Efficient amine oxidation using metal–organic framework photocatalysts for aminoalkyl radical-mediated halogen-atom transfer

Abstract

The utilization of amines, typically serving as electron-donating sacrificial agents, to boost catalytic performance using MOFs is very common in various photocatalytic reactions. However, the reactivity of the oxidized amine in the system has been rarely studied thus far. Inspired by the very recent development of halogen-atom transfer (XAT) reactions using α-aminoalkyl radicals, we report herein a systematic study focusing on the use of heterogeneneous MOF photocatalysts to achieve XAT reactions, by means of amine photooxidation generating α-aminoalkyl radicals. A series of classical MOF photocatalysts with excellent stability, including porphyrinic Zr-MOFs of MOF-525, MOF-545, PCN-221 and PCN-223, as well as amino-functionalized UiO-66-NH₂ and MIL-125-NH₂, were examined for the method. Photocatalytic experiments show that the porphyrinic MOFs are efficient and recyclable photocatalysts to achieve diverse XAT reactions such as dehalogenation, hydroalkylation and polyfluoroarylation. Mechanism studies clearly show the radical nature of the reaction, including the generation of α-aminoalkyl radicals due to the amine photooxidation by MOFs and the further activation of halides via the XAT process forming reactive carbon radicals. The work provides a new exploration of the rational utilization of the common amine oxidation of MOFs to achieve diverse XAT reactions, displaying the great potential of MOFs for radical chemistry via photocatalysis.