Anodically Generated Radical Cations

Aromatics, aromatic amines, phenols, and electron-rich olefins can be dimerized via radical cations according to Eq. (170). 

Thereby either substituted dimers 82 are formed in a ECCjq-sequence or dehydro dimers 83 in a ECCg-sequence. Adams 2S 29 has given a detailed discussion of these pathways, which needs not be duplicated here. Coupling of substrates via anodically generated radical cations is a powerful synthetic method. To illustrate this some representative examples from the wealth of available material are given in Table 13. More extensive compilations of data may be found in Ref.10). 

0-Xylene (H2O, acetone, H2SO4) Bi-o-xylyl (39%) + other products 371) 

Butadiene trans, frans- 1,8-Dime thoxy-2,6-octadie tie (13%), Irani-l,6-dimethoxy-2,7-octadiene (13%), 3,6-di-methoxy-l,7-octadiene (13%), dimethoxydodeca-  

1-Bis (dimethyl- amino )e thy lene (CH3CN, NaC104) 1,1,4,4-Tetrakis (dimethylamino) butadiene 386) 

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The structure of diphosphallenic radical cations, generated from the allene ArP=C=PAr by electrochemical oxidation, has been examined using EPR spectroscopy. Ab initio calculations including correlation effects at the MP2 and MCSCF levels have determined that two rotamers exist compatible with Jahn-Teller distortion of the allene.146 Anodically generated radical cations of alkyl phosphites [(RO P] and silylphosphites [(RO)2POSiMe3] reacted with alkenes by initial attack at the C=C bond followed by electron transfer, deprotonation, and elimination of an alkyl or trimethylsilyl cation to form identical alkyl phosphate adducts.147 The electron ionization-induced McLafferty rearrangement of n-hexylphosphine afford the a-distonic radical cation CTEPH, the distinct reactivity of which suggests there is no... 

Nucleophilic substitution of an anodically generated radical cation (Eq. (91)) ... 

In Table 17 representative examples of anodically generated radical cations are compiled. 

Related to these coupling reactions is the reaction between an anodically generated radical cation, for example, derived from anthracene or 9-phenyl anthracene, and a carbon nucleophile such as anisole, toluene or benzene [Eq. (71)] [248]. 

Phenoxanthin, 68 X = S Y = O, is prepared by the electrochemical oxidation of diphenyl ether in dichloromethane and trichloroacetic acid containing tetraethyl-ammonium perchlorate at a composite anode of carbon and sulphur. The anode generates sulphur cations, which carry out electrophilic substitution on the benzene ring [237], Phenoxathiin radical-cation, formed at the potential of the fust oxidation wave, has been characertised by esr spectroscopy [238],... 

Anodic oxidation has been used for generating radical-cations from amines such as i 7, .Af,iV,i f -tetramethylbenzidine (Fritsch and Adams, 1965) and triethylenediamine (McKiimey and Geske, 1965), and from aromatic ethers such as jj-dimethoxybenzene (Zweig et al., 1964). 

In addition to chemical oxidations and reductions, electrochemical processes are an important means of generation of charged-radical species. One-electron oxidation at the anode of an electrolysis cell generates radical cations, while one-electron reduction at the cathode generates radical anions. 

Moeller and co-workers published the electron transfer cyclization of silyl enol ethers initiated by anodic oxidation. The in situ generated radical cation of 54 intramolecularly attacks the double bond. The resulting benzylic radical position is further oxidized and attacked by a solvent molecule as well as the remaining cationic center to yield 55. Cleavage of the SiO bond leads to the formation of 56 (Scheme 12). 

Scheme 9 Reaction pathways of anodically generated aromatic radical cations.

Intramolecular coupling reactions between nucleophilic olefins have also proven to hold potential as synthetically useful reactions. The first example of this type of reaction was reported by Shono and coworkers who examined the intramolecular coupling reaction of an enol acetate and a monosubstituted olefin (Scheme 41) [50]. This reaction was conducted in an effort to probe the nature of the radical cation intermediate generated from the anodic oxidation of... 

The radical cations generated at the anode surface can dimerize before they diffuse into solution. The dimeric compound formed can then be cyclized in solution via formation of a carbon-heteroatom bond. The anodic oxidation of enaminones reported earlier [46] and also recently [47] could serve as an illustration of this type of intermolecular cyclization. 

Conversion of toluenes to the benzoic acid is also accomplished by anodic oxidation in acetic acid containing some nitric acid. It is not clear if this reaction involves the aromatic radical-cation or if the oxidising agents are nitrogen oxide radicals generated by electron transfer from nitrate ions [66, 67]. Oxidation of 4-fluorotoluene at a lead dioxide anode in dilute sulphuric acid gives 4-fluorobenzoic acid in a reaction which involves loss of a proton from the aromatic radical-cation and them in further oxidation of the benzyl radical formed [68]. 

Radical cations of 2-alkylidene-l,3-dithianes can be generated electrochemically by anodic oxidation using a reticulated vitreous carbon (RVC) anode <2002TL7159>. These intermediates readily react with nucleophiles at C-1. Upon removal of the second electron, the sulfur-stabilized cations were trapped by nucleophilic solvents, such as MeOH, to furnish the final cycloaddition products. Hydroxy groups <20010L1729> and secondary amides <2005OL3553> were employed as O-nucleophiles and enol ethers as C-nucleophiles (Scheme 50) <2002JA10101>. 

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