Higher Cyclic Alkenes

Higher Cyclic Alkenes. The effect on the reactivity of cyclo-octatetraene and of cycloheptatriene upon co-ordination to —Fe(CO)s has been investigated. Product analysis indicates that nucleophilic attack of penta-fluorothiophenoxide at Fe(dienyl)(CO)3 takes place at the dienyl ligand rather than at the iron atom, for the cyclohexa- and cyclohepta- as well as the cyclopenta-dienyl compounds.  

The addition of electrophilic dienophiles to cyclo-octatetraene coordinated to — Fe(CO)3 or to — Ru(CO)s does not occur directly, since it is a thermally forbidden process. Rather, the dienophile co-ordinates to the metal, with subsequent transfer to the cycloalkene to give the observed 1 1 product. An analogue of the second stage of this mechanism is provided by the transfer of acetylacetone from platinum to the unsaturated ligand in the reaction (47) - (48). This is internal nucleophilic addition of acetyl- 

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The reaction of norbomene yields the cis exo diester (equation 66).93 This exo isomer is not obtained directly by Diels-Alder chemistry. Other cyclic alkenes such as cyclopentene yield cis diesters, but isomers are obtained as a result of (3-hydride elimination-readdition from intermediates such as (23) prior to CO insertion (equation 67). Thus the palladium walks around the ring to some extent, but always stays on the same face. The extent of rearrangement can be minimized by higher CO pressures since CO insertion becomes more competitive with (3-elimination. This rearrangement becomes a critical problem in the dicarboxylation of 1-alkenes, since a variety of diesters are formed and the reaction is not particularly useful. These reactions were carried out with catalytic amounts of palladium and stoichiometric amounts of copper chloride. 

The catalytic properties of Del-Ti-MWW have been compared with those of other titanosilicates in the epoxidation of cyclic alkenes (Table 4.4). The TON decreased sharply for TS-1, Ti-beta and 3D Ti-MWW with increasing molecular size of cyclic alkenes. Ti-MCM-41 with mesopores, however, showed higher TONs for cyclooctene and cyclododecene. This implies that the reaction space is extremely important for the reactions of bulky molecules. The delamination of Ti-MWW increased the TON greatly for not only cyclopentene but also bulkier cycloalkenes. Especially, the catalytic activity of Del-Ti-MWW was about 6 x higher than that of Ti-MWW for cyclooctene and cyclododecene. Del-Ti-MWW even turned out to be superior to Ti-MCM-41 in the epoxidation of bulky substrates. This should be due to the high accessibility of Ti active sites in Del-Ti-MWW. Thus the delamination was able to change Ti-MWW into an effective catalyst applicable to reactions of bulky substrates. 

The first large-scale application was the Phillips Triolefin Process (1966) in which propene was converted into ethene and 2-butene. Due to market changes the reverse process, in which propene is produced, became more attractive later. This process has been in operation since 1985. Another process is the Shell Higher Olefin Process (SHOP) in which ethene is oligomerized and the products are metathesized into detergent range olefins. The same company developed a process in speciality chemicals in which alpha-, omega-dienes are formed from cyclic alkenes. 

Under the conditions of stoichiometric (eq. (4)) or catalytic (Scheme 2) reactions, propylene is oxidized to isopropenyl acetate as the main reaction product, along with allyl and cis- and trans -n-propenyl acetates. Higher acyclic alkenes C4-C10 are converted to mixtures of allyl and vinyl esters [5]. Cyclic alkenes also produce homoallylic esters [6, 7]. 

In addition to propene and ethene many other alkenes (higher alkenes, styrenes, cyclic alkenes) have been polymerized with these new catalysts and a great variety of new polymers and oligomers have been synthesized, including plastic materials with melting points as high as 500°C, novel rubber materials etc. Several new polymers can now be made by catalyst design. Basically, the con-... 

One important difference in activation energies, however, is apparent for Pi, which has been associated with "soft" coke probably adjacent to metal sites, where the activation energy of the cyclohexene-coked catalyst is ca. 15 kJ mok higher than for spent and 1-hexene coked catalysts. This difference may be related to the dynamics of hydrocarbon adsorption. For linear alkenes only one end of the chain may be attached to the catalyst surface, while for small cyclic alkenes, which have a more compact structure, the entire molecule may be adsorbed onto or electronically affected by the catalyst surface. That is, adsorbed cyclohexene which may lie... 

