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In this study, we carried out preparative and mechanistic studies on the photochemical reaction of a series of 3-acylestrone derivatives in confined and sustainable micellar environment under steady-state conditions and the results were compared with those obtained in cyclohexane solution. The aim of this work is mainly focused to show whether the nature of the surfactant (cationic, neutral and anionic) leads to noticeable selectivity in the photoproduct formation. The 3-acylestrone derivatives underwent the photo-Fries rearrangement, with concomitant homolytic fragmentation of the ester group and [1;3]-acyl migration. This pathway afforded the ortho-acyl estrone derivatives, the main photoproducts together with estrone. However, epimerization of the ortho regioisomer 2-acetylestrone and estrone through Norrish Type I photoreaction occurred involving the fragmentation of the C-α at the carbonyl group (C-17) of the steroid. UV–visible and 2D-NMR (NOESY) spectroscopies have been employed to measure the binding constant Kb and the location of the steroids within the hydrophobic core of the micelle.

The photochemistry of tris(2,4-dibromophenyl)amine was investigated via time-resolved nanosecond spectroscopy. The tris(2,4-dibromophenyl)amine radical cation ("Magic Green") was immediately detected after the laser pulse; this intermediate then cyclizes to N-aryl-4a,4b-dihydrocarbazole radical cation. The latter transient reacted with molecular oxygen to provide the corresponding hydroperoxyl radical, which smoothly co-oxidize sulfides into sulfoxides. On the other hand, the photogenerated "Magic Green" was exploited to promote the co-oxidation of nucleophilic triarylphosphines to triarylphosphine oxides through an electron transfer process preventing the amine cyclization. In this case, the intermediate Ar3 POO•+ was found to play a key role in phosphine oxide formation.

The photoreactivity of triphenylamine in homogeneous media has been investigated by means of laser flash photolysis spectroscopy and preparative experiments. The goal of this study consists in the evaluation of the photophysical and photochemical behavior of transient N-phenyl-4a,4b-dihydrocarbazole (DHC0) under polar and non-polar solvents which was not performed yet. The kinetic parameters of such a key intermediate including lifetime (τd, τR and τox) and rate constants (kd, kR and kox) have been measured and the intermediacy of DHC0 in the co-oxidation of nucleophilic substrates such as sulfides and phosphines was investigated by a combined time-resolved and steady-state approach. Two distinctly oxidizing intermediates, a hydroperoxy radical and a hydroperoxide derivative were formed from transient DHC0 in the presence of molecular oxygen. Radical I oxidizes triarylphosphines, whereas intermediate II converts sulfides into the corresponding sulfoxides.

In this study, we carried out preparative and mechanistic studies on the photochemical reaction of a series of p-substituted phenyl benzoates in confined and sustainable micellar environment. The aim of this work is mainly focused to show whether the nature of the surfactant (ionic or nonionic) leads to noticeable selectivity in the photoproduct formation and whether the electronic effects of the substituents affect the chemical yields and the rate of formation of the 5-substituted-2-hydroxybenzophenone derivatives. Application of the Hammett linear free energy relationship (LFER) on the rate of formation of benzophenone derivatives, on the lower energy band of the UV-visible absorption spectra of the aryl benzoates and 5-substituted-2-hydroxybenzophenone derivatives allows a satisfactory quantification of the substituent effects. Furthermore, UV-visible and 2D-NMR (NOESY) spectroscopies have been employed to measure the binding constant Kb and the location of the aryl benzoates within the hydrophobic core of the micelle. Finally, TD-DFT calculations have been carried out to estimate the energies of the absorption bands of p-substituted phenyl benzoates and 5-substituted-2-hydroxybenzophenone derivatives providing good linear correlation with those values measured experimentally.

Preparative and mechanistic studies on the photochemical reaction of a series of p-substituted benzanilides in polar and nonpolar solvents have been carried out. The aim of this work is mainly focused to show whether the reaction solvent and the electronic effects of the substituents affect the product distribution, the chemical yields and the rate of formation of the 5-substituted-2-aminobenzophenone derivatives. Application of the Hammett linear free energy relationship (LFER) on the rate of formation of 2-aminobenzophenone derivatives, on the lower energy band of the UV-visible absorption spectra of the benzanilides and 5-substituted-2-aminobenzophenone derivatives allows a satisfactory quantification of the substituent effects. Furthermore, the solvent effect was also analyzed on the photoreaction by means of the Reichardt's solvent parameter (E

