1,721,002 research outputs found
Structural and solvent effects on the C−S bond cleavage in aryl triphenylmethyl sulfide radical cations
No full textSteady-state and laser flash photolysis (LFP) studies of a series of aryl triphenylmethyl sulfides [1, 3,4-(CH3O)2-C6H3SC(C6H5)3; 2, 4-CH3O-C6H4SC(C6H5)3; 3, 4-CH3-C6H4SC(C6H5)3; 4, C6H5SC(C6H5)3; and 5, 4-Br-C6H4SC(C6H5)3] has been carried out in the presence of N-methoxyphenanthridinium hexafluorophosphate in CH3CN, CH2Cl2, CH2Cl2/CH3CN, and CH2Cl2/CH3OH mixtures. Products deriving from the C–S bond cleavage in the radical cations 1•+–5•+ have been observed in the steady-state photolysis experiments. Time-resolved LFP showed first-order decay of the radical cations accompanied by formation of the triphenylmethyl cation. A significant decrease of the C–S bond cleavage rate constants was observed by increasing the electron-donating power of the arylsulfenyl substituent, that is, by increasing the stability of the radical cations. DFT calculations showed that, in 2•+ and 3•+, charge and spin densities are mainly localized in the ArS group. In the TS of the C–S bond cleavage an increase of the positive charge in the trityl moiety and of the spin density on the ArS group is observed. The higher delocalization of the charge in the TS as compared to the initial state is probably at the origin of the observation that the C–S bond cleavage rates decrease by increasing the polarity of the solvent
Involvement of alkoxyl radical intermediates in the photolysis of 1-alkylcycloalkanols in the presence of bis(pyridine) iodonium tetrafluoroborate - Comparison with the (diacetoxyiodo)benzene/I-2 system
No full textThe product distributions observed after visible light irradiation of a series of 1-alkylcycloalkanols (1-3) and bis(pyridine)iodonium tetrafluo-. roborate (IPy2BF4), have been compared with those observed after irradiation of the same substrates in the presence of (diacetoxy)iodobenzene (DIB) and I-2, i.e. under bona fide conditions for alkoxyl radical generation. The observation of very similar product distributions with the two systems strongly supports the previous hypothesis that alkoxyl radical intermediates are formed after photolysis of substrates 1-3 in the presence Of IPy2BF4. (c) 2006 Elsevier B.V. All rights reserved
N-Hydroxyphthalimide: a hydrogen atom transfer mediator in hydrocarbon oxidations promoted by nonheme iron(IV)-Oxo complexes
No full textThe oxidation of a series of hydrocarbons by the nonheme iron(IV)-oxo complex [(N4Py)FeIV═O]2+ is efficiently mediated by N-hydroxyphthalimide. The increase of reactivity is associated to the oxidation of the mediator to the phthalimide N-oxyl radical, which efficiently abstracts a hydrogen atom from the substrates, regenerating the mediator in its reduced form
Reaction of Singlet Oxygen with Thioanisole in Ionic Liquid-Acetonitrile Binary Mixtures
No full textA study of the reaction of thioanisole with singlet oxygen in different ionic liquid acetonitrile binary mixtures has shown that ILs are able to accelerate the thioanisole sulfoxidation when used as additives. With imidazolium ILs, the maximum efficiency is reached at X-IL similar to 0.1-0.2, whereas for the pyrrolidinium IL a plateau is reached. These results are discussed in terms of the ILs' tendency to form ionic aggregates and of differences in sulfoxidation reaction mechanism
Singlet Oxygen Promoted Carbon-Heteroatom Bond Cleavage Reactions in Dibenzyl Sulfides and Tertiary Dibenzylamines. Structural Effects and the Role of Exciplexes
No full textThe C-heteroatom cleavage reactions of substituted dibenzyl sulfides and substituted dibenzylcyclohexylamines
promoted by singlet oxygen in MeCN have been investigated. In both systems, the cleavage
reactions (leading to benzaldehyde and substituted benzaldehyde) were slightly favored by electronwithdrawing
substituents with F values of +0.47 (sulfides) and +0.27 (amines). With dibenzyl sulfides,
