Cyclopentene, carbon atom reactions

Cyclopentenes behave differently and often act through radical mechanisms this can lead to photoreduction to cyclopentanes, or photoaddition of the kind exemplified by norborneneand propan-2-ol 12.57). The photoadduct in this process is linked through the carbon atom of the alcohol, and not the oxygen atom. A related addition to acetonitrile 12.58) takes place when norbornene is irradiated in the presence of a silver(i) compound. It is likely thal a metal complex of the alkene is the real irradiation substrate, and the same may be true for copper(i)-promoted additions of haloalkanes to electron-deficient alkenes (2.59). When dichloromelhane is used in such a reaction the product can be reduced electrochemically to a cyclopropane (2.60), which is of value because the related thermal addition of CH.I, to alkenes in the presence of copper does not succeed with electron-poor compounds. 

Neither C5- nor C6-cyclization involve carbonium-ion intermediates over platinum metal. The rates of the -propylbenzene - indan reaction (where the new bond is formed between a primary carbon atom and the aromatic ring) and the n-butylbenzene- 1-methylindan reaction (which involves a secondary carbon atom) are quite similar (13). Furthermore, comparison of the C6-cyclization rates of -butylbenzene and n-pentylbenzene (forming naphthalene and methylnaphthalene, respectively) over platinum-on-silica catalyst shows that in this reaction a primary carbon has higher reactivity than a secondary carbon (Table IV) (29). Lester postulated that platinum acts as a weak Lewis acid for adsorbed cyclopentenes, creating electron-deficient species that can rearrange like carbonium ions (55). The relative cyclization rates discussed above strongly contradict Lester s cyclization mechanism for platinum metal. 

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. 

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]. 

The sequence of cyclohexene cleavage and aldol reaction on the dicarbonyl product gives ring-contracted cyclopentenes. This proved particularly valuable when Iwata6 wanted to make subergorgic acid 41 that has three five-membered rings awkwardly joined around a quaternary carbon atom. So crowded are these compounds that they are difficult to draw clearly. Ozono-lysis of the synthetic cyclohexene 38 gave the unstable dialdehyde 39 that cyclised by an aldol condensation to 40 and hence could be oxidised to 41. 

From a preparative point of view, the acylation of ketones via enamines is of particular interest. In comparison with pyrrolidine and piperidine enamines, the less reactive morpholine enamines give better yields, as found by Hiinig et al.2iZ j8-Diketones are the products of acylation with an acyl halide followed by acid hydrolysis, whereas with ethyl chloroformate, /J-ketoesters are obtained.212 Hiinig and his collaborators242-247 have used the acylation of 1-morpholino-l-cyclopentene and 1-morpholino-l-cyclohexene to lengthen the chains of acids by five and six carbon atoms, respectively. The reaction may... 

These highly reactive yet stable species are strong electrophiles of tetraphilic character, since nucleophiles may attack three different carbon atoms (a,/ ,a ) and iodine. In most reactions the first step is a Michael addition at fi-C with formation of an alkenyl zwitterionic intermediate (ylide) which normally eliminates iodoben-zene, generating an alkylidene carbene then, a 1,2-shift of the nucleophile ensues. The final result is its combination with the alkynyl moiety which behaves formally as an alkynyl cation. The initial adduct may react with an electrophile, notably a proton, in which case alkenyl iodonium salts are obtained also, cyclopentenes may be formed by intramolecular C-H 1,5-insertion from the alkylidenecarbenes ... 

Intramolecular oxonium ylide formation is assumed to initialize the copper-catalyzed transformation of a,p-epoxy diazomethyl ketones 341 to olefins 342 in the presence of an alcohol The reaction may be described as an intramolecular oxygen transfer from the epoxide ring to the carbenoid carbon atom, yielding a P.y-unsaturated a-ketoaldehyde which is then acetalized. A detailed reaction mechanism has been proposed. In some cases, the oxonium-ylide pathway gives rise to additional products when the reaction is catalyzed by copper powder. If, on the other hand, diazoketones of type 341 are heated in the presence of olefins (e.g. styrene, cyclohexene, cyclopentene, but not isopropenyl acetate or 2,3-dimethyl-2-butene) and palladium(II) acetate, intermolecular cyclopropanation rather than oxonium ylide derived chemistry takes place... 