Irradiation of a series of 3-acylestrones under a nitrogen atmosphere in cyclohexane, acetonitrile (MeCN), and methanol (MeOH) was investigated under steady-state conditions. The molecules underwent the photo-Fries rearrangement, with concomitant homolytic fragmentation of the ester group and [1;3]-acyl migration. This pathway afforded the ortho-acyl estrone derivatives, the main photoproducts, together with estrone. During the irradiation of 3-benzoyl estrone, epimerization of estrone through the Norrish type I reaction occurred, providing lumiestrone as the photoproduct. This photoreaction involves the fragmentation of the C-α at the carbonyl group (C-17) of the steroid. On the other hand, epimerization of ortho-regioisomer 2-acetyl estrone occurred during the irradiation of 3-acetyl estrone. Photosensitization with acetone and chemical quenching with N, N, N, N-tetramethyldiazetinedioxide of the photo-Fries reaction confirmed that the photoreaction took place from the singlet excited state while the Norrish type I reaction proceeds efficiently from the triplet excited state. Solvent effects, as well as the nature of the acyl group on the photoreactions, were also studied.

This chapter deals with photo-rearrangement reactions useful in organic synthesis that can be carried out upon direct irradiation of the substrate, under photocatalytic conditions with metal complexes or photoorganocatalysts and also under photosensitization via energy transfer as well as electron transfer proccesses.

The direct irradiation of diphenyl sulfide and p-substituted thioanisoles in the presence of oxygen was investigated by means of both steady state and laser flash photolysis experiments. Two competitive pathways took place from the triplet excited state of thioanisoles, C-S bond cleavage, finally leading to aryl sulfinic acid and sensitized oxidation leading to S-oxidation. Co-oxidation of dodecyl methyl sulfide occurred efficiently implying that an S-persulfoxide intermediate is involved during the sensitized oxidation. On the other hand, triplet state of diphenyl sulfide also showed competitive C-S bond cleavage giving phenyl sulfinic acid and ionization to diphenyl sulfide radical cation that in turn led to diphenyl sulfoxide. The rate constants of the above reactions were determined by time-resolved experiments. Fil: Bonesi, Sergio Mauricio. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones en Hidratos de Carbono. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones en Hidratos de Carbono; Argentina. PhotoGreen Lab; Italia Fil: Crespi, Stefano. PhotoGreen Lab; Italia Fil: Merli, Daniele. Department of Chemistry; Italia Fil: Manet, Ilse. ISOF-CNR; Italia Fil: Albini, Angelo. PhotoGreen Lab; Italia

Irradiation of a series of p-substituted aryl 10 benzoates under N2 atmosphere in homogeneous and micellar 11 media was investigated by means of steady-state condition and 12 of time-resolved spectroscopy. A notable selectivity in favor of 13 the 2-hydroxybenzophenone derivatives was observed in 14 micellar media. The benzophenone derivatives were the 15 main photoproduct. On the other hand, in homogeneous 16 media (cyclohexane, acetonitrile, and methanol) the observed 17 product distribution was entirely different, viz. substituted 2- 18 hydroxybenzophenones, p-substituted phenols, benzyl and 19 benzoic acid were found. The binding constants in the 20 surfactant were also measured and NOESY experiments showed that the aryl benzoates were located in the hydrophobic core of 21 the micelle. Laser flash photolysis experiments led to the characterization of both p-substituted phenoxy radical and substituted 22 2-benzoylcyclohexadienone transients in homogeneous and micellar environment. Fil: Siano, Gastón. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones en Hidratos de Carbono. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones en Hidratos de Carbono; Argentina Fil: Crespi, Stefano. Universita Degli Studi Di Pavia; Italia Fil: Mella, Mariella. Universita Degli Studi Di Pavia; Italia Fil: Bonesi, Sergio Mauricio. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones en Hidratos de Carbono. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones en Hidratos de Carbono; Argentina. Universita Degli Studi Di Pavia; Italia

The photochemistry of tris(p-bromophenyl)amine was investigated in a nitrogen- and oxygen-flushed solution under laser flash photolysis conditions. The detected intermediates were the corresponding amine radical cation ("Magic Blue") and the N-phenyl-4a,4b-dihydrocarbazole radical cation that, under an oxygen atmosphere, is converted to the corresponding hydroperoxyl radical. The role of the last species was supported by the smooth co-oxidation of sulfides to sulfoxides. On the other hand, co-oxidation of nucleophilic triarylphosphines to triarylphosphine oxides arose from an electron transfer between the photogenerated "Magic Blue" and phosphine that prevented the amine cyclization. In this case, intermediate Ar3POO•+ was found to play a key role in phosphine oxide formation. Fil: Bonesi, Sergio Mauricio. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones en Hidratos de Carbono. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones en Hidratos de Carbono; Argentina Fil: Protti, Stefano. Universita Di Pavia; Italia Fil: Albini, Angelo. Universita Di Pavia; Italia

