Olefins radical cation

Although these substrates are both inert to singlet oxygen, olefin oxidative cleavage was observed with superoxide. Furthermore, an oxidative equilibration of the olefin radical cations... 

Each of these reactants is thought to occur through a common intermediate, the olefin radical cation, but the environment in which this species is generated apparently controls its observed chemistry. 

The above facts indicate that on excitation of the semiconductors the olefins transfer an electron to the photogenerated positive hole in the initiation process to give the olefin radical cation, and concurrently the electron excited to the conduction band is... 

To get insight into the reactivity of olefin radical cations toward oxygen, anodic oxidation of olefins under oxygen was attempted. DPE was electrolyzed at 1.5 V vs SCE in a mixture of... 

In some cases the nucleophilic capture of a radical cation is followed by coupling with the radical anion (or possibly with the neutral acceptor), resulting ultimately in an aromatic substitution reaction. Thus, irradiation of 1,4-dicyanobenzene in acetonitrile-methanol (3 1) solution containing 2,3-dimethylbutene or several other olefins leads to capture of the olefin radical cation by methanol, followed by coupling of the resulting radical with the sensitizer radical anion. Loss of cyanide ion completes the net substitution reaction [144]. This photochemical nucleophile olefin combination, aromatic substitution (photo-NOCAS) reaction has shown synthetic utility (in spite of its awkward acronym). 

In summary, a variety of interesting bifunctional radical cation structures have been established. At least one species (152) features an orthogonal arrangement of spin and charge, whereas another (155) clearly meets the definition of a distonic radical cation. The fulvene radical cations (154), on the other hand, are bifunctional and may deviate to some extent from planarity however, they appear to be ordinary olefin radical cations rather than perpendicular ones. 

However, in most cases, the detailed mechanism is not yet known, i.e., whether CIP, SSIP or even free radical ions are scavenged by the nucleophile. Arnold and Snow [62] suggest an attack of methanol to solvated olefin radical cations, whereas Mariano observed a highly stereoselective example of a direct scavenging of a radical cation — radical anion pair by methanol [63]. Although this process has received relatively little attention, it is obvious that scavenging of different types of radical ion intermediates is not only possible but may be used to differentiate between the various types of radical ion pairs (CIP and SSIP). 

Whereas a triplet 1,4-biradical has been assigned as the most probable intermediate at that time, later work on intramolecular trapping reaction favored the assumption of radical ion pairs [46]. Efficient lactonization reaction to form 51 during irradiation of pent-4-enoic acid 50 and 48 accounts for an olefin radical cation which undergoes electrophilic addition towards the carboxyl group. Another type of rearrangement has been detected in the photoreaction of tetramethylallene and 48 [47]. 5-Hydroxy-indan-2-ones 52 are formed in high yields probably via instable spiro-oxetanes 49b as intermediates. 

The pulse radiolysis studies of liquid alkanes have relevance to the radiolysis of polyethylene and related polymers. In liquid alkanes at ambient temperature, the reaction intermediates such as alkane radical-cations, olefin radical-cations, olefine dimer-cations, excited states, and alkyl radicals have been observed after the electron-pulse irradiation [90-93]. According to the nanosecond and subnanosecond studies by Tagawa et al., the observed species were alkane radical cations, excited states, and alkyl radicals in n-dodecane excited states and cyclohexyl radical were observed in cyclohexane, and only radicals in neopentane [91, 93]. Olefin radical-cations were also detected in cyclohexane containing carbon tetrachloride [92],... 

Reactions of PET-generated alkene radical cations have been one of the important areas of research over the years and several reviews have been written on this subject [2, 5,11], A vivid summary of this topic has been provided by Mattay [10] recently. However, we will discuss here some representative examples of synthetic interest from olefin radical cations. The reactivity profiles of alkene radical cations may be illustrated on the lines of Mattay [10b] as shown below. 

Among the suggestions for the [2 4- 2] cycloaddition mechanism of 02 to olefins, Foote etal. [39,40] proposed a nonconcerted pathway involving a rate-limiting electron-transfer process with the formation of an olefin radical cation and superoxide ion complex (Scheme 1). 

In the effort to find confirmation on Foote s original mechanistic proposal [84] and discriminate among these two different pathways, a great deal of experimental proofs were achieved. First of all, the DCA and/or 9-cyanoanthracene (CNA)-sensitized reactions on aryl-olefins were studied under inert atmosphere by flash spectroscopic techniques obtaining clear evident for the formation of both olefin radical cations and cyanoaromatic radical anions [95]. In the presence of oxygen, the cyanoaromatic radical anions were rapidly removed, supporting the very rapid formation of superoxide ion and so its direct involvement in these photoinduced oxygenations. 

However, as we will see later on, other modes of evolution of the primary intermediate radical ions can be suggested to explain some oxidation reactions mediated by electron-transfer processes. In fact, several exceptions to the Foote s BQ-controlled photooxygenation procedure have been reported during the last years on several electron-rich substrates. Thus, the involvement of superoxide ion, as an oxygen active species, in all of the DCA-sensitized photooxygenations, remains questionable [96,105,112,115,128]. Schaap and co-workers [98] recorded under inert atmosphere the characteristic ESR spectrum of the (DCA ) radical anion. On the other hand, the involvement of aryl-olefin radical cations and their reactions with superoxide ion was easily observed by quenching experiments with compounds exhibiting lower oxidation potentials than those of aryl-olefins [85, 95, 98],... 

