Cyclohexene radical

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. ... 

Another DFT study on hydroxylation of cyclohexene catalysed by manganese oxoporphyrin suggests the physisorption of the substrate over the catalyst molecule followed by the abstraction of H from the C—H bond by the oxo group of the catalyst. The cyclohexene radical is readily bound to the OH group formed at the active site of the catalyst. Alcohol is formed on the catalyst active site via oxygen rebound mechanism without the dissociation of cyclohexene radical. ... 

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. 

The primary and secondary products of photolysis of common diazirines are collected in Table 4. According to the table secondary reactions include not only isomerization of alkenes and hydrogen elimination to alkynes, but also a retro-Diels-Alder reaction of vibrationally excited cyclohexene, as well as obvious radical reactions in the case of excited propene. 

An example of this reaction is the reaction of cyclohexene with t-butyl perbenzoate, which is mediated by Cu(I). " The initial step is the reductive cleavage of the perester. The t-butoxy radical then abstracts hydrogen from cyclohexene to give an allylic radical. The radical is oxidized by Cu(II) to the carbocation, which captures benzoate ion. The net effect is an allylic oxidation. 

There are three sorts of C-H bonds in cyclohexene, and Table 5.3 gives an estimate of their relative strengths. Although a typical secondary alkyl C-H bond has a strength of about 400 kj/mol (96 kcal/mol) and a typical vinylic C-H bond has a strength of 445 kj/mol (106 kcal/mol), ail allylic C-H bond has a strength of only about 360 kj/mol (87 kcal/mol). An allylic radical is therefore more stable than a typical alkyl radical with the same substitution by about 40 kj/mol (9 kcal/mol). 

Vinyl radicals can also participate in 6-exo cyclizations. In pioneering work, Stork and his group at Columbia University showed that stereoisomeric vinyl bromides 20 and 21 (see Scheme 3) can be converted to cyclohexene 22.7 The significance of this finding is twofold first, the stereochemistry of the vinyl bromide is inconsequential since both stereoisomers converge upon the same product and second, the radical cyclization process tolerates electrophilic methoxycarbonyl groups. The observation that the stereochemistry of the vinyl bromide is inconsequential is not surprising because the barrier for inversion of most vinyl radicals is very low.8 This important feature of vinyl radical cyclization chemistry is also exemplified in the conversion of vinyl bromide 23 to tricycle 24, the key step in Stork s synthesis of norseychellanone (25) (see Scheme 4).9 As in... 

It has been reported that molecnlar oxygen plays an important role in the allylic oxidation of olefins with TBHP (25, 26). Rothenberg and coworkers (25) proposed the formation of an alcoxy radical via one-electron transfer to hydroperoxide, Equation 4, as the initiation step of the allylic oxidation of cyclohexene in the presence of molecnlar oxygen. Then, the alcoxy radical abstracts an allylic hydrogen from the cyclohexene molecnle. Equation 5. The allylic radical (8) formed reacts with molecular oxygen to yield 2-cyclohexenyl hydroperoxide... 

In Entry 9, the initial reaction involves 5-exo addition of the aryl radical to the more-substituted end of the cyclohexene double bond, followed by a 6-endo addition to the phenylthiovinyl group. The reaction is completed by elimination of the phenylthio radical. The product is an intermediate in the synthesis of morphine. 

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... 

Allylic rearrangements with 3,3,6,6-dj-cyclohexene occurred in 20% of the MMO hydroxylation products compared to 33% for cytochrome P-450. These two experiments suggest that, with M. trichospor-ium OB3b, a rebound reaction must occur with a greater rate constant than with cytochrome P-450, in accord with the radical clock substrate work. 

Epoxide-derived radicals are generated under very mild reaction conditions and are therefore valuable for intermolecular C-C bond-forming reactions [27,29]. The resulting products, 5-hydroxyketones, 5-hydroxyesters or 5-lactones constitute important synthetic intermediates. The first examples were reported by Nugent and RajanBabu who used a variety of epoxides, such as cyclohexene oxide and a Sharpless epoxide (Scheme 7). 

Upon reaction of (1010) with Nil2, trinuclear [Ni3(/r3-I)2(dppm)3] (1012) is formed. The latter can also be obtained directly from [Ni(cod)2], Nil2 and dppm. (1012) photoreduces carbon dioxide to its radical anion, which can be trapped by cyclohexene.2455... 

Monochloroamine adds to cyclohexene to produce Pms-chlorocyclo-hexylamine and other products from radical recombination<94> ... 

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