Cationic cyclohexene

Several mechanisms for the polymerization of vinyl ether and epoxies have been suggested [20,22,23,25,27,28,33-35]. On irradiation with gamma rays or electrons, pure epoxies polymerize via a cationic mechanism [35]. However, this cationic polymerization is inhibited by just traces of moisture, as shown below for cyclohexene oxide in reaction 5. 

The first successful hydrogenation reactions in ionic liquids were studied by the groups of de Souza [45] and Chauvin [46] in 1995. De Souza et al. investigated the Rh-catalyzed hydrogenation of cyclohexene in l-n-butyl-3-methylimidazolium ([BMIM]) tetrafluoroborate. Chauvin et al. dissolved the cationic Osborn complex [Rh(nbd)(PPh3)2][PFg] (nbd = norbornadiene) in ionic liquids with weakly coordinating anions (e.g., [PFg] , [BFJ , and [SbF ] ) and used the obtained ionic catalyst solutions for the biphasic hydrogenation of 1-pentene as seen in Scheme 5.2-7. 

In the present instance, protonation of the C1-C2 double bond gives a carbo-cation that can react further to give the 1,2 adduct 3-chloro-3-methylcyclohexene and the 1,4 adduct 3-chloro-L-methylcyclohexene. Protonation of the C3-C4 double bond gives a symmetrical carbocation, whose two resonance forms are equivalent. Thus, the 1,2 adduct and the 1,4 adduct have the same structure 6-chloro-l-methyl-cyclohexene. Of the two possible modes of protonation, the first is more likely because it yields a tertiary allylic cation rather than a secondary allylic cation. 

The most radiation-stable poly(olefin sulfone) is polyethylene sulfone) and the most radiation-sensitive is poly(cyclohexene sulfone). In the case of poly(3-methyl-l-butene sulfone) there is very much isomerization of the olefin formed by radiolysis and only 58.5% of the olefin formed is 3-methyl-l-butene. The main isomerization product is 2-methyl-2-butene (37.3% of the olefin). Similar isomerization, though to a smaller extent, occurs in poly(l-butene sulfone) where about 10% of 2-butene is formed. The formation of the olefin isomer may occur partly by radiation-induced isomerization of the initial olefin, but studies with added scavengers73 do not support this as the major source of the isomers. The presence of a cation scavenger, triethylamine, eliminates the formation of the isomer of the parent olefin in both cases of poly(l-butene sulfone) and poly(3-methyl-1-butene sulfone)73 indicating that the isomerization of the olefin occurred mainly by a cationic mechanism, as suggested previously72. 

A potential major benefit of a cyclohexene-based template was the capacity to mimic more closely the putative transition state sialosyl cation by the placement of a double bond in the correct position. While the ring conformation in Neu5Ac2en 4 is very similar to the sialosyl cation transition state 5, these two stracmres are essentially isomers due to the fact that the double bond is either between the ring oxygen and C-2 in 5 or C-2 and C-3 in 4. The synthesis and biological evaluation of... 

In nucleophilic solvents, products that arise from reaction of the solvent with the cationic intermediate may be formed. For example, reaction of cyclohexene with hydrogen bromide in acetic acid gives cyclohexyl acetate as well as cyclohexyl bromide. This occurs because acetic acid acts as a nucleophile in competition with the... 

Several pieces of mechanistic evidence implicate allylic radicals or cations as intermediates in these oxidations. Thus 14C in cyclohexene is distributed in the product cyclohexenone indicating that a symmetrical allylic intermediate is involved at some stage.159... 

A further interesting example of a steric effect was recently published106. The sterically shielded 2.2.6.6-tetramethyl piperidinium radical cation adds to cyclohexene by almost three powers of ten slower than the piperidinium radical cation itself107. ... 

There are many reports for nitration of alkenes using various nitrating agents, which proceeds via an ionic or radical addition process.49 Nitration of cyclohexene with acetyl nitrate gives a mixture of 3-and y-nitrocyclohexenes, 1,2-nitroacetate, and 1,2-nitronitrate. This reaction is not a simple ionic or radical process instead, [2+2] cycloaddition of nitryl cation is proposed.50... 

Cationic complexes, such as 55 and 56, catalyze the polymerization of propylene oxide, cyclohexene oxide, and of e-caprolactone with substantially higher activities than neutral zinc complexes. 

Irradiation of mixtures of cyclohexene with 1,3-cyclohexadiene leads to high yield of dimers (G = 6.3)86. Schutte and Freeman87 found that radiolysis of 1,3-cyclohexadiene dissolved in various solvents gives dimers mainly via cationic Diels-Alder addition,... 

