Photolysis of diaryliodonium salts (UV/Vis, EPR and GC/MS investigations) (original) (raw)
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Tetrahedron, 1999
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Magnetic Resonance in Chemistry, 2010
Photolysis of trifluoromethyl ketones (TFMKs) 1a-1e versus the non-fluorinated ketones 2a-2b in the presence of radical initiators by electron paramagnetic resonance spectroscopy has been studied for the first time. The transient radicals generated after irradiation of the ketones were identified by trapping with 2-methyl-2-nitrosopropane (MNP) and 2,4,6-tritert-butylnitrosobenzene (TTBNB) as spin traps. TTBNB is a powerful, particularly useful spin trap in these kinds of processes producing anilino and nitroxyl spin adducts due to the ambivalent reactivity on the N and O atoms. In the presence of t-butylperoxide, short-chain TFMKs, such as 1,1,1-trifluoroacetone (1d) and hexafluoroacetone (1e), give rise to detection of the elusive trifluoromethyl radical. In contrast, long-chain TFMKs did not provide clues to prove formation of the trifluoromethyl radical but instead to radicals derived by abstraction of one α-methylene proton to the carbonyl. Although TFMKs are quite stable to photodegradation in the absence of initiator, methyl ketone 2b and phenyl ketone 3 produce radicals resulting from abstraction of a γ -hydrogen to the carbonyl group.
Chemistry – An Asian Journal, 2008
A series of nonfluorinated and fluorinated aryl azides with varied functionality patterns were irradiated in 2,2,2-trifluoroethanol with either a high-pressure or a low-pressure mercury lamp. Interestingly, one of the major products in these reactions was the result of the recombination of anilino and alkyl radicals to form the corresponding hemiaminal compounds. The structure of the recombination products was assigned unambiguously after proton/deuterium exchange experiments followed by MS and MS/MS analysis.
Radical fluoroalkylation reactions of (hetero)arenes and sulfides under red light photocatalysis
All reactions were carried out in an argon atmosphere under anhydrous conditions. Reaction solvents such as N,N´-dimethylformamide (DMF), acetonitrile (MeCN), methanol (MeOH), were chromatography quality and were not further purified. Chromatography and extraction solvents dichloromethane, chloroform, isooctane, n-hexane, n-heptane, ethyl acetate, acetone, dichloromethane (DCM), and ethanol were purchased from commercial suppliers. Ascorbic acid was 99% pure and used as received from the supplier. 2,4,6-trimethylcollidine, and sodium acetate were used as received from the suppliers. Fluorinated reagents 1-iodo1,1,2,2,3,3,4,4,4-nonafluorobutane (perfluorobutyl iodide), was a commercial reagent and used without further purification, except when the Stern Volmer experiment was conducted, in which case it was filtered through a small neutral alumina column to remove traces of iodine dissolved. Heteroaromatic compounds (2-methyl-indole, Gramine, 2amino-5-methylpyridine, 2-amio-5-bromopyridine, 2,4,6-triaminopyrimidine, 2methoxy-9-methylcarbazole, 2-mercaptobenzothiazole, 2mercaptobenzoimidazole, 5-mercaptopurine) and aromatic substrates (aniline, N-methylaniline, N,N-dimethylaniline, N,N-dimethylamino-a-naphthalene, 2methylaniline, 2,5-dimethoxyaniline and anisole) were vacuum-distilled before use. 2,2,6,6-Tetramethyl-1-piperidinyloxy (TEMPO) and 1,4-dinitrobenzene were ultra-pure-grade reagents. Dye Eosin Y (2-(2,4,5,7-tetrabromo-6-oxido-3oxo-3 3H-xanthen-9-yl)benzoate), was 99.9% pure and used as received from the supplier. Methylene Blue (3,7-bis(dimethylamino)phenothiazin-5-ium), 29H,31H-phthalocyanine 1 , and the zinc salt of phthalocyanine 2 were commercially available and used as received from the supplier. Photocatalyst 2,9(10),16(17),23(24)-tetrakis-(1-adamantylsulfanyl) phthalocyaninatozinc(II) 3, was synthesized according to literature procedures (see section V.