Literatura científica selecionada sobre o tema "Triplet aryl cations"

The article traces the development of the epr and optical spectroscopic characterisation and progressive theoretical understanding of the aryl cation, starting from the initial cryogenic epr study of 2,5-dialkoxyl-4-morpholinophenyl cation, which defined unambiguously its triplet state character, through simple MO, ab initio and DFT calculations on substituted aryl cations, to picosecond laser photolysis studies in fluorinated solvents, noting especially the early and vital contribution of Martyn Symons.

Light-related adverse side-effects of drugs are now an important source of concern. In order that the mechanism underlying to such effects is recognised, an in-depth photochemical study must be carried out. The case of some fluoroquinolone antibiotics (norfloxacin, enoxacin, lomefloxacin) is discussed as a representative example. These drugs undergo heterolytic fragmentation of the C—F bond leading to aryl cations. Quantum yields in neutral water range from 0.001 to 0.5 depending on the charge transfer degree of theππ*state. Lower values are obtained at both acidic and basic pH. There is indication that the excited state involved in most of this reactions is a relatively long-lived triplet. The aryl cations undergo intra- or intermolecular reactions, and are presumably involved in the reported photo-toxic effect.

An inexpensive and highly efficient metal-free alternative to commonly used Ru- and Ir-based catalysts was proposed. It was shown that the new 2,7-di-tert-butyl-5,10-bis(4-trifluoromethylphenyl)-5,10-dihydrophenazine outcompeted the iridium phenylpyridyl complex in photoredox activity in the alkylation of silyl enol ethers yielding aryl alkyl ketones. The reaction occurred under visible light irradiation at room temperature and was also applicable to drug derivatives (ibuprofen and naproxen). In-depth photophysical, electrochemical, and quantum chemical studies showed that the aforementioned N,N-diaryldihydrophenazine exhibited enhanced properties that were essential for the photoredox catalysis (a long-lived triplet excited state, strong reducing ability, high stability of the radical cations formed in single-electron-transfer event, and chemical inertness of the catalyst with respect to reactants). Importantly, the substituted N,N′-diaryldihydrophenazines could be obtained directly from diaryl amines; a facile, easily handled and scaled-up one-pot synthetic procedure was elaborated.

The photochemistry of the (E)-cinnamyl acetates ((E)-1-aryl-3-propenyl acetates, 8a–8e) with substituents H, 4-CH3O, 3-CH3O, 4-CF3, and 3-CF3, respectively, was examined in both cyclohexane and methanol solvents. Alkene isomerization (E to Z) occurred more efficiently than other reactions and evidence is presented that this process occurs from the excited triplet state. In a slower process, 1,3-migration of the acetoxy group led to the rearranged 3-aryl-3-propenyl acetate isomers (9a–9e) as the major pathway, particularly in cylohexane. In methanol, the isomeric ethers 3-aryl-3-methoxypropene (14) and 1-aryl-3-methoxypropene (15) were formed by reaction of methanol with the photochemically generated cation. The combined yield of 14 and 15 (95% and 5%, respectively) was quantitative for the 4-methoxyphenyl compound (8b). Independent irradiations of the isomers 9a–9c demonstrated that the ethers 14 and 15 were primary photoproducts from 8 and not secondary photoproducts from 9. Fluorescence quantum yields and excited singlet state lifetimes indicated that the reactions, other than the E to Z isomerization, are from the excited singlet state.Key words: cinnamyl acetates, photochemistry, E/Z isomerization, photocleavage, fluorescence.

Les systèmes polycycliques constituent la structure de base de nombreux composés pharmaceutiques aux propriétés biologiques variées. Pour obtenir ces structures, les réactions chimiques les plus souvent utilisées sont la cyclisation, la cycloaddition et, dans certains cas, l’annélation. Différentes méthodes d’activation peuvent être employées pour réaliser ces transformations comme l’activation thermique, photochimique, hyperbare, électrochimique, sonochimique ou par les micro-ondes par exemple, chacune possédant ses avantages et ses inconvénients. Par ailleurs, un objectif récurrent pour les chimistes est la production de composés complexes utilisant des méthodes de synthèse à faible coût, avec une bonne économie d’atomes, et en utilisant pas ou peu d’additifs.Dans ce contexte, la cyclisation activée par les UV est une des méthodes permettant d’obtenir des composés polycycliques. Dans les deux premiers chapitres de cette thèse, nous examinons la réactivité de différents cations aryles triplet, générés à partir des (ortho-iodobenzyl)-β-tetralones, des (ortho-iodobenzyl)indanones et des tetrahydroquinolines. Les résultats obtenus montrent qu’il est possible de synthétiser, dans des conditions d’économie d’atomes élevée et sans additifs, des composés contenant une structure de phénanthrène et de phénanthridine.Dans les deux derniers chapitres, l’activation des réactions organiques par la pression est étudiée. L’application de cette méthode pour activer une réaction de Diels-Alder à demande électronique normale, avec divers dérivés de 5-nitroquinolines en présence de diènes enrichis, s’est révélée très efficace pour donner des produits polycycliques désaromatisés. De plus, la combinaison de la haute-pression et d’un diène cyclique a démontré que la diastéréosélectivité de la réaction de Diels-Alder pouvait être contrôlée. Enfin, une réaction multicomposante (4+2)/(3+2) a également été étudiée dans des conditions hyperbares. Les résultats de cette réaction ont montré que des produits nitrosoacétaliques complexes pouvaient aussi être facilement obtenus, avec une économie d’atomes complète, toujours en l’absence d’additifs
Polycyclic systems are the base structure to many pharmaceutical compounds with varying biological properties. The most straightforward reaction pathways to achieve these structures largely revolve around cyclisation, cycloaddition and, in some cases, annulation. There are different methods (high-pressure chemistry, thermochemistry, sonochemistry, microwave chemistry, electrochemistry, and photochemistry) to activate the reactions to obtain these compounds with each its advantages and drawbacks. Furthermore, a recurring need in organic chemistry is the production of complex compounds using methods with high atom-efficiency, reagentless and low cost. In this context, cyclisation activated by UV-light is a simple method to obtain polycyclic compounds. Hence, in the first two chapters of this thesis, we examine the reactivity of different triplet aryl cations generated from (ortho-iodobenzyl)-β-tetralones, (ortho-iodobenzyl)indanones and tetrahydroquinolines. The results showed that products with either a phenanthrene and phenanthridine skeleton can be obtained in atom-efficient and reagentless conditions. High-pressure is another activation method explored in the last two chapters. Its application to a Normal-Electron-Demand Diels-Alder reaction with various 5-nitroquinoline derivatives and electron-rich dienes, proved to be very efficient at giving 3D polycyclic products. Furthermore, the combination of high-pressure and a cyclic diene demonstrated that the diastereoselectivity of the Diels-Alder reaction can be controlled. A (4+2)/(3+2) domino reaction was also explored under hyperbaric conditions. The results from this multicomponent reaction showed that complex nitrosoaketal products could be achieved with ease and high atom-efficiency