Cation-radical intermediates

The cation—radical intermediate loses a proton to become, in this case, a benzyl radical. The relative rate of attack (via electron transfer) on an aromatic aldehyde with respect to a corresponding methylarene is a function of the ionization potentials (8.8 eV for toluene, 9.5 eV for benzaldehyde) it is much... 

It is important to note that the efficiency of the various cycloaddition reactions presented above arises from a rapid cleavage of the resulting cation radical intermediates, which renders the back electron transfer process ineffective. 

The donor-induced disproportionation in equation (91) leads to the EDA complex, i.e., [D, NO+]NO as the (first) directly observable intermediate. The critical role of the nitrosonium EDA complex in the electron-transfer activation in equation (92) is confirmed by the spectroscopic observation of the cation-radical intermediates (i.e., D+ ) as well as by an alternative (low-temperature) photochemical activation with deliberate irradiation of the charge-transfer band252 (equation 95). 

Note that under these conditions the thermal reaction in equation (90) is too slow to compete.) Finally, the stoichiometry for the oxygen-atom transfer from NO to the donor cation radical in equation (93) is independently established by the reaction of isolated cation radical intermediates with NO. 251,252... 

The fact that the anodic oxidation of allylsilanes usually gives a mixture of two regioisomers suggests a mechanism involving the allyl cation intermediate (Scheme 3). The initial one-electron transfer from the allylsilane produces the cation radical intermediate [9], Although in the case of anodic oxidation of simple olefins the carbon-allylic hydrogen bond is cleaved [28], in this case the... 

Closely related reactions have been accomplished by photoelectron-transfer reactions of allylsilanes and benzylsilanes, and a similar mechanism involving the cation radical intermediate is suggested [29]. Chemical oxidation of allylsilanes [27] and ferrocenylsilanes [30] also cleaves the C-Si bond and mechanism of these reactions seem to closely relate to that of the electrochemical process. 

Suda and coworkers described the anodic oxidation of 2-silyl-l,3-dithianes which have two sulfur atoms on the carbon adjacent to silicon [42], In this case, however, the C Si bond is not cleaved, but the C-S bonds are cleaved to give the corresponding acylsilanes (Scheme 12). Although the detailed mechanism has not been clarified as yet, the difference in the anode material seems to be responsible for the different pathway of the reaction. In fact, a platinum plate anode is used in this reaction, although a carbon anode is usually used for the oxidative cleavage of the C-Si bond. In the anodic oxidation of 2-silyl-l,3-dithianes the use of a carbon anode results in a significant decrease in the yield of acylsilanes. The effects of the nature of the solvent and the supporting electrolyte may also be important for the fate of the initially formed cation radical intermediate. Since various 2-alkyl-2-silyl-l,3-dithianes can be readily synthesized, this reaction provides a convenient route to acylsilanes. 

The mechanism shown in Scheme 23 has been suggested The first step involves the transfer of an electron from the acylsilane to produce the cation-radical intermediate. Attack of methanol at the silicon cleaves the C-Si bond to give the acyl radical intermediate, although there is no direct evidence for the acyl radical intermediate. The acyl radical is then oxidized anodically to the acyl cation, which reacts with methanol to give the corresponding methyl ester. 

The [Fe =0(TMP+ )]+ complex exhibited a characteristic bright green color and corresponding visible absorbance in its UV-vis spectrum. In its NMR spectrum, the meta-proton doublet of the porphyrin mesityl groups were shifted more than 70 ppm downfield from tetramethylsilane (TMS) because they were in the presence of the cation radical, while the methyl protons shift between 10 and 20ppm downfield. In Mossbauer spectroscopy, the isomer shift, 5 of 0.06 mm/s, and A q value of 1.62mm/s were similar to those for other known Fe(IV) complexes. Electron paramagnetic resonance (EPR), resonance Raman (RR), and EXAFS spectroscopies provided additional indications of an Fe =0 n-cation radical intermediate. For instance,... 

Cation-Radical Intermediates in Metabolism of Furan Xenobiotics... 

Mechanisms for other reactions of non-phenolic substrates catalyzed by the enzyme are also understood on the basis of cation radical intermediates (15,52). 

Thus, almost all the reactions of lignin substructure model dimers by the enzyme are explained on the basis of cation radical intermediates and their subsequent reactions with nucleophiles such as H2O and intramolecular hydroxyl groups, and with radicals such as di oxygen (for non-phenolic substrates), or on the basis of phenoxy radical intermediates (for phenolic substrates). 

In 1999, Zhou and Clennan [76] reported a type IIaRH oxidation of 1,5-dithiacyclooctane, 26, in CaY. They suggested (Fig. 24) that 26 was activated by an electron transfer to give a radical cation, which subsequently reacted with oxygen to give a peroxysulfonium cation radical intermediate. This scenario is supported by the ease of oxidation of 26 [77] and by the reported propensity of... 

