Cyclopentenes special

Most Kaminsky catalysts contain only one type of active center. They produce ethylene—a-olefin copolymers with uniform compositional distributions and quite narrow MWDs which, at their limit, can be characterized by M.Jratios of about 2.0 and MFR of about 15. These features of the catalysts determine their first appHcations in the specialty resin area, to be used in the synthesis of either uniformly branched VLDPE resins or completely amorphous PE plastomers. Kaminsky catalysts have been gradually replacing Ziegler catalysts in the manufacture of certain commodity LLDPE products. They also faciUtate the copolymerization of ethylene with cycHc dienes such as cyclopentene and norhornene (33,34). These copolymers are compositionaHy uniform and can be used as LLDPE resins with special properties. Ethylene—norhornene copolymers are resistant to chemicals and heat, have high glass transitions, and very high transparency which makes them suitable for polymer optical fibers (34). 

Like benzocyclobutadiene, benzazetes are reactive dienophiles, but fail to react as dienes except in the special case of dimerization. Thus 2-phenylbenzazete is inert to cyclopentene, but-2-yne, iV-phenylmaleimide and iV,iV-dimethylaminopropyne. 

Since 1,5-enediones are usually obtained via pyrylium salts, syntheses of the type found in Section B, 2, a have a rather theroetical interest, save for a few special syntheses. There exist several direct syntheses of l,5-enediones, e.g., from j8-chlorovinyl ketones and j8-diketones or j8-keto esters special pathways to 1,5-enediones have also been described, namely, oxidation with lead tetraacetate or with periodic acid of cyclopentene-l,2-diols. ... 

The vinylcyclopropane rearrangement is an important method for the construction of cyclopentenes. The direct 1,4-addition of a carbene to a 1,3-diene to give a cyclopentene works only in a few special cases and with poor yield. The desired product may instead be obtained by a sequence involving the 1,2-addition of a carbene to one carbon-carbon double bond of a 1,3-diene to give a vinylcyclopropane, and a subsequent rearrangement to yield a cyclopentene ... 

The preparation described here of 3-cyclopentene-1-carboxylic acid from dimethyl malonate and cis-1,4-dichloro-2-butene is an optimized version of a method reported earlier3 for obtaining this often used and versatile building block.6 The procedure is simple and efficient and requires only standard laboratory equipment. 3-Cyclopentene-1-carboxylic acid has previously been prepared through reaction of diethyl malonate with cis-1,4-dichloro(or dibromo)-2-butene in the presence of ethanolic sodium ethoxide, followed by hydrolysis of the isolated diethyl 3-cyclopentene-1,1-dicarboxylate intermediate, fractional recrystallization of the resultant diacid to remove the unwanted vinylcyclopropyl isomer, and finally decarboxylation.2>7 Alternatively, this compound can be obtained from the vinylcyclopropyl isomer (prepared from diethyl malonate and trans-1,4-dichloro-2-butene)8 or from cyclopentadiene9 or cyclopentene.10 In comparison with the present procedure, however, all these methods suffer from poor selectivity, low yields, length, or need of special equipment or reagents, if not a combination of these drawbacks. 

Cyclopentene and norbornene are highly flammable and harmful in contact with skin, eyes and the respiratory system. For poly(nor-bornene) no special hazards are reported. On handling, the usual precautions should be applied. 

Apart form the aforementioned highly enantioselective hetero-Diels-Alder reactions, that proceed with very low catalyst loadings, the catalytically accessible enolates have also been used for related intramolecular Michael reactions (Philips et al. 2007) and for the desym-metrization of 1,3-diketones yielding cyclopentenes via an intramolecular aldol reaction (Wadamoto et al. 2007). The formation of cyclopentenes, however, presents a special case, so—depending on the stereochemical nature of the enone substrates (s-cis or s-trans) and the stereochemistry of the final products—two different mechanisms are discussed in the literature. Whereas /ran.v-cycl open (cries are proposed to be available upon conjugate addition of a homoenolate to chalcones,... 

