Iodonium photolysis

A very simple synthesis of coumestrol (228) has been described by Kappe and coworkers (Scheme 46) (74ZN(B)292). It is based upon dehydrogenation of 4-hydroxy-3-phenyl-coumarins to coumestans (720PP233). A number of 2 -hydroxy 3-phenylcoumarins were oxidized with lead tetraacetate to the corresponding coumestans 3-(l-acetoxy-4-methoxy-2-oxo-3,5-cyclohexadienyl)coumarins were obtained as by-products (76BCJ1955). Coumes-tan itself (226) has been obtained by photolysis of the phenol ether (232), which is in turn available from 4-hydroxycoumarin (229) and (diacetoxyiodo)benzene (Scheme 47) (78CB3857) via an iodonium ylide (231). 

Generation of triplet carbenes from photolysis of iodonium ylides has been discussed in a short review.9... 

The photochemistry of diphenyl- and bis(4-methylphenyl)iodonium salts has been investigated481,482. Diphenyliodonium halides (140, X = Cl, Br, I) exist as tight ion pairs in acetonitrile. Their photolysis gives almost exclusively iodobenzene by a homolytic cleavage from a charge-transfer excited state. In aqueous acetonitrile, however, the ion pairs are solvent-separated and substantial amounts of 2-, 3- and 4-iodobiphenyls (141) are formed in addition to iodobenzene (142), benzene (143), acetanilide (144) and biphenyl (145) (equation 126). In this medium the photodecomposition occurs via initial heterolysis of the molecule in its excited state, leading to iodobenzene and phenyl cation. 

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. 

Similar to pyridinium salts, iodonium salts can be reduced by silyl radicals generated by photolysis of polysilanes. However, due to the tail absorption bands of iodonium salts at about 300 nm, polysilanes with relatively longer... 

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]. 

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. 

Using fluorescence and laser flash techniques one can show that the lipophilic substituted iodonium hexafluoroantimonates react efflcently with singlet and triplet states of several sensitizers. Furthermore, one observes the decay of the electron donor, which was investigated with 9,10-diphenylanthracene (DPA) and 9,10-dimethylanthracene as sensitizers, which act as electron donor. Using the CIDNP-technique and laser flash photolysis one can prove that electron transfer takes place to the iodonium salts [9], which results in formation of a proton (see Scheme 3). 

The photochemical hydration of one of the triple bonds of naphthyl-1,3-butadiynes is reported to yield naphthyl substituted ynones if the reaction is carried out in methanol rather than water then the primary products include (240) which result from addition of solvent or photoreduction. These compounds undergo secondary photochemistry and are found to photocyclise to the phenanthrenes (241). A similar reaction occurs for phenyl substituted 1,3-butadiynes. Photolysis of the phenyl iodonium ylid of dibenzoylmethane, i.e. (242), with terminal alkynes yields (243) which photocyclises in a similar manner to (240) to yield benzoyl-naphthols (244). °... 

A systematic study of the product distributions resulting on photolysis of diphenyl- and substituted diphenyliodonium cations as functions of both solvent and counterion was carried out by Timpe and Schikowsky [71]. Unlike the earlier results of Pappas, these authors found that, taking into consideration all organic products, quantum yields of the disappearance of iodonium cations and of formation of protic acid are within experimental error. Significantly, light intensity, /, dependence of the quantum yields was observed in this study. Timpe and Schikowsky s principal observations are summarized in Table 3. 

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). 

Electron Transfer Sensitization. Iodonium and sulfonium salts undergo sensitized (electron transfer) photolysis with a variety of photosensitizers, D, meeting the requirements of the Weller equation [59] for exergonic electron transfer to an acceptor, A (Eq. (28)) [98]. No coulombic term is included owing to the cationic nature of iodonium and sulfonium moieties, that is, there is no free energy contribution due to formation of an ion pair. 

Some differences in ratios of phenylated anthracene photoproducts have been reported [70,91a, 94]. These differences are fairly minor and may be attributed to differences in chemical conversion and analysis or differences in photolysis conditions Hacker et al. [94] obtained their product distributions at relatively high anthracene concentration (0.0025 M), compared to 4x10 4M in the study of DeVoe et al. [91a], The quantum yield for anthracene sensitized photolysis of iodonium salts apparently increases with anthracene concentration. 

Iodonium salt photolysis sensitized by dialkoxyanthracenes is thus sensitive to solvent viscosity in respect to bimolecular quenching [100]. 

