which has been jointly developed by Institute Francais du Petrole and Chinese Petroleum Corp., the C4 feed is mainly composed of 2-butene (1-butene does not favor this reaction but reacts differently with olefins, producing metathetic by-products). The reaction between 1-butene and 2-butene, for example, produces 2-pentene and propylene. The amount of 2-pentene depends on the ratio of 1-butene in the feedstock. 3-Hexene is also a by-product from the reaction of two butene molecules (ethylene is also formed during this reaction). The properties of the feed to metathesis are shown in Table 9-1. Table 9-2 illustrates the results from the metatheses reaction at two different conversions. The main by-product was 2-pentene. Olefins in the range of Ce-Cg and higher were present, but to a much lower extent than C5. 

The most thoroughly studied reactions are the metathesis of propene to ethene and 2-butene, and the metathesis of 2-pentene to 2-butene and 3-hexene. Generally, the thermodynamic equilibrium ratio of the trans and cis components of the products is obtained. The reacting alkene molecules need not be identical, two different alkenes react with each other in the same way. 

It has been shown that halogen-substituted alkenes can participate in the metathesis reaction, e.g. 5-bromo-l-pentene reacts with 2-pentene 11). A very interesting reaction is the conversion of methyl-9-octa-decenoate into 9-octadecene and dimethyl-9-octadecenedioate 12) ... 

Typical Examples of Catalyst Systems for the Homogeneous Metathesis of Pentene... 

Induction periods are also found in studies of homogeneous systems (St, 69, 88, 97f). For the system MoCMNO CsHsN -tiHsAlCh this has been demonstrated by Hughes (69), who observed that the mixture of catalyst components achieved maximum activity towards the metathesis of 2-pentene after a reaction time of about one hour. 

Product Distribution Obtained by Metathesis of an Equimolar Mixture of Cydopente.ne (CJ t) and 2-Pentene (CsHl0) in the Presence of... 

The reaction is called metathesis of alkenes. In the example shown above, 2-pentene (either cis, trans, or a cis-trans mixture) is converted to a mixture of 50%... 

Fig. 2 Quantification in the gas phase of 3,3-dimethylbutene and 4,4-dimethyl-2-pentene during propene metathesis (500 equiv.) catalyzed with 13(1 equiv.) at 25 °C...

The preponderance of stereochemical data in the literature has been obtained from studies using 2-pentene, which now appears to have been a rather poor substrate for emphasizing steric aspects of the reaction. Recent experiments utilizing 4-methyl-2-pentene (76) have given much clearer indications of steric control in metathesis reactions (vide infra). 

Soluble metathesis catalysts yield trans products in reactions with // / v-2-pentene, but generally are not very stereospecific with c/.v-2-pen-tene. In the latter case, the initially formed butenes and hexenes are typically about 60 and 50% cis, respectively. Basset noted (19) that widely diverse catalyst systems behaved similarily, and so it was suggested that the ligand composition about the transition metal was not a significant factor in the steric course of these reactions. Subsequently, various schemes to portray the stereochemistry have been proposed which deal only with interactions involving alkyl substituents on the reacting olefin or on the presumed metallocyclobutane intermediate. 

Fortunately, steric control arising from interactions of alkyl moieties derived from reacting olefins can be enhanced and observed by selection of appropriate reactants. This effect was demonstrated in the work of Lawrence and Ofstead (76), who studied the metathesis of 4-methyl-2-pentene induced by a WCl6Et2OBu4Sn catalyst. This catalyst is not particularly unique, for the steric course of the metathesis of m-2-pen-tene with this system was found to be essentially equivalent to that previously observed (18) with a conventional catalyst employing an or-ganoaluminum cocatalyst. 

Compositions of metathesis reaction mixtures obtained over a range of conversions starting with cis- and trans-4-methyl-2-pentene are shown in Figs. 1 and 2, respectively. Certain important differences are evident in comparisons with the course of reactions of 2-pentene isomers (18). 

Fig. 1. The metathesis of m-4-methyl-2-pentene. Effect of conversion on products structure (76).