A new series of decyl chain [-(CH2)9CH3] pterin conjugates have been investigated by photochemical and photophysical methods, and with theoretical solubility calculations. To synthesize the pterins, a nucleophilic substitution (SN2) reaction was used for the regioselective coupling of the alkyl chain to the O site over the N3 site. However, the O-alkylated pterin converts to N3-alkylated pterin under basic conditions, pointing to a kinetic product in the former and a thermodynamic product in the latter. Two additional adducts were also obtained from an N-amine condensation of DMF solvent molecule as byproducts. In comparison to the natural product pterin, the alkyl chain pterins possess reduced fluorescence quantum yields (F) and increased singlet oxygen quantum yields (-). It is shown that the DMF-condensed pterins were more photostable compared to the N3- and O-alkylated pterins bearing a free amine group. The alkyl chain pterins efficiently intercalate in large unilamellar vesicles, which is a good indicator of their potential use as photosensitizers in biomembranes. Our study serves as a starting point where the synthesis can be expanded to produce a wider series of lipophilic, photooxidatively active pterins. Fil: Vignoni, Mariana. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina. City University of New York; Estados Unidos Fil: Walalawela, Niluksha. City University of New York; Estados Unidos. The Graduate Center of the City University of New York; Estados Unidos Fil: Bonesi, Sergio Mauricio. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones en Hidratos de Carbono. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones en Hidratos de Carbono; Argentina Fil: Greer, Alexander. City University of New York; Estados Unidos. The Graduate Center of the City University of New York; Estados Unidos Fil: Thomas, Andrés Héctor. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina

A facile photochemical preparation of 5-, 6- and 7-substituted chroman-4-ones from aryl 3-methyl-2-butenoate esters is described. The two-phase base-catalyzed method relies upon two consecutive processes in one-pot reaction through a photo-Fries rearrangement and a based-catalyzed intramolecular oxa-Michael addition to afford the desired products. Fil: Iguchi, Daniela. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones En Hidratos de Carbono; Argentina Fil: Erra Balsells, Rosa. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones En Hidratos de Carbono; Argentina Fil: Bonesi, Sergio Mauricio. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones En Hidratos de Carbono; Argentina

The oxidation of triphenylphosphine in the presence of various photocatalytic systems (dicyanoanthracene/biphenyl, N-methylquinolinium, triphenylpyrylium, and thiatriphenylpyrylium tetrafluoroborate) was investigated by means of both steady state and laser flash photolysis experiments. The effect of different additives (including 1,4- benzoquinone, diphenylsulfoxide, tetramethylethylene, and sodium azide) on the photosensitized oxidation was investigated in order to fully characterize the involved intermediates. Photoinduced electron transfer and final regeneration of the catalyst occur when dicyanoanthracene and N-methylquinolinium are used, while in cage oxygen transfer to the photoexcited (thio)pyrylium derivatives have been characterized in the last two cases. Fil: Bonesi, Sergio Mauricio. PhotoGreen Lab; Italia. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones en Hidratos de Carbono. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones en Hidratos de Carbono; Argentina Fil: Protti, Stefano. PhotoGreen Lab; Italia Fil: Albini, Angelo. PhotoGreen Lab; Italia

Several aryl 3-methyl-2-butenoate esters upon irradiation lead to the formation of [1,3]-migrated photoproducts, phenol and, surprisingly 2,2-dimethylchroman-4-one derivatives. The starting photochemical reaction takes place from the singlet excited state of the ester and as a total mechanism two consecutive reaction pathways are proposed. The former involves the photo-Fries rearrangement of the esters in all the solvents studied and, depending on the proticity of the solvent, the latter involves an ESIPT process followed by thermal 6π-electrocyclic reaction and/or thermal (intramolecular oxa-Michael addition) cyclization of the ortho regioisomers photoformed. This second pathway is responsible of the formation of the 2,2-dimethylchroman-4-one derivatives. Fil: Iguchi, Daniela. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones en Hidratos de Carbono. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones en Hidratos de Carbono; Argentina Fil: Erra Balsells, Rosa. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones en Hidratos de Carbono. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones en Hidratos de Carbono; Argentina Fil: Bonesi, Sergio Mauricio. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones en Hidratos de Carbono. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones en Hidratos de Carbono; Argentina

Photocatalytic methods for fluoroalkyl-radical generation provide more convenient alternatives to the classical perfluoroalkyl-radical (Rf) production through chemical initiators, such as azo or peroxide compounds or the employment of transition metals through a thermal electron transfer (ET) initiation process. The mild photocatalytic reaction conditions tolerate a variety of functional groups and, thus, are handy to the late-stage modification of bioactive molecules. Transition metal-photocatalytic reactions for Rf radical generation profit from the redox properties of coordinatively saturated Ru or Ir organocomplexes to act as both electron donor and reductive species, thus allowing for the utilization of electron accepting and donating fluoroalkylating agents for Rf radical production. On the other hand, laboratory-available and inexpensive photoorgano catalysts (POC), in the absence of transition metals, can also act as electron exchange species upon excitation, resulting in ET reactions that produce Rf radicals. In this work, a critical account of transition metal and transition metal-free Rf radical production will be described with photoorgano catalysts, studying classical examples and the most recent investigations in the field.