-). Photocatalyst [Ir(dF(CF3)ppy)2(dtbbpy))PF6 , (dF = 2-(2.4-difluorophenyl)-5-(trifluoromethyl)pyridine, dtbbpy = 4,4´-di-tert-butyl-2,2´-bipyridine) was used as received from the supplier. Zinc acetate was reagent grade. Yields were referred to as isolated yields of analytically pure materials unless otherwise noted, as the case of yields calculated from 19 F NMR and 1 H NMR spectral integration through the use of internal standard (benzotrifluoride). Reactions were magnetically stirred and monitored by thin-layer chromatography (TLC) using Silica gel 60 F254 pre-coated plates (0.25 mm, Merck), and revealed by UV-light. Purification of the reaction products was carried out by column chromatography using Ultra-Pure Silica Gel (230-400 mesh), standard silica-gel for column chromatography (60 mesh) or silica-gel for thin layer preparative chromatography with fluorescent indicator (rhodamine). The light sources were commercially available LEDs (red light, 4 x 5 Watt each, total 20 Watt), household 60-watt fluorescent light bulb (CFL), a 5-Watt blue LED, and green LED (5 Watt). 1 H NMR spectra were recorded on a Bruker Avance 500 (500 MHz), or a Bruker Avance 600 (600 MHz) spectrometers, and are reported in ppm using the solvent residual peak resonance as the internal standard (CDCl3 at 7.28 ppm). 1 H NMR data are reported as follows: chemical shift; number of hydrogens; multiplicity; coupling constants (Hz). Multiplicity is abbreviated as follows: s = singlet, d = doublet, t = triplet, dd = double doublet, m = multiplet, br = broad. Proton-decoupled 13 C NMR spectra were recorded on
Journal of the American Chemical Society, 2008
Aryloxenium ions 1 are reactive intermediates that are isoelectronic with the better known arylcarbenium and arylnitrenium ions. They are proposed to be involved in synthetically and industrially useful oxidation reactions of phenols. However, mechanistic studies of these intermediates are limited. Until recently, the lifetimes of these intermediates in solution and their reactivity patterns were unknown. Previously, the quinol esters 2 have been used to generate 1, which were indirectly detected by azide ion trapping to generate azide adducts 4 at the expense of quinols 3, during hydrolysis reactions in the dark. Laser flash photolysis (LFP) of 2b in the presence of O 2 in aqueous solution leads to two reactive intermediates with λ max 360 and 460 nm, respectively, while in pure CH3CN only one species with λmax 350 nm is produced. The intermediate with λ max 460 nm was previously identified as 1b based on direct observation of its decomposition kinetics in the presence of N 3 -, comparison to azide ion trapping results from the hydrolysis reactions, and photolysis reaction products (3b). The agreement between the calculated (B3LYP/6-31G(d)) and observed time-resolved resonance Raman (TR 3 ) spectra of 1b further confirms its identity. The second intermediate with λ max 360 nm (350 nm in CH3CN) has been characterized as the radical 5b, based on its photolytic generation in the less polar CH 3CN and on isolated photolysis reaction products (6b and 7b). Only the radical intermediate 5b is generated by photolysis in CH 3CN, so its UV-vis spectrum, reaction products, and decay kinetics can be investigated in this solvent without interference from 1b. In addition, the radical 5a was generated by LFP of 2a and was identified by comparison to a published UV-vis spectrum of authentic 5a obtained under similar conditions. The similarity of the UV-vis spectra of 5a and 5b, their reaction products, and the kinetics of their decay confirm the assigned structures. The lifetime of 1b in aqueous solution at room temperature is 170 ns. This intermediate decays with firstorder kinetics. The radical intermediate 5b decomposes in a biphasic manner, with lifetimes of 12 and 75 µs. The decay processes of 5a and 5b were successfully modeled with a kinetic scheme that included reversible formation of a dimer. The scheme is similar to the kinetic models applied to describe the decay of other aryloxy radicals.