The authors, on the basis of their experimental results, pointed out that olefin radical cations-allylically methylated and/or stabilized by heteroatoms or n conjugation [136] do not react with an active oxygen species, suggesting, in spite of their low oxidation potentials, that several factors may contribute to determine this behavior. Thus, one of the important factors is the presence of an independent 7t-system in which odd-electron density is not delocalized, whereas, a second and a third factor can be the insufficient steric hindrance to block the attack of oxygen... 

Reactions with stable radical species, e.g. oxygen and NO2, occur readily, although oxygenation rates of aryl olefin radical cations (ca 10 s [310]) are two or... 

Organic and inorganic radical formation in zeolites can occur spontaneously, on adsorption of molecules into a suitably activated zeolite, or as the result of radiolysis of adsorbed species. Once a radical is formed, EPR spectroscopy can be used to follow its subsequent reactions. For example, Trifunac et al have recently described the use of variable temperature EPR to investigate reactions of olefin radical cations generated in ZSM-5 zeolites. [4]. This work shows clearly the facile rearrangement of radical cations produced by irradiation of... 

Photochemical addition reactions may also occur as electron-transfer reactions involving a radical ion pair. An illustrative example is the photochemical reaction of 9-cyanophenanthrene (154) with 2,3-dimethyl-2-butene, which, in nonpolar solvents, gives good yields of a [2 + 2] cycloadduct via a singlet exciplex, while in polar solvents radical ions are formed in the primary photochemical process. The olefin radical cation then undergoes deprotonation to yield an allyl radical or suffers nucleophilic attack by the solvent to produce a methoxy alkyl radical. Coupling of these radicals with... 

Polyelectrolytes and soluble polymers containing triarylamine monomers have been applied successfully for the indirect electrochemical oxidation of benzylic alcohols to the benzaldehydes. With the triarylamine polyelectrolyte systems, no additional supporting electrolyte was necessary [91]. Polymer-coated electrodes containing triarylamine redox centers have also been generated either by coating of the electrode with poly(4-vinyltri-arylamine) films [92], or by electrochemical polymerization of 4-vinyl- or 4-(l-hydroxy-ethyl) triarylamines [93], or pyrrol- or aniline-linked triarylamines [94], Triarylamine radical cations are also suitable to induce pericyclic reactions via olefin radical cations in the form of an electron-transfer chain reaction. These include radical cation cycloadditions [95], dioxetane [96] and endoperoxide formation [97], and cycloreversion reactions [98]. 

Silver ions cause perturbation of the (E)-(Z) photoisomerization pathway for both stilbene and azobenzene . The efficiency of silver ions in this respect is compared with the effect of Nal which can only induce a heavy atom effect. Ag+ clearly forms complexes with both compounds. Observation of cis-trans conversion in olefin radical cations shows that electron transfer can bring about isomerization of stilbene derivatives. The efficiency of such processes obviously depends on the presence and nature of any substituents. Another study deals with photochemical generation, isomerization, and effects of oxygenation on stilbene radicals. The intermediates examined were generated by electron transfer reactions. Related behaviour probably occurs through the effect of exciplex formation on photoisomerization of styrene derivatives of 5,6-benz-2,2 -diquinoyE. ... 

Behavior of olefin radical cations has not been vjell investigated until recently. Lewis reported that 9,10-dicyanoanthracene (DCA) sensitized irradiation of stilbene led to its cis to trans isomerization (33). The resulting c - from electron transfer from cis-isomer to DCA isomerizes to t , v hich subsequently accepts an electron from cis to give trans and regenerate leading to a chain process. The quantum yield for cis to trans isomerization increases with increasing cis-isomer concentration. 

However, let us now extend Simons logic to gain understanding of olefin radical cations as well as of olefins. More precisely, planar o2ir1 C2H4+ correlates... 

Although the first case puts no demands on the energetics because escape is always a feasible process, no direct observation by a time-resolved CIDNP experiment seems to have been successful so far instead, it has repeatedly been reported that time-resolved photo-CIDNP experiments showed olefin radical cations or anions to be configurationally stable. The second and third cases are obviously only possible if the energy of the olefin triplet or the biradical lies below the energy of the radical ion pair. 

Usually, as the formation of a radical-cation from a neutral substrate is associated with an increase in its acidity, facile deprotonation can take place [6, 7]. In a majority of instances, proton transfer takes place between radical-cation/radical-anion pairs, with the net result being the formation of two radicals and consequently a bimolecular coupling product (Scheme 3). This process is encountered in benzyl radical-cations, olefin radical-cations, and amine radical-cations. 

Nucleophilic addition to organic radical-cations is one of the most common pathways to produce radicals. Irradiation of a wide variety of A-(2-alkenyl)- and N-(3-alkenyl)phthalimides and A-alkenylphthalimides [26] with a remote alkenyl double bond [27] afford new 5-, 6- and medium-size ring systems. Irradiation in methanol-acetonitrile triggers intramolecular electron transfer from the olefin double bond to the excited phthalimide carbonyl group. Because of the highly nucleophilic character of methanol, the olefin radical-cation can be trapped. The derived radical combines with the radical-anion to yield coupling products (Scheme 15). 

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