Oxidation of a series of sulfides R′ SR (R′,dodecyl,phenethyl,benzyl,cumyl,benzhydryl; R,ethyl,phenyl) under photoinduced electron transfer conditions (sensitizers: 9,10-dicyanoanthracene,triphenylpyrylium tetrafluoroborate) yields sulfoxides,products of trapping of the alkyl cation (for the benzhydryl and cumyl derivatives),and mainly aldehydes or ketones that result from α -deprotonation. The competing paths from the sulfide radical cation are discussed in this paper.

The photosensitized electron-transfer oxidation of a series of ethyl sulfides RSEt (1, R = C12H25; 2, PhCH2CH2; 3, PhCH2; 4, PhCMe2; 5, Ph2CH) has been examined in acetonitrile and the product distribution discussed on the basis of the mechanisms proposed. In nitrogen-flushed solutions, cleaved alcohols and alkenes are formed, whereas under oxygen, in reactions that are 10–70 times faster, sulfoxides and cleaved aldehydes and ketones are formed in addition to the aforementioned products. Two sensitizers are compared, 9,10-dicyanoanthracene (DCA) and 2,4,6-triphenylpyrylium tetrafluoroborate (TPP+BF4–), the former giving a higher proportion of the sulfoxide, the latter of cleaved carbonyls. The sulfoxidation is due to the contribution of the singlet oxygen path with DCA. Oxidative cleavage, on the other hand, occurs both with DCA and with TPP+ which is known to produce neither singlet oxygen nor the superoxide anion. This process involves deprotonation from the α position of the sulfide radical cation, but the TPP+ results suggest that O2·– is not necessarily involved and non-activated oxygen forms a weak adduct with the radical cation promoting α-hydrogen transfer, particularly with benzylic derivatives.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Solid photocatalysts in which the 2,4,6-triphenylpyrylium (TP) or 2,4,6-triphenylthiapyrylium cation (TTP) are encapsulated within zeolite Y or deposited on mesoporous silica are used as photosensitizers for the oxidation of sulfides, both aromatic and aliphatic, in solvents of various polarity. Contrary to the same cations in solution, these solid sensitizers are not significantly degraded during operation. An effective oxygenation takes place leading to sulfoxides, disulfides, sulfinic and sulfenic esters as well as sulfonic acids. This large scope method shows a limited dependence on the substrate structure and on conditions and is suitable for the abatement of sulfur-containing pollutants.

The photophysical behaviour of 1-nitrocarbazole, 3-nitrocarbazole, 1,6-dinitrocarbazole and 3,6-dinitrocarbazole in acetonitrile was studied by using spectroscopic, luminescence and photoacoustic techniques. Absorption spectra and triplet state transient spectra were recorded and the ϕ T and τ T values were determined. Phosphorescence emission at 77 K and the dynamic properties in terms of ϕ p , k p o and k nr for the mono and dinitrocarbazoles were derived. Results are discussed in terms of the optimised structure of 1-nitrocarbazole, 3-nitrocarbazole, 1,6-dinitrocarbazole and 3,6-dinitrocarbazole and are compared with that of carbazole. The nitro group attached to the carbazole moiety induces important changes in the photophysical behaviour of these compounds.

Irradiation (254 nm) of five alkyl and benzyl ethyl sulfides causes efficient ( Φ r 0.27–0.90) homolytic cleavage of the C–S bond. Of the resulting fragments, thiyl radicals mainly couple, while alkyl radicals abstract hydrogen, disproportionate or couple when stabilized (benzyl). Selective trapping of either of the two types of radicals occurs in the presence of nucleophilic (methyl vinyl ether and 1-hexene) and, respectively, electrophilic (acrylonitrile) alkenes. When an easily oxidized radical is formed, e.g. cumyl, secondary electron transfer leads to the corresponding cation.