The simplest model compound is cyclohexene oxide III. Monomers IV, V and VII represent different aspects of the ester portion of I, while monomers VII and VIII reflect aspects of both the monomer I and the polymer which is formed by cationic ring-opening polymerization. Monomers IV-VII were prepared using a phase transfer catalyzed epoxidation based on the method of Venturello and D Aloisio (6) and employed previously in this laboratory (7). This method was not effective for the preparation of monomer VIII. In this specific case (equation 4), epoxidation using Oxone (potassium monoperoxysulfate) was employed. 

However, styrene and cyclohexene gave complex product mixtures, and 1-octene did not react under the same reaction conditions. Thus, the activity of this catalyst is intrinsically low. Jacobs and co-workers [159,160] applied Veturello s catalyst [PO WCKOj ]3- (tethered on a commercial nitrate-form resin with alkylammonium cations) to the epoxidation of allylic alcohols and terpenes. The regio- and diastereoselectivity of the parent homogeneous catalysts were preserved in the supported catalyst. For bulky alkenes, the reactivity of the POM catalyst was superior to that of Ti-based catalysts with large pore sizes such as Ti-p and Ti-MCM-48. The catalytic activity of the recycled catalyst was completely maintained after several cycles and the filtrate was catalytically inactive, indicating that the observed catalysis is truly heterogeneous in nature. 

When 4-/-butylcyclohex-1 -enyl(phenyl)iodonium tetrafluoroborate (3) is heated at 60 °C in chloroform, 1-fluorocyclohexene 4, 1-chlorocyclohexene 5 and l-(o-iodophenyl)cyclohexene 6 are formed with accompanying iodobenzene leaving group (eq 2).3 These three substitution products are best accounted for by formation of an ion pair involving cyclohexenyl cation 7. The cyclohexenyl cation 7 formed picks up fluoride from tetrafluoroborate and chloride from chloroform solvent, and recombines with the iodobenzene generated (eq 3). This kind of reactions with a counteranion and solvent are characteristic of unstable carbocations and are known in the case of phenyl cation generated from the diazonium salt in the Schiemann-type reaction.4... 

We are being somewhat disingenuous here. If performed and interpreted correctly and with the appropriate ancillary phase-change enthalpy information, the enthalpy of formation of an arbitrary species by ion-molecule reaction chemistry and by combustion calorimetry must be the same. That the ionization potential of quinuclidine is higher than l,4-diazabicyclo[2.2.2]octane simply says that there is a stabilizing effect in the radical cation of the latter not found in the former. This information does not say that there is a stabilizing effect in the neutral molecular form of the latter not found in the former. After all, we trust the reader is not bothered by the fact that the ionization potential order of the cyclohexenes increases in the order 1,3-diene < 1,4-diene < 1-ene < 1,3,5-triene (benzene). 

The mechanism for the photoreaction between 133 and cyclohexene can be summarized as in Scheme 8. The initiating electron transfer fluorescence quenching of 133 by cyclohexene resulted in the formation of an w-amino radical-radical cation pair 136. Proton transfer from the 2-position of the cyclohexene radical cation to the nitrogen atom of the a-amino radical leads to another radical cation-radical pair 137. Recombination of 137 at the radical site affords the adduct 134, while nucleophilic attack at the cation radical of 136 leads to another radical pair 138 which is the precursor for the adduct 135. 

These results have been interpreted in terms of HOMO-LUMO interactions. As a result of the orbital perturbation, the interaction of the HOMO of the cyclohexene double bond with the LUMO of the developing cation may become effective. At the first stage of this interaction, an overlap of the LUMO of the cyclobutyl cation with the p lobe of the double bond located close to the cation center is probably important. However, when the reaction progresses, the interaction with the p lobe on the remote carbon atom has been assumed to increase significantly. 

Scheme 4 Anodic substitution and addition with cyclohexene (15) via a radical cation.

Despite many applications of the iron-mediated carbazole synthesis, the access to 2-oxygenated tricyclic carbazole alkaloids using this method is limited due to the moderate yields for the oxidative cyclization [88,90]. In this respect, the molybdenum-mediated oxidative coupling of an arylamine and cyclohexene 2a represents a complementary method. The construction of the carbazole framework is achieved by consecutive molybdenum-mediated C-C and C-N bond formation. The cationic molybdenum complex, required for the electrophilic aromatic substitution, is easily prepared (Scheme 23). 

This bimolecular mechanism also applies to cycloalkanes which can be activated by intermolecular hydride transfer to small carbenium ions to form cyclohexyl cations prior to cracking. Alternately, the cyclohexyl cations can deprotonate and form cyclohexene. With two similar intermolecular hydride transfers an aromatic can also form [46]. 

The prototype hole-catalyzed Diels-Alder reaction between the butadiene radical cation and ethylene has also been studied by Bauld [53]. He finds strongly exothermic formation of a l-hexene-3,6-diyl radical cation intermediate without activation energy followed by a weakly activated (activation energy 2.3 kcal mol ) closure of the second C-C bond to form the cyclohexene radical cation, The reaction shows no overall activation energy relative to the... 

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