Benedicte Lallemand

Catalysts that can be regulated in terms of activity and selectivity by external stimuli may allow the efficient multistep synthesis of complex molecules and pharmaceuticals. Herein, we report the light-color regulation of the redox potential of a photocatalyst to control the activation of chemical bonds. Light-color control of the redox power of a photocatalyst introduces a new selectivity parameter to photoredox catalysis: Instead of changing the catalyst or ligand, alteration of the color of the visible-light irradiation adjusts the selectivity in catalytic transformations. By using this principle, the selective activation of aryl–halide bonds for C−H arylation and the sequential conversion of functional groups with different reduction potentials is possible by simply applying different colors of light for excitation of the photocatalyst.

I see your two colors, shining through: The xanthene dye rhodamine 6G is a moderately reducing photoredox catalyst if irradiated with green light, but provides an high reduction power of −2.4 V vs. SCE upon blue-light irradiation. This allows control of the selectivity of photocatalytic C−H arylation reactions through changing the color of the light.

The development of new CH functionalization protocols based on inexpensive cobalt catalysts is currently attracting significant interest. Functionalized 8-aminoquinoline compounds are high-potential building blocks in organic chemistry and pharmaceutical compounds and new facile routes for their preparation would be highly valuable. Recently, copper has been applied as catalyst for the functionalization of 8-aminoquinoline compounds and found to operate through a single electron transfer (SET) mechanism, although requiring elevated reaction temperatures. Herein, we described the first example of a cobalt-catalyzed remote CH functionalization of 8-aminoquinoline compounds operating through a SET mechanism, exemplified using a practical and mild nitration protocol. The reaction uses inexpensive cobalt nitrate hexahydrate [Co(NO₃)₂⋅6 H₂O] as catalyst and tert-butyl nitrite (TBN) as nitro source. This methodology offers the basis for the facile preparation of many new functionalized 8-aminoquinoline derivatives.