Cyclohexenes radical attack

Spin trapping methods were also used to show that when carotenoid-P-cyclodextrin 1 1 inclusion complex is formed (Polyakov et al. 2004), cyclodextrin does not prevent the reaction of carotenoids with Fe3+ ions but does reduce their scavenging rate toward OOH radicals. This implies that different sites of the carotenoid interact with free radicals and the Fe3+ ions. Presumably, the OOH radical attacks only the cyclohexene ring of the carotenoid. This indicates that the torus-shaped cyclodextrins, Scheme 9.6, protects the incorporated carotenoids from reactive oxygen species. Since cyclodextrins are widely used as carriers and stabilizers of dietary carotenoids, this demonstrates a mechanism for their safe delivery to the cell membrane before reaction with oxygen species occurs. 

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. 

Thermolytic Reactions. At the higher temperatures, nonoxidative reactions also occur as for example the formation of tetrasubstituted cyclohexenes via Diels-Alder reaction, or the formation of dehydrodimers and mono- or polycyclic dimers via combination of alkyl free radicals or free radical attack on double bonds. 

Little is known of the chemistry of alkyl-substituted cycloalkanes such as C6H11CH3. The presence of one tertiary C—H bond will not noticeably increase the overall rate of radical attack because of the abundance of secondary C—H bonds. Attack at a ring C—H position will tend to give a similar product distribution to that observed for cyclohexane with high yields of the various isomeric cyclohexenes at most temperatures between 600 and 1000 K. Loss of a hydrogen atom at the /3 position to the R group would induce homolysis to give cyclohexene and R radicals, particularly at temperatures above 800 K. 

The first reaction was found by Levy and Szwarc to be predominant when methyl radicals attacked isooctane. The second reaction is predominant, however, for aromatic hydrocarbons. The free radicals formed in the above two reactions will react with each other, with other free radicals, or with impurities. The affinity of the methyl radical to attack an aromatic increases in the following order benzene, diphenyl ether, pyridine, diphenyl, benzophenone, naphthalene, quinoline, phenanthrene, pyrene, and anthracene. The ability of free alkyl radicals to interact with isopropylbenzene and cyclohexene decreases in the following order methyl, ethyl, propyl, butyl, isopropyl, sec-butyl, and tertiary butyl. 

The reduction of hydroxylamine by titanous salts in water produces the free amino radical, a reaction analogous to the formation of triphenylmethyl from the carbinol and a reducing agent.138 The amino radical will attack benzene to give diaminocyclohexadiene and di-(aminocyclohexadienyl) it converts cyclohexene into cyclohexyl-amine.139... 

A mathematical program developed and checked for this purpose (35) has been used to determine the three parameters rA, rB, and which determine the shapes of the curves. Table IV uses as examples the values obtained (6) for rA, rB, and in a homogeneous family such as ethers which are treated in co-oxidation with cumene, Tetralin, cyclohexene, and among themselves. The reactivity in the propagation stage depends much more on the substrate attacked than on the attacking radical. This hypothesis, moreover, is confirmed by the values of the product of rA X t b, values which are and have to be quite close to 1 (18,19). 

We have demonstrated that intermolecularly, amidyl radicals preferentially abstract an allylic hydrogen rather than add to a TT bond of olefins such as cyclohexene and 1,3-pentadiene (33). This reactivity pattern is completely reversed in intramolecular reactions as shown in the following examples of alkenyl mitro-samide photolysis. In every case, the amidyl radicals generated from photolysis preferentially attack the ir bonds intramolecu-... 

The photoreaction of cyclohexene with carbonyl compounds showed that attack at the allylic position of the cyclohexene molecule as well as addition to the double bond occurred (7). An attack at the allylic position of this compound is common in free radical reactions. It is therefore noteworthy that in the reaction of cyclohexene with formamide only addition products of formamide to cyclohexene were detected under these reaction conditions (i.e. temperature and concentrations of the various reagents employed). The concentration of the various reagents, or the facile addition step of the carbamoyl radical towards double bonds may account for these results. 

All this was later put on a sound basis as a result of more precise measurements of rate constants and of activation energies. However, it did not require precise measurements to predict which chlorinated hydrocarbons would decompose by a radical chain mechanism and which by the unimolecular mechanism. Clearly, if the chlorinated hydrocarbon, or the product from the pyrolysis of the chlorinated hydrocarbon reacted with chlorine atoms to break the chain then the chain mechanism would not exist. Such chlorinated hydrocarbons would decompose by the unimolecular mechanism. Mono-chlorinated derivatives of propane, butane, cyclohexane, etc. would afford propylene, butenes, cyclohexene, etc. All these olefins are inhibitors of chlorine radical chain reactions because of the attack of chlorine atoms at their allylic positions to give the corresponding stabilized allylic radicals which do not carry the chain. 

Co(II) carboxylates were well documented, and for cyclohexene as substrate the cyclohexenyl hydroperoxide is formed in situ by attack of 02 on the allylic radical produced by allylic hydrogen abstraction, Reaction 12 (4, 48). The products, usually those shown in Reaction 13, are formed via the metal-catalyzed decomposition of the hydroperoxide, and any 02 coordination at the metal is incidental there is... 

In the first propagation step of the Wohl-Ziegler bromination, the bromine atom abstracts a hydrogen atom from the allylic position of the olefin and thereby initiates a substitution. This is not the only reaction mode conceivable under these conditions. As an alternative, the bromine atom could attack the C=C double bond and thereby start a radical addition to it (Figure 1.25). Such an addition is indeed observed when cyclohexene is reacted with a Br2/AIBN mixture. 

The dynamics of the reactions of 0( P) with cyclohexane, cyclohexene, and cyclohexa-1,4-diene have been studied by measurement of the product OH(X II) internal state distributions in a molecular beam/LIF apparatus. The rotational state distributions were found to be similar for all three reactions and consistent with small (1—3%) partitioning of the available energy, indicating that H-abstraction occurs only when the O atom is collinear with the C-H bond under attack. Comparisons with model predictions suggested that some of the extra energy available in the more exoergic reactions between 0( P) and the unsaturated hydrocarbons is released into internal excitation of the hydrocarbon radical product, resulting in only a modest increase in OH vibrational excitation. 

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... 

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]. 

Another route to alkenyl radicals is by addition of radicals to alkynes. An application of this procedure, which serves as a model for the synthesis of the CD ring system of cardiac aglycones was reported by Stork and co-workers (4.55). The initial alkyl radical, formed selectively from the bromide 61 attacks the alkyne regioselectively to give an intermediate alkenyl radical, which reacts further with the alkene of the cyclohexene to give the product 62. A mixture of alkene stereoisomers is produced owing to the ease of E-Z alkenyl radical isomerization. 

The oxidation of cyclohexene in the presence of copper, cobalt and manganese carboxylates has continued to receive attention in recent years [430-441]. The stable monomeric products of reaction are largely 2-cyclohexene-l-one and 2-cyclo-hexene-l-ol with smaller amounts of cyclohexene oxide. Cyclohexenyl hydroperoxide formed by attack of dioxygen on the allylic radical produced by allylic-hydrogen abstraction, has been established to be the reaction intermediate. The product profile has been found to vary somewhat with the metal complex used. It was found [431] that with Co(II) or Mn(II) carboxylates reaction rate and selectivity to 2-cyclohexene-l-one were maximal at 46 °C. 

Both Ru02 and Pt(0) hydrogen evolution catalysts were used in this work [51].Similar rates of attack by the excited state decatungstate and t-butoxy radical on cyclohexene by Hou and Hill were consistent with the radical nature of the substrate attack process [75, 88],... 

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