Ion radical intermediates

Emission spectra have been recorded for four aryl-substituted isoindoles rmder conditions of electrochemical stimulation. Electrochemiluminescence, which was easily visible in daylight, was measured at a concentration of 2-10 mM of emitter in V jV-dimethylformamide with platinum electrodes. Emission spectra due to electrochemi-luminescence and to fluorescence were found to be identical, and quantum yields for fluorescence were obtained by irradiation with a calibrated Hght source. Values are given in Table X. As with peak potentials determined by cyclic voltammetry, the results of luminescence studies are interpreted in terms of radical ion intermediates. ... 

Other evidence cited for SET mechanisms has been detection of radical or radical ion intermediates by ESR or CIDNP the finding that such reactions can take place at 1 -norbomyl bridgeheads and the formation of cyclic side products when the substrate has a double bond in the 5,6 position (such substrates are called radical probes). 

When catechol was oxidized with Mn04 under aprotic conditions, a semiquinone radical ion intermediate was involved. For autoxidations (i.e., with atmospheric oxygen) a free-radical mechanism is known to operate. 

Rule 1 applies to homogeneous processes and states that those reactions which are sensitive to the sonochemical effect are those which proceed via radical or radical-ion intermediates. This statement means that sonication is able to effect reactions proceeding through radicals and that ionic reactions are not likely to be modified by such irradiation. 

The aza-di-TT-methane (ADPM) rearrangement of aza-1,4-dienes via radical-cat-ions suggests the possibility that other radical-ion intermediates (e.g., radical-anions) could also be responsible for this rearrangement reaction. In order to test this proposal, the azadiene 101 was irradiated for 20 min in acetonitrile using A,iV-dimethylaniline (DMA) as an electron-donor sensitizer. The reaction leads to formation of the cyclopropylimine 102. Separation of product mixture by column chromatography on silica gel affords the aldehyde 34 (21%) resulting from hydrolysis of the imine 102, (Scheme 18) [70]. 

The above results lead to the conclusion that rearrangement of 124 to the cyclopropane 125, occurring in the reactions sensitized by DMA, must take place via radical-anion intermediates. Considering that the C—double bond in compounds 101, 117, 118, and 119 should be a better electron acceptor than the diphenylvinyl unit in 124, it is logical to assume that the rearrangement of the 1-azadienes also takes place via the same types of radical-ion intermediates. 

The rapid rates of reduction of the oxalato (10) (k = 450 + 1,000 (H+)) and of the pyruvate (2) complexes (2A x 103at 25°C. and (H+) = 0.1) can hardly be understood as caused by chelation. Binoxalate does not chelate unless the proton is lost, and the rate law for the reduction of the complex shows that it brings a proton into the activated complex. Pyruvate almost certainly is not chelated in the product. Both groups are rapidly reduced by Craq.+2 when they are feee from the cobalt center. (The reduction of H2C2O4 by Craq+2 was explored by R. Milburn and the present author (29). The observations on pyruvate were made by R. Butler (2)). The complexes of pyridine-2-carboxylate and pyridine-4-carboxylate are rapidly reduced by Cr+2 at least in the forms which present the nitrogen without associated protons. Radical ion intermediates for these structures are not unreasonable. In fact, a stable free radical derived from AT-ethyl-4-carbethoxypyridinyl has been... 

Harry Gray Two points in Prof. Taube s paper quoted as experiments in progress haven t been mentioned. Both are concerned with the mechanism of electron transfer, because the transmission in the ligand, wherever the attack is, is through the 7r-system, and in cobalt(III) in the detectable radical ion intermediate, because of the improbability of resonance transfer from tt to electron resonance experiment in which one tries the reduction by chromous and looks for the ESR signal of the radical ion. 

A further significant mechanistic pathway for aromatic nitration can involve a single electron-transfer reaction to an initial radical ion intermediate ... 

Quenching of singlet cyanoanthracenes by t-1 in acetonitrile solution results in the formation of the cyanoanthracene anion radical and t-1 cation radical. The formation and decay of these radical ion intermediates have been investigated by transient absorption (143) and Raman (TR3) spectroscopy (89). The yield of ionic photodissociation from the dicyanoanthracene-t-1... 

Apparently, surface adsorption of the radical ion intermediates so stabilizes and directs their chemical reactivity that combination with superoxide rather than deprotonation of the aryl radical cation are observed. 

This reaction involves the reductive homo-coupling of a carbonyl compound to produce a symmetrically substituted 1,2-diol. TheJQrst step is single electron transfer of the carbonyl bond, which generates radical ion intermediates that couple via carbon-carbon bond formation to give a 1,2-diol. The example depicted above shows the preparation of pinacol itself. 

