Cation radical probes

Absolute Reaction Rates and Cation Radical Probes... 

This substrate, along with the corresponding substrate 6, which has a cyclo-butanation rate of 1.2 x 10 , have been proposed as sensitive cation radical probes, for detecting the presence of cation radical intermediates in various reactions [74]. An interesting example is the reaction of tetracyanoethylene (TCNE) with electron rich alkenes, a reaction for which an electron transfer mechanism had been considered (Scheme 39) [75],... 

Scheme 39. Cation radical probes quantitatively rule out an et mechanism for the addition of tetracyano-ethylene to electron rich alkenes.

Though the detailed mechanism of olefin epoxidation is still controversial, Scheme 8 depicts possible intermediates, metallacycle (a), K-cation radical (b), carbocation (c), carbon radical (d), and concerted oxygen insertion (e) [2, 216, 217]. As discussed above, the intermediacy of metallacycle has been questioned. One of the most attractive mechanism shown in Scheme 8 is the involvement of one electron transfer process to form the olefin 7C-cation radicals (b). Observation of rearranged products of alkenes, known to form through the intermediacy of the alkene cation radicals, in the course of oxidation catalyzed by iron porphyrin complexes is consistent with this mechanism [218, 219]. A -alkylation during the epoxidation of terminal olefins is also well explained by the transient formation of olefin cation radical [220]. A Hammett p value of -0.93 was reported in the epoxidation of substitute styrene by Fe (TPP)Cl/PhIO system, suggesting a polar transition state required for cation radical formation [221] Very recently, Mirafzal et al. have applied cation radical probes as shown in Scheme 9 to... 

Scheme 9. Examples of cation radical probes prepared by Mirafzal et al. [222,223].

Haddaway, K., Somekawa, K., Fleming, R, TosseU, J.A., and Mariano, P.S., The chemistry of aUene cation radicals probed by the use of theoretical and electron-transfer photochemical methods, /. Org. Chem., 52, 4239,1987. 

The obtained data clearly show that the g-anisotropy of the triplet states is larger than that of the respective cation-radical. A similar effect has been observed for the triplet states of the primary donors in PS II231 and in the bacterial RC.111112114 This can be explained by the fact that the triplet electrons probe the spin distribution in two different orbitals (HOMO and LUMO), and the latter has a rather large spin density at the nitrogens and the central magnesium (cf. references 216, 218), by which the spin-orbit coupling and the g-anisotropy is increased. 

In general, the ethylene bond in organic cation radicals is weakened, and the barrier to rotation becomes significantly less than that of the neutral ethylene derivative. This particular property of the ethylene bond in cation radicals has been used to probe for the mechanism of many reactions (Todres 1987). 

Horhold found that the action spectrum of phenyl-substituted polyarylene-vinylenes shifts substantially to longer wavelengths when the photoconductor probe is simultaneously irradiated by UV light from another source31. The action spectrum of photoconductivity of the polymer subjected to UV radiation is the same as the action spectrum of photoconductivity of the cation radical of the same polymer... 

The reaction of 6 with TCNE yields only the conventional adduct corresponding to the uncyclized probe and none of the product expected from the cation radical cyclization. That the probe cyclization of the cation radical of 6 would have been observed in the context of an ET mechanism, if it had been involved, was demonstrated by generating the contact ion radical pair of 6+7TCNE via excitation of the charge transfer complex of 6 and TCNE. The cyclobutane cyclization product of the probe reaction was easily detected under these conditions. Consequently, an ET mechanism for this reaction can be confidently excluded. In a similar manner, the epoxidation of 5 and 6 by oxidized metalloporphyrins provides strong evidence against a cation radical mechanism for these reactions [76]. 

Goff and coworkers extensively used paramagnetic NMR in identifying the nature of coupling present between the unpaired electron present on the metal and porphyrin ring in metalloporphyrin it-cation radicals [224-228], NMR spectroscopy does probe the environment of protons at the extreme periphery of... 

C. Stradowski, Methylviologen cation radical as probe of oxygen diffusion through polymer, J. Appl. Polym. Sci., 1990, 41, 2511 B. Maiti and S. Schlick, Oxygen permeation in perfluorinated ionomers based on the reaction with the methylviologen cation radical. An ESR and optical study, Chem. Mater., 1992, 4, 458. 

On the other hand, Newcomb et al. proposed a cationic pathway for P-450 catalyzed C-H hydroxylation, based on experiments using various radical probes [6]. The results suggest that no intermediate is formed during the reactions. Recently Collman et al. proposed that alkane makes a complex with oxo species and oxygen transfer occurs in a stereospecific manner [7],... 

Although SET photochemistry is often a practical method to promote cation radical reactions, such a variant had not been probed until we recently discovered the SET-photoinduced degenerate methylenecyclopropane rearrangement of 2,2-diaryl-1-methylenecyclo-propanes." Below, we describe in detail the results of our studies of this process including the chemical and spectroscopic identification... 

The measured rate constant for decay of the initial monomer radical cation from probe 1 of 1.2 x 10 s" agrees reasonably well with Bauld s earlier estimate, although the agreement appears to be fortuitous. By contrast, the rate constant for the cyclization of the monomer radical cation of probe 2 is almost 3 orders of magnitude faster than both the earlier estimate from product studies for this system (>3 x 10 s ) and the measured rate constant for 1. It is possible that some of the discrepancy may be due to cleavage of the initial adduct radical cation to regenerate the monomer radical cation, which would mean that the rate constant measured in the laser experiments does not reflect only the initial cyclization rate. The apparently slower... 

These results provide the first detailed calibration for a series of intramolecular radical cation probes based on cycloaddition chemistry. The cyclization rate constants cover several orders of magnitude in timescale, an ideal case for using 1—3 as probes for radical cations of different lifetimes. However, the time-resolved experiments demonstrate that the application of radical cation probes, at least those based on aryl alkene cycloaddition chemistry, may be considerably less straightforward than similar experiments with free radical probes or clocks. Some of the problems that need to be addressed include the variation of products with the reaction conditions and method of radical cation generation, and the possibility of reversibility of the initial adduct formation. Furthermore, at least some radical cation reactions are quite sensitive to solvent and this may mean that calibrations for radical cation cycloadditions will have to be done in a variety of solvents. 

Abstract. A -ethyl-A -hexadecyl-4,4 -bipyridinium bromide (Cj6VBr2) and A -ethyl-A -octadecyl-4,4 -bipyridinium bromide (CigVBr2) were used as electroactive probes to assess the interactions between surfactants and cyclodextrins. Cyclic voltammetry, visible spectroscopy, fluorescence spectroscopy and surface tension techniques were used to detect the formation of complexes between the surfactant viologen probes and a- and -cyclodextrins. The voltammetric results suggest the formation of inclusion compounds in which the hydrophobic tail of the surfactant viologens penetrate the cyclodextrin cavity. The dimerization of the viologen cation radicals is essentially suppressed by the presence of a-cyclodex-trin (ACD) while no effects are observed in the presence of )5-cyclodextrin (BCD). The observed results are best explained by the relative solubility in aqueous media of each of the inclusion complexes in the several accessible viologen oxidation states. 

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