Diphenyliodonium salts, electron transfer

N-Arylation. Only highly activated aryl halides react with pyridines. Thus, 2,4-dinitrochlorobenzene with pyridine forms l-(2,4-dinitrophenyl)pyridinium chloride (Zinckes salt) active heteroaryl halides such as 2-chloropyr-imidine react similarly. To N-phenylate pyridine, diphenyliodonium ions are needed Pf UBFzj + pyridine 1-phenylpyridinium BF4 +PhI. This reaction may involve initial electron transfer. 

The photochemical activation of the phosphonium salt (Eq. 55) has its thermal counterpart in the facile (dark) conversion of the iodonium salt Ph2l+ Mn(CO)s . Owing to the greatly enhanced reduction potential of diphenyliodonium (Ered 0 V relative to the SCE [181]) as compared to tetraphenylphosphonium (Ered 2.3 V relative to the SCE [140]), electron transfer in the iodonium-metallate ion pair is now energetically feasible (AGet 0 eV). As a result, complete conversion of the charge-transfer salt to the electron-transfer products Mn2(CO)io and PhMn(CO)s is observed within minutes upon mixing of the two components [140]. 

Diphenyliodonium salts [IPh2]X E° — -0.9 V relative to FeCp2 [285]) are commercially available (X = Cl, Br, I) and the BF4, PFe, AsFe and SbFe can be made [286]. As aryldiazonium salts, they decompose, after electron transfer, to the phenyl radical which dimerizes to biphenyl and abstracts a H atom from the solvent. 

Electron transfer photosensitization of iodonium salts and sulfonium salts is similar primary differences are a result of the stronger oxidizing power of iodonium salts compared to sulfonium salts ( (red) = —0.7 V and —1.2V versus SCE for diphenyliodonium and triphenylsulfonium salts respectively, see below). Triplet sensitization is again similar, including the triplet energies, and has been discussed above. 

Scheme 4 describes the electron transfer photosensitization of iodonium salts by anthracene [61,70,91-94]. Singlet anthracene reacts with diphenyliodonium cation by diffusion controlled electron transfer in acetonitrile solution. In-cage decomposition of diphenyliodo radical competes with rapid back electron transfer to yield the singlet radical pair of anthracene cation... 

DeVoe et al. have reported quantitative aspects of photosensitization of diphenyliodonium salt and bis(4-dimethylaminobenzylidene)acetone (DMBA) [101]. This ketone is a bis-vinylog of Michler s Ketone, which is a well-known sensitizer for onium salt initiated free radical polymerizations [102,103], The reaction with DMBA is an example of electron transfer sensitization gated by conformational relaxation of the sensitizer. The ratio of iodonium salt consumption to aminoketone consumption is two, the second iodonium salt equivalent is consumed by a second reducing equivalent from the aminoalkyl radical on the oxidized photosensitizer. 

Some triplet photosensitizers also undergo electron transfer with iodo-nium and sulfonium salts. Manivannan et al. [104] have demonstrated that ketocoumarins sensitize photolysis of diphenyliodonium salts exclusively from their triplet states. An electron transfer mechanism was inferred from observation of a light absorbing transient assignable to the ketocoumarin radical cation, on laser flash photolysis of a ketocoumarin onium salt mixture in methanol. 

A variety of other ketones have been shown to photosensitize iodonium and sulfonium salts from triplet excited states by the electron transfer mechanism [102,103], Rates of electron transfer from triplet 3-(2-isoxazolinyl)-phenyl ketone, XI, to a series of onium salts in aq. acetonitrile have been shown to scale with the free energy change according to the Weller equation (Eq. (28)) [105], In the same study, the rates of triplet quenching of a series of ketones by diphenyliodonium cation in the same solvent were evaluated the results were not tractable in terms of usual linear free energy... 

Alternatively hydrogen atom abstraction can compete unproductively with electron transfer sensitization of onium salt photolysis. Consider the sensitization of diphenyliodonium cation by phenanthrenequinone. In the presence of H-atom donors, for example, glycidyl ether monomers, phen-anthrenesemiquinone radicals are formed on photolysis these radicals disproportionate to starting quinone and 9,10-dihydroxyphenanthrene without being intercepted by diphenyliodonium cation [110]. 

A similarly detailed study of the polymerization of methyl methacrylate by diphenyliodonium chloride in aq. AN using various aromatic ketone sensitizers has recently been carried out by Timpe s group [103a]. On the one hand the results were not complicated by monomer quenching of excited ketone sensitizer, but were complicated by operation of simultaneous singlet and triplet pathways for the electron-transfer sensitization. In this study the data were analyzed without reference to the possibility of triplet energy transfer sensitization of the iodonium salt photolysis [22], or the possible involvement of charge transfer complexes. 

Still another example is an initiating system composed of 7-diethylamino-3 -(2 -N-methyl-benzimidazolyl)-coumarin and diphenyliodonium hexafluorophosphate. This composition initiates the polymerization of methyl methacrylate in visible light. After the dye absorbs flic light energy, quick electron transfer takes place from the dye to the iodonium salt to produce free radicals. " The light induced reaction is claimed to occur mainly through the excited singlet state of the coumarin and results in low sensitive to O2. The fluorescence of the coumarin compound was reported to be quenched efficiently by the iodonium salt. " The reaction was observed to be in accord with the Stem-Volmer equation. The influence of the concentration of coumarin on the polymerization rate of methyl methacrylate led to the conclusion that the free radicals from coumarin act mainly as chain terminators. ... 

A particularly graphic example of electron-transfer photosensitization is the interaction of diphenyliodonium salts with perylene. Exposure of a dichloromethane solution of a mixture of these two compounds to UV light for 2 s results in the formation of the intensely blue-colored and relatively stable perylene eation-radical. The blue color remains in solution in air for a considerable time until it slowly fades. When this photolysis reaction is conducted in the presence of a cationically polymerizable monomer, the blue color of the perylene cation-radical is very transient. 

A wide assortment of additional electron-transfer PSs for diaryliodonium salts have been described in the journal and patent literature. These include ketocoumarins [FOU 88], 9,10-phenanthraquinone [BAU 86], Mannich bases [DE 88b], 1,3-indanediones [TEH 13], benzoquinonylsulfanyl derivatives [SUG 03], acridinediones [SEE 01] and dimethylaminobenzylidine derivatives [ICH 87], In addition, the use of dyes such as eosine and Rhodamine [DE 88a, DE 89] have been employed to provide photosensitization in the visible region of the spectrum. A particularly interesting system devised by Yagci et al. [AYD 08] is the dithienothiophene, 48, used with diphenyliodonium hexaflurophosphate to carry out the cationic photopolymerizations of cyclohexene oxide, 3,4-epoxycyclohexylmethyl 3, 4 -epoxycyclohexane carboxylate, N-vinyl carbazole, n-butyl vinyl ether and styrene. These investigators have further applied this system to the preparation of metallic silver-filled epoxy nanocomposites [YAG 11]. 

TABLE 5. ELECTRON TRANSFER REACTION OF RADICALS WITH DIPHENYLIODONIUM SALTS... 

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