Photostimulated Reactions Research Articles

Elementary processes triggered in curcumin molecule by gas-phase resonance electron attachment and by photoexcitation in solution.

Electron-driven processes in isolated curcumin (CUR) molecules are studied by means of dissociative electron attachment (DEA) spectroscopy under gas-phase conditions. Elementary photostimulated reactions initiated in CUR molecules under UV irradiation are studied using the chemically induced dynamic nuclear polarization method in an acetonitrile solvent. Density functional theory is applied to elucidate the energetics of fragmentation of CUR by low-energy (0-15eV) resonance electron attachment and to characterize various CUR radical forms. The adiabatic electron affinity of CUR molecule is experimentally estimated to be about 1eV. An extra electron attachment to the π1* LUMO and π2* molecular orbitals is responsible for the most intense DEA signals observed at thermal electron energy. The most abundant long-lived (hundreds of micro- to milliseconds) molecular negative ions CUR- are detected not only at the thermal energy of incident electrons but also at 0.6eV, which is due to the formation of the π3* and π4* temporary negative ion states predicted to lie around 1eV. Proton-assisted electron transfer between CUR molecules is registered under UV irradiation. The formation of both radical-anions and radical-cations of CUR is found to be more favorable in its enol form. The present findings shed some light on the elementary processes triggered in CUR by electrons and photons and, therefore, can be useful to understand the molecular mechanisms responsible for a variety of biological effects produced by CUR.

Развитие исследований по ИК-спектроскопии поверхности в Санкт-Петербургском университете

The paper describes the history of spectral studies of surface processes at St. Petersburg University from the first pre-war experiments of Terenin and Kasparov to the present day. The development of experimental technique and advances in the studies of surface acid and basic sites, functional groups, and adsorption complexes are considered. Spectroscopy in a wide temperature range enables us to study such phenomena as lateral interactions between adsorbed molecules and linkage isomerism, to obtain information about the mechanism of dark and photostimulated reactions. Attention is paid to the application of computer modeling and quantum chemistry to study processes on the surface.

Samarium Iodide Showcase: Unraveling the Mechanistic Puzzle

ConspectusSmI2 was introduced to organic chemistry as a single electron transfer agent in 1977. After ca. 15 years of latency, the scientific community has realized the high potential of this reagent, and its chemistry has started blooming. This versatile reagent has mediated a myriad of new bond formations, cyclizations, and other reactions. Its popularity stems largely from the fact that three different intermediates, radical anions, radicals, and anions, depending on the ligand or additive used, could be obtained. Each of these intermediates could in principle lead to a different product. While these options vastly enrich the repertoire of SmI2, they necessitate a thorough mechanistic understanding, especially concerning how appropriate ligands direct the SmI2 to the desired intermediate. Our first paper on this subject dealt with the reduction of an activated double bond. The results were puzzling, especially the H/D isotope effect, which depended on the order of the reagents addition. This seminal paper was fundamental to an understanding of how the SmI2 works and enabled us to later explain various phenomena. For example, it was found that in a given reaction, when MeOH is used as a proton source, a spiro compound is obtained, while a bicyclic product is obtained when t-BuOH is used. Our contribution culminated in formulating guidelines for the rational use of proton donors in SmI2 reactions.The need to understand the complexity of the effect of additives on various processes is nicely demonstrated in photoinduced reactions. For example, hexamethylphosphoramide (HMPA) enhances the reduction of anthracene while hampering the reaction of benzyl chloride. The mechanistic understanding gained enabled us also to broaden the scope of photostimulated reactions from substrates reacting by a dissociative electron transfer mechanism to normal reductions, which are difficult to accomplish at the ground state. Harnessing the classical knowledge of proton transfer mechanisms to our SmI2 research enabled us to decipher an old conundrum: why does the combination of water and amine have such an enhancing effect on the reactivity of SmI2, which is not typical of these two when used separately. In our studies on the affinity of ligands to SmI2, we discovered that, in contradistinction to the accepted dogma, SmI2 is much more azaphilic than it is oxophilic. On the basis of the size difference between Sm3+ and Sm2+, we developed a simple diagnostic tool for the nature of the steps following the electron transfer. The reduction of imines showed that substrate affinity to SmI2 plays also a crucial role. In these reactions, new features such as autocatalysis and catalysis by quantum dots were discovered. Several studies of the ligand effect lead to a clear formulation of when an inner sphere or outer sphere electron transfer should be expected. In addition, several reactions where proton-coupled electron transfer (PCET) is the dominant mechanism were identified. Finally, the surprisingly old tool of NMR “shift reagents” was rediscovered and used to directly derive essential information on the binding constants of ligands and substrates to SmI2.

