Arene radical cations, intramolecular

Pandey and co-workers have generated arene radical cations by PET from electron-rich aromatic rings [119]. The photoreaction is apparently initiated by single-electron transfer from the excited state of the arene to ground state 1,4-dicyanonaphthalene (DCN) in an aerated aqueous solution of acetonitrile. Intramolecular reaction with nucleophiles leads to anellated products regio-specifically. The author explains the regiospecifidty of the cyclization step from... 

Pandey, G., Karthikeyan, M., and Mumgan, A. (1998) New intramolecular a-arylation strategy of ketones by the reaction of silyl enol ethers to photosensitized electron transfer generated arene radical cations construction of benzannulated and benzospiroannulated compounds. Journal of Organic Chemistry, 63, 2867-2872. 

A versatile strategy for efficient intramolecular oc-arylation of ketones was achieved by the reaction of silyle enol ethers with PET-generated arene radical cations. This strategy involved one-electron transfer from the excited methoxy-substituted arenes to ground-state DCN [42]. Pandey et al. reported the construction of five- to eight-membered benzannulated as well as benzospiroannulated compounds using this approach (Sch. 20) [42a]. The course of the reaction can be controlled via the silyl enol ether obtained... 

Likewise, -nucleophiles react well in intramolecular cychzations to trap olefin and arene radical cations [278]. 

These two concepts can be combined by using benzyl trialkylsilanes as electron donors in intramolecular PET-reactions. Excitation of the iminium chromophore and electron transfer leads to arene radical cations which rapidly cleave the C-Si bond to give a 1,6-biradical which combines to give the indolizidine skeleton (Scheme 36) [140]. 

The success of the strategy is further applied for the synthesis of carbo-and spiro-aimulated aromatic compounds [146,147] by the intramolecular cyclization of silyl enolethers to PET-generated arene radical cations. Two types of carbocyclic compounds (170 and 173), varying in ring sizes, may be synthesized [146] starting from the same ketone (i.e., 169), as two types of silyl enol ethers can be produced using either thermodynamic or kinetic enolisation procedures. The core spiro structure (177) of the anticancer antibiotic ffed-ericamycin is also prepared [147] by the PET cyclization of 176 (Scheme 36). 

The photolysis of donor-acceptor systems shows a reaction pattern of unique synthetic value. Direct irradiation of the donor-acceptor pairs, such as arene-amine, leads by intramolecular electron transfer, to amine radical cations and arene radical anions. The generated radical cation and radical anion intermediates undergo cyclization reactions providing efficient synthetic routes to N-heterocycles with a variety of ring sizes. 

Aryl radical cations generated by electron-transfer processes from methoxy substituted arenes to DCN, tethered by oxygen, nitrogen as well as carbon nucleophile leads to intramolecular cyclizations (Scheme 8.67). The synthetic potentials of... 

Whereas intermolecular attack of a tz nucleophile is rather rare [331] (e.g. disulfide + -f- arene [332], and diselenide + + dienes [333]), intramolecular cycliza-tions of amine radical cations have been useful in the preparation of five-membered rings [334]. 

G. Pandey. Upon electron transfer sensitization of the donorsubstituted arene by means of dicyanonaphthalene oxidation occurs to form a radical cation which is intramolecularly trapped by the acid group to form the coumarin. 

Dicyanobenzene was converted into 3-cyanobenzoic acid by using the bacterium Rhodococcus rhodochrocus ". Intramolecular electron transfer involving isomeric forms of dicyanobenzene was investigated in detaiP and the mechanisms of electroreduction of all three isomers were studied. When dicyanobenzene was used as catalyst in photoamination reactions of arenes, the first step of the reaction was the formation of an aryl radical cation by electron transfer from the aromatic hydrocarbon to m-dicyano-benzene. ... 

Mariano and coworkers have exploited this PET bond cleavage chemistry in intramolecular as well as intermolecular cyclization reactions [38]. The combined iminium-benzylsilane functionalities undergo intramolecular PET to provide an intramolecular amine radical/benzyl silane cation-radical pair by exciting either the iminium salt or the arene chromophore [38]. Cleavage of the benzyl C—Si bond presumably takes place with assistance of moderately weak n ldeophiles such as methanol. The diradical couples intramolecularly to provide an indolizidine derivative [38] in 90% yield. 

The failure of tertiary (AI,IV-dimethylaminoalkyl)arenes and stilbenes to undergo intramolecular addition may reflect structural differences between inter- vs. intramolecular exciplexes. Polar solvents are generally required for the observation of in-termolecular addition reactions of tertiary amine exciplexes. Equilibration between solvent-separated and radical ion pairs may be necessary in order to achieve an appropriate reaction trajectory for a-C-H proton transfer. In the case of intramolecular exciplexes with short chain linkers, electron transfer in polar solvents may occur in extended geometries which are inappropriate for proton transfer and chain folding may not compete effectively with exciplex decay. The exceptions to these generalizations, benzene and styrene, form more localized anion radicals which undergo both inter- and intramolecular reactions with tertiary amine cation radicals in nonpolar solvents. 

---