Photoredox chemistry of nitrobenzyl alcohols in aqueous solution. Acid and base catalysis of reaction

Abstract

PETER WAN and KEITH YATES. Can. J. Chem. 64, 2076Chem. 64, (1986. The photochemistry of several m-and p-nitrobenzyl a!cohols (1-5) has been studied in aqueous solution. These compounds react via an intramolecular photoredox pathway to give reduced and oxidized moieties of the substituent groups. The reaction is an example of a new type of photoreaction of nitro-substituted aromatic derivatives that is not observed in organic solvents, the presence of water being essential. This effect is exemplified by measuring the quantum efficiency as a function of mol% water in aqueous acetonitrile, methanol, and formamide: the reaction efficiency decreases rapidly as water is depleted in the mixture. Catalytic effects due to the hydronium and hydroxide ions were studied: the para derivatives exhibited hydroxide ion catalysis; the meta derivatives exhibited hydronium ion catalysis. Quantum yields, solvent isotope effects, and a-deuterium isotope effects are reported for the parent derivatives.PETER WAN et KEITH YATES. Can. J. Chem. 64, 2076 (1986).On a CtudiC la photochimie de plusieurs alcools m-et p-nitro benzyliques (1-5) en solution aqueuse. Ces composCs rkagissent selon un processus photoredox intermediaire pour donner des fragments de groupes substituCs oxydCs et reduits. C'est un exemple d'un nouveau type de rCactions photochimiques des dCrivCs aromatiques nitrCs que l'on n'observe pas dans des solvants organiques. L'eau est donc indispensable. On a mis en evidence le r61e de I'eau en mesurant le rendement quantique de la rCaction en fonction du pourcentage molaire d'eau dans des solutions aqueuses d'acetonitrile, de mCthanol et de formamide. Le rendement de la rCaction diminue rapidement lorsque la teneur en eau du mClange diminue. On a Ctudie les effets catalytiques dus aux ions hydronium et hydroxydes. Les rkactions des dCrivCs para sont catalyses par les ions hydroxydes tandis que celles des derives substituks en me'ta sont catalysees par les ions hydroniums. On rapporte les rendements quantiques, les effets isotopiques du solvant et les effets isotopiques du deutCrium en a des derives apparent&.[Traduit par la revue]

A Zinc(II) Photocage Based on a Decarboxylation Metal Ion Release Mechanism for Investigating Homeostasis and Biological Signaling

“…Based on the mechanistic studies of photoactive meta-substituted compounds, [27,28] we expected the α-carbanion formed after the decarboxylation to be oxidized to an imine radical-anion (Scheme S1 in the Supporting Information). Hydrolysis of the imine would then yield iminodiacetic acid (3, IDA) and m-nitrobenzaldehyde photoproducts (Figure 1).…”

Photooxygenation of nitrobenzyl derivatives. Mechanisms of photogeneration and hydrolysis of α-hydroperoxy nitrobenzyl ethers

“…Both m-and p-nitrobenzyl alcohols 1 and 7, respectively, are commercially available. In addition, their aqueous photochemistry under deaerated conditions has been reported by us (6,7). Nitrobenzyl ethers 2-5 and 8 are available from their corresponding nitrobenzyl chlorides, via the appropriate Williamson ether synthesis.…”

“…Recently, we (5-7) and others (8,9) have noted that some m-and p-nitro-substituted benzenes display unexpected reactivity when irradiated in aqueous solution, especially in the presence of hydronium and hydroxide ions as catalysts for the transformations (5)(6)(7)(8). Whereas it is well known (1-4) that o-nitrobenzyl systems are photolabile due to the proximity to the substituent of the reacting nitro chromophore, resulting in initial intramolecular hydrogen abstraction via the n, T" triplet excited state, it had been assumed for many years that the corresponding meta and para derivatives should not be photolabile, except for simple photoreduction.…”

“…Whereas it is well known (1-4) that o-nitrobenzyl systems are photolabile due to the proximity to the substituent of the reacting nitro chromophore, resulting in initial intramolecular hydrogen abstraction via the n, T" triplet excited state, it had been assumed for many years that the corresponding meta and para derivatives should not be photolabile, except for simple photoreduction. However, in aqueous solution, a number of m-and p-nitro-substituted derivatives have been found to react via new photochemical processes (5)(6)(7)(8)(9). In a preliminary report (10) we had demonstrated that molecular oxygen can react with photoexcited nitrobenzyl derivatives, which represent an additional example of the rich and sometimes unpredictable photochemistry of m-and p-substituted nitroaromatic compounds.…”

“…"reduction" product in the overall photoredox pathway, the mechanism of which has been discussed in some detail (6,7). However, when 1 is irradiated in oxygenated aqueous solution, the only product is m-nitrobenzaldehyde, with a concurrent 20% increase in quantum efficiency (from 0.087 to 0.10).…”