Radical intermediates ether transfer reactions

The corrinoid-mediated reduction of polyhaloethenes has been the subject of a recent study, which reports reaction via homolytic C-halogen bond fission. The elimination of a further halogen radical affords haloalkynes, which lead to acetylene itself.56 The electron transfer-induced reductive cleavage of alkyl phenyl ethers with lithium naphthalenide has been re-examined in a study which showed that it is possible to reverse regioselectivity of the cleavage (i.e. ArOR to ArH or ArOH) by introduction of a positive charge adjacent to the alkyl ether bond.57 A radical intermediate has been detected by ESR spectroscopy in the reduction of imines to amines with formic acid58 which infers reacts takes place via Lukasiewicz s mechanism.59... 

An analogous strategy was applied for annelation of ring D in Kuehne s syntheses of 20-epi- ((/-vincadifformine (96) and yz-vincadifformine (97). Upon slow addition of n-BujSnH and AIBN via syringe pump to the phenylselenyl ether 94,20-epi- vincadifformine (96) and vincadifformine (97) were formed in a 1 2 ratio. The separated products did not epimerize under the reaction conditions, indicating a facial preference in the hydrogen transfer to the pentacyclic radical intermediate 95. The ethyl substituent blocked 5-exo-trig cyclization. 

Processes involving a single-electron transfer (SET) step and cation-radical intermediates can occur in the reactions of X - or X -iodanes with electron-rich organic substrates in polar, non-nucleophilic solvents. Kita and coworkers first found that the reactions of p-substituted phenol ethers 29 with [bis(trifluoroacetoxy)iodo]benzene in the presence of some nucleophiles in fluoroalcohol solvents afford products of nucleophilic aromatic substitution 31 via a SET mechanism (Scheme 1.5) [212,213]. On the basis of detailed UV and ESR spectroscopic measurements, it was confirmed that this process involves the generation of cation-radicals 30 produced by SET oxidation through the charge-transfer complex of phenyl ethers with the hypervalent iodine reagent [213,214],... 

Kita et al. further developed PIFA-induced CDC reactions between phenyl ether derivatives and cyclic 1,3-dicarbonyl compounds as nucleophiles (Scheme 8.2). The reactions with 1 equiv. of PIFA in hexafluoro-2-propyl alcohol attach nucleophiles onto the ort/jo-position of para-substituted phenyl ethers to afford the dehydrogenative coupling products 8 in moderate yields. UV and electron spin resonance (ESR) spectroscopic studies support a reaction mechanism involving the formation of the charge-transfer complex 9 followed by the generation of the cation radical intermediate 10. This is the first example of the reaction of aromatic compounds with PIFA that involves the formation not of diatyliodonium(m) salt 11 but the cation radical intermediate 10 as a key intermediate. 

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