Isobutylene metathesis

On the other hand, the reaction of CpfLnR with propylene did not afford any polymers but rather an allyl complex, Cp Ln(f/3-allyl), via a cr-bond metathesis reaction [56,117]. One molecule of propylene can insert itself into the Lu-Me bond of CpfLuMe to give the corresponding isobutylene complex. The successive insertion of propylene is 1000-fold slower than the first insertion [57]. The gas-phase reaction of Sc(CH3)2 with propylene also produces a... 

Cyclopropanes in low yield were first noted in 1964 by Banks and Bailey (12) during the disproportionation of ethylene, but little significance was attached to that observation until recently, because such products had no obvious relevance to early mechanistic concepts based on pairwise rearrangements of bisolefin complexes. However, the subsequent adoption of carbenelike species as metathesis intermediates (4) provided a foundation for later development of cyclopropanation concepts. The notable results of Casey and Burkhardt (5) made an impact which seemed rather neatly to unify mechanistically the interconversion of cyclopropanes and metathesis olefins, although the reactions which they observed were stoichiometric rather than catalytic [see Eq. (4)]. Nevertheless, their work indicated a net redistribution of =CPh2 and =CH2 from (CO)5W=CPh2 and isobutylene, respectively, to form CH2=CPh2. Dissociation and transfer of CO yielded W(CO)6. Unfortunately, the fate of the isopropylidene moiety remained unknown. In 1976,... 

Pampus and co-workers (65) established the relative reactivity of a series of olefins to be 1-butene > 2-butene > isobutylene. This order of reactivity has been confirmed by others, and exactly parallels the reported order of stability of transition metal (Rh) complexes with these olefins (66), thus clearly implicating precomplexation of the olefin with the transition metal prior to metathesis. On a limited scale, Schrock observed a similar order of reactivity for olefins in reactions with (175-C5H5 )TaCl2[=CH(CH3 )3 ], which is known to possess a nucleophilic car-bene carbon (64). This complex also provides the requisite empty coordination site needed for precomplexation. In that study, cyclopropanes or metathesis olefins were not observed as products. 

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). 

A number of new processes exploiting metathesis have been developed by Phillips. A novel way to manufacture lubricating oils has been demonstrated.145 The basic reaction is self-metathesis of 1-octene or 1-decene to produce Ci4-C28 internal alkenes. The branched hydrocarbons formed after dimerization and hydrogenation may be utilized as lubricating oils. Metathetical cleavage of isobutylene with propylene or 2-butenes to isoamylenes has a potential in isoprene manufacture.136,146 High isoamylene yields can be achieved by further metathesis of C6+ byproducts with ethylene and propylene. Dehydrogenation to isoprene is already practiced in the transformation of isoamylenes of FCC C5 olefin cuts. 

Complex 4 can be regarded as titanium carbene Cp2Ti = CH2 coordinated by Me2AlCl. This is the first example of a well-defined metal carbene that catalyzes olefin metathesis and is re-isolated in high yield at the end of the reaction, Eq. (10). In a comparatively slow degenerative metathesis reaction (near equilibrium after 47 h at 51 °C). The 13C label of isobutylene was shown to grow into the methylene group of methylene cyclohexane [29]. 

Description Ethylene feedstream (plus recycle ethylene) and butenes feedstream (plus recycle butenes) are introduced into the fixed-bed, metathesis reactor. The catalyst promotes reaction of ethylene and 2-butene to form propylene and simultaneously isomerizes 1-butene to 2-butene. Effluent from the metathesis reactor is fractionated to yield high-purity, polymerization-grade propylene, as well as ethylene and butenes for recycle and small byproduct streams. Due to the unique nature of the catalyst system, the mixed C4 feed stream can contain a significant amount of isobutylene without impacting performance of the OCT process. A variation of OCT—Automet Technology—can be used to generate ethylene, propylene and the comonomer—hexene-1—by metathesis of n-butenes. 

Description Crude C4 streams are converted into propylene and an isobutylene-rich stream in three IFP process steps (1) butadiene and C4 acetylenes selective hydrogenation and butenes hydroisomerization, (2) isobutylene removal via distillation or MTBE production and (3) metathesis (Meta-4). 

Grabbs reported the use of (4a) in the synthesis of symmetrical trisubstituted olefins, wherein isomer selectivity is not an issue. Isolated yields using neat isobutylene or 2-methyl-2-butene as both reactant and solvent and a variety of functionalized metathesis partners were good to excellent under low catalyst loadings, providing a viable alternative to the Wittig reaction (equation 21). 

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 process is quite atom economical because the isobutylene is recycled to undergo dimerization to the starting mixture of trimethylpentene isomers, and the metathesis catalyst, typically a mixture of W03/silica and MgO, also promotes isomerization of the 1-pentene to the desired 2-isomer.44... 

The preparation of [Bu (H)Ga(/u,-NEt2)]2 was accomplished by the metathesis reaction between Bu Li with [Cl2Ga(/u,-NEt2)]2- Evidence suggests that two Bu groups are lost as isobutylene and result in Ga-H bond formation. The gallium hydride demonstrates high stability... 

Consider now the reactions of 1,1-disubstituted ethylenes. Since monosubstituted ethylenes react selectively to interchange their more highly substituted halves, 1,1-disubstituted ethylenes should react even more selectively in the same way [Eq. (22)]. This increased selectivity manifests itself in isobutylene s reaction with (Tol)2C=W(CO)5 yielding even less than the 0.06% of more highly substituted 1,1-ditolylethylene yielded by 1-pentene (23). But to demonstrate that metathesis catalysts will induce the reaction in Eq. (22) selectively is not easy, for the... 

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. 

Metathesis/Disproportionation (Cation exchange resin) tetramethyl ethylene from isobutylene... 

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