Photocatalytic methods for fluoroalkyl-radical generation provide more convenient alternatives to the classical perfluoroalkyl-radical (Rf) production through chemical initiators, such as azo or peroxide compounds or the employment of transition metals through a thermal electron transfer (ET) initiation process. The mild photocatalytic reaction conditions tolerate a variety of functional groups and, thus, are handy to the late-stage modification of bioactive molecules. Transition metal-photocatalytic reactions for Rf radical generation profit from the redox properties of coordinatively saturated Ru or Ir organocomplexes to act as both electron donor and reductive species, thus allowing for the utilization of electron accepting and donating fluoroalkylating agents for Rf radical production. On the other hand, laboratory-available and inexpensive photoorgano catalysts (POC), in the absence of transition metals, can also act as electron exchange species upon excitation, resulting in ET reactions that produce Rf radicals. In this work, a critical account of transition metal and transition metal-free Rf radical production will be described with photoorgano catalysts, studying classical examples and the most recent investigations in the field. Fil: Barata Vallejo, Sebastian. Universidad de Buenos Aires. Facultad de Farmacia y Bioquímica. Departamento de Química Orgánica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Bonesi, Sergio Mauricio. Universidad de Buenos Aires. Facultad de Farmacia y Bioquímica. Departamento de Química Orgánica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones en Hidratos de Carbono. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones en Hidratos de Carbono; Argentina Fil: Postigo, Jose Alberto. Universidad de Buenos Aires. Facultad de Farmacia y Bioquímica. Departamento de Química Orgánica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina

Product distribution, total quenching rate (kT), and rate of chemical reaction (kr) with singlet oxygen have been determined for some alkyl, benzyl, α-methylbenzyl, and cumyl sulfides. Their contributions depend on the steric hindering around the sulfur atom. In protic solvents, the sulfoxide is the main product via a hydrogen-bonded persulfoxide. In apolar solvents, intramolecular α-H abstraction leads to oxidative C–S bond cleavage, with varying efficiency. The behavior of sulfides is compared to that of alkenes and amines.

The oxidation of diethyl and diphenyl sulfide photosensitized by dicyanoanthracene (DCA), N-methylquinolinium tetrafluoroborate (NMQ(+)), and triphenylpyrylium tetrafluoroborate (TPPI) has been explored by steady-state and laser flash photolysis studies in acetonitrile, methanol, and 1,2-dichloroethane. In the Et2S/DCA system sulfide-enhanced intersystem crossing leads to generation of O-1(2), which eventually gives the sulfoxide via a persulfoxide; this mechanism plays no role with Ph2S, though enhanced formation of (3)DCA has been demonstrated. In all other cases an electron-transfer (ET) mechanism is involved. Electron-transfer sulfoxidation occurs with efficiency essentially independent of the sulfide structure, is subject to quenching by benzoquinone, and does not lead to Ph2SO cooxidation. Formation of the radical cations R2S center dot+ has been assessed by flash photolysis (medium-dependent yield, dichloroethane >> CH3CN > CH3OH) and confirmed by quenching with 1,4-dimethoxybenzene. Electron-transfer oxidations occur both when the superoxide anion is generated by the reduced sensitizer (DCA(center dot-), NMQ center dot) and when this is not the case (TPP center dot). Although it is possible that different mechanisms operate with different ET sensitizers, a plausible unitary mechanism can be proposed. This considers that reaction between R2S+ and O-2(center dot-) mainly involves back electron transfer, whereas sulfoxidation results primarily from the reaction of the sulfide radical cation with molecular oxygen. Calculations indeed show that the initially formed fleeting complex RS2+center dot center dot center dot O-O-center dot adds to a sulfide molecule and gives strongly stabilized R2S-O-center dot-O+-SR2 via an accessible transition state. This intermediate gives the sulfoxide, probably via a radical cation chain path. This mechanism explains the larger scope of ET sulfoxidation with respect to the singlet-oxygen process.

The preparation, isolation and characterization by elemental analysis and 1H-NMR, 13C-NMR, and MS data of the bromo derivatives of N-substituted carbazoles, i.e., of 9-methyl-9H-carbazole (1), 9-phenyl-9H-carbazole (2), 9-benzyl-9H-carbazole (3), 2-methoxy-9-methyl-9H-carbazole (4), and of C-substituted carbazoles, i.e., of 2-(acetyloxy)-9H-carbazole (5) and 3-nitro-9H-carbazole (6), are reported, in part for the first time. As brominating reagents, N-bromosuccinimide (NBS) or NBS/silica gel in CH2Cl2, NBS in AcOH, KBrO3/KBr in EtOH doped with a catalytic amount of H2SO4, or KBrO3/KBr in AcOH were employed, and their uses were compared. Semi-empirical PM3 calculations were performed to predict the reactivity of the N-substituted and C-substituted carbazoles and of their bromo derivatives and found to verify the experimental results. The UV-absorption and fluorescence and phosphorescence emission spectra of the bromocarbazole derivatives in MeCN solution at 298 K and in a solid matrix at 77 K were compared with those of the corresponding carbazoles 1–6. The dynamic properties of the lowest excited singlet and triplet states (τf, τp, ϕf, and ϕp) were measured under the same experimental conditions. The intramolecular spin–orbital-coupling effect of the Br-atom and NO2 group on the spectroscopic data, photophysical parameters, and on the photo reactivity were also briefly analyzed.