Benzene formed from photolysis of the 1 1 complex is a cage-escape product from 3(Jul-CHO+ /Ph ). Benzene formed from the photolysis of the 2 1 complex is an in-cage product from 3((Jul-CHO)2 /Ph ). The formation of 2 1 complexes of amino-substituted ketones and iodonium salts has been suggested to account for the high photosensitivity of polymeric Mannich bases with iodonium salts [102]. Formation of 2 1 donor iodonium cation complexes has been rationalized by consideration of the crystal structures of diphenyliodonium halides, which crystallize as dimers with square planar iodine atoms with two bridging halide counterions [102,108]. 

Photochemical Sensitization. Photolysis of diaryliodonium salts in the presence of benzoin ethers results in efficient reaction of the iodonium salt [96,97]. Scheme 5 illustrates the mechanism of photolysis according to Ledwith [96] and Timpe [92], Accordingly, photocleavage of benzoin ethers yields easily oxidized ketyl radicals (and acyl radicals which can also initiate radical polymerization). That only ketyl radicals participate in photochemical sensitization of onium salt decomposition was confirmed by ESR spin trapping with benzylidene-tcrt-butylamine-AT-oxide [10b]. As the chemistry... 

The photoinitiation of polymerization of pentaerythritol tetraacrylate using phenyl-(p-anisyl)-iodonium triflate or triphenylsulfonium hexafluorophosphate, sensitized with either l,6-diphenyl-l,3,5-hexatriene or 1,3-diphenyl-2-pyrazoline, was illustrated by Smith [111b] in 1981. Under his conditions, direct photolysis of the onium initiators failed to initiate polymerization. Baumann and co-workers, however, found conditions for initiation of radical polymerization on direct irradiation of onium salts [18,122], consistent with the hypothesized generation of radicals capable of cage escape in direct photolysis. 

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. 

Mechanistic studies on systems related to commercial photoinitiators for polymerization include work on arene diazonium salts,sensitized photolysis of iodonium salts,and the use of excited states of ketones as electron donors in iodonium salt systems employed as photoinitiators. 

CIDNP spectroscopy has been applied to both direct and sensitized photoreactions of onium salts (diaryliodonium and triarylsulfonium salts) [134] see Chart XVIII. In a recent investigation [134f] ambiguities of earlier studies on sensitized photolysis of iodonium salts [134a, 134b], where S-T -type polarizations had to be postulated to rationalize the polariz-... 

It should be pointed out that the photolysis of the diaryliodonium and triaryl-sulfonium salts proceeds differently from the photolysis of the diazonium systems. The reaction mechanism of the photolysis of iodonium salts is described by Reactions [6.46] and [6.47]. 

Triplet sensitization of sulfonium salts proceeds exclusively by the homolytic pathway, and that the only arene escape product is benzene, not biphenyl or acetanilide. However, it is difficult to differentiate between the homolytic or heterolytic pathways for the cage reaction, formation of the isomeric halobiaryls. Our recent studies on photoinduced electron transfer reactions between naphthalene and sulfonium salts, have shown that no meta- rearrangement product product is obtained from the reaction of phenyl radical with diphenylsulfinyl radical cation. Similarly, it is expected that the 2- and 4-halobiaryl should be the preferred products from the homolytic fragments, the arene radical-haloarene radical cation pair. The heterolytic pathway generates the arene cation-haloarene pair, which should react less selectively and form the 3-halobiaryl, in addition to the other two isomers. The increased selectivity of 2-halobiaryl over 3-halobiaryl formation from photolysis of the diaryliodonium salts versus the bromonium or chloronium salts, suggests that homolytic cleavage is more favored for iodonium salts than bromonium or chloronium salts. This is also consistent with the observation that more of the escape aryl fragment is radical derived for diaryliodonium salts than for the other diarylhalonium salts. 

In the same groundbreaking paper [21], CriveUo and coworkers also demonstrated that the anion plays no role in determining the photosensitivity of the iodonium salt and the photolysis rates of diaryliodonium salts having the same cations but different non-nucleophilic counterions (BF4, PFs, AsFs", or SbFe ) are identical. Likewise, the cation structure has little effect on the photodecomposition of diaryliodonium salts. The utility of iodonium salts as photoinitiators has been demonstrated in several cationic polymerizations using olefins, epoxides, cycUe ethers, lactones and cyclic sulfides as the monomers [21],... 

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