Sensitized Photolysis of Iodonium Salts Studied by CIDNP. Solvent Dependence and Influence of Lipophilic Substituents

Abstract

The sensitized photoreactions of diphenyliodonium salt and four of its technically relevant derivatives bearing lipophilic substituents were investigated by pseudo-steady-state and time-resolved CIDNP experiments. Singlet (naphthalene, diphenylanthracene, dimethylanthracene) and triplet (xanthone, thioxanthone, Michler's ketone) sensitizers were used in a variety of solvents (acetonitrile, dimethyl sulfoxide, dimethylformamide, methanol, chloroform, tetrahydrofurane, diethylene glycol dimethyl ether, dioxane, toluene). Under all conditions, the primary step leading to CIDNP was found to be electron transfer from the excited sensitizer *Sens to the onium cation On+. All spin-polarization effects could be explained consistently within the framework of radical pair theory (S−T0-type CIDNP). Pair substitution, i.e., the transformation of the primary radical pairs RP 1 into secondary pairs RP 2 ( where Ph• is the phenyl or an aryl radical) plays a key role for the CIDNP effects and even leads to a field dependence of the polarization phases for the system diphenyliodonium cation/naphthalene in dioxane. Decreasing solvent polarity causes an increase of the rate RP 1 → RP 2. The introduction of a long lipophilic side chain into the onium salt has the same effect, presumably owing to self-solvation.

Reaction of Reduced Flavins and Flavoproteins with Diphenyliodonium Chloride

“…Further elution of the Sep-Pak cartridges with 2.5-3% CH 3 CN afforded a pale yellow nonfluorescent solution with absorption maximum at 364 nm. This fraction corresponds to FIG.…”

“…The known capability of iodonium compounds to undergo radical reaction (1)(2)(3)(4), the fact that most of the known enzymes to be severely inhibited, such as neutrophil NADH oxidase (5,6), cytochrome P-450 reductase (7), xanthine oxidase (8), nitric-oxide synthase (9), and sulfite reductase (10), involve radical chemistry in their mechanisms has led to the concept that inhibition by iodonium compounds is a marker of flavoprotein enzymes functioning by radical mechanisms, particularly because other flavoproteins, such as D-and L-amino acid oxidases, glucose oxidase, and glutathione reductase, which do not involve radicals in their reaction, are not inhibited (8). Previous work has demonstrated that the enzyme flavin needs to be reduced for inhibition to occur, and that the reaction of the reduced flavin with iodonium compounds results in phenyl adducts of the flavin (8,10).…”

Intermolecular Olefin Functionalisation Involving Aryl Radicals Generated from Arenediazonium Salts

“…The aerobic, oxidative generation of aryl radicals from arylhydrazines might even play a key role in the chemical defense of the mushroom Stephanospora caroticolor. [16] Other recently developed methods for the generation of aryl radicals include the irradition of iodonium salts [17] and the treatment of arylboronic acids with the strong oxidant manganeseA C H T U N G T R E N N U N G (III) acetate. [18] Among all the reaction principles mentioned, ar-A C H T U N G T R E N N U N G enediazonium salts probably offer the most convenient and cost-effective access to aryl radicals.…”

Mechanistic Study of Tetrahydrofuran Polymerization Photoinitiated by a Sulfonium Salt/Thioxanthone System

“…[25] Stable iodonium and sulfonium salts [1,11,26,27] possess considerable importance as initiators for both cationic and radical photopolymerization, especially in relation to polymer-based photoimaging. [28] Sensitization by energyor electron-transfer is often used to overcome the unfavorable absorption properties of these compounds. The addition of the sensitizers results in complex mechanisms involving intermediates of various types.…”