Oxidative C-H and C-C Bond Functionalization Catalyzed by Dirhodium Caprolactamate

Abstract

Dirhodium tetrakiscaprolactamate, Rh2(cap)4, was found to be an effective catalyst for allylic oxidation in combination with tert-butyl hydroperoxide (t-BuOOH). Substituted cyclic olefins were converted to enones and enediones with 0.1 - 1.0 mol% catalyst loading. Rh2(cap)4 was also found to be an effective catalyst for benzylic oxidation in combination with t-BuOOH. Benzylic carbonyl compounds were obtained across a range of substrates including those that contained nitrogen and acid-labile functionality. A formal synthesis of palmarumycin CP2 was achieved using this methodology. Spectroscopically, it was determined that Rh2(cap)4 (Rh24+) undergoes a 1- electron oxidation upon treatment with t-BuOOH to give a higher valent dirhodium(II,III) complex (Rh25+).

A mild, efficient, and selective aziridination catalyzed by Rh2(cap)4 was discovered. Using p-toluenesulfonamide (TsNH2), N-bromosuccinimide (NBS), and potassium carbonate (K2CO3), aziridines were obtained with as little as 0.01 mol% Rh2(cap)4. Aziridine formation occurred through a Rh25+-catalyzed amidobromination and subsequent base-induced ring closure. An X-ray crystal structure of a Rh25+ halide complex, formed from the oxidation of Rh2(cap)4 with N chlorosuccinimide, was obtained.

Finally, Rh2(cap)4 was found to be a catalyst for C-H oxidation of tertiary amines using t-BuOOH. An oxidative Mannich reaction was realized when the C-H oxidation was conducted in the presence of 2-siloxyfurans as nucleophiles. Reactions were performed efficiently (as low as 0.1 mol% loading) in MeOH using 70% t BuOOH in H2O (T-HYDRO). Synthetically useful aminoalkyl butenolides were obtained.