Cyclohexenes radical cations

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. 

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... 

For example, EPR evidence showed that cyclohexane-1,4-diyl, generated by radiolysis of hexadiene, rearranged to cyclohexene radical cation. Similarly, ant/-5-methylbicyclo[2.1.0]-pentane radical cation (33) rearranged to 1-methylcy-clopentene radical cation (34) via a 1,2-shift of the syn-5-hydrogen. ... 

Many studies used radiation chemistry to produce the radical and radical cations and anions of various dienes in order to measure their properties. Extensive work was devoted to the radical cation of norbomadiene in order to solve the question whether it is identical with the cation radical of quadricyclane . Desrosiers and Trifunac produced radical cations of 1,4-cyclohexadiene by pulse radiolysis in several solvents and measured by time-resolved fluorescence-detected magnetic resonance the ESR spectra of the cation radical. The cation radical of 1,4-cyclohexadiene was produced by charge transfer from saturated hydrocarbon cations formed by radiolysis of the solvent. In a similar system, the radical cations of 1,3- and 1,4-cyclohexadiene were studied in a zeolite matrix and their isomerization reactions were studied. Dienyl radicals similar to many other kinds of radicals were formed by radiolysis inside an admantane matrix. Korth and coworkers used this method to create cyclooctatrienyl radicals by radiolysis of bicyclo[5.1.0]octa-2,5-diene in admantane-Di6 matrix, or of bromocyclooctatriene in the same matrix. Williams and coworkers irradiated 1,5-hexadiene in CFCI3 matrix to obtain the radical cation which was found to undergo cyclization to the cyclohexene radical cation through the intermediate cyclohexane-1,4-diyl radical cation. 

However, the presence in the anolyte of an anion, which oxidises more readily than the organic substrate and which appears in the substituted product, does not preclude ionisation of the substrate as the operative mechanism. Thus Parker and Burget showed that cyanation of anisole does not proceed at potentials where only cyanide is electroactive ionisation of the anisole is a prerequisite for substitution. " A further example is the chlorination of cyclohexene where, by operating at high electrode potentials at which the olefin is electroactive, chloride ion can be intercepted en route to the anode surface by cyclohexene radical cations diffusing away from it. ... 

The observation that pulse radiolysis of NaO-saturated methylcyclohexane gives the solvent radical cation but that the argon-saturated solution gives the olefinic methyl-cyclohexene radical cation is attributed to the formation of a common excited-state precursor which then either fragments (Ar) or is quenched (NaO). Rate constants for the various processes have been measured. 

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. ... 

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). 

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

Products from the electrochemical oxidation of cyclohexene (Scheme 2.1) illustrate the general course of reaction [28, 29]. The radical-cation either undergoes loss of an allylic proton or reacts, at the centre of liighest positive charge density, with a nucleophile. Either reaction leads to a carbon radical, which is oxidised to the carbonium ion. A Wagncr-Meerwein rearrangement then gives the most stable carbonium ion, which subsequently reacts with a nucleophile. 

Oxidation of cyclohexene by peroxydisulfate in the presence of copper(II) salts results in the formation of cyclopentanecarboxaldehyde as the main product in an aqueous acetonitrile solution (equation 261), and 2-cyclohexenyl acetate in an acetic acid solution (equation 262).588,589 Reaction (261) has been interpreted as the formation of a radical cation (186) by oxidation of cyclohexene with S2Og, followed by hydrolysis of (186) to the /3-hydroxy alkyl radical (187), which is oxidized by copper(II) salts to the rearranged aldehydic product (188 equation 263).589... 

Nuclear597 or side-chain588,598 acetoxylation of arenes can be performed with good yields by persulfate and copper(II) salts in acetic acid (equations 268 and 269). As previously shown for cyclohexene (equation 263), persulfate oxidizes the aromatic ring to a radical cation which loses a proton to give a carbon radical, which is further oxidized by copper(II) acetate to the final acetoxylated product. 

