Isomerization of but-l-ene

Bell et al. (81) presented forced diffusion calculations of butene isomers in the zeolite DAF-1. DAF-1 (82) is a MeALPO comprising two different channel systems, both bounded by 12-rings. The first of these is unidimensional with periodic supercages, while the other is three-dimensional and linked by double 10-rings. The two channel systems are linked together by small 8-ring pores. It is a particularly useful catalyst for the isomerization of but-l-ene to isobutylene (S3) its activity and selectivity are greater than those of ferrierite, theta-1, or ZSM-5. 

For all reactions, CePd3 was the most active catalyst, and at the same time the least selective. For the isomerization of but-l-ene into but-2-ene, as compared to the formation of butane, the order of selectivity was... 

The isomerization of but-l-ene to but-2-enes on WO3 supported on titania has been shown to occur readily and proceed via a carbenium ion intermediate. The activity of this reaction has been shown to be related directly to the density of acid sites with materials having low acidity (WO3 supported on silica and WO3 supported on magnesium oxide) being inactive. 

Alkenes.— The isomerization of but-l-ene to cis- and tra j-but-2-ene by [NiLi] (L=tertiary phosphine) in acid solution exhibits a rate dependence on the nature of L, is preceded in many cases by an induction period, and is drastically retarded by... 

Many other compounds have been shown to act as co-catalysts in various systems, and their activity is interpreted by analogous reactions [30-33]. However, the confidence with which one previously generalised this simple picture has been shaken by some extremely important papers from Eastham s group [34], These authors have studied the isomerization of cis- and Zraws-but-2-ene and of but-l-ene and the polymerization of propene and of the butenes by boron fluoride with either methanol or acetic acid as cocatalyst. Their complicated kinetic results indicate that more than one complex may be involved in the reaction mechanism, and the authors have discussed the implications of their findings in some detail. 

Although at sufficiently low temperatures all the individual linear alkenes preserve their identity as chemisorbed species, one of the clear-cut conclusions from work to date is that—even for the relatively unreactive Pt surface—at room temperature the finely divided metals cause virtually complete isomerization between but-l-ene and the cis- and /ra .v-but-2-enes. The extent to which this can even occur on the close-packed Pt( 111) face at lower coverages at 300 K is a current question. Attempts to identify intermediates in the isomerization process, possibly of an allylic nature, would be a matter of priority on single crystals or finely divided metals. 

Scheme 1 Reactions of but-l-ene on MoS. 1, isotopic exchange via n-butenyl (la) and s-butenyl (lb) intermediates on MH sites refs. 27, 31, 3S) 2, isomerization on MH sites refs. 28, 29, 31) 3, hydrogenation on MH sites refs. 26, 27)...

The reaction of 3,3-disubstituted cyclopropenes with mono- and 1,2-disubstituted alkenes proceeds only with difficulty and leads to low yields of cyclopropanes. In the case of but-l-ene, an 8% yield, with hex-1-ene and hept-l-ene between 5 and 10% yield, and with cyclooctene about 10% of the cyclopropane product is formed. In these cases, the major product is the formal dimer of the intermediate ethenylcarbene complex, i.e. the corresponding (fj-hexatriene. When copper(I) chloride is used as catalyst rather than the copper halide/phosphane or phosphite system, about half the yield of the [2-f-1] cycloadduct is obtained along with an increased amount of the hexatriene. Mechanistically, these acyclic trienes could also be formed from an (alk-l-enyl)bicyclo[1.1.0]butane intermediate without any carbene being involved. Bicyclo[1.1.0]butanes are low yield (< 20%) byproducts of the thermal dimerization reaction of methyl 3,3-dimethylcyclopropenecarboxylate (1). On the other hand, bicyclo[l. 1. Ojbutanes, such as 3, are known to undergo isomerization to form 1,3-dienes. ... 

The metathesis of propene on WOs/Si02 is speeded up by pretreatment of the catalyst with HCl (Pennella 1974 Aliev 1977) but the product but-2-ene undergoes considerable isomerization to but-l-ene (Aliev 1978). The inclusion of 1% cycloocta-1,5-diene (COD) in the propene stream also increases the rate of metathesis and reduces the break-in time from 20 min to less than 5 min at 500°C. The latter effect disappears when the additive is removed, so it is not due to reduction of the catalyst. The effects of both HCl and COD have been attributed to a favourable modification in the metal d-orbital levels as the result of the presence of new ligands (Pennella 1973, 1974). Pretreatment with hexamethyldisilazane (HMDS) at 250°C also has a remarkable effect on the activity, increasing it as much as 140 times for the metathesis of propene at 427°C. The same treatment of silica alone completely eliminates its capacity to isomerize rra/j5-but-2-ene at 427°C, so it is concluded that the Bronsted acidic hydroxyl groups, poisoned by HMDS, are not likely to be the precursors for the active sites in propene metathesis over W03/Si02 (van Roosmalen 1980a, 1982). 

On Mo03/) -Al203 the cis/trans isomeratization of but-2-ene follows second-order kinetics, but is accompanied by isomerization to but-l-ene and secondary cross-metathesis reactions (Davie 1972a Engelhard 1982e). 

Reactions with the lower olefins are described in the preceding sections. The crossmetathesis of but-l-ene with cis- or trans-hut-2-ene on Re207/Al203 at 30-40°C is accompanied by cis/trans isomerization of the but-2-ene, but not by self-metathesis... 

Figure 4 Variation of rate of but-l-ene isomerization with mercury coverage, dug at o, 0°C , 21 °C , 69°C on a Rh-SiO catalyst (Reproduced by permission from / Catal., 1972, 26, 229)...

Cis-trans isomerism occurs when the arrangement of the bonds prevents free internal rotation about bonds within the molecule. Such restricted rotation occurs, for example, in alkenes and many cyclic compounds. Consider the structures of but-l-ene and but-Z-ene (Figure 20.48). 

The allylic resonance may give rise to formation of a mixture of isomeric allylic bromides, e.g. 6 and 8 from but-l-ene. The product ratio depends on the relative stability of the two possible allylic radical species 5 and 7 ... 

This means that the isopropyl group stabilises the secondary ion sufficiently (compared to but-l-ene) for hydride transfer reactions to be suppressed, at least at low temperature. At about -120 °C a high polymer of structure (IV) is formed. This is a true phantom polymer, since there exists no corresponding monomer. Evidently, at the very low temperature the propagation reaction which would lead to structure (III) becomes much slower than the isomerization reaction ... 

Chloroheptafluorobut-l-ene (3) isomcrizes in the presence of antimony(V) fluoride to give an E Z mixture of but-2-ene 4.8 4-Chloroheptafluorobut-l-ene also gives the isomeric but-2-cne (see also Section 5.1.2.2.). 

Researchers at Daikin Kogyo have reported that perfluorobut-2-ene (2a) can be obtained in 98% yield by passing but-l-ene la over alumina at 250 C.21,22 The isomerization of per-fluoro(3-phenylprop-l-ene) (Id) to 2d has also been achieved quantitatively with the DuPont catalyst aluminum chlorofluoride (ACF).14... 

The realization that the closely similar spectra from but-l-ene and the but-2-ene denoted isomerization on Ni/Si02 also led Campione and Ekerdt (54) to search for evidence of surface intermediates. Before pumping off the but-l-ene gas phase, they observed three weak absorptions at 1560, 1545, and 1535 cm 1... 

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