Practical and efficient oxygenation of benzylic C-H bonds with dioxygen catalyzed by two-dimensional porous organic frameworks possessing porphyrin cobalt(II) units

Abstract

Metalloporphyrin T(4-Br)PPCo was used in the construction of two-dimensional porous organic framework materials (2D POFs) in order to achieve the direct benzyl C-H bonds oxygenation. In order to prevent the disordered diffusion of free radicals in 2D POFs, a restricted reaction channel was constructed, and heterogeneous catalysis was applied to decrease the deep oxygenation of partial oxygenation products due to central metal sites. All 2D POFs were characterized as amorphous microporous materials containing highly and uniformly dispersed Co2+. It was used to partially oxygenate C-H bonds at benzyl position employing dioxygen in solvent-free condition, and the conversion and selectivity were improved at the same time, especially when residual -B(OH)2 and -Br were capped. Compared with T(4-Br)PPCo, the substrate conversion increased from 39 % to 66 %, and the selectivity for 4-nitroacetophenone increased from 66 % to 91 %. The reaction mechanism, apparent kinetics and substrate compatibility were also studied. In this work, it provided a guiding reference for the effective C-H bonds oxygenation and other free radical based chemical reactions. The use of POFs in catalytic chemistry was further broadened, and a successful method for improving the catalytic characteristics of POFs by capping the remaining active groups was suggested.