Electronic spectra (absorption, fluorescence and phosphorescence emission spectra) of nitro-β- carboline derivatives (nitronorharmanes, nitroharmanes and nitroharmines) in acetonitrile and in methanol solution at 298 K and in solid matrix at 77 K have been recorded and compared with those of the corresponding β-carbolines and chloroharmine derivatives. The electronic absorption spectra of triplet transients of these β-carbolines, the triplet lifetime, τT, and the intersystem crossing quantum yield, φisc, were measured by Laser Flash Photolysis at 298 K. The dynamic properties of the lowest excited singlet and triplet states (τf, τp, φf and φp) have been measured in the same experimental conditions. From these data, the radiative and radiationless rate constants (k o f , kisc and kic) were derived. The intramolecular spin - orbital coupling effect on the spectroscopic data and photophysical rate constant were also analyzed.

1H and 13C nmr spectra of several N- and C-substituted carbazoles (Series 1, 2, 3 and 4) were measured. Correlations between chemical shifts and substituent constants show that these parameters describe properly the substituent effect on the nmr phenomena. Atomic charge densities for carbazoles of Series 1, 2, 3 and 4 were calculated by using the semi empirical PM3 method. These values also show a linear correlation with the 13C chemical shifts. The synthesis of several carbazole derivatives 1a – 1g, 2a – 2g, 3a – 3j and 4a – 4g have been carried out according to literature procedures. The carbazoles 3i, 3j and 4c have been synthesized and fully characterized for the first time.

Electronic spectra (absorption, fluorescence and phosphorescence emission spectra) of chlorocarbazoles 1a–e, 2a–b, 3a, 4a–b, 5a, 6a, 7a, 8a–c, 9a and 9b in acetonitrile and in solid matrix have been recorded at 298 and 77 K. The dynamic properties of the lowest excited singlet and triplet states in term of fluorescence and phosphorescence lifetime, τf and τp, fluorescence and phosphorescence quantum yield, φf and φp have been measured in the same experimental conditions and from these data the radiative and the radiationless rate constants (kf0, kisc, kf0(77), kisc(77), kp0 and knr0) and the intersystem crossing quantum yield, φisc, were derived. The intramolecular heavy atom effect (HAE) on the spectroscopic data and photophysical rate constant was analyzed and the incorporation of chlorine atoms to the carbazole moiety proved their ability to quench the fluorescence emission by spin-orbital coupling. The values of the HOMO and LUMO energy, the oscillator strength (f) and the λmax(abs) associated to the electronic transitions, the heat of formation of the chlorocarbazoles and the corresponding radical cation (ΔHf and ΔHf(RC)) and the adiabatic ionization potential (Ia) were also calculated by using the semiempirical PM3 method after HF/3-21G geometrical optimization, and were compared with the spectroscopic and photophysical data as well as with the one electron oxidation potential data (Ep/2).

Irradiation of triphenylamine (Ph3N) in nitrogen-flushed solution leads to 9-phenylcarbazole and two tetrahydroderivatives (1,2,3,4- and 1,2,7,8-) via disproportionation of the corresponding 4a,4b-dihydrocarbazole. In oxygen-equilibrated solution oxidative cyclization occurs through the intermediacy of a triplet peroxy diradical, which either abstracts a hydrogen atom intramolecularly or (mainly) cleaves back to the reagents. The role of the key intermediates is supported by DFT calculations and by trapping by triarylphosphines (that are thus efficiently oxidized, while preventing the cyclization of Ph3N). The hydroperoxide, on the other hand, causes inefficient co-oxidation of sulfides. Fil: Bonesi, Sergio Mauricio. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones en Hidratos de Carbono; Argentina Fil: Dondi, Daniele. Universita Di Pavia; Italia Fil: Protti, Stefano. Universita Di Pavia; Italia Fil: Fagnoni, Maurizio. Universita Di Pavia; Italia Fil: Albini, Angelo. Universita Di Pavia; Italia