Combined Experimental and Theoretical Studies on the Radical Nucleophile Addition Reaction for Sulfide- and Selenide-Centered Anions.

The reactivity of sulfur- and selenium-centered nucleophiles toward 1-naphthyl radicals was studied in dimethylsulfoxide. The photostimulated reaction of sulfide anions, -SC(NH)C6H5 (1), -SC(NH)NH2 (2), and -SC(NH)CH3 (3), renders, after the addition of MeI, methyl 1-naphthylsulfide as a main product together with bis(1-naphthyl) sulfide and naphthalene under irradiation. Concordantly, the reaction of selenide anions, -SeC(NH)C6H5 (4), -SeC(NH)NH2 (5), and -SeCN (6), produces methyl 1-naphthyl selenide, bis(1-naphthyl) selenide, and naphthalene in the presence of potassium tert-butoxide anion (entrainment conditions). Absolute rate constants for the coupling of ions 1-6 to 1-naphthyl radicals were determined; as a general trend, the selenide-centered nucleophiles enhance in 2 times the reactivity of their sulfide analogues. From the mechanistic study, it is proposed that the unstable radical anion produced by the addition of the nucleophile to 1-naphthyl radical affords, after fragmentation, 1-naphthylsulfide/selenide anion. In addition, experimental results are discussed in terms of density functional theory calculations. There is a generally good agreement between the experimental and the calculated reactivities, the spin density being the main parameter to describe the difference found among the anions under study. Moreover, the calculations predict that anion -SeC(NH)CH3 (7) would be a good candidate for the synthesis of selenide derivatives.

ChemInform Abstract: Synthesis of Dibenzosultams by “Transition‐Metal‐Free” Photoinduced Intramolecular Arylation of N‐Aryl‐2‐halobenzenesulfonamides.

A new and general synthetic route to prepare dibenzosultams is here reported. This approach involves the synthesis of N-aryl-2-halobenzenesulfonamides (3), followed by intramolecular C–C photoinduced arylation under soft conditions without the use of “Transition Metal”. The photostimulated reactions exhibit very good tolerance to different substituent groups with good to excellent isolated yields (42–98%) of products. Moreover, it is shown that LED (λ = 395 nm) is an efficient light energy source to initiate efficiently the reactions. Theoretical inspection of the mechanism was made to probe the involvement of the radical-anion SRN1 process.

Synthesis of Dibenzosultams by "Transition-Metal-Free" Photoinduced Intramolecular Arylation of N-Aryl-2-halobenzenesulfonamides.

A new and general synthetic route to prepare dibenzosultams is here reported. This approach involves the synthesis of N-aryl-2-halobenzenesulfonamides (3), followed by intramolecular C-C photoinduced arylation under soft conditions without the use of "Transition Metal". The photostimulated reactions exhibit very good tolerance to different substituent groups with good to excellent isolated yields (42-98%) of products. Moreover, it is shown that LED (λ = 395 nm) is an efficient light energy source to initiate efficiently the reactions. Theoretical inspection of the mechanism was made to probe the involvement of the radical-anion SRN1 process.

New synthesis and biological evaluation of benzothiazole derivates as antifungal agents.

In search of new antifungal agrochemicals that could replace commercially available, aryl-2-mercaptobenzothiazoles were synthesized. They were prepared by two methodologies, using both photostimulated reaction and microwave assisted reaction. These reactions took place without the use of metallic catalyst by a one-pot procedure with excellent yields (70-98%). Synthesized compounds were evaluated for fungal growth inhibition against Botrytis cinerea. Most of the compounds have an excellent antifungal activity, and three of these showed a superior inhibitory effect to commercial fungicide Triadimefon. IC50 values observed for 2-(phenylthio)benzothiazole, 2-(2-chlorophenylthio)benzothiazole, and 2-(3-chlorophenyl thio)benzothiazole were 0.75, 0.69, and 0.65 μg mL(-1), respectively.

“Transition-metal-free” synthesis of carbazoles by photostimulated reactions of 2′-halo[1,1′-biphenyl]-2-amines.

An efficient and simple protocol for the preparation of a series of 9H-carbazoles by photostimulated SRN1 substitution reactions is presented. Substituted 9H-carbazoles were synthesized in low to excellent yields (up to 96%) through an intramolecular C–N bond formation of 2′-halo[1,1′-biphenyl]-2-amines by the photoinitiated SRN1 mechanism under mild and “transition-metal-free” conditions. The biphenylamines used as substrates were obtained with isolated yields ranging from 21% to 84% by two approaches: (A) the cross-coupling Suzuki–Miyaura reaction and (B) the radical arylation of anilines. Some key aspects of the proposed mechanism were evaluated at the B3LYP/6-311+G* level.

