Construction of a Clock Catalytic System: Highly Efficient and Self‐Indicating Synthesis of Benzoheterocycles at Ambient Temperature

Abstract

Inspired by the mechanism of the clock reaction, we rationally constructed a novel phosphomolybdenum blue (PMB) clock catalytic system for a highly efficient synthesis of benzimidazoles and benzothiazoles simply at ambient temperature. The current PMB clock catalytic system exhibits a sharp decoloration event to announce the depletion of the intermediateconstraint, making this synthetic approach selfindicating and TLC-free. 31 P NMR and XPS analysis of PMBcatalyst showed that only two Mo 6 + atoms are reduced to Mo 5 + atoms in the Keggin structure due to the moderate reducibility of benzimidazoline and benzothiazoline intermediates. Thus,the active Keggin-type POM cluster could be well maintained in DMSO during the redox cycling of phosphomolybdic acid (PMA) and PMB. 1 H NMR tracing experiment not only confirmed the proposed reaction mechanism but also showed that PMB exerts Lewis acid catalytic activityat the early phase of the reaction other than the expected redox catalytic activity.

A peroxo‐Mo(VI)/Mo(VI)‐mediated redox synthesis of quinazolin‐4(3H)‐ones and their aggregation‐induced emission property and mechanism

“…According to the procedures established in benzimidzole synthesis, [ 35 ] the reaction of 1.0 equiv of o ‐aminobenzamide, 1.1 equiv of benzaldehyde, 1 mol% PMA, and 1.2 equiv of tert ‐butyl hydrogenperoxide (TBHP) was conducted in DMSO at ambient temperature. TLC monitoring indicated that it involves two phases, which is very similar to what happens in benzimidazole synthesis.…”

“…It is noteworthy that the sharp decoloration event of the PMB clock‐catalyzed benzimidazole/benzothiazole synthesis [ 35 ] was not observed in quinazolin‐4(3 H )‐one synthesis (Figure 1A). Originally, we ascribed the constant color to the fact that the reaction rate of oxidative dehydrogenation by PMA (Mo(VI) → Mo(V)) is relatively slow than that of the oxidation of PMB back to PMA by TBHP (Mo(V) → Mo(VI)).…”

“…Recently, inspired by the mechanism of “clock reaction,” our research group rationally constructed a novel redox catalytic system for highly efficient synthesis of benzimidazoles simply at ambient temperature. [ 35 ] In this method, the oxidation of benzimdazoline intermediate by phosphomolybdic acid (PMA) is ultrafast, and the reduced phosphomolybdnum blue (PMB) is then quickly oxidized back to PMA by TBHP. Therefore, the Mo(VI)–Mo(V) redox cycle could drastically accelerate the formation of benzimidazole product under mild conditions.…”