Iodonium cation radical

Sonochemically induced cation radical intramolecular cyclization upon the action of an iodonium salt was also demonstrated (Arizawa and others 2001). Being oxidized with phenyliodonium bis(trifluoroacetate), l-(3-anisyl)-2-(l,3-cyclohexadien-2-yl) ethane forms the cation-radical and then 5 -methoxyspiro[cycloxehane-l,l -indan]-2,6-dione. The yield of this final product is high enough. 

Another similar example, also used in polymerization initiation, is the case of the iodonium salts of Rose Bengal (RB2-) [224, 225]. Methylene chloride solutions of these salts bleach in a few seconds in room light through an electron transfer photoinduced from the excited RB2- to the iodonium cation the resulting phenyl radicals were reported to initiate polymerization of acrylate. 

The proposed mechanism was identical with that in acid-catalyzed reactions except for the initiation step. Photolysis of the iodonium salt yields cations and cation radicals that react with traces of water or the monomer to form HX [23]. The Bronsted acid HX then functions similarly to other Bronsted acids in the polymerization reactions. 1,3-Diisopropenylbenzene has also been polymerized in a photoinitiated cationic reaction using 70 as the initiator [Eq. (14)] [9]. 

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. 

Crivello has published extensively on onium compounds and their function as catalysts (3,4). Generally, such compounds are photocatalysts, but are stable in the absence of light. Irradiation at the proper wavelength leads to a complex photodecomposition of the onium ion in which reactive cation radicals and neutral radicals are formed as short lived intermediates. Also formed are acids, corresponding to the anion of the onium compound, which catalyze the epoxy polymerization. Equations 2 and 3 show the decomposition of iodonium cations. 

In consideration of the effects of the irreversible electrochemical reduction of iodonium cation and the estimated lifetime of diphenyliodo radical of 200ps [61], the thermodynamic one-electron reduction potential of diphenyliodonium cation is likely to be 300-600 mV [62] more negative than that measured by cyclic voltametry, that is, red = —0.9 to —1.3V versus SCE. 

Recently, Ledwith (68) continuing his interest in the chemistry of cation radicals (69. 70) demonstrated that the photoinitiation by triarylaminium, sulfonium, and iodonium salts occurs by a mechanism that is different from that proposed by Crivello. 

The proposed mechanism for these catalytic oxidations includes two catalytic redox cycles (i) initial oxidation of iodobenzene with Oxone, producing hydroxy(phenyl)iodonium ion and hydrated iodosylbenzene and (ii) the oxidation of iron(III)-porphyrin to the oxoiron(IV)-porphyrin cation-radical complex by the intermediate iodine(III) species (Scheme 4.58) [93]. The oxoiron(IV)-porphyrin cation-radical complex acts as the actual oxygenating agent toward aromatic hydrocarbons. The presence of the [PhI(OH)]+ and PhI(OH)2 species in solutions containing Phi and Oxone has been confirmed by ESI mass spectrometry [93]. 

In addition to iodonium, sulfonium and selenonium compounds, onium salts of bromine, chlorine, arsenic, and phosphoras are also stable and can act as sources of cation radicals as well as Bronsted acids, when irradiated with light. Performance of diaryl chloronium and diaryl bromonium salts was studied by Nickers and Abu. Also, aryl ammonium and aryl phosphonium, and an alkyl aryl sulfonium salt were investigated. It appears that the general behavior of these materials is similar to diphenyl iodonium and triphenyl sulfonium salts. These are formations of singlet and triplet states followed by cleavages of the carbon-onium atom bonds and in-cage and out of cage-escape reactivity. The anions of choice appear to be boron tetrafluoride, phosphorus hexafluoride, arsenic hexafluoride, and antimony hexafluoride. 

An ultraviolet light photoinitiator, diphenyliodonium 9-acridinecarbo-xylate shows different absorption and fluorescence profiles and photochemical properties when irradiated with near-UV light. The anion absorbs the radiation and sensitizes the photolysis of the iodonium cation and formation of a cationic photoinitiator. At the same time, the free radicals thus formed initiated polymerization of vinyl monomers The structure of ion pairs influences the rate and efficiency of the intra-ion-pair electron transfer and the polymerization. 

The second type of reaction is exemplified in Eq. (32) in which the phenyliodinium cation-radical coupling with iodobenzene to give a proton and a new iodonium salt. 