sulfones were also obtained whereas sulfoxide formation became negligible when the reactions were
carried out in the presence of a base. Through a careful product study for the oxidation of dibenzyl
sulfide, in the presence and in the absence of Ph2SO, it was established that sulfone and cleavage product
(benzaldehyde) do not come by the same route (involving the persulfoxide and the hydroperoxysulfonium
ylide) as required by the generally accepted mechanism (Scheme 1) for C-heteroatom cleavage reactions
of sulfides promoted by singlet oxygen. On this basis and in light of the similar structural effects noted
above it is suggested that dibenzyl sulfides and dibenzylamines form benzaldehydes by a very similar
mechanism. The reaction with singlet oxygen leads to an exciplex that can undergo an intracomplex
hydrogen atom transfer to produce a radical pair. With sulfides, collapse of the radical pair leads to an
R-hydroperoxy sulfide than can give benzaldehyde by an intramolecular path as described in Scheme 3.
With amines, the radical pair undergoes an electron-transfer reaction to form an iminium cation that
hydrolyzes to benzaldehyde. From a kinetic study it has been established that the fraction of exciplex
converted to aldehyde is ca. 20% with sulfides and ca. 7% with amines
Oxidations of aromatic sulfides promoted by the phthalimide N-oxyl radical (PINO)
No full textThe oxidation of a series of alkyl aryl sulfides promoted by the phthalimide N-oxyl radical (PINO) has been investigated by kinetic and product analysis. Sulfoxides are formed as major reaction products in the oxidation of thioanisoles and benzyl phenyl sulfides. The observation of fragmentation products in the oxidation of 2-phenyl-2-propyl phenyl sulfide and diphenylmethyl phenyl sulfide indicates that the reaction involves an initial electron transfer reaction from the sulfide to PINO with the formation of aryl sulfide radical cations and the anion PINO-. Combination of the species then leads to a radical adduct precursor of sulfoxides while the rapid C–S cleavage occurs with aryl sulfide radical cations that can form the stable 2-phenyl-2-propyl or diphenylmethyl carbocation
Fragmentation reactions of radical cations
No full textThis paper summarizes most of the recent work by our group on the kinetic aspects of a variety of fragmentation reactions of radical cations involving the cleavage of C-H, O-H, C-C, and C-S bonds. In particular, the problems that have been addressed concern: (a) the carbon acidity/oxygen acidity dichotomy in the fragmentation of aryl- and thioarylalkanol radical cations; (b) the decarboxylation of aryl and thioarylacetic acid radical cations; (c) the structural factors influencing the rate of C-S bond cleavage in the radical cations of phenyl alkyl sulfides. The results presented and discussed have allowed us to recognize the important role played by the electron reorganization energy, associated with the intramolecular electron transfer from the scissile bond to the SOMO, with respect to the dynamics and mechanism of the fragmentation process in the radical cation. Copyright (c) 2006 John Wiley & Sons, Ltd
Structural effects on the beta-scission reaction of alkoxyl radicals. Direct measurement of the absolute rate constants for ring opening of benzocycloalken-1-oxyl radicals
No full textThe absolute rate constants for beta-scission of a series of benzocycloalken-1-oxyl radicals and of the 2-(4-methylphenyl)-2-butoxyl radical have been measured directly by laser flash photolysis. The benzocycloalken-1-oxyl radicals undergo ring opening with rates which parallel the ring strain of the corresponding cycloalkanes. In the 1-X-indan-1-oxyl radical series, ring opening is observed when X = H, Me, whereas exclusive C-X bond cleavage occurs when X = Et. The factors governing the fragmentation regioselectivity are discussed
