DIRHODIUM CAPROLACTAMATE CATALYZED OXIDATIONS BY TERT-BUTYL HYDROPEROXIDE. FROM MECHANISMS TO NEW METHODOLOGIES
Ratnikov, Maxim O.
Doyle, Michael P.
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Dirhodium caprolactamate, Rh<sub>2</sub>(cap)<sub>4</sub>, is a very efficient catalyst for the generation of the <italic>tert</italic>-butylperoxy radical from <italic>tert</italic>-butyl hydroperoxide, and the <italic>tert</italic>-butylperoxy radical is a highly effective oxidant for phenols and anilines. These reactions are performed with 70% aqueous <italic>tert</italic>-butyl hydroperoxide using dirhodium caprolactamate in amounts as low as 0.01 mol % in toluene and chlorobenzene to oxidize <italic>para</italic>-substituted phenols to 4-(<italic>tert</italic>-butyldioxy)cyclohexadienones. Electron rich and electron poor phenolic substrates undergo selective oxidation in good to excellent yields, but steric influences from bulky para substituents force oxidation onto the ortho position resulting in <italic>ortho</italic>-quinones. Comparative results with RuCl<sub>2</sub>(PPh<sub>3</sub>)<sub>3</sub> and CuI are provided, and mechanistic comparisons are made between these catalysts that are based on diastereoselectivity (reactions with estrone), regioselectivity (reactions with <italic>p</italic>-<italic>tert</italic>-butylphenol) and chemoselectivity in the formation of 4-(<italic>tert</italic>- butyldioxy)cyclohexadienones. The data obtained are consistent with hydrogen atom abstraction by the <italic>tert</italic>-butylperoxy radical followed by radical combination between the phenoxy radical and the <italic>tert</italic>-butylperoxy radical. Under similar reaction conditions <italic>para</italic>-substituted anilines are oxidized to nitroarenes in good yield. An efficient one-pot tandem sequence forming heterocycles is developed. The method consists of an efficient phenol oxidation