4-Cyclopentenyl cation

The mechanism of reaction with steroids has not been elucidated. Various nonquantitative reactions occur simultaneously. Cyclopentenyl cations have been postulated as intermediates which condense with anisaldehyde to yield colored compounds [4]. It is probable that triphenylmethane dyes are also formed with aromatic compounds. 

Upon treatment of a divinyl ketone 1 with a protic acid or a Lewis acid, an electrocyclic ring closure can take place to yield a cyclopentenone 3. This reaction is called the Nazarov cyclization Protonation at the carbonyl oxygen of the divinyl ketone 1 leads to formation of a hydroxypentadienyl cation 2, which can undergo a thermally allowed, conrotatory electrocyclic ring closure reaction to give a cyclopentenyl cation 4. Through subsequent loss of a proton a mixture of isomeric cyclopentenones 5 and 6 is obtained ... 

Enol ether additives were used to probe the protonation of 3-cyclopen-tenylidene (127). Treatment of A-nitroso-A-(2-vinylcyclopropyl)urea (124) with sodium methoxide generates 2-vinylcyclopropylidene (126) by way of the labile diazo compound 125 (Scheme 25). For simplicity, products derived directly from 126 (allene, ether, cycloadduct) are not shown in Scheme 25. The Skat-tebpl rearrangement of 126 generates 127 whose protonation leads to the 3-cyclopentenyl cation (128). In the presence of methanol, cyclopentadiene (130) and 3-methoxycyclopentene (132) were obtained.53 With an equimolar mixture of methyl vinyl ether and methanol, cycloaddition of 127 (—> 131)... 

DFT-calculated 13C NMR chemical shifts for the pentamethylcyclopentadienyl cation and the (CH3)5-cyclopentenyl cation 97 provide conclusive evidence that the... 

Instead the allyl cation 97 was obtained. The reported experimental 13C NMR data (<513C(exp) 8 — 250/243, 153, 60 ppm) are in agreement with the cyclopentenyl cation structure 97, but differ significantly from the calculated 13C NMR chemical shift for both Jahn-Teller distorted valence isomeric cyclopentadienyl cation structures 95 and 96. 

The resultant cycloalkenyl carbenium ions, especially the cyclopentenyl cations, are very stable (103,104) and can even be observed as free cations in zeolites 105,106). These ions can oligomerize further and, within zeolites, irreversibly block the acidic hydroxyl groups. With liquid acids, the oligomers will dilute the acid and thus lower its acid strength. 

Prins reaction, heteropolyacid catalysis, 41 156 Probe molecules, 42 119 acidic dissociation constant, 38 210 NMR solid acidity studies, 42 139-140 acylium ions, 42 139, 160 aldehydes, 42 162-163 alkyl carbenium ions, 42 154-157 allyl cation, 42 143-144 ammonia, 42 172-174 arenium ions, 42 150-154 carbonium ions, 42 157-160 chalcogenenonium ions, 42 161-162 cyclopentenyl cations, 42 140-143 indanyl cations, 42 144-147 ketones, 42 162,163-165 nitrogen-containing compounds, 42 165-170... 

More recently Miron and Lee (1962) analysed the hydrocarbons removed from strong acid catalysts in some detail, and suggested unsaturated cyclic structures. These structures contain from one to five five-membered rings with various methyl and alkenyl substituents and a minimum of two double bonds per molecule. However, during their drowning procedures, as the acid is diluted, considerable polymerization occurs. This conclusion is based on work by Hodge (1963), who showed that cyclopentenyl cations are rapidly destroyed by alkylation at 10 m concentrations in 35% H2SO4. 

Reaction pathways for the addition of ethylene to butadiene radical cation involving H-shifts have been investigated at the coupled cluster UCCSD(T)/DZP//UMP2(fc)/DZP-b ZPE level of theory.Several rearrangement reactions have been found to occur below the energy limit of separated ethylene and butadiene radical cation. The cyclopentenyl cation ( 5117)+ in the gas phase may originate from various pathways. 

Data from Olah et al., 1972b, Olah and Liang, 1972, and Olah and Porter, 1971 (ext. TMS, conv. to CSj = 194-6). The data for the cyclohexenyl and cyclopentenyl cations were transposed in the original article. 