Reactivity of Saturated Hydrocarbon Anchoring Arms on Si(100) upon White Light Photoactivation: Experimental Evidence and Theoretical Insights

In the frame of the much debated issue of photostimulated reaction mechanisms on Si-oriented surfaces, the first report is presented here where the presence of saturated anchoring arms in organometallic and organic molecules unexpectedly leads to covalent bonding on H-terminated Si(100). The molecular substrates tested (methyl-, ethyl-, and n-propylferrocene, 4-chlorotoluene, and 1-chloro-4-ethylbenzene) were chosen considering that the carbon atom directly attached to the benzene ring, C1, sits at a peculiar state of charge also by virtue of σ–π hyperconjugation effect and that it can be sterically hindered in its interaction to a surface if it becomes part of a longer chain. The anchoring reaction was conducted via exposure to low-intensity white light, and the resulting hybrid species were characterized by means of X-ray photoelectron spectroscopy (XPS), electrochemical measurements by cyclic voltammetry (CV), atomic force microscopy (AFM), and density functional theory (DFT) calculations. At variance with the so far proposed theories on the light-induced hydrosilylation of Si surfaces, which is invariably considered as necessary for a covalent bonding in the presence of unsaturated carbon groups as the anchoring moiety to the surface, all the aromatic molecules tested here successfully anchored to H–Si(100). As a countercheck, attempts to anchor onto H–Si(100) two species with a sterically hindered C1 atom, n-butylferrocene, and a nonaromatic structure, 1-chlorodecane, led to a surface coverage close to zero. XPS, CV, and AFM results showed that the molecular monolayer coverage on the Si surface decreases as the sterical hindrance of C1 increases. DFT calculations proved that the adhesion ability of the different species diminishes as the length of the alkyl chain increases, while the breakage of the C1–H bond is always favored, irrespective of chain length. In light of our previous theoretical studies, a reaction mechanism is envisaged as involving radical species via a homolytic cleavage of the C–H bond at the C1 atom of the molecule, induced through white light irradiation.

ChemInform Abstract: Synthesis of 6‐Substituted 2‐Pyrrolyl and Indolyl Benzoxazoles by Intramolecular O‐Arylation in Photostimulated Reactions.

AbstractIrradiation of amides of type (I), (IIII), and (V) allows a simple and efficient access to pyrrolyl and indolyl benzoxazoles.

Synthesis of 6-Substituted 2-Pyrrolyl and Indolyl Benzoxazoles by Intramolecular O-Arylation in Photostimulated Reactions

The synthesis of a series of 6-substituted 2-pyrrolyl and 2-indolyl benzoxazoles by photostimulated C-O cyclization of anions from 2-pyrrole carboxamides, 2-indole carboxamides, or 3-indole carboxamides has been found to proceed in good to excellent yields (41-100%) in DMSO and liquid ammonia. The pyrrole and indole carboxamides are obtained in good to very good isolated yields by an amidation reaction of different 2-haloanilines with 2-carboxylic acid of pyrrole and 2- or 3-carboxylic acid of indole. To explain the regiochemical outcome of these reactions (C-O arylation vs C-N or C-C arylation), a theoretical analysis was performed using DFT methods and the B3LYP functional.

Direct Imaging of Site-Specific Photocatalytical Reactions of O2 on TiO2(110).

Photostimulated reactions of single O2 molecules on reduced TiO2(110) surfaces were directly observed at an atomic level with high-resolution scanning tunneling microscopy at 50 K. Two distinct reactions of O2 desorption and dissociation occur at different active sites of terminal Ti atoms and bridging O vacancies, respectively. Two reaction channels follow very different kinetics. While hole-mediated O2 desorption is promptly and fully completed, electron-mediated O2 dissociation is much slower and is quenched above some critical O2 coverage. Evidently, the O2 photochemistry on TiO2(110) is quite more complex than thought previously. Density functional theory calculations indicate that both coordination and charge state of an O2 molecule chemisorbed at the specific site largely determine a particular reaction pathway.

Luminescent and photochemical processes in CdBr2 : AgCl crystals

This paper reports on the results of the investigation into the optical luminescence properties of photochromic crystals CdBr2 : AgCl grown by the Bridgman-Stockbarger method. It has been shown that, under X-ray, optical, and N2-laser excitations of the grown crystals, there occurs emission due to Ag+ impurities in addition to emission from centers characteristic of CdBr2. The photostimulated chemical reactions occurring in CdBr2 : AgCl lead to a weakening of the luminescence and to a change in its spectral composition. Models of photosensitive centers and centers of photochemical coloring have been proposed. The mechanisms of the photochromic effect have been considered. The nature of luminescent and trapping centers has been discussed.

