Achieving the additive-free oxidative self-coupling of o-aminophenols (OAPs) to form pharmacologically active aminophenoxazinones (APXOs) via a heterogeneous process under an oxygen (O2) atmosphere is of great significance for sustainable development. Herein, two isostructural polyoxometalate-based coordination polymers with the general formula M (i-Pr-Im) 4M (i-Pr-Im) (Phen) 22HPMo8V6O42·nH2O (M = Co (1) or Ni (2) ; i-Pr-Im = N-isopropylimidazole, Phen = 1, 10-phenanthroline) have been synthesized and thoroughly characterized. Needing no additives, both compounds are capable of effectively catalyzing the oxidation of OAP to APXO using O2 as an oxidant, with yields of 96. 0% for 1 and 82. 0% for 2. Experimental findings demonstrate that the higher catalytic performance of 1 is attributed to the synergistic work between the Co sites and the PMo8V6 units in its structure, with the catalytic process involving superoxide radicals. Catalyst 1 can be easily recovered by centrifugation and reused at least five times without compromising performance, and the successful scale-up to a gram-scale reaction (100 mmol, 10. 9 g) resulted in a 93. 0% yield of APXO (46. 5 mmol, 9. 86 g) and a turnover number of 6200. This work represents the first application of polyoxometalate-based heterogeneous catalysis for additive-free, O2-driven APXO synthesis and provides a promising platform for sustainable oxidative coupling.
Yu et al. (Wed,) studied this question.