Propylene double-bond isomerization

Such a mode of adsorption provides a mechanism for double-bond isomerization which we can follow by use of labeled propylenes (13). For example, on the basis of the foregoing, we expect the following sequence for adsorbed CH3—CH=CD2 ... 

Double-bond isomerization was once used in the multistep synthesis of isoprene developed by Goodyear.266-268 2-Methyl-1-pentene produced by the dimerization of propylene was isomerized to 2-methyl-2-pentene over a silica-alumina catalyst at 100°C. The product was cracked to isoprene and methane. Because of the lower cost of isoprene isolated from naphtha or gas oil-cracking streams, synthetic isoprene processes presently are not practiced commercially. 

Clark and Cook 71) disproportionated [1-14C] propylene and [2-14C] propylene over cobalt oxide-molybdate-alumina catalyst. At 60 °C their results were consistent with those reported Mol and coworkers, confirming the four-center mechanism. At temperatures above 60 °C, double-bond isomerization activity of the cobalt-molybdate catalyst became a factor and at 160 °C nearly one-half of [l-l4C] propylene had isomerized to [3-14C] propylene prior to disproportionation. The authors note that at temperatures where isomerization does not occur, the possibility of a jr-allyl intermediate appears to be excluded however, at higher temperatures, the 77-allyl mechanism cannot be so easily dismissed. 

Woody, Lewis, and Wills 72> studied the disproportionation of [1-14C] propylene over cobalt oxide-molybdate-alumina at 149 and 177 °C. Approximately equal amounts of radioactivity were found in the approximately equal molar quantities of ethylene and butene. These results are in agreement with those of Clark and Cook showing that double-bond isomerization was a factor in this temperature region. Woody and coworkers suggest that since the isomerization of the 2-butene product was negligible, an explanation of double-bond mobility as simple isomerization is probably an oversimplification. 

They proposed that 1-butene is obtained from ethylene by intermediate (I) in which breaking one bond and hydrogen shift occurs. 2-Butene is produced by double-bond isomerization, and propylene is formed from ethylene and 2-butene by breaking ring bonds Cj —C2 and C3 C4 of structure (II). 

In view of the difference of a factor of 10 or more in peak delay between butene and thiophene at similar temperatures, butene adsorption was checked to see that chemisorption was in fact occurring below 200° C. Using the 50-foot propylene carbonate column, it was found that some butane was formed in spite of the H2S present down to 150° C. (without H2S butene was almost completely hydrogenated at this temperature) and both cis-trans and double bond isomerization of the butenes went to completion at temperatures below 100° C., indicating that chemisorption of butene must have occurred. It is therefore felt that extrapolation of the butene sorption results obtained to the temperature range of the desulfurization reaction (above 200° C.) should be valid. 

In small pore zeolite systems, the polymerization of propylene (118), isobutylene (120), vinyl chloride (121), and styrene (121) over Linde 5A, the polymerization of propylene and isobutylene over chabazite (19), and the double bond isomerization of 2-methyl-l-pentene over Linde 5A (122) have been reported. Since most of these reactants and the products derived from them cannot pass through the 4-5 A entry pores, it is assumed that these reactions occurred on the external surface of the zeolite. 

These two characteristics are not always encountered, especially in the cases of addition polymerization of monomers with double bonds. Isomerization of the monomers may occur, or false bonding of monomeric units into the chain may take place during the actual step of adding monomer onto the polymer chain. Consequently the assumed chemical structure of the polymer must always be carefully verified by analytical methods. Analysis is especially important with industrial polymer production, since the preparation history is not often known exactly. The chemical names of industrial or commercial polymers are often nothing more than a kind of generic name. Commercial poly(ethylenes), for example, despite the ascribed name, are often not homopolymers, but copolymers of ethylene and propylene. As well as that, commercial polymers practically always contain additives such as antioxidants, uv absorbers, fillers, etc. In the addition polymerization of monomers with multiple bonds, head-to-head and tail-to-tail structures are always to be expected together with the normal head-to-tail bonding, as can be seen, for example, with vinyl compounds such as CH2=CHR ... 

A selective dimerization of propylene to 2,3-dimethylbutene catalyzed by R4P[(i-Pr,P)NiCl3] with Et3Al2Cl3 in a toluene medium has been reported 1617]. The increasing temperature (—20 to +20°C) leads to the formation of C, olefins at the expense of 4-methyl-1-pentene. This suggests a secondary codimerization of the product olefin with propylene. Most of the olefins are the thermodynamically less favored a-olefins, indicating the absence of double-bond isomerization under these conditions. The Al/Ni ratio, although having a predominant effect on reaction rate and yield at low values, has no influence on the catalyst selectivity. 

The presence of a variety of reaction intermediates described above can be clearly demonstrated by detailed tracer studies in the case of deuterium exchange of propylene, which is dosely related to double-bond isomerization of olelins. ... 

Elastomers. Ethylene—propylene terpolymer (diene monomer) elastomers (EPDM) use a variety of third monomers during polymerization (see Elastomers, ethyiene-propylene-diene rubber). Ethyhdenenorbomene (ENB) is the most important of these monomers and requires dicyclopentadiene as a precursor. ENB is synthesized in a two step preparation, ie, a Diels-Alder reaction of CPD (via cracking of DCPD) with butadiene to yield 5-vinylbicyclo[2.2.1]-hept-2-ene [3048-64-4] (7) where the external double bond is then isomerized catalyticaHy toward the ring yielding 5-ethyhdenebicyclo[2.2.1]-hept-2-ene [16219-75-3] (ENB) (8) (60). 