Kinetic studies on the photoinduced electron transfer reduction of a variety of halomethanes in acetonitrile and ethanol at 298 K are reported in terms of the quenching rate constants (kq) with a series of reductants (carbazoles and anthracenes) whose one-electron oxidation potentials (Eox) have been measured again in the present work vs. SCE and independently vs. ferrocene. The Rehm–Weller Gibbs energy relationship is applied to determine the fundamental parameters for the one-electron reduction, i.e., the one-electron reduction potentials (ERX/RX−˙) of the halomethanes and the intrinsic barrier for the electron transfer reduction (ΔGG‡o). The ERX/RX−˙ values obtained were related to the concerted electron transfer–bond breaking reduction potentials (ERX/R˙ + X−) and the standard free enthalpies of dissociation of RX−˙ (ΔGo, dissRX−˙) were estimated in each solvent. Additionally, the one-electron reduction potential (ERX/RX−˙) values estimated in acetonitrile were also related to different thermodynamic parameters such as electron affinity (EA), LUMO–HOMO energy differences (ΔE) and the bond dissociation energy (DRX). Optimized geometry, EA and ΔE for halomethanes were calculated by an ab initio method at the B3LYP level using 3-21G, 6-31+G(d,p) and G-311+G(3df,2pd) basis sets. In all these cases good linear correlations were obtained. The ΔGG‡o values obtained are compared with those calculated using the equation ΔGG‡o = λ/4 with λ = λi + λo where the solvent reorganization energy (λo) and the inner-sphere reorganization energies (λi) associated with the structural change upon the electron-transfer process were calculated, the former by using the Marcus–Hush model and the latter by using semiempirical and ab initio molecular modeling and QSAR properties. Results obtained from the preparative irradiation of carbazoles in the presence of halomethanes, which are consistent with a photoinduced electron transfer mechanism are also discussed.

Singlet oxygen adds to benzyl ethyl sulfide (5, total quenching rate ca. 1 × 107 M-1 s-1, little dependent on the solvent) to ultimately give benzaldehyde (6) and a small amount of the sulfone (8) in aprotic media (rate of the chemical reaction in benzene 5.5 × 106 M-1 s-1) and mainly the sulfoxide (7) in protic media (1.1 × 107 M-1 s-1 in methanol). In the presence of small amounts (0.002−0.3 M) of protic additives (alcohols, phenol, carboxylic acids), the sulfoxide becomes the main product in benzene also. Various evidence support the formation of two intermediates in aprotic solvents. The first one is an exciplex or a syn persulfoxide. It undergoes intramolecular hydrogen abstraction to give a ylide and finally benzaldehyde. Such rearrangement is either a concerted or a radicalic process and not a proton transfer (as indicated by the deuterium effect observed with the α-d benzyl sulfide and the occurrence of the process with the p-nitro and p-methoxy derivatives, 5‘ and 5‘ ‘). This intermediate is not ...

The photochemistry of two N -acyl carbazoles, N -acetyl and N -benzoyl carbazole, in different pure and mixed organic solvents is studied. Depending on the properties of the medium, photo-Fries rearrangement and photoinduced single electron transfer (PSET) processes are observed yielding the former 1-acyl and 3-acyl carbazoles and the latter 3-chloro- N -acyl carbazole. k SV , k Q and φ for fluorescence emission, conversion of N -acyl carbazole and product formation yields have been measured as well as the properties of the N -acyl carbazole radical cations formed during the PSET process (laser flash photolysis experiments). The Rehm-Weller equation is used in order to evaluate the ΔG ° ET of the PSET processes.

The biphasic base catalysis-mediated photo-Fries rearrangement reaction of aryl 3-methy-2-butenoate esters in room temperature cyclohexane—10% KOH system was investigated. This mild photochemical reaction leads to the formation of chroman-4-one derivatives in good to high yield and in short reaction times (30–120 min) in a one-pot photochemical reaction. Also, the photochemical reaction, as a convenient, versatile, and general method, applies efficiently to polycyclic and heterocyclic 3-methy-2-butenaote esters.

The biphasic base catalysis-mediated photo-Fries rearrangement reaction of aryl 3-methy-2-butenoate esters in room temperature cyclohexane - 10% KOH system was investigated. This mild photochemical reaction leads to the formation of chroman-4-one derivatives in good to high yield and in short reaction times (30-120 min) in a one-pot photochemical reaction. Also, the photochemical reaction, as a convenient, versatile, and general method, applies efficiently to polycyclic and heterocyclic 3-methy-2-butenaote esters. Fil: Samaniego Lopez, Cecilia. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones en Hidratos de Carbono. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones en Hidratos de Carbono; Argentina Fil: Erra Balsells, Rosa. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones en Hidratos de Carbono. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones en Hidratos de Carbono; Argentina Fil: Bonesi, Sergio Mauricio. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones en Hidratos de Carbono. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones en Hidratos de Carbono; Argentina

The photochemical reaction of N-acetyl carbazole was studied in ethanol and dichloromethane solution. The reaction products consist of the usual photo-Fries rearrangement products (1-acetyl and 3-acetyl carbazole), carbazole, 4-acetyl carbazole and 3,N-diacetyl carbazole. The photoproducts were identified by gas chromatography-mass spectrometry. They were isolated and analysed by UV and IR spectroscopy, mass spectroscopy and nuclear magnetic resonance spectroscopy, which provided evidence for their structural identification. Electron donor—acceptor (EDA) complex formation was observed in ethanol and dichloromethane and the relationship between the photochemical reactivity and the EDA formation is briefly discussed.