In general, since epoxidation involves electrophilic reagents, double bonds with electron-donating substituents are more reactive. Thus, internal or cyclic alkenes give higher conversions than do terminal olefins. Conversely, in general, electron-withdrawing substituents increase the activity of electrophilic catalysts. 

Alkenes lead to 1,4-dihydropyridazines (347) or the spiro derivatives (348). Cyclic alkenes, such as cyclopropenes give rise to diazanorcaradienes (349) or higher homologues, and reaction with alkynes yields pyridazines (350). Hetero-27r-systems such as nitriles, imines, thioketones, and N-sulflnylamines open the way to heterocyclic six-membered systems (351), (353), (357), and (356). (4-1-1) Addition reactions of isocyanides and carbenes make isopyrazoles (354) and (355) available. Reduction processes lead to 1,4- or 1,6-dihydrotetrazines (340) or (352). The reactions of 1,4-dihydrotetrazines (340) were briefly collected in Scheme 57. 

The resulting nucleophilic alkoxymethyl radical may be trapped by an electron-deficient alkene. Reduction of the adduct radical (3 by DCA radical anion and protonation of the resulting anion, confirmed by deuterium incorporation from methanol-OD, gives the final product (3%). The diastereoselectivity shown has its origin in a preference for protonation, under kinetic control, from the less hindered side. For acyclic alkenes such as methyl 2-cyanocrotonate or dimethyl maleate, free rotation within (395) results in a low cisJrans ratio of 1.8-2.5 1 whereas for cyclic alkenes such as N,3-dimethylmaleimide or 3-methylmaleimide the cisitrans ratio is considerably higher at 86 14. ... 

Most of what will be said in the Section will relate to ethene (and ethene-d4) much of it will also probably apply to higher linear and cyclic alkenes (see Further Reading list), the only additional factor being that activation of allylic methyl or methylene groups adjacent to the double-bond causes easy dissociation of a C—H bond thereon, with the formation of a delocalised three-centre jr-bond the adsorbed state is then a r-alkenylic species (see Table 4.3). 

To conclude this section, we should note what has not been discussed -and why. The emphasis has been on trying to understand how the nature of the surface metal atoms influences the type of hydrocarbon structure formed, and although there is a great deal of information available on the structures formed by higher and cyclic alkenes, alkadienes, alkynes and benzene, some of which will be touched on below, it is only with ethene that a sufficiently wide range of surfaces has been investigated to make comparisons between them possible. A further omission, to be remedied in Section 4.7, is any attempt to provide a theoretical basis for the observations any more profound than that outlined above. The reason for this delay is that further useful information comes from studying the detailed structures of certain adsorbed molecules (Section 4.5) and the manner of their thermal decomposition (Section 4.6). 

It has been shown that this catalyst is selective in epoxidation of linear alkenes the linear epoxide yield was two to four times higher than in catalysis by ordinary porphyrin. It was also demonstrated that, in catalysis by the dendrimers, cyclic alkenes are oxidized three times more rapidly than similar linear 1-alkenes are. The catalyst activity decreases only by 10% at a turnover number (TON) of 1000, which is much higher than that for the monomolecular analogue. A cobalt complex with dendrimer phthalocyanine was much more stable, while remaining active, in... 

Scheme 25) was observed when cyclic alkenes (e.g., 214) were treated with ruthenium carbene complex 18 in the presence of terminal alkynes (e.g., 215). A mechanism involving initial ROM, followed by alkyne insertion of the intermediate carbene complex, followed by ROM from intermediate 217, was proposed. In order to account for the unexpectedly high yield (the yield is higher than the anticipated E Z selectivity in the formation of 217) of the process, a second source of the observed product involving metathesis of an additional mole of cyclopentene from intermediate 217 was suggested. 

The relative rates of hydroboration [7] of cyclic alkenes depend on the presence of strain [11] on the carbon-carbon double bond. The double bond of cyclopen-tene is considerably more strained than the double bond of cyclohexene, and this strain is responsible for higher reactivity of cyclopentene. 

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