The third radical cation structure type for hexadiene systems is formed by radical cation addition without fragmentation. Two hexadiene derivatives were mentioned earlier in this review, allylcyclopropene (Sect. 4.4) [245] and dicyclopropenyl (Sect. 5.3) [369], The products formed upon electron transfer from either substrate can be rationalized via an intramolecular cycloaddition reaction which is arrested after the first step (e.g. -> 133). Recent ESR observations on the parent hexadiene system indicated the formation of a cyclohexane-1,4-diyl radical cation (141). The spectrum shows six nuclei with identical couplings of 11.9G, assigned to four axial p- and two a-protons (Fig. 29) [397-399]. The free electron spin is shared between two carbons, which may explain the blue color of the sample ( charge resonance). At temperatures above 90 K, cyclohexane-1,4-diyl radical cation is converted to that of cyclohexene thus, the ESR results do not support a radical cation Cope rearrangement. 

Cyclohexadiene produces 1,2- and 1,4-diacetoxycyclohexene 2SS cyclohexene forms 3-acetoxycyclohexene 28S and 2-methyl-2-butene yields 2-methyl-3-acetoxy-l-butene 286 on electrolysis in acetic acid/sodiumacetate as SSE, presumably via radical cations. 

Enamines have unusually low oxidation potentials and readily undergo one-electron oxidation to a radical cation in the presence of a suitable oxidant. For example, enamines l-(Af,Af-dimethylamino)cyclohexene (41), 2,5-dimethyl-1-(TV,7V-dimethylamino)cyclohex-ene (42) and l-(A, A -dimethylamino)-l-phenylethene (43) have oxidation potentials of... 

So far, only a few examples of cationic photopolymerizations using PET corresponding to Scheme 3 have been described [10,13,165]. In the ternary system cyclohexene oxide, 9.10-dicyano anthracene and polynuclear aromatics, the polymerization of the former is initiated by the radical cations of the aromatic hydrocarbons formed via the PET with the dicyano compound. 

In another example, Yildirim et al. photochemically generated anthracene radical cations in the presence of TEMPO [29]. TEMPO immediately trapped the radical to form the TEMPO-anthracene cation, which was subsequently used to initiate cationic polymerization of cyclohexene oxide (CHOX). The resulting alkoxyamine-functional polycyclohexene oxide was used to macroinitiate styrene polymerization, resulting in the formation of S-6/-CHOX (Scheme 8.9). 

Bauld and coworkers, especially, developed the analogous Diels-Alder (4 + 2) cycloaddition reactions. These reactions are conveniently catalyzed by tris(4-bromophenyl)aminium hexachloroantimonate (78) or by photosensitization with aromatic nitriles. The radical cation-catalyzed Diels-Alder reaction is far faster than the uncatalyzed one, and leads to some selectivity for attack at the least substituted double bond for the monoene component (Scheme 18, 79 —> 80), but only modest endo selectivity (e- and x-80) [105]. Cross reactions with two dienes proved to be notably less sensitive to inhibition by steric hindrance of alkyl groups substituted on the double bonds than the uncatalyzed reactions, as cyclohexadiene adds detectably even to the trisubstituted double bond of 2-methylhexadiene (82), producing both 83 and 84. Dienes such as 85 react with donor-substituted olefins (86) to principally give the vinylcyclobutene products 87, but they may be thermally rearranged to the cyclohexene product 88 in good yield [105]. Schmittel and coworkers have studied the cation radical catalyzed Diels-Alder addition of both... 

The one-electron oxidation of cyclohexenes by S04 in aqueous solution has been studied from kinetic and stereochemical standpoints [49]. It was found that the alkene oxidation proceeds by an addition-elimination mechanism with 804" adding to the C=C double bond in the first step followed by C -OSOs" heterolysis to give a solvent-separated alkene radical cation-sulfate ion pair (SnI mechanism) (Scheme 9). 

Attack of water on the radical cation occurs before the sulfate group has completely departed, the sulfate group hindering the approach of water from one side of the cyclohexene skeleton. The lifetime of the solvent-separated radical cation was estimated to be in the 10-100 ps range [49]. 

The lifetime of the radical cation is <20 ns, as deduced from 193 nm photoionization experiments of the cyclohexene in aqueous solution. 

The radical cation formed in the heterolysis can also be produced by 193 nm photoionization of cyclohexene in aqueous solution, see Scheme 8 S. Steenken, unpublished material. 

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