The photosensitized oxygenation of diphenyl (1), di-tert-butyl (2) and phenyl tert-butyl sulfide (3) was studied. Bimolecular rate constants of singlet oxygen quenching are low (1 to 5 x 10(4) M(-1)s(-1)) since the sulfides are poor nucleophiles due to sterical hindrance (2, 3) or the HOMO on the sulfur atom being a less accessible p(z) orbital (1). The quenching is mainly physical, but chemical reaction leading to sulfoxides also takes place in methanol and, to a lower degree, in acetonitrile. Catalysis by carboxylic acids considerably enhances the rate of sulfoxidation. Inefficiency in the chemical reaction is again due to the poor nucleophilicity of the sulfides, which limits oxygen transfer by electrophilic intermediates such as the protonated persulfoxide.

The photochemistry of two 2-acyloxycarbazoles, 2-acetyl- and 2-benzoyloxycarbazole, in different solvents has been studied. Irradiation of the 2-acyloxycarbazoles in organic media at 254 or 313 nm yields the [1,3]-migrated photoproducts, 1-acyl-2-hydroxycarbazole, 3-acyl-2-hydroxycarbazole and 2-hydroxycarbazole. The effects of the solvent, the atmosphere and the intensity of the light source on the photochemistry of 2-acyloxycarbazole have been studied. Laser flash photolysis as well as photosensitization experiments were performed in order to determine the photoreactive excited state. Electronic spectra (absorption, fluorescence and phosphorescence emission spectra) of the 2-acyloxycarbazoles have been recorded in homogeneous media at 298 K and in solid matrices at 77 K. The dynamic properties of the lowest singlet excited state in terms of fluorescence lifetime and fluorescence quantum yield have been measured in different organic solvents at room temperature. The photo-Fries rearrangement as a mild and clean one-pot reaction for the preparation of an advanced intermediate precursor in the total synthesis of carbazole alkaloids is described.

The photophysical behaviour of 1-nitrocarbazol and 3-nitrocarbazol in different organic solvents has been studied by using spectroscopic, luminescence and photoacoustic techniques. Absorption spectra and triplet state transient spectra were recorded and formation quantum yields and lifetimes determined. Phosphorescence emission at 77 K and laser induced luminescence in acetonitrile solution at 298 K for the nitrocarbazoles were studied. Results are discussed in terms of the optimized structure of 1 -nitrocarbazole and 3-nitrocarbazole and compared with that of carbazole. The nitro group attached to the carbazole moiety induces important changes in the photophysical behavior of those compounds., Facultad de Ciencias Exactas

The sluggish (kr < 10kq) photooxygenation of diethyl sulfide in both benzene and other aprotic solvents such as acetone and acetonitrile is made efficient by addition of small amounts of alcohols and, with a much more conspicuous increase, of phenols and carboxylic acids (≪0.1% additive is sufficient in this case). A kinetic analysis shows that the effect is accounted for by interaction of the protic additives with the first formed intermediate, the persulfoxide, in competition with cleavage to the components. The thus obtained rate constants kH linearly correlate with the acid strength of the additives, and the effect is rationalized as a general acid catalysis. Hydrogen bonding of the persulfoxide under this condition accounts in an economic way for the observed data, including co-oxygenation of Ph2SO in mixed solvents.

Benzyl ethyl sulfide (6a) is photo-oxidized to benzaldehyde in benzene, whereas diethyl sulfide is known to give inefficiently the sulfoxide under these conditions. Oxidative CS cleavage is the main process also with benzhydryl ethyl sulfide (6c), but not with α-methylbenzyl ethyl sulfide (6b), which mainly gives the sulfoxide. The carbonyl derivatives reasonably arise from S-hydroperoxy ylides (3). Consistently with this finding, calculations at the PM3 level suggest that the first intermediate, the persulfoxide (1), undergoes intramolecular hydrogen transfer when an activated α-hydrogen is available and gives 3. This is the case for the above benzyl sulfides (ΔH ≠ for the process decreases with decreasing CH BDE). However, only some of the persulfoxide conformations are correctly oriented for this rearrangement, and this may slow this process and make other reactions compete, as happens with 6b. Copyright © 1999 John Wiley & Sons, Ltd.