Additive-free oxidation of isochromans with molecular oxygen synergistically catalyzed by mixed-addendum polyoxometalate-based coordination polymers

Abstract

Attaining the selective oxidation of isochroman into isochromanone in a molecular oxygen (O₂) environment without any additives, via a heterogeneous oxidation process, is highly desirable and challenging work. Herein, we prepare two mixed-addendum polyoxometalate-based coordination polymers of the general formula [H_xM_1-x(i-Pr-Im)₄][H₂N(CH₃)₂]₄[HPMo₈V₆O₄₂] (M = Co 1, Ni, 2; i-Pr-Im = 1-isopropyl-1H-imidazole). Needing no additives, they can catalyze the selective oxidation of isochroman to isochromanone with O₂ as an oxidant, with yields of 91.5% (1) and 46.8% (2), respectively. Mechanistic studies indicate that the excellent performance of catalyst 1 is attributed to the synergistic operation of [Co(i-Pr-Im)₄)] complex and PMo₈V₆ unit, and that the catalytic reaction is a radical pathway involving superoxide radicals. Additionally, the catalyst 1 can be recycled and reused at least four times with uncompromised performance. These results provide fundamental guidelines for designing efficient and multi-site heterogeneous catalysts for the selective oxidation of benzyl C(sp³)-H bonds by activating O₂.

Graphical Abstract