Iodonium salt reduction

The most recent work of the General Electric group has addressed the development of diaryliodonium salts as thermal (15) or redox (16) initiators of cationic polymerization. These systems contain Cu (added as such or generated by reduction of added Cu ), which serves to reduce the iodonium salt. Reduction of the iodo-nium salt produces the cationating agent (either H+ or Ar" ) which initiates chain growth. The authors suggest eq. 2 as the mechanism of initiation (15). 

Weissmann, M., S. Baranton, and C. Coutanceau. Modification of carbon substrates by aryl and alkynyl iodonium salt reduction. Langmuir 26, 2010 15002—15009. 

Electron transfer to the xanthenes, particularly reduction with amines, has been used for a number of years to initiate acrylate polymerization. A typical system is that reported to form volume holograms—lithium or zinc acrylate, triethanolamine and Eosin, Erythrosin, or Rose Bengal [290], Similar mixtures are used to form printing plates photoreducible dye, phenylac-ridine, and acrylate monomer [292], A recent patent application discloses aryl iodonium salts, Rose Bengal, and oxidizable triazines such as 2-methyl-4,6-bis(trichloromethyl)-s-triazine to polymerize acrylates [292],... 

The formation of carbon-heteroatom bonds can be effected by reactions of hypervalent iodine reagents with a wide range of organic substrates and inorganic nucleophiles, and represents one of the most popular applications of organoiodine(III) compounds [1-10]. Except for C-I(III) bond forming reactions used for the synthesis of iodanes and iodonium salts, C-heteroatom bond formation is almost always accompanied by reduction of the hypervalent iodine reagents to iodine(I) compounds. 

Alternatively, chloroarenes can be activated via the formation of manganese, chromium, iron or ruthenium 7r-complexes that react at low temperature with phenoxides to yield diaryl ethers [14], Higher temperatures (DMF, 90 °C) require the formation of diaryl ethers from iodonium salts and phenoxides [15] and the coupling of bromo benzoquinones with phenoxides (DMF, 100-110 °C) followed by a subsequent reduction with dithionite [16]. 

Reductive coupling of iodonium salts catalysed by a palladium-zinc system also produced biaryls in good yield [38]. Also very effective was the palladium-catalysed cross-coupling of iodonium salts with sodium tetraphenylborate in water [39]. The reaction of 3-indolyl phenyliodonium trifluoroacetate with several alkyl and aryl lithium reagents gave 3-substituted indoles [40] ... 

Azides are formed by the reaction of lithio derivatives with />-toliicncsulfonyl azide, and these in turn can be converted into the corresponding amino compounds by a variety of reductive procedures. Nitro compounds are available by a novel reversal of the general pattern of reaction with electrophiles. This approach requires the initial conversion of the lithio compound into an iodonium salt followed by reaction with nitrite ion. This is illustrated by the preparation of 3-nitrothiophene (Scheme 145). Other nucleophiles, such as thiocyanate ion which yields the 3-thiocyanate, can be employed. The preparative significance of these reactions is again that products not accessible by electrophilic substitution can be obtained. 

The first step in the mechanism involves the reduction of Cu(II) to Cu(I) by ascorbyl-6-hexadeeanoate giving dehydroascorbic acid and a weak acid HY benzoic acid). In fact this stage of the process has no importance since Cu(I) benzoate may directly be used to initiate the polymerization by reducing the pyridinium salt. The strong Bronsted acid formed attacks the monomer and initiates the polymerization. Notably, lower polymer yields were obtained by using pyridium salt rather than iodonium salt. 

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]. 

Iodonium and sulfonium salts undergo irreversible one electron electrochemical or chemical reduction [51,52], Reduction of diaryliodonium salts in water exhibit two to four waves in the polarogram depending upon the concentration of iodonium salt, type of electrode, nature and concentration of the supporting electrolyte, and the maximum suppressor [51,53-56]. Reductive electrolysis of diphenyliodonium salts in water at mercury yields mixtures of diphenylmercury, iodobenzene, and benzene, depending upon the potential used during the electrolysis [51,53-55]. Reduction at platinum or glassy carbon electrodes occurs without appearance of the first wave (see below) [54,56,57]. The mechanism shown in Scheme 1 was proposed for aqueous electrolysis of diphenyliodonium salts [51a] ... 