Hamers and coworkers showed that undoped diamond (001) surfaces undergo [2 + 2] cycloaddition reactions with a prototypical alkene, cyclopentene [95]. The (001) surfaces of diamond, silicon, and germanium share a similar bonding motif in which pairs of atoms can bond to each other via a strong a bond and a weaker tt bond forming a dimer. Normally, the formation of dimers on the diamond surface is prevented by hydrogen chemisorption however, the diamond (001) surface was made reactive for the addition reaction by heat treatment in the UHV at 1375 K. At these temperatures, the surface is cleaned of impurities and surface hydrogen desorbs. This leads to the formation of carbon atoms paired into C=C dimers with a partial 7t bond [95]. This surface is essentially an extended array of alkene-like C=C... 

The donor-acceptor acyl chloride-aluminium chloride complex is not only an electrophilic reagent but also a hydride ion acceptor (oxidizing reagent). The best hydride donors are the saturated hydrocarbons, specially those containing a tertiary carbon atom. The acylation of cyclohexane is an old and well-known reaction giving acetyl-methyl-cyclopentene and/or -cyclopentane. 26,27 the yields are low because the solvent used was either cyclohexane itself or small quantities of chloroform. in our hands, we found that the reaction carried out in methylene chloride solution ( IM) led to the l-acetyl-2-methylcyclopentene and l,3-diacetyl-2-methylcyclopentene Z in good yields. A cleaner reaction also occurred from methylcyclopentane. The ratio of ys could be changed by modification of the reaction conditions (see Table 1) (overall yields of the hydrocarbons functionalization were around 75-80 %). 

The reaction of norbomadiene with an acetylene may follow different reaction channels, depending on the reaction conditions. Using a cobalt/phosphine catalyst, an acetylene adds to the two unsaturated front carbon atoms of norbomadiene, giving a five-membered cyclopentene ring. Simultaneously, a three-membered cyclopropane ling is formed at the backside of the molecule. The polycyclic skeleton obtained in this homo Diels-Alder reaction is called the deltacyclene skeleton (Scheme 4). 

The results of studies on the copolymerization of labeled cycloolefins have been successfully applied to the elucidation of the basic mechanism of metathesis polymerization [38]. In this regard, reaction of cyclooc-tene with cyclopentene, isotopically labeled at one carbon atom of the double bond, in the presence of the catalytic system WOCl4/Et2AlCl/benzoyl peroxide, proved that the process occurred by the cleavage of the carbon-carbon double bond ... 

In the case of ethylene itself the cyclopentene technique is obviously inapplicable and the relative rate constants have in this case been obtained in other ways, for example, by measuring yield of carbon monoxide formed by fragmentation of the products of reaction of oxygen atoms with ethylene. In this case the total pressure has to be kept approximately constant although some variation is not too important in view of the relatively small effect of pressure on CO yield at pressures normally used. 

Cyclopentene is a symmetrical alkene, which takes on meaning when it is compared to the reaction of an unsymmetrical alkene such as 2-methyl-2-butene. An unsymmetrical alkene will have different atoms or groups attached to the carbons of the C=C unit. In 2-methyl-2-butene, one carbon of the C=C unit has a methyl and a hydrogen, and the other carbon has two methyl groups. If 2-methyl-2-butene reacts with HBr in the same way as cyclopentene, the intermediate is a carbocation, and subsequent reaction with the nucleophilic bromide ion will form an alkyl bromide. However, the reaction may generate two different products—4 and 5— via two different carbocation intermediates. When the percentage yield of products from this reaction is determined experimentally, it is clear that 5 is the mqjor product. Why An analysis of the mechanism for formation of 4 and also for formation of 5 will provide a prediction of the major product based on rmderstanding each carbocation intermediate. 

The first step is a simple displacement of two molecules of carbon monoxide. In the second step the acidic hydrogen of 7r-bonded cyclopentadiene shifts to the iron atom to give w-cyclopentadienyliron dicarbonyl hydride, a known compound. Two molecules of this iron hydride then add hydrogen to cyclopentadiene to form cyclopentene and the dimer of w-cyclopentadienyl-iron dicarbonyl. This reaction proceeds at temperatures between 150° and 220° C. Above 220° C, the remaining carbon monoxide molecules are displaced by a second molecule of cyclopentadiene forming ferrocene [42). 

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