Enhanced Structural Variety of Nonplanar N-Oxyl Radical Catalysts and Their Application to the Aerobic Oxidation of Benzylic C-H Bonds.

Abstract

The design and synthesis of structurally variable, nonplanar N-oxyl radical catalysts and their application to the aerobic oxidation, etherification, and acetoamidation of benzylic C-H bonds are described. The catalytic oxidation of C-H bonds represents a powerful tool to synthesize oxygenated functional molecules from simple hydrocarbons in a straightforward way. Electron-deficient N-oxyl radical catalysts, such as phthalimidoyl N-oxyl (PINO) radical, generated from N-hydroxyphthalimide (1), have attracted much attention because of their applications in the oxidation of C-H bonds with high bond dissociation energy (BDE). However, a few sites in 1 are available for structural modifications and improvements of the catalytic performance. By replacing one carbonyl group in 1 with a trifluoromethyl (CF3)-substituted sp(3)-carbon, we generated an additional tunable site and a nonplanar backbone, while retaining the desirable electron-withdrawing properties and increasing the lipophilicity with respect to 1. We synthesized a variety of N-hydroxy precatalysts containing such a CF3 moiety, and investigated their utility in the aerobic oxidation of benzylic C-H bonds. Precatalysts with electron-withdrawing substituents, such as trifluoroethoxy and the acetophenone moieties, afforded higher yields than a corresponding methoxy-substituted analogue. The introduction of substituents at the aromatic ring was also effective, as evident from the performance of 7-CF3 and 4,5,6,7-tetrafluoro precatalysts. Especially the combination of trifluoroethoxy- and 4,5,6,7-tetrafluoro substitution afforded a superior performance. These catalyst systems exhibited high functional group tolerance during the aerobic oxidation of C-H bonds, and benzylic etherification and Ritter-type reactions could be carried out at room temperature when a selected precatalyst and N-bromosuccinimide (NBS) were used.