Radical cation production

Methylbenzenes lose a proton from a methyl group to form a benzyl radical. In aqueous M-percbloric acid this reaction is fast with a rate constant in the range 10 lO s and the process is not reversible [24]. The process becomes slower as the number of methyl substituents increases, Hexaethylbenzene radical cation is relatively stable. When the benzyl radical is formed, further reactions lead to the development of a complex esr spectrum. Anodic oxidation of hexamethylbenzene in trifluoroacetic acid at concentrations greater than 1 O M yields the radical-cation I by the process shown in Scheme 6.1 [14], Preparative scale, anodic oxidation of methylbenzenes leads to the benzyl carbonium ion by oxidation of the benzyl radicals formed from the substrate radical-cation. Products isolated result from further reactions of this carbonium ion. 

An additional feature observed upon micellization is an increase in the relative yield of radical cation production. From Table IV can be seen that at low concentration of Q, the amount of radical cation produced is essentially nil and that triplet production is unity. Above CMC the radical cation production becomes comparable to the... 

The complexes [Ru(Pc)(CO)L] and [Ru(Pc)L2] (L = py, 4-methylpyridine, 4-tert-butylpyridine, imid, MeCN, DMF, DMSO) each show a reversible one electron oxidation in CH2C12 El — + 0.91 V and + 0.77 vs. SCE for [Ru(Pc)COpy] and [Ru(Pc)py2] respectively.2635 Quantitative oxidation gives the it radical cation products [Ru(Pc+)(CO)L] and [Ru(Pc+)L2] which have been characterized by electronic and ESR spectroscopy 26 the redox activity of ruthenium phthalocyani-nes have been compared to the corresponding porphyrin analogues.2635... 

Figure 6.25. The oxidation of a highly oxidizable aromatic probe substituted with multiple electron-donating substituents yields a detectable radical cation product.

The photoinduced anti-Markovnikov addition of methanol to 1,1-diphenylethene reported by Arnold and co-workers in 1973 provides the first example of the addition of a nucleophile to an arylalkene radical cation. There are now a number of studies that demonstrate the generality of nucleophilic addition of alcohols, amines, and anions such as cyanide to aryl- and diaryl-alkene radical cations. Product studies and mechanistic work have established that addition occurs at the 3-position of I-aryl or 1,1 -diarylalkene radical cations to give arylmethyl or diaryl-methyl radical-derived products as shown in Scheme I for the addition of methanol to 1,1-diphenylethene. For neutral nucleophiles, such as alcohols and amines, radical formation requires prior deprotonation of the 1,3-distonic radical cation formed in the initial addition reaction. The final product usually results from reduction of the radical by the sensitizer radical anion to give an anion that is then protonated, although other radical... 

Polymeric Viologen Systems. The write-erase efficiency of an ECD using for example aqueous MV would be low since both the dicationic and radical-cation states are very soluble. Improved MV-based systems may be made by retarding the rate at which the radical-cation product of electron transfer diffuses away from the electrode by use of an anionic polyelectrolyte such as poly(2-acrylamido-2-methylpropane-sulfonic acid) (polyAMPS) (46,47) or the sulfonated perfluorinated polyether, Nafion (48). An alternative approach is the surface modification of an electrode using silanization methodology (49), a preformed polymeric viologen salt (50) or oxidative electropolymerization of a pyrrole-substituted viologen (51) (Fig. 5). 

Anisole in trifluoroacetic acid-methylene chloride electrolyzed in a 2-compartment cell with Pt-gauze electrodes, after oxidation the polarity of the electrodes reversed to reduce the intermediate radical cations product. Y almost 100% conversion 50%. F. e. s. K. Bedigaard et al., Tetrah. Let. 1972, 2271. 

The radical cation of 1 (T ) is produced by a photo-induced electron transfer reaction with an excited electron acceptor, chloranil. The major product observed in the CIDNP spectrum is the regenerated electron donor, 1. The parameters for Kaptein s net effect rule in this case are that the RP is from a triplet precursor (p. is +), the recombination product is that which is under consideration (e is +) and Ag is negative. This leaves the sign of the hyperfine coupling constant as the only unknown in the expression for the polarization phase. Roth et aJ [10] used the phase and intensity of each signal to detemiine the relative signs and magnitudes of the... 

Figure 8, Wavepacket dynamics of the butatriene radical cation after its production in the A state, shown as snapshots of the adiabatic density (wavepacket amplitude squared) at various times. The 2D model uses the coordinates in Figure Ic, and includes the coupled A andX states, The PES are plotted in the adiabatic picture (see Fig. lb). The initial structure represents the neutral ground-state vibronic wave function vertically excited onto the diabatic A state of the radical cation,...

Aromatic ethers and furans undergo alkoxylation by addition upon electrolysis in an alcohol containing a suitable electrolyte.Other compounds such as aromatic hydrocarbons, alkenes, A -alkyl amides, and ethers lead to alkoxylated products by substitution. Two mechanisms for these electrochemical alkoxylations are currently discussed. The first one consists of direct oxidation of the substrate to give the radical cation which reacts with the alcohol, followed by reoxidation of the intermediate radical and either alcoholysis or elimination of a proton to the final product. In the second mechanism the primary step is the oxidation of the alcoholate to give an alkoxyl radical which then reacts with the substrate, the consequent steps then being the same as above. The formation of quinone acetals in particular seems to proceed via the second mechanism. ... 

Use geometries, electrostatic potential maps and spin densities to help you draw Lewis structures for butanal radical cation, the transition state and product. Where is the positive charge and the unpaired electron in each Is the positive charge (the unpaired electron) more or less delocalized in the transition state than in the reactant In the product ... 