Mechanisms which involve the cation radical intermediate were also proposed for the cleavage of Ca-C0 and 0-0-4 bonds of 0-0-4 lignin substructure models by the enzyme (26,27). Thus, mechanisms for most of the... 

Scheme 13. Mechanism of oxidation of the five-coordinate form of HRP by [Ru(bpy)3]3 + via the formation of -cation radical intermediate (160).

Since silyl enol ethers have a silyl group ji to the jr-system, anodic oxidation of silyl enol ethers takes place easily. In fact, anodic oxidation of silyl enol ethers proceeds smoothly to provide the homo-coupling products, 1,4-diketones (equations 37 and 38)42. This dimerization of the initially generated cation radical intermediate is more likely than the reaction of acyl cations formed by two electron oxidation of unreacted silyl enol ethers in these anodic reactions. 

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... 

The oxidation of phenol ethers 26 by [bis(trifluoroacetoxy)iodo]benzene in the presence of external or internal nucleophiles affords products of nucleophilic substitution 28 via the intermediate formation of the cation radical intermediate 27 according to Scheme 12 [21,27 - 30]. 

Electrolysis offers an alternative route for organic synthesis via the formation of anion and cation radical intermediates. However, traditional electrolytic methods suffer from a number of limitations such as heterogeneity of the electric field, thermal loss due to heating and obligatory use of supporting electrolytes. These factors either hamper electrosynthetic efficiency or make the separation process cumbersome. The combination of electrosynthesis and microreaction technology effectively overcomes these difficulties. 

The exposure of aromatic ethers to BTIB in (CF3)2CHOH or CF3CH2OH (TFE) leads to arene cation-radical intermediates and permits the introduction of nucleophiles (N, AcO-, NCS-, ArS-) into the alkoxyarene nucleus (94JA3684, 95JOC7144). For example, the treatment of / -substituted anisoles with BTIB in the presence of trimethylsilyl azide or -isothiocyanate affords the 2-anisyl azides 98 or 2-anisyl thiocyanates 99, respectively (Scheme 30). 

The EPR and ENDOR spectroscopy was used for studies of catalytic intermediates in native and mutant cytochrome P450cam in cryogenic temperatures (6 and 77K) (Davydov et al., 2001). The ternary complex of camphor, dioxygen, and ferrous-enzyme was irradiated with y-rays to inject the second electron. This process showed that the primary product upon reduction of the complex is the end -on intermediate. This species converts even at cryogenic temperatures to the hydroperoxo-ferriheme form and after brief annealing at a temperature around 200 K, causes camphor to convert to the product. In spite of conclusions derived from x-ray analysis (Schlichtich et al., 2000) no spectroscopic evidence for the buildup of a high-valance oxyferryl/porphyrin rc-cation radical intermediate during the entire catalytic circle has been obtained. 

Cation radical intermediate with electron delocalized over the aromatic groups. Absorbs light at 652 nm (appears blue)... 

In the electrooxidation, one electron is transferred from the enamine to the anode to form a cation radical intermediate which is very unstable, its lifetime being less than 0.2 ms. When the anodic oxidation is carried out in a nucleophilic solvent such as methanol, the intermediate reacts rapidly with the solvent2. The anodic oxidation of morpholinoenamines (la,b) in methanol, for instance, yields two types of enamine,... 

The cation radical intermediates formed from the enamines may be trapped by nucleophiles other than the solvent when these nucleophiles are electrochemically less oxidizable than the enamines. Indeed, the cation radical intermediates formed from morpholino-, piperidino-and pyrrolidinoenamines are trapped by carbanions derived from active methylene compounds such as methyl acetoacetate, acetylacetone and dimethyl malonate with moderate yields (equation 2)3. The products are easily transformed to the corresponding a-substituted ketones by hydrolysis with dilute hydrochloric acid. 

One-electron oxidation of an olefin, arene, or a bibenzyl group can lead to C—H or C—C bond cleavage to produce an allyl or benzyl radical [40, 41]. This area has been pioneered by Arnold [41], The PET reaction of 1,1,2,2-tetraphenylethane and methyl-3,3-diphenylethyl ether have been reported by Arnold and coworkers [41] to provide heterolytic C—C bond cleavage through an intermediate tetra-phenylethane cation-radical. The cation-radical intermediate fragments to di-phenylmethyl radical and diphenylmethyl carbocation. 

Enol silyl ethers can lead to a-chloro ketones on treatment with anhydrous copper(II) chloride in DMF or iron(lll) chloride in acetonitrile (equation 13, Table 1). The chlorination of (36 equation 14) proceeds through a cation radical intermediate formed by an electron-transfer process with metal halides. 

The reaction of indole (38) with copper(II) chloride also proceeds dirough a cation radical intermediate (39) to give mainly 2-chloroindole (40), together with a dimeric by-product (41), as shown in equation (15) ... 

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