The analysis of similar processes with benzophenone (1) and benzil (7) requires a higher time resolution of the experimental setup. Using ns-laser flash photolysis, we observed the formation of radical ion intermediates, depending on solvent polarity, added salts and competing H-abstraction [36]. Summarizing all these experiments, one can draw the following conclusions (cf. Figs. 3—5, see also Ref. [33]) ... 

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). 

These experiments clearly show the involvement of at least two different types of radical ion intermediates which can be designed CIP and SSIP/FRI. The origin of the different reactivity of CIP and SSIP remains unclarified. One speculative explanation may be concluded in anology to the triplet sensitized dimerization of 1,3-cyclohexadiene, which mainly leads to type-1 products, however in different... 

I reasoned that it may be possible to change the pathway for breakdown of radical ion intermediates such as XVII by either changing the acetoxy leaving group in II to one which would leave more readily (e.g., halide, SCN or pyridinium) or by changing the electronic character of the sulfur atom (e.g., by oxidation to the sulfoxide). The use of the sulfoxide of II as the electrochemical reduction substrate proved to be most successful, eliminating both of the unwanted pathways B and C in Scheme 10. Work at CSU on the other major practical objectives (increasing... 

The results obtained in these studies on the photoreactivity of different 3,y-unsaturated systems under SET-sensitization have opened new lines of research in the area of di-7r-methane photochemistry. However, further studies will be necessary to determine the scope and synthetic potential of these new reactions of the di-7r-methane type via radical-ion intermediates. 

Among the first examples of photo-induced reactions through radical-ion intermediates was the dimerization of VCZ, discovered by Ellinger [66], and later thoroughly investigated by Ledwith [67, 68]. 

The carbonyl-carbon kinetic isotope effect (KIE) and the substituent effects for the reaction of lithium pinacolone enolate (112) with benzaldehyde (equation 31) were analyzed by Yamataka, Mishima and coworkers ° and the results were compared with those for other lithium reagents such as MeLi, PhLi and AllLi. Ab initio (HF/6-31-I-G ) calculations were carried out to estimate the equilibrium isotope effect (EIE) on the addition to benzaldehyde. In general, a carbonyl addition reaction (equation 32) proceeds by way of either a direct one-step polar nucleophilic attack (PL) or a two-step process involving electron transfer (ET) and a radical ion intermediate. The carbonyl-carbon KIE was of primary nature for the PL or the radical coupling (RC) rate-determining ET mechanism, while it was considered to be less important for the ET rate-determining mechanism. The reaction of 112 with benzaldehyde gave a small positive KIE = 1.019),... 

Study of excited-state and radical-ion intermediates in SET reactions using time-resolved techniques... 

Direct evidence for the formation of radical ion intermediates in the benzophenone-amine system was also obtained by Peters et al. [153-156]. Picosecond laser-flash photolysis studies have indicated the formation of ketyl radical anions concomitant with the decay of the benzophenone triplet. For 1.0 M dime-thylaniline and diethylaniline the rate of electron transfer to the benzophenone triplet was 3.6 x 10 and 4.2 x 10 m s , respectively. On the basis of their studies Peters et al. proposed a mechanism in which a solvent-separated ion pair... 

Silverman s studies on mechanism based MAO inactivation have provided overwhelming support for the role of electron transfer in the MAO catalyzed dealkylation of amines. It must be mentioned however that spectroscopic attempts for detecting the radical ion intermediates have hitherto been unsuccessful. Yasanobu and coworkers could not find EPR spectral evidence for radical intermediates in MAO-catalyzed oxidation of benzylamine [205]. Miller et al. failed to observe the flavin semiquinone or an amine-flavin adduct in rapid-scan-stopped flow spectroscopy [206]. The only time-dependent absorption change observed in this study was the bleaching of the oxidized flavin. Furthermore, no influence of a magnetic field up to 6500 G was observed on the rate of MAO B reduction. The reaction rates of systems with kinetically significant radical pair intermediates are known to be altered... 

Yuan, H., Yan, B. Photochemical [2+2] cycloaddition via radical ion intermediates. A CIDNP evidence. Chin. Chem. Lett. 1992, 3, 25-28. 

Generally it is difficult to distinguish between electrophilic addition to an aromatic ring and one electron removal. The first process gives a cationic and the second a radical cationic intermediate. Substituent effects are expected to influence both kinds of reactions in the same way.129) The same can be said for nucleophile versus one electron addition to an aromatic substrate to yield an anion and a radical anion, respectively. Reactions proceeding by a pair of one electron transfer reactions to give radical ion intermediates rather than by processes which yield purely ionic intermediates may be more common than is realized.129-132 New research will provide an answer. 

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