When equimolar quantities of 80a and its dication 110 are combined in acetonitrile, single electron transfer occurs and the coproportionation product was obtained (95TL2741).Tliis deeply red-colored, air-sensitive radical cation 111 showed a strong ESR signal (g = 2.0034). On the other hand, the excellent electron donor 80a could be prepared by electrolytic reduction starting from 110. It was necessary to carry out the reduction with scrupulous exclusion of oxygen. Tlius, the electrolysis of 110 at -1.10 V initially gave rise to an intense red color, which was presumably due to the formation of 111. Upon further reduction, the red color faded and the tetraaza-fulvalene 80a was isolated at a 62% yield (Scheme 45). 

Formation of an intimate ion pair of OH " and aminium radical cation was also proposed for the intermediate step before deprotonation. The presence of the above radical was verified through UV analysis of the polymer formed with the characteristic band on the end group. Through chromatographic analysis of the TBH-DMT reaction products, H2O was detected as the above mechanism proposes after deprotonation. 

A comparison of the reactivity of F2 and CH3C02F with uracil, using 18F as a tracer, revealed that apart from 5-F and 5,5-difluoro products, two others, (3) and (4), which depended on the solvent, were obtained. This dependency was believed to arise from the fate of the radical cation... 

When 10-phenylphenothiazine (104) (and 10-phenylphenoxazine) was brominated in acetic acid a number of products were isolated. Pyridine perbromide, though, only brominated the phenyl substituent (Scheme 47). The suggestion that acetic acid bromination might involve the radical cation of the substrate (104) was confirmed by generating the radical cation of the substrate (104) with perchloric acid prior to bromination. Again a 43% yield of the 3-bromo product and multiple bromination products were observed (Scheme 47). The reaction of 10-phenylphenoxazine with pyridine perbromide appeared to be at least partially electrophilic the products... 

The mechanism proposed for the production of radicals from the N,N-dimethylaniline/BPO couple179,1 involves reaction of the aniline with BPO by a Sn-2 mechanism to produce an intermediate (44). This thermally decomposes to benzoyloxy radicals and an amine radical cation (46) both of which might, in principle, initiate polymerization (Scheme 3.29). Pryor and Hendrikson181 were able to distinguish this mechanism from a process involving single electron transfer through a study of the kinetic isotope effect. 

Simple mechanistic considerations easily explain why heterolytic dissociation of the C — N bond in a diazonium ion is likely to occur, as a nitrogen molecule is already preformed in a diazonium ion. On the other hand, homolytic dissociation of the C —N bond is very unlikely from an energetic point of view. In heterolysis N2, a very stable product, is formed in addition to the aryl cation (8.1), which is a metastable intermediate, whereas in homolysis two metastable primary products, the aryl radical (8.2) and the dinitrogen radical cation (8.3) would be formed. This event is unlikely indeed, and as discussed in Section 8.6, homolytic dediazoniation does not proceed by simple homolysis of a diazonium ion. 

The UV spectra suggest that the equilibrium between the diazonium ion and the solvent, on the one hand, and an electron donor-acceptor complex (8.58) on the other, lies on the side of the complex. The latter may possibly exist also as a radical pair (8.60) or a covalent compound (8.59). Dissociation of this complex within a cage to form an aryl radical, a nitrogen molecule, and the radical cation of DMSO is slow and rate-determining. Fast subsequent steps lead to the products observed. 

Elegant evidence that free electrons can be transferred from an organic donor to a diazonium ion was found by Becker et al. (1975, 1977a see also Becker, 1978). These authors observed that diazonium salts quench the fluorescence of pyrene (and other arenes) at a rate k = 2.5 x 1010 m-1 s-1. The pyrene radical cation and the aryldiazenyl radical would appear to be the likely products of electron transfer. However, pyrene is a weak nucleophile the concentration of its covalent product with the diazonium ion is estimated to lie below 0.019o at equilibrium. If electron transfer were to proceed via this proposed intermediate present in such a low concentration, then the measured rate constant could not be so large. Nevertheless, dynamic fluorescence quenching in the excited state of the electron donor-acceptor complex preferred at equilibrium would fit the facts. Evidence supporting a diffusion-controlled electron transfer (k = 1.8 x 1010 to 2.5 X 1010 s-1) was provided by pulse radiolysis. 

Ce4+ is a versatile one-electron oxidizing agent (E° = - 1.71 eV in HC10466 capable of oxidizing sulfoxides. Rao and coworkers66 have described the oxidation of dimethyl sulfoxide to dimethyl sulfone by Ce4+ cation in perchloric acid and proposed a SET mechanism. In the first step DMSO rapidly replaces a molecule of water in the coordination sphere of the metal (Ce v has a coordination number of 8). An intramolecular electron transfer leads to the production of a cation which is subsequently converted into sulfone by reaction with water. The formation of radicals was confirmed by polymerization of acrylonitrile added to the medium. We have written a plausible mechanism for the process (Scheme 8), but there is no compelling experimental data concerning the inner versus outer sphere character of the reaction between HzO and the radical cation of DMSO. 

It is evident from the nature of the products, especially those formed with toluene present, that the photoreaction in weakly acidic medium involves incursion of a radical species. The complete suppression of reactions leading to the above products, in the presence of oxygen, strongly suggests that it is an excited triplet trityl ion which undergoes reaction. It is postulated that the primary photochemical process is the abstraction of a hydrogen atom by the triplet trityl ion to form the radical cation 90, which was proposed as an intermediate in the dimerization reactions carried out in strong acid (Cole, 1970). 

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