Electron transfer cation reactive intermediates

The chemical pathways leading to acid generation for both direct irradiation and photosensitization (both electron transfer and triplet mechanisms) are complex and at present not fully characterized. Radicals, cations, and radical cations aH have been proposed as reactive intermediates, with the latter two species beHeved to be sources of the photogenerated acid (Fig. 20) (53). In the case of electron-transfer photosensitization, aromatic radical cations (generated from the photosensitizer) are beHeved to be a proton source as weU (54). 

The wide diversity of the foregoing reactions with electron-poor acceptors (which include cationic and neutral electrophiles as well as strong and weak one-electron oxidants) points to enol silyl ethers as electron donors in general. Indeed, we will show how the electron-transfer paradigm can be applied to the various reactions of enol silyl ethers listed above in which the donor/acceptor pair leads to a variety of reactive intermediates including cation radicals, anion radicals, radicals, etc. that govern the product distribution. Moreover, the modulation of ion-pair (cation radical and anion radical) dynamics by solvent and added salt allows control of the competing pathways to achieve the desired selectivity (see below). 

Furthermore, kinetic analysis of the decay rate of anthracene cation radical, together with quantum yield measurements, establishes that the ion-radical pair in equation (76) is the critical reactive intermediate in osmylation reaction. Subsequent rapid ion-pair collapse then leads to the osmium adduct with a rate constant k 109 s 1 in competition with back electron-transfer, i.e.,... 

Thioglycosides can also be activated by a one-electron transfer reaction from sulfur to the activating reagent tris-(4-bromophenyl)ammoniumyl hexachloroanti-monate (TBPA+) [102,103]. The use of this promoter was inspired by an earlier report where activation was achieved under electrochemical conditions to give an intermediate S-glycosyl radical cation intermediate [104], and the reactivity and mechanism have also been explored [105,106]. 

The mechanistic conundrum presented by such a dichotomy between electron-transfer and electrophilic processes can only be rigorously resolved by the experimental proof of whether the cation radical (or the electrophilic adduct) is, or is not, the vital reactive intermediate. However, in a thermal (adiabatic) reaction between arene donors and the nitrosonium cation, such reactive intermediates cannot be formed in sufficient concentrations to be observed directly by conventional experimental methods since their rates of follow-up reactions must perforce always be faster than their rates of formation, except when they are formed in a reversible equilibrium like the... 

Photoinduced electron transfer promoted cyclization reactions of a-silyl-methyl amines have been described by two groups. The group of Pandey cyclized amines of type 135 obtaining pyrrolidines and piperidines 139 in high yields [148]. The cyclization of the a-silylated amine 140 leads to a 1 1 mixture of the isomers 141 and 142 [149]. The absence of diastereoselectivity in comparison to analogous 3-substituted-5-hexenyl radical carbocyclization stereochemistry [9] supports the notion that a reaction pathway via a free radical is unlikely in this photocyclization. The proposed mechanism involves delocalized a-silylmethyl amine radical cations as reactive intermediates. For stereochemical purposes, Pandey has investigated the cyclization reaction of 143, yielding... 

The difference between the two reactions of Scheme 2.9 may also be considered in terms of the complete electron transfer in both cases. If the a-nitrostilbene anion-radical and metallocomplex cation-radical are formed as short-lived intermediates, then the dimerization of the former becomes doubtful. The dimerization under electrochemical conditions may be a result of increased concentration of reactive anion-radicals near the electrode. This concentration is simply much higher in the electrochemical reaction because all of the stuff is being formed at the electrode, and therefore, there is more dimerization. Such a difference between electrode and chemical reactions should be kept in mind. In special experiments, only 2% of the anion-radical of a-nitrostilbene were prepared after interruption of controlled-potential electrolysis at a platinum gauze electrode. The kept potential was just past the cathodic peak. The electrolysis was performed in the well-stirred solution of trani -a-nitrostilbene in AN. Both processes developed in this case, namely, trans-to-cis conversion and dimerization (Kraiya et al. 2004). The partial electrolysis of a-nitrostilbene resulted in redox-catalyzed equilibration of the neutral isomers. 

Ion-radical organic reactions of the Sj j l type are less sterically restricted than classical Sj reactions. Generally, the nucleophilic (not Sj j ) reactivity varies with the steric demand at the reaction center. The electron-transfer reactivity does not depend on steric effects. To illustrate this, one can compare electron transfer and nucleophilic reactivity between ketene silyl acetals and cationic electrophiles (Fukuzumi et al. 2001). Nevertheless, space strains may determine the overall results of these reactions if either intermediate radicals or forming products are sterically hindered. 

Recently chloromethylated polystyrene (CMS), a highly sensitive, high resolution electron resist with excellent dry etching durability, was developed. Very recently reactive intermediates in irradiated polystyrene, which is a starting material of CMS, have been studied and the transient absorption spectra of excimer (2-4), triplet states (2,5), charge-transfer complexes, and radical cations (6) of polystyrene have been measured. The present paper describes the cross-linking mechanism of the high sensitivity CMS resist and compares it to that of polystyrene on the basis of data on reactive intermediates of polystyrene and CMS. 

We conclude that oxidative transformations of organic substrates can be readily understood as emanating from either photogenerated surface adsorbed radical cations or from radicals formed by activated oxygen radicals (surface oxides or adsorbed hydroxy, hydroperoxy or peroxy radicals). Photoelectrochemical methods not only generate the reactive species by interfacial electron transfer, but also control the subsequent activity of the surface adsorbed intermediate. 

Schmittel M, Ghorai MK (2001) Reactivity patterns of radical ions - a unifying picture of radical-anion and radical-cation transformations. In Balzani V (ed) Electron transfer in chemistry, vol 2. Organic molecules. Wiley-VCH, Weinheim, pp 5-54 Schoneich C, Bonifacic M, Dillinger U, Asmus K-D (1990) Hydrogen abstraction by thiyl radicals from activated C-H-bond of alcohols, ethers and polyunsaturated fatty acids. In Chatgilialoglu C, Asmus K-D (eds) Sulfur-centered reactive intermediates in chemistry and biology. Plenum, New York, pp 367-376... 

Pyridylarenes undergo Cu(II)-catalysed diverse oxidative C-H functionalization reactions. The tolerance of alkene, alkoxy, and aldehyde functionality is a synthetically useful feature of this reaction. A radical-cation pathway (Scheme 4) has been postulated to explain the data from mechanistic studies. A single electron transfer (SET) from the aryl ring to the coordinated Cu(II) leading to the cation-radical intermediate is the rate-limiting step. The lack of reactivity of biphenyl led to the suggestion that the coordination of Cu(II) to the pyridine is necessary for the SET process. The observed ortho selectivity is explained by an intramolecular anion transfer from a nitrogen-bound Cu(I) complex.53... 

Electron-transfer oxidation of organic compounds involves multiple steps with transient radicals as key reactive intermediates.14 The electron-transfer oxidation of a neutral, diamagnetic organic donor (RH), having an even number of electrons, produces a radical cation, as shown in Eq. (7). 

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