Photopolymerization reactions are widely used for printing and photoresist appHcations (55). Spectral sensitization of cationic polymerization has utilized electron transfer from heteroaromatics, ketones, or dyes to initiators like iodonium or sulfonium salts (60). However, sensitized free-radical polymerization has been the main technology of choice (55). Spectral sensitizers over the wavelength region 300—700 nm are effective. AcryUc monomer polymerization, for example, is sensitized by xanthene, thiazine, acridine, cyanine, and merocyanine dyes. The required free-radical formation via these dyes may be achieved by hydrogen atom-transfer, electron-transfer, or exciplex formation with other initiator components of the photopolymer system. 

Because visible light is not energetic enough to break chemical bonds, direct production of free radicals by the photoinitiator does not occur. Instead when cationic initiation is needed, as for reaction with epoxies, DIBF is used in conjunction with an iodonium compound such as 4-octyloxyphenyl-phenyliodonium hexaf luoroantimonate (OPPI). It has been proposed that when irradiated, DIBF and OPPI interact to form a cationic species. 

Although radical cations are generated in some electron-irradiated monomers (e.g., vinyl ethers or epoxies), efficient cationic polymerization is not observed. Under certain conditions (addition of iodonium, sulfonium, or sulfoxonium salts, cationic polymerization with the use of electron beam irradiation can be induced. Several studies on radiation cross-linking of elastomers support the concept of ionic mechanism. ... 

Anodic iodination 24°) involves an iodonium intermediate, probably N-iodo-acetonitrilium perchlorate (29) undergoing electrophilic aromatic substitition (Eq. (100) ). A radical cation (28) as intermediate is improbable in this case. Electrolysis of iodine and aromatics in CH3CN/LiC104 yields the corresponding... 

Analogous experimental conditions (i.e. Cl, 0.1-1 Torr) allowed for the detection by tandem mass spectrometry of the collision complexes formed in the ion-molecule reactions of several aromatic radical cations M+ (M = C6H5X, X= Me, N02, Cl pyridine, furan, thiophene) and neutral iodoalkanes RI (R= n-Pr, 2-Pr, n-Bu, 2-Bu, etc.) en route to areni-um ions34,35. The collision complexes are covalently bonded species, namely nonisomeriz-ing iodonium radical positive ions 4 which dissociate to arenium ions 5 via reductive elimination of I (Scheme 7)34. 

Free radical promoted, cationic polymerization also occurs upon irradiation of pyridinium salts in the presence of acylphosphine oxides. But phosphonyl radicals formed are not oxidized even by much stronger oxidants such as iodonium ions as was demonstrated by laser flash photolysis studies [51, 52]. The electron donor radical generating process involves either hydrogen abstraction or the addition of phosphorus centered or benzoyl radicals to vinyl ether monomers [53]. Typical reactions for the photoinitiated cationic polymerization of butyl vinyl ether by using acylphosphine oxide-pyridinium salt combination are shown in Scheme 10. 

In the first study 2,2-dimethoxy-2-phenyl acetophenone was photolysed at 366 ran in n-butylvinyl ether in the presence of di-p-tolyl iodonium hexafluorophosphate as oxidising salt. The free radicals produced in the photolysis were transformed into cationic active species for the polymerisation of the vinyl ether by the electron transfer to the iodonium ion. In the second report, various radical sources were photolysed in the presence of the monomer and silver hexafluorophosphate, the latter acting as one-electron oxidant. 

We note later some cations containing exclusively halogen atoms. Iodine also forms ions (R-I-R)" in which the groups R are usually aromatic radicals (Ar). The halides (Ar-I-Ar)X are not very stable, but the hydroxides are strong bases, proving dissociation to (Ar-I—Ar) ions. These iodonium compounds are the analogues of ammonium and sulphonlum compounds ... 

Under conditions of nanosecond laser-flash photolysis, a long-lived transient absorption assigned to VI, A 465 nm, analogous to V, the intermediate proposed in formation of iodobiphenyls from iodonium salts, is observed [83], The diphenylsulfinyl radical cation, /Lmai340, 750 nm, is similarly observed in acetone sensitized laser-flash photolysis. Photo-CIDNP observations, namely emissive polarization for benzene, suggest that the homolytic cleavage pathway also operates under conditions of direct photolysis, accounting, in part, for the diphenylsulfide product [83], as well as the photochemistry of polymeric triarylsulfonium salts [81] (see 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... 

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