ROMP processes are unique in that they offer unsaturated polymer products with properties between those of saturated polyethylene and the highly unsaturated polybutadienes. These polyalkenamers have been the subject of intense study by companies dealing in speciality polymers. In the 1970 s the ROMP product of cyclopentene attracted attention as a replacement for natural rubber, due to its good strength and ageing properties. Although the elastomer was never commercialized, as its overall characteristics did not meet requirements, the work stimulated research into ROMP of other cyclic alkene... 

Asymmetric Acylation of me o-Diols. Both cyclic and acyclic me o-1,2-diols are desymmetrized by acylation in the presence of a stoichiometric amount of this ligand with modest-to-excellent enantioselectivity (eq 4). In a special case, cis-5,5-dimethyl-2-cyclopentene-l,4-dior was monobenzoylated in the presence of a catalytic amount of this ligand in good yield and with perfect enantioselection (87%, >99.5% ee). 

The thermal expansion of a vinylcyclopropane to a cyclopentene ring is a special case of a [l,3]-sigmatropic migration of carbon, although it can also be considered an internal [ 2 + 2]-cycloaddition reaction (see 15-63). It is known as a vinylcyclopropane rearrangementThe reaction has been carried out on many vinylcyclopropanes bearing various substituents in the ring or... 

Flavourings that are useful for aromatising bakery and chocolate food products can be made from sulphur-free amino acids by the reaction with cyclic ketones (Fig. 3.36) such as 4-hydroxy-2,5-dimethyl-3(2//)-furanone (26), maltol (63) or 2-hydroxy-3-methyl-2-cyclopenten-l-one (64, cyclotene) [109]. Amino acids of special interest are leucine, valine, proline and hydroxyproline. The reaction is carried out favourably in fat or propylene glycol. 

The typical Nazarov strategy 51 followed by 53 disconnects the rest of the ring from the double bond, but you cannot be quite sure where the double bond will end up in the ring. Another special method, the Pauson-Khand reaction,24,25 differs in both respects. It adds the enone portion of the ring to the rest of the molecule and you can be quite certain where the new double bond will be. The reaction is between an alkene, say cyclopentene 97, an alkyne, and Co2(CO)8 to form a cyclopentenone26 98 in one step. A complex 99 is first formed between the alkyne and the cobalt atoms with the two n-bonds replacing two CO molecules (both are two-electron donors). You may see this complex drawn as 99a but it really has a tricyclic tetrahedrane structure 99b composed of three-membered rings and with a Co-Co bond. 

In this regard, the Hoveyda catalyst 71 was next examined. To our great delight, it afforded the desired cyclopentene 68 in an excellent 92% yield after overnight heating in toluene at 110 °C (Entry 4). Importantly, the reaction could be successfully performed, without any special precautions, in non-distilled commercial solvent in a reaction vessel that was open to the air. 

The 1,4-addition of carbenes to 1,3-dienes to give cyclopentenes is extremely rare, since 1,2-addition to give a vinylcyclopropane is much more favorable. Unfortunately, vinylcyclopropanes can be converted into cyclopentenes on heating at higher temperature. In special conditions direct 1,4-addition reactions are observed, but only in poor yield (Scheme 5.40). 

Very recently, it has been shown that on the basis of the energetic criterion of antiaromaticity and the proton affinity of 3-cyclopropenyl anion (13) this ion does not merit being differentiated from other aUylic anions and is therefore best thought of as non-aromatic. Cyclopropene is the smallest cycloafkene, and its conjugate base at C3 is considered to be a special anion that is destabihzed due to the presence of 4jt electrons in this fuUy conjugated monocycHc species. Its acidity, however, follows the same correlation as for cyclobutene, cyclopentene, cyclohexene, and propene. No additional parameter beyond the central C—C—C bond angle is needed to explain or account for the weak acidity of cyclopropene. 

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