A second observation was the fact that isomerization of the starting asymmetric olefin was much faster than the formation of new symmetric olefins. In fact, 40% of the initial cis olefin (Fig. 1) had isomerized to trans after only 4% conversion to new olefins. This result formally parallels the highly selective regenerative metathesis of a-olefins (60, 61), except that steric factors now prevail, because electronic effects should be minimal. Finally, the composition of the initially formed butene from r/j-4-methyl-2-pentene was essentially identical to that obtained when cA-2-pentene was used (18). When tra .v-4-methyl-2-pentene was metath-esized (Fig. 2), the composition of the initially formed butenes indicated a rather high trans specificity. 

From a consideration of data provided in studies limited to the metathesis of 2-pentene, several views of the stereochemistry have been recently advanced. For the most part, they deal only with steric influences caused by alkyl groups coming from the reacting olefin. 

Structure A leads to cis products, structure B to trans products. Neither structure was favored in Basset s scheme, and this was consistent with typically observed nonstereospecific metathesis of cw-2-pentene. Structure C favors formation of trans products, as is widely observed. 

Casey and co-workers visualized the stereochemistry of 2-pentene metathesis in terms of the relative stabilities of the various substituted metallocycle derivatives (14). As in Katz s scheme, a puckered ring was assumed, but Casey emphasized only that reaction pathways were favored which provided conformations possessing the fewest axial substit-... 

With these guidelines, the results from the metathesis of 4-methyl-2-pentene now appear to be reasonably accountable for. Most importantly, the lack of formation of c7s-2,5-dimethyl-3-hexene indicates that cis- 1,2-diisopropyl-substitution on the ring is highly unfavored, and trans-1,2-substitution leading to adjacent equatorial substituents is acceptable ... 

The rapid isomerization of c/s-4-methyl-2-pentene relative to productive metathesis suggests further information regarding the isomerization process. If regenerative metathesis proceded selectively via an isopropyl carbene. and assuming that the isopropyl groups maintained equatorial... 

The moderate specificity for forming m-2-butene initially (see Fig. 1) is again consistent with equatorial orientation of isopropyl the rather low cis specificity indicates only a moderate preference for equatorial orientation of the a-methyl, probably because of the offsetting weak repulsions caused by cis- 1,2-dimethyl-substitution. This effect is absent in the metathesis of tra i-4-methyl-2-pentene, and trans specificity for... 

It would be tempting to apply the same rationale to the metathesis of 2-pentene isomers, but clearly, the steric requirements of methyl and ethyl are much less than that of isopropyl, and trade-offs involving cis-1,2-disubstitution vs. axial orientation are not clear neither is the important role of catalyst ligand influence. 

Figure 16.4. Metal alkylidene mechanism for cyclopentene/2-pentene metathesis...

The final stereochemistry of a metathesis reaction is controlled by the thermodynamics, as the reaction will continue as long as the catalyst is active and eventually equilibrium will be reached. For 1,2-substituted alkenes this means that there is a preference for the trans isomer the thermodynamic equilibrium at room temperature for cis and trans 2-butene leads to a ratio 1 3. For an RCM reaction in which small rings are made, clearly the result will be a cis product, but for cross metathesis, RCM for large rings, ROMP and ADMET both cis and trans double bonds can be made. The stereochemistry of the initially formed product is determined by the permanent ligands on the metal catalyst and the interactions between the substituents at the three carbon atoms in the metallacyclic intermediate. Cis reactants tend to produce more cis products and trans reactants tend to give relatively more trans products this is especially pronounced when one bulky substituent is present as in cis and trans 4-methyl-2-pentene [35], Since the transition states will resemble the metallacyclobutane intermediates we can use the interactions in the latter to explain these results. 

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],... 

Scheme 141 Electrogenerated metathesis catalyst and reaction with 2-pentene.

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. 

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). 

Substituted malonates and (5-keto-esters have also been successfully used as pronucleophiles (Scheme 9.11) [25a, 36]. From P-ketoesters, approximately 1 1 mixtures of epimers are generally formed. Products derived from 2-alkenylmalo-nates have been subjected to Ru-catalyzed ring-closing metathesis to give cyclo-pentene derivatives in good yield [25a, 36]. With the ester-amide displayed in Scheme 9.11 as pronucleophile, 1 1 mixtures of epimers were also formed [44]. 

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