Similarly, photogeneration of other reducing radicals results in reduction of diaryliodonium salts. Hydrogen abstraction from ethers and alcohols by ketone triplets [97], formation of 1,4-biradical by reaction of ketones with unsaturated compounds such as acrylates [97,103], all yield radicals which reduce diaryliodonium salts. Hydrogen atom abstraction from ethers and amines provides a chain process for iodonium salt decomposition (Scheme 6), wherein quantum yields as high as 5 have been reported [60a]. 

Alkynyl(phenyl)iodonium salts react with certain carbon nucleophiles to yield products which are converted into carbenes by reductive elimination of iodobenzene. The carbenes... 

A general mechanistic manifold for Pd -catalyzed direct arylations involves (i) Pd"-mediated C—H activation to afford 1, (ii) oxidation of 1 to Pd -aryl complex 3, and (iii) C—C bond formation via reductive elimination to release the product and regenerate the Pd" catalyst (Scheme 24.3). Aryl iodides (Scheme 24.3a) or aryl iodonium salts (Scheme 24.3b) are the most commonly employed... 

Other examples include 4-bromosydnones,< ) diaryl iodonium salts and bromonaphthalenes.< > The first wave in the polaro-graphic curve of 4-bromosydnones corresponds to the reduction of the C-Br bond, while the second is practically identical to the wave of the parent sydnone. The first two waves of diaryl iodonium... 

Similarly, the direct mode of the SECM allowed the local electrografting of a gold substrate with organic moieties by the local generation of radical from an iodonium salt (an onium salt whose reduction behaves similarly to that of diazonium salts Figure 8.12). Its extension to diazonium salts and to bromoethylbenzenediazonium should allow the patterning of an electrode surface with an ATRP inihator for subsequent positive transfer of patterns of polymer brushes. 

Some examples in which carbonylative and cychzation steps work separately in a one-pot procedure dealing with the synthesis of oxadiazoles 34 [43,44], quinolines, and naphthyridines 35 [45] have been reported. Oxadiazoles were prepared via palladium-catalyzed cyclocarbonylation of aryliodides or diaryliodonium salts with amidoximes (Scheme 13.16). The acylpalladium complex generated from oxidative addition-insertion reaction between aryliodide/iodonium salt, Pd(0), and carbon monoxide reacts with the amidoxime giving rise, after reductive elimination of Pd(0), to an O-acyl-amidoxime, which in turn undergoes cydodehydratation to oxadiazole in moderate to good yields. 

The divergent outcome between the reactions of diaryliodonium salts with charged nucleophiles and with neutral nucleophiles was also explained by this model. In principle, the 10-1-3 intermediate, which is formed in the first step of the overall process, can decompose by two routes. When intramolecular rotation is possible, ligand coupling takes place easily. The second possibility is the homolytic cleavage of the iodine-nucleophile bond, leading to a pair of radicals formed by one electron reduction of the iodonium cation. 

The iodonium and sulfonium salts have found the most application in both the photoinitiation of polymerization and polymer-based photoimaging. Amenability of these salts to spectral sensitization by either an electron-transfer [21] or a triplet energy transfer pathway [22] is an important factor. The reduction potential of the phosphonium salts, -2.1 to —1.55 V versus SCE [10], is too negative for efficient electron transfer sensitization from most sensitizers of practical interest (see below). As a result, most of the fundamental studies have focused on the iodonium and sulfonium salts, as will this review. 

In addition to diazonium salts, there are many other species that are able, once electrochemicaUy activated, to modify the surface with aromatics, unsaturated and saturated groups. These surface modifications can be achieved by (i) oxidation (amines, hydrazines, alcohols, carboxylates, carbamates, carbanions, Grignard reagents) (ii) reduction, some being closely related to diazonium salts (iodoniums, sulfonium) and other different species (vinylics, alkyl halide) (iii) spontaneously, chemically or pho-tochemically (alcenes, azides, peroxides). Note that electrochemical oxidation methods can be applied only to the materials that can withstand quite positive potentials such as carbon or Pt. This section will describe these modifications [357]. 

Datsenko, S., N. Ignat ev, P. Earthen, H.-J. Frohn, T. Scholten, T. Schroer, and D. Welting. Electrochemical reduction of pentafluorophenylxenonium, -diazonium, -iodonium, -bromonium, and -phosphonium salts. Z. Anorg. Allg. Chem. 624, 1998 1669-1673. 

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