Behavioral features of photostimulated processes in the heterogeneous composition of polymer–semiconductor–salt of a metal

In the current study experimental data about changes in the photosensitive composition consisting of polyvinyl alcohol, zinc oxide and bismuth chloride (PVA–ZnO–BiCl 3) under UV-irradiation are presented. There are both organic (PVA) and inorganic compounds of the named composition whose changes are determined. The main products of the photostimulated reactions are metal bismuth, bismuth oxide and polyene structures of various lengths in the form of a polyvinyl alcohol. In this research a heuristic scheme of photostimulated reactions in the photosensitive composition of PVA–ZnO–BiCl 3 and in similar compositions was suggested.

Synthesis of ε-oxo acids by photostimulated reactions of 2-(2-iodophenyl)acetate ion with carbanions by the SRN1 mechanism. Synthesis of novel 3-benzazepin-2-ones

Fil: Guastavino, Javier Fernando. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Cordoba. Instituto de Investigaciones en Fisico-quimica de Cordoba. Universidad Nacional de Cordoba. Facultad de Ciencias Quimicas. Instituto de Investigaciones en Fisico-quimica de Cordoba; Argentina

Reactivity of endo-3-bromocamphor with sulfur-centered nucleophiles by an electron transfer mechanism. Electrophilic behaviour of the 3-camphoryl radical

The photostimulated reaction of arylthiolate ions with endo-3-bromocamphor produced both reduction and substitution products. The pK(a) and proton affinities of the conjugated acids were found to be good indicators of the reactivity.

ChemInform Abstract: Reactivity of Ketone Enolates with 3-Halo-2-amino Derivatives of Benzo( b)thiophene Under Photostimulated SRN1 Reaction Conditions.

ChemInform Abstract: Reactivity of Ketone Enolates with 3-Halo-2-amino Derivatives of Benzo( b)thiophene Under Photostimulated SRN1 Reaction Conditions.

Photochemical naphthylation at α-carbon atom of carbonyl and its equivalences

The photostimulated reaction of halonaphthalene with a series of carbanions derived from propionic acid derivatives in liquid ammonia led to the naphthylation at α-carbon of the carbanions in an isolated yield ranging from 25% to 86%. In all cases, the dehalonaphthalenes were found to be by-products and the reaction was inhibited by p-dinitrobenzene. An electron transfer from the carbanion to the halonaphthalene followed by ejection of halogen led to naphthyl radical, as predicted by comparing the LUMOs of the carbanions and the halonaphthalenes, was involved in the process. Absence of the alkylnaphthalene and 1,2-dinaphthylalkane in the products indicates the smooth electron transfer between (Naph-Nu)− and Naph-X. C-vs. O-naphthylation is fully addressed in terms of MNDO calculations and acid-base principle.

Thermo- and photostimulated processes of polarization and depolarization in CdI2: Ag

Currents of thermogradient polarization and depolarization of an electret state that arises in a photochromic crystal CdI2: Ag during one-side cooling of the sample in the dark from 325 to 90 K in the presence of a temperature gradient directed along the crystallographic axis C6 have been found. At 90 K, the crystal polarized in the thermogradient electromotive force field is characterized by the photosensitivity in the near-edge, impurity, and infrared spectral regions. It is revealed that the electret state in the CdI2: Ag crystal is also formed at room temperature during photolysis under irradiation of the samples by integrated light from a xenon lamp. Models of photosensitive centers formed upon doping of the CdI2 crystal from the melt by the Ag+ impurity and during the occurrence of thermo- and photostimulated chemical reactions are proposed. The mechanism of the photochromic effect, including the change in the charge state of silver impurity ions, is considered.

Development of Highly Regioselective Amidyl Radical Cyclization Based on Lone Pair−Lone Pair Repulsion

The substituent effect on the reactivity and regioselectivity of N-(4-pentenyl)amidyl radical cyclization was investigated. Exclusive 6- endo cyclization was observed for N-(4-pentenyl)amidyl radicals with internal vinylic heteroatom substitution (Cl, Br, I, OMe, SEt). The substituent on the carbonyl group also showed a significant influence on the reactivity of amidyl radicals, which increases in the order of Ph < Me < OEt. As a result, the photostimulated reactions of N-(4-halopent-4-enyl)amides and carbamates (X = Cl, Br, I) with DIB/I 2 or Pb(OAc) 4/I 2 led to the efficient and exclusive formation of the corresponding piperidines while those of N-(5-halopent-4-enyl)amides afforded the pyrrolidine products only. The halogen-substitution effect also allowed the 6- exo and 7- endo amidyl radical cyclization to proceed in a highly regioselective manner. The above experimental results, in combination with theoretical analyses, revealed that the lone pair-lone pair repulsion between the nitrogen radical and the vinylic heteroatom played an important role in controlling the regioselectivity of cyclization.