Electron-transfer mechanism in the N-demethylation of N,N-dimethylanilines by the phthalimide-N-oxyl radical
No full textThe reactivity of the phthalimide N-oxyl radical (PINO) toward the N-methyl C-H bond of a number of 4-X-substituted N,N-dimethylanilines (X = OMe, OPh, CF3, CO2Et, CN) has been investigated by product and kinetic analysis. PINO was generated in CH3CN by reaction of N-hydroxyphthalimide (NHPI) with Pb(OAc)(4) or, for the kinetic study of the most reactive substrates (X = OMe, OPh), with tert-butoxyl radical produced by 266 nm laser flash photolysis of di-tert-butyl peroxide. The reaction was found to lead to the N-demethylation of the N,N-dimethylaniline with a rate very sensitive to the electron donating power of the substituent (rho(+) = -2.5) as well as to the oxidation potential of the substrates. With appropriately deuterated N,N-dimethylanilines the intermolecular and intramolecular deuterium kinetic isotope effects (DKIEs) were measured for some substrates (X = OMe, CO2Et, CN) with the following results. First, intramolecular DKIE [(k(H)/k(D))(intra)] was found to be always different and higher than intermolecular DKIE [(k(H)/k(D))(inter)]; second, no intermolecular DKIE [(k(H)/k(D))(inter) = 1] was observed for X = OMe, whereas substantial values of (k(H)/k(D))(inter) were exhibited by X = CO2Et (4.8) and X = CN (5.8). These results, while are incompatible with a single step hydrogen atom transfer from the N-C-H bond to the N-oxyl radical, as proposed for the reaction of PINO with benzylic C-H bonds, can be nicely interpreted on the basis of a two-step mechanism involving a reversible electron transfer from the aniline to PINO leading to an anilinium radical cation, followed by a proton-transfer step that produces an alpha-amino carbon radical. In line with this conclusion the reactivity data exhibited a good fit with the Marcus equation and a lambda value of 37.6 kcal mol(-1) was calculated for the reorganization energy required in this electron-transfer process. From this value, a quite high reorganization energy (> 60 kcal mol-1) is estimated for the PINO/NHPI(-H)(-) self-exchange reaction. It is suggested that the N-demethylated product derives from the reaction of the alpha-amino carbon radical with PINO to form either a cross-coupling product or an alpha-amino carbocation. Both species may react with the small amounts of H2O present in the medium to form a carbinolamine that, again by hydrolysis, can be eventually converted into the N-demethylated product
A kinetic study of the electron-transfer reaction of the phthalimide-N-oxyl radical (PINO) with ferrocenes
No full textA kinetic study of the one-electron oxidation of a series of ferrocenes (FcX: X = H, CO2Et, CONH2, CH2CN, CH2OH, Et, and Met) by PING generated in CH3CN by reaction of N-hydroxyphthalimide (NHPI) with the cumyloxyl radical produced by 355 nm laser flash photolysis of dicumyl peroxide has been carried out. Ferrocenium cations were formed, and the reaction rate was determined by following the decay of PINO radical at 380 nm as a function of the FcX concentration. Rate constants were very,sensitive to the oxidation potential of the substrates and exhibited a good fit with the Marcus equation, from which a lambda value of 38.3 kcal mol(-1) was calculated for the reorganization energy required in the PINO/ferrocenes electron-transfer process. Knowing the ferrocene/ferrocenium self-exchange reorganization energy it was possible to calculate a value of 49.1 kcal mol(-1) for the PINO/PINO- self-exchange reaction in CH3CN. Moreover, from the Marcus cross relation and the self-exchange rates of ferrocene and dimethylferrocene, the intrinsic reactivity of PING in electron-transfer reactions has been calculated as 7.6 x 10(2) M-1 s(-1). The implications of these values and the comparison with the electron-transfer self-exchange reorganization energies of peroxyl radicals are briefly discussed
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