Many research groups have attributed the isomerization to a series of additions and eliminations of a cobalt carbonyl hydride. However, it has been shown that aldehydes may be found with formyl groups attached to a carbon atom other than the two of the double bond even under non-isomerizing conditions. Piacenti and co-workers (44, 45) studied the hydroformylation of [l-14C]propylene and of a>-deuterated a-olefins. Even for a-olefins with chain lengths up to C6, the formyl group was attached to all possible carbon atoms in the product mixture. However, in the deuterated experiments, deuterium was present only on carbons 2, 3, and a) of the resulting aldehydes. These results were explained by pro-... 

Orchin and Roos (108) examined the isomerization of allylbenzene by HCo(CO)4 and DCo(CO)4 at ambient temperature and pressure. Both HCo(CO)4 and DCo(CO)4 catalyzed isomerization to propenylbenzene at the same rate, and when DCo(CO)4 was used as catalyst 5% of the propenylbenzene produced was found to contain a deuterium atom. Hydroformylation of propylene with residual DCo(CO)4, after an isomerization of allylbenzene, yielded RCDO with no detectable RCHO. The authors chose to reject a mechanism involving addition of D—Co to the olefinic double bond, on the grounds that the lack of an isotope effect indicated breaking of D—Co, or H—Co, was not the rate-determining step, and that only a relatively minor amount of deuterium was incorporated into the isomerized reaction product. Instead, the authors favored a mechanism expressed as... 

On the contrary, similar EHT calculations of possible intermediate structures in the isomerization reaction (also exemplified by propylene) (91) predicted the stabilization of the carbocation on a surface that contradicted the experimental data (92). This was likely due to the limitations of EHT as applied to the total energy computations, especially of the charged forms. Essentially the same conclusion, that EHT overestimated the stability of the cationic form of propylene, was drawn by Schliebs et al. (93), who compared different mechanisms of the double-bond migration in zeolites using the EHT calculations for the cluster composed of four Si(Al)04 tetrahedra. 

From previously reported studies then, several different products are possible. The initial attack by the oxygen moiety may apparently be vinylic (on either of the two carbons of the double bond) or allylic (on the carbon next to the doubly bonded carbons). Distinction must be made between allylic attack as described here and allylic products which can arise either by true allylic attack or by vinylic attack followed by olefinic isomerization. Thus it is not clear whether such products as 2-hexen-l-yl acetate(II) (58) have been formed by vinylic attack upon hexene followed by olefinic isomerization, by olefin isomerization of hexene to 2-hexene followed by allylic attack, or by some type of synchronous mechanism in which oxygen attack and olefin isomerization occur simultaneously. This last possibility could be visualized as involving some type of 7r-allylic complex (Reaction 2). This involvement of TT-allylic complex can be ruled out only in the production of isopropenyl acetate from propylene since a mechanism such as this followed by olefin isomerization could not be used in that case. For the butenes and higher... 

There is hindered rotation about any carbon-carbon double bond, but it gives rise to geometric isomerism only if there is a certain relationship among the groups attached to the doubly-bonded carbons. We can look for this isomerism by drawing the possible structures (or better yet, by constructing them from molecular models), and then seeing if these are indeed isomeric, or actually identical. On this basis we find that propylene, 1-butene, and isobutylene should not show... 

Olefin metathesis is a useful reaction for the production of propylene from ethylene and butenes using certain transition-metal compound catalysts. The two main equilibrium reactions that take place simultaneously are metathesis and isomerization. Metathesis transforms the carbon-carbon double bond, a functional group that is unreactive toward many reagents that react with many other functional groups. New carbon-carbon double bonds are formed at or near room temperature even in aqueous media from starting materials. Because olefin metathesis is a reversible reaction, propylene can be produced from ethylene and butene-2. Metathesis can be added to steam crackers to enhance the production of propylene by the transformation of ethylene and the cracking of mixed butenes. Fig. 3 shows a schematic flow diagram of a typical metathesis process. Examples of metathesis... 

Depending on the chemical reactivity of the stilbene derivatives other oxidizing agents such as diphenyl selenide [73] and, with superior results, the recently discovered system iodide/propylene oxide in an inert gas atmosphere [74] have been applied. The absence of air diminishes photooxidative side reactions and the trapping of photogenerated hydrogen iodine with propylene oxide prevents the reduction of the double bonds in the stilbenes. An alternative to the complete aromatization is the partial aromatization to other dihydrophenanthrenes by isomerization (hydrogen shifts, [67a-c]). 

During the catalytic reduction of allyl alcohol,(2) the saturation of the double bond (and propanol formation) may also be accompanied by isomerization (with formation of propionaldehyde) and C-0 or C-C bond cleavage (destruction) via hydrogenolysis, yielding gaseous products (propylene, propane, ethylene, acetylene). 

The aldehydes and alcohols produced are a mixture of normal and iso-compounds. This is due not only to the orientation of the hydrogen with respect to the C-CO bonds in the initial reaction complex but also to the isomerization of the olefin under the process conditions. It may be significant that nickel carbonyl does not readily shift the olefin double bond under the 0X0 process conditions, and nickel compoimds are very poor catalysts for the process. From isooctene 32% n-nonyl alcohol, and from propylene 50% n-butyl alcohol are obtained, the remainder of the products being isoalcohols. In general, using a-olefins as raw material, one obtains about 60% isoalcohols. The synthesis will not occur unless a labile hydrogen atom is available in the olefin reactant. With diolefins the reaction takes place at only one double bond. 

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