Acetylene/cyclopentene

Because of the similarity between the structures of the Ge(100)-2 x 1 and Si(100)-2x1 surfaces, cycloaddition products like those observed on Si(100)-2 x 1 are also expected to form at the Ge surface. Indeed, studying butadiene, Teplyakov et al. [240] showed that a similar Diels-Alder product formed on the surface of Ge(100)-2 x 1. Studies of alkenes have also revealed the formation of [2 + 2] cycloaddition products on germanium. For example, cyclopentene has been shown to form the [2 + 2] cycloaddition product on both surfaces [224,296,297]. In further studies of several other dienes and alkenes (including ethylene [298-303], acetylene [304-306], and cyclohexadiene [307]), cycloaddition products were found for Ge(100)-2 x 1 similar to those observed for Si(100)-2 x 1. 

Ruthenium catalysts, coordinated with an N-heterocyclic carbene allowed for the ROMP of low-strain cyclopentene and substituted cyclopentenes (10,23). Suitable ruthenium and osmium carbene compounds may be synthesized using diazo compounds, by neutral electron donor ligand exchange, by cross metathesis, using acetylene, cumulated olefins, and in an one-pot method using diazo compounds and neutral electron donors (24). The route via diazo compounds is shown in Figure 1.7. 

Triazines are generally more reactive in [2 + 4] cycloaddition in comparison with 1,2,3-tria-zines. The wide variety of dienophiles can be employed enamines, enaminones, vinyl silyl ethers, vinyl thioethers, cyclic ketene jV,O-acetals, /V-phenylmaleimide, 6-dimethylaminopentafulvene, 2-alkylidene-imidazolidines (cychc ketene aminals), cyclic vinyl ethers, arynes, benzocyclopropene, acetylenes, and alkenes like ethylene, (Z)-but-2-ene, cyclopentene, cyclooctene and bicyclo[2.2.1]hept-2-ene, hexa-1,5-diene, cycloocta-1,5-diene, diallyl ether, cyclododeca-l,5,9-triene,... 

Ethynylcyclopropanes, like normal acetylenes, react with dicobalt octacarbonyl in ether to form stable dinuclear cluster-like hexacarbonyl complexes (equation 170)236. The complex with l-chIoro-2,2,3,3-tetramethylethynylcyclopropane reacts stereo- and regioselec-tively with norbomene in a typical Pauson-Khand reaction to give the exn-2-cyclopropyl substituted cyclopentenone (equation 171). Similarly, the reaction of 2-ethoxycyclo-propylacetylene with cyclopentene in the presence of Co2(CO)8 under CO gave 3-(2-ethoxycyclopropyl)-cw-bicyclo[3.3.0]oct-3-en-2-one (equation 172)242. 

Reactions of the recoil C1] with several olefins have been studied, including ethylene, propylene, cyclopentene, and cfs-butene-2, as well as with several paraffins. The type of products observed indicated the existence of several general modes of interaction, such as CH bond insertion, interactions with CC double bonds, formation of methylene-C11. The most important single product in all systems is acetylene, presumably formed by CH insertion and subsequent decomposition of the intermediate. Direct interaction with double bonds is shown by the fact that, for example, in the case of propylene, yields of stable carbon atom addition products were significantly higher than in the case of propane. The same was true for ethylene and ethane. 

SCS-MP2 and the new perturbative B2-PLYP density functional methods provide accurate reaction barriers and outperform MP2 and B3-LYP methods when applied to the 1,3-dipolar cycloaddition reactions of ethylene and acetylene.39 Phosphepine has been shown to catalyse the asymmetric 3 + 2-cycloaddition of allenes with a variety of enones (e.g. chalcones) to produce highly functionalized cyclopentenes with good enantiomeric excess.40 The AuPPh3SbF6 complex catalysed the intramolecular 3 + 2- cycloaddition of unactivated arenyne- (or enyne)-yne functionalities under ambient conditions.41 A review of the use of Rh(I)-catalysed 3 + 2-cycloadditions of diaryl-and arylalkyl-cyclopropenones and aryl-, heteroaryl-, and dialkyl-substituted alkynes to synthesise cyclopentadienones for use in the synthesis of natural products, polymers, dendrimers, and antigen-presenting scaffolds has been presented.42... 

When (67) was treated with a wide variety of cycloaddition reagents under various conditions, it behaved as a diene or a dienophile but not as a 1,3-dipole. As a dienophile it reacted with 2,3-dimethyl-1,3-butadiene to give (70) and with cyclopentadiene to give an analogous product. As a diene it reacted with [2.2.1] bicycloheptene to give (72), presummably via (71), by loss of carbon monoxide and hydrogen. No products were isolated when (67) was treated with maleic anhydride, dimethyl acetylene-dicarboxylate, diphenylacetylene, dimethyl fumarate, carbon disulfide, isobutyl vinyl ether, cyclohexene, and cyclopentene. 

In 1986, we found that alkynyl-A3-iodanes serve as good Michael acceptors toward soft nucleophiles, because of the highly electron-deficient nature of the /3-acetylenic carbon atom. This conjugate addition of nucleophiles constitutes a key step of a highly versatile cyclopentene annulation of alkynyl-A3-iodanes via the tandem Michael-carbene insertion (MCI) reaction [Eq. (103)] [185]. 

A further subtlety is the cyclization of the initial 2,5-hexadienoyl complex to form a six-membered ring, as an alternative to the cyclopentene system. In the reaction of allyl chloride with acetylene and carbon monoxide in the presence of Ni(CO)4, only traces of six-membered ring systems... 

Interesting evidence supporting the mechanism of polymerisation of acetylenes via carbene species is provided by the block and random copolymerisation of acetylenic monomers with cycloolefins. For instance, block copolymers of acetylene and cyclopentene with the WC —AlEtCT catalyst [41] and block copolymers of acetylene and norbornene with the (MeA. Oj2W(=NAr)= CHMe3 catalyst [42] have been obtained moreover, random copolymers of phenylacetylene and norbornene with the WC16 catalyst have also been obtained [149, 150],... 

Besides the common oleftnic dipolarophiles, other unsaturated systems have been evaluated in cycloaddition reactions of zwitterionic TMM-Pd complexes, including polyenes and acetylenes. While acyclic electron-poor dienes generally gave mixtures of five- and seven-membered rings [48], a limited number of selective [3 + 4] and [3 + 6] cycloaddition reactions have been achieved with cyclic polyenic substrates as illustrated by formation of cycloadducts 41 and 42 from pyrone [49] and tropone [50], respectively (Scheme 16). On the other hand, activated alkynes have failed to produce the corresponding cyclopentene derivatives [51]. 

Isobutane and light olefins are the desired hydrocarbon feeds. Unfortunately, impurities such as acetylenes, dienes, sulfur- and oxygen-containing hydrocarbons, cyclopentene, and water are also often present. Purification of the feeds is expensive, but is sometimes cost-effective as a means of reducing the buildup in the acids of conjunct polymers. Dry hydrocarbon feeds are preferred, especially with HE. The water transfers to the HE and is a concern relative to metal corrosion. Solid adsorbents are often used for drying of feedstocks. 

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