Propene on HY was, therefore, selected for the first in situ variable-temperature study using the CAVERN method. These experiments were carried out in early 1988 and published in 1989 (93). The central features of the CAVERN experiments were that the propene was introduced into the zeolite at cryogenic temperature and the sample was manipulated so that spectral acquisition could commence with an unreacted sample. Additional spectra were then acquired as the sample was slowly raised to room temperature. Detailed experiments of this sort were carried out for propene-2-l3C and propene-7-13C and less extensive experiments were performed for propene-3-13C. These experiments showed, among other things, that the 250 ppm peak was formed coincident with a second peak at ca. 156 ppm and the relative intensities of these peaks were 2 1. A careful study of the literature of carbenium ion chemistry in sulfuric acid and superacid solution media suggested the assignment of these resonances (250 and 156 ppm) to alkyl-substituted cyclopentenyl cations similar to 4. 

Fig. 12. 75.4-MHz 13C CP/MAS spectra showing the formation of cyclopentenyl cations 8 from cycIopentene-13Ci (random) on zeolite HZSM-5 and 4 from propene-2-,3C on zeolite UY.

The allyl cation (9) is the simplest member of the class of resonance-stabilized cations that includes the alkyl-substituted cyclopentenyl cations. But one could also say that the carbenium ion (CH3) is the simplest member of a class of cations that includes the trityl cation. In each case, 10 or so orders of magnitude of acidity separate the primitive member from its more elaborate derivatives. 

The principal components of the trityl cation in zeolite HY are <5 = 282 ppm and <5j = 55 ppm. It is instructive to tabulate all of the 13C principal component data measured for free carbenium ions in zeolites as well as for a few carbenium ions characterized in other solid acid media (Table III). The zeolitic species, in addition to the trityl cation (119), are the substituted cyclopentenyl cation 8 (102), the phenylindanyl cation 13, and the methylindanyl cation 12 (113). Values for the rert-butyl cation 2 and methylcyclopentyl cation 17 (prepared on metal halides) (43, 45) are included for comparison. Note that the ordering of isotropic chemical shifts is reasonably consistent with one s intuition from resonance structures i.e., the more delocalized the positive charge, the smaller the isotropic shift. This effect is even more apparent in the magnitudes of the CSA. Since... 

Summary of Chemical Shift Parameters for the Trityl Cation 16, Phenylindanyl Cation 13, Cyclopentenyl Cation 8, Methylindanyl Cation 12, tert-butyl Cation 2, and Methylcyclopentyl Cation 17... 

Experimental work using a pulse-quench catalytic reactor650 to probe transition between induction reactions and hydrocarbon synthesis on a working H-ZSM-5 catalyst has resulted in the suggestion that stable cyclopentenyl cations are formed during the induction period from small amounts of olefins formed in an induction reaction 647 One study reports a surprising observation, namely, enhanced aromatic formation over the physical mixture of Ga203 and H-ZSM-5 (1 1) (18.2% of benzene and methylbenzenes).651... 

If both cr-bonds form at the same time, the reaction is pericyclic, but carbocations are capable of reacting with alkenes, and so the first bond may form to give the cyclopentenyl cation 2.51 in one step, with the second bond formed in a separate step 2.51 (arrows). The reaction remains a cycloaddition whether both bonds are formed at the same time or not, but it is pericyclic only if they are both formed in the same step, as is probable in this case. Fig. 2.6 shows a small selection of other reactions that may similarly be pericyclic. >... 

Fig. 4.2 illustrates the first few members of the series of equilibria of conjugated ions. In cations, they are the equilibria between the allyl 4.11 and the cyclopropyl cation 4.12, the pentadienyl 4.13 and the cyclopentenyl cation 4.14, and the heptatrienyl 4.15 and cycloheptadienyl cation 4.16, In anions, they are between the allyl 4.17 and the cyclopropyl anion 4.18, the pentadienyl 4.19 and the cyclopentenyl anion 4.20, and the heptatrienyl 4.21 and cycloheptadienyl anion 4.22. There are heteroatom-containing analogues, with nitrogen and oxygen lone pairs rather than a carbanion centre, and the systems can again have substituents and fused rings. 

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