ABSTRACT
Demonstrated here is an external photo-sensitizer-free (auto-sensitized) singlet oxygen-enabled solvent-dependent tertiary hydroxylation and aryl-alkyl spiro-etherification of C3-maleimidated quinoxalines. Such "reagent-less" photo-oxygenation at Csp3-H and etherification involving Csp3-H/Csp2-H are unparalleled. Possibly, the highly π-conjugated N-H tautomer allows the substrate to get excited by irradiation, and subsequently, it attains the triplet state via ISC. This excited triplet-state sensitized molecule then transfers its energy to a triplet-state oxygen (3O2) generating reactive singlet oxygen (1O2) for hydroxylation and spirocyclization depending on the solvent used. In HFIP, the generated alkoxy radical accepts a proton via HAT giving hydroxylated product. In contrast, in an aprotic PhCl it underwent a radical addition at the ortho-position of the C2 aryl to provide spiro-ether. An unprecedented orthogonal spiro-etherification was observed via the displacement of o-substitutents for ortho (-OEt, -OMe, -F, -Cl, -Br) substituted substrates. The order of ipso substitution follows the trend -OMe>-OEt>-F>-H>-Cl>-Br. Both these oxygenation reactions can be carried out with nearly equal ease using direct sunlight without the requirement of any elaborate reaction setup. Demonstration of large-scale synthesis and a few interesting transformations have also been realized. Furthermore, several insightful control experiments and quantum chemical computations were performed to unravel the mechanism.
ABSTRACT
The photochemistry of 2-iodobenzothiazole (IBT) was investigated in an argon matrix at 4 K. Irradiating matrix-isolated IBT at λ = 254 nm led to the formation of 2-isocyanophenylthiyl radical 3 and benzothiirene 4; surprisingly, the 2-dehydrobenzothiazole radical 1 was not detected. A subsequent 365 nm irradiation or annealing of matrix containing 3 and iodine atom (at 30 K) reverted the precursor IBT, indicating an iodine-triggered thiyl radical-based cyclization, which was confirmed by computations using density functional theory. For gaining mechanistic insights, similar investigations were performed using benzothiazole BT as a precursor. Once again, the 254 nm light-induced C-S scission accompanied by H-shift led to 2-isocyanothiophenol 7. A conformational analysis of 2-isocyanothiophenol 7 and its photochemistry was also carried out with the help of IR spectroscopy under cryogenic conditions and computations. Considering the importance of thiyl radicals in biomolecular pathways, the investigations on its photochemical generation and a rare, reversible phenomenon of iodine-mediated cyclization under cryogenic conditions are insightful for fundamental photochemistry and sulfur chemistry.
ABSTRACT
We report a new class of arylazopyrazolium-based ionic photoswitches (AAPIPs). These AAPIPs with different counter ions have been accessed through a modular synthetic approach in high yields. More importantly, the AAPIPs exhibit excellent reversible photoswitching and exceptional thermal stability in water. The effects of solvents, counter ions, substitutions, concentration, pH, and glutathione (GSH) have been evaluated using spectroscopic investigations. The results revealed that the bistability of studied AAPIPs is robust and near quantitative. The thermal half-life of Z isomers is extremely high in water (up to years), and it can be lowered electronically by the electron-withdrawing groups or highly basic pH.
ABSTRACT
A modular approach has been adopted to synthesize a wide range of visible light-driven photoswitchable azoheteroarenes. In this regard, we considered ortho substitution of cyclic amines in the aryl ring and varied substitution patterns. Using detailed spectroscopic studies, we established a relationship between structure and photoswitching ability and also half-lives of the Z-isomers. Through this, we envision tunable and bidirectional longer wavelength photoswitches.
Subject(s)
LightABSTRACT
Considering the growing interest in azoheteroarene photoswitches, the molecular level process underlying photochemistry is intriguing. In our earlier investigation on phenylazo-3,5-dimethylisoxazole, besides E-Z photoisomerization, we also observed light-induced phase transition that supports the possible conformational changes under neat conditions. Furthermore, hydrogen bond-forming groups such as -OH, at the ortho position to the azo chromophore, can potentially hamper the isomerization through tautomerism. All of them develop a curiosity in the photochemical outcome of 2-hydroxyphenylazo-3,5-dimethylisoxazole (HPAI, 1). Herein, we report the photochemistry of HPAI in an argon matrix at 4 K, followed by infrared spectroscopy. Through experiments and computations, we identified the E-Z photoisomerization in HPAI as the only observed channel among the above-mentioned possibilities.
Subject(s)
Exploratory Behavior , Hydrogen Bonding , Isoxazoles , Photochemistry , Spectrophotometry, InfraredABSTRACT
Light-responsive molecular systems with multiple photoswitches in C3 -symmetric designs have enormous application potential. The design part of such molecular systems is critical due to its influence in several properties associated with the photoswitches. In order to tune, and in the evaluation of the design-property relationship, we synthesized 18 tripodal systems with variations in the core, linkers, connectivity, and azo(hetero)arene photoswitches. Through extensive spectroscopic and computational studies, we envisaged the factors controlling near-quantitative photoisomerization in both the directions (bistability) and the thermal stability of the metastable states. Furthermore, we also evaluated the impact of designs in obtaining reversible photo-responsive sol-gel phase transitions, solvatochromism, photo- and thermochromism.
