Anode-generated radicals

Unsymmetrical dimers are accessible through the coelectrolysis of different olefins (Table 7, number 11). Coupling can also be achieved by the addition of anodically generated radicals with subsequent dimerization of the adduct radicals (additive dimerization, see Section 5.3.3). 

Scheme 5 Addition of anodically generated radicals to alkenes.

The influence of mechanism and kinetic data on yields and selectivities in addition reactions of anodically generated radicals to olefins has been calculated and the predictions tested in preparative electrolyses [118]. 

The results show that, in the coupling of anodically generated radicals, good diastereoselectivities can only be achieved when the intermediate radical... 

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... 

Kuraray361 used the addition of anodically generated radicals of 1,3-dicarbonyl compounds for C—C coupling in the preparation of intermediates for P-blockers. 

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

The primary adduct 53 (Eq. (117) ) of the anodically generated radical R undergoes a series of follow-up reactions a) hydrogen abstraction to 54, b) dimerization to additive dimers 55, c) coupling with R to 1,2-disubsti-tuted monomers56, d) le-oxidation to a carbonium ion that either solvolyzes to 57 or, when 1,3-dicarbonyl compounds are added cyclizes intramolecularly to tetra (58) - or dihydrofuran derivatives (59). Product control is possible in some cases by suitable choice of the anode potential. With a high anode potential,... 

Anodic coupling can be achieved in two ways. These are a) Coupling of anodically generated radicals (Eq. (150) ) ... 

In the chemical reaction Conway etaL 21 assumes metal adsorbed radicals as reactive intermediates, while Eberson 324-) advocates free radical intermediates. Though reactions via adsorbed radicals cannot be rigorously ruled out, such interactions must be very weak as the anodically generated radicals react simi-... 

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). 

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

Table 10. Selected Examples of Coupling of Anodically Generated Radicals from Carbanions ... 

Table 11 Additions of Anodically Generated Radicals to Alkenes Number Radical precursor Olefin RCH=CYR Conditions... 

Electrolysis of a methanol solution of methyl oxalate with ethylene under pressure yielded 70-90% of the dimethyl esters of succinic, adipic, suberic, and sebacic acids. Decrease in the ethylene pressure or increase of the current density led to a decrease in the higher esters in the product mixture [241]. The influence of mechanism and kinetic data on yields and selectivities in addition reactions of anodically generated radicals to olefins has been calculated and predictions have been tested in preparative electrolyses [244]. 

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]. 

Schafer and coworkers [494] reported interesting diasteroselective cross-coupling of anodically generated radicals bearing chiral amide groups. 

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