Butene double bond migration

This is the most dubious in the case of Pd catalysts, which have high activity in isomerization and double-bond migration. From studies of the half hydrogenation and the isomerization of isoprene130 with Pd, Pt and Ni, the Pd catalyst led to the highest extent of isomerization. From the results of the reduction of isoprene it appears that 1,4-addition as well as 1,2- and 3,4-additions took place, because a significant amount of 2-methyl-2-butene was formed with all catalysts. 

Migration of the double bond of terminal alkenes to internal position is favored by the equilibria. Thus 1-butene in the presence of activated clay, silica gel, alumina, or phosphoric acid on pumice may yield equilibrium product mixtures comprised of about 20% 1-butene and 80% 2-butenes.91 The main transformation of branched 1-alkenes under mild conditions is also double-bond migration. For example, 2,4,4-trimethyl-1-pentene is isomerized to the equilibrium mixture92 with 20% 2,4,4-trimethyl-2-pentene when treated with silica gel at 25°C. 

Skeletal isomerization requires higher temperature and stronger acid catalysts than do double-bond migration and cis-trans isomerization. Butylenes, for example, are transformed to isobutylene over supported phosphoric acid catalysts.98 The equilibrium mixture at 300°C contains approximately equal amounts of straight-chain and branched butenes. Similar studies were carried out with pentene isomers.99 Side reactions, however, may become dominant under more severe conditions.100... 

The relative contribution of the two mechanisms to the actual isomerization process depends on the metals and the experimental conditions. Comprehensive studies of the isomerization of n-butenes on Group VIII metals demonstrated179-181 that the Horiuti-Polanyi mechanism, the dissociative mechanism with the involvement of Jt-allyl intermediates, and direct intramolecular hydrogen shift may all contribute to double-bond migration. The Horiuti-Polanyi mechanism and a direct 1,3 sigma-tropic shift without deuterium incorporation may be operative in cis-trans isomerization. 

Among other nonaddition processes, adiponitrile may be manufactured by the direct hydrocyanation of 1,3-butadiene (DuPont process).169 172,187 196 A homogeneous Ni(0) complex catalyzes both steps of addition of HCN to the olefinic bonds (Scheme 6.4). The isomeric monocyano butenes (20 and 21) are first formed in a ratio of approximately 1 2. All subsequent steps, the isomerization of 20 to the desired 1,4-addition product (21), a further isomerization step (double-bond migration), and the addition of the second molecule of HCN, are promoted by Lewis acids (ZnCl2 or SnCl2). Without Lewis acids the last step is much slower then the addition of the first molecule of HCN. Reaction temperatures below 150°C are employed. 

One of the earliest studies of n-butene hydrogenation was that reported by Twigg [121] who observed that, for the reaction of l butene with hydrogen over a nickel wire between 76 and 126°C, both hydrogenation and double-bond migration occurred. Hydrogenation and double-bond migration followed the same kinetic rate law, namely... 

The metal-catalysed hydrogenation of the higher olefins exhibit general features which are similar to those observed with the n-butenes. Thus, for example, the hydrogenation of hex-1-ene over Adams platinum catalyst [144] is accompanied by very low amounts of double-bond migration the relative rates of isomerisation and hydrogenation are in the ratio 0.03 1. Similarly, in the liquid phase hydrogenation of the n-pentenes over platinum—charcoal and iridium—charcoal [145], little or no isomerisation... 

To distinguish between 4PN or 3PN precursors to MGN, further experiments were done using DCN, followed by analysis of dinitrile products and recovered 3PN by GC/MS and H NMR. The simplest results were obtained with ZnCl2, where little or no deuterium incorporation in the recovered 3PN was observed. The ability of this NiL /DCN/ZnC catalyst system to isomerize double bond migration without deuterium incorporation in the isomerized olefin is reminiscent of the isomerization of 1-butene with Ni[P(0Et)3]4/D2S04, where the ratio of isomerization to deuteration rates was 170 (at 0°C). Figure 12 shows the possible pathways to DN-d, products from PN-d0 precursors. Essentially all of the MGN comes from wrong-way addition of DCN to 4PN, rather than from 3PN. This is consistent with rate measurements which show that addition of HCN to 4PN is 500-600 times faster than addition to 3PN (47). 

Fig. S. Hydrogen promoting effects on the cis to trans isomerization (a) and the double bond migration (b) of n-butenes on MoS2 catalyst at room temperature (36).

Direct irradiation of tetramethylethylene 27 afforded octamethylcyclobutane 28 (Scheme 8) in good yield31. Stereo specific dimerization was obtained in the irradiation of concentrated solutions of Z- and E-2-butenes 29 and 33, respectively32. Irradiation of 29 afforded a mixture of the all-c product 30 and the cis-anti-cis 31 along with double bond migration (32) and Z E isomerization (33). Irradiation of E-2-butene 33 afforded 31 and trans-anti-trans 34 as the dimeric products. 

In isomerization reactions, an alkene is deprotonated to form an allyl anion, which is reprotonated to give the more stable alkene (double-bond migration). The most simple example is the isomerization of 1-butene producing a mixture of cis- and trans-2-butene (Scheme 3). Because the stability of the cis-allyl anion formed as an intermediate is greater than for the trans form, a high cis/trans ratio is observed for base-catalyzed reactions whereas for acid-catalyzed reactions the ratio is close to unity. Thus, the cis/trans ratio of the products has frequently been used as an indication of base-catalyzed reaction mechanisms. The carbanions formed in the course of such superbase reactions are not freely mobile in solution,... 

Decomposition of 1-butene is more complex than that for the 2-butene isomers. Double-bond migration is rapid at the lower end of the temperature range of these experiments and becomes a minor reaction path as the temperature increases. At all temperatures and conversions, products heavier than the feed are produced these make only a minor contribution to the products from the 2-butenes. While some carbon is formed under all of the experimental conditions, significant amounts are not observed below about 50% conversion with either of the isomers. 

Fully substituted alkenes are hydrosilylated with aluminum chloride without double-bond migration. The products have tertiary alkyl groups and thus may be utilized as silyl protecting groups for alcohols and related functional groups. " For example, chlorodimethylthexylsilane (49) is readily prepared from 2,3-dimethyl-2-butene (equation 42). 

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