SiO2-supported HClO4 catalyzed synthesis of (Z)-thiazolylhydrazonoindolin-2-ones and their electrochemical properties

In this paper, an environmentally benign silica-supported perchloric acid (HClO4-SiO2) catalyzed green FCDR strategy has been developed for the synthesis of (Z)-THIs (6) with high stereospecificity via an intramolecular hydrogen bond (IHB) directed approach, involving the reaction of methyl ketones (1), N-bromosuccinimide (NBS) (2), isatins (4) and thiosemicarbazide (5) in ethanol at reflux temperature for 45-60 min in one-pot. The reaction proceeds through the construction of C-Br (alpha-bromination), C-S & C-N (heterocyclization), and C--N (condensation) bonds in one pot. The absolute structure of the compound (Z)-3-(2-(4-(4-bromophenyl)thiazol-2-yl)hydrazono)indolin-2-one (6e) has been confirmed by single-crystal XRD analysis. Further, the role of IHB on Z-configuration of the synthesized (Z)-THIs is proved by single-crystal XRD and 1H NMR studies. Wide substrate scope, good functional group tolerance, scalability, improved safety since the method circumvents the use of highly lachrymatric alpha-bromoketones as starting materials, high product yields (up to 98%), short reaction times, reusable solid Bronsted acid catalyst (HClO4-SiO2), and products that do not require column chromatography purification are all attractive features of this FCDR strategy. Electrochemical properties of THIs (6) are examined by cyclic voltammetry. The HOMO and LUMO energy level of THIs, 6a, 6c, 6d, 6j, 6o-6v, 6y, and 6aa are comparable with the reported ambipolar materials, and the HOMO levels of other THIs, 6b, 6e-6i, 6n, 6w, 6x, 6z and 6 ab-6ae are similar with the most commonly used hole transporting materials (HTMs).

Automated summary

An environmentally friendly FCDR strategy has been developed for the synthesis of (Z)-THIs using silica-supported perchloric acid as catalyst, with high stereospecificity. The method offers a wide substrate scope, high product yields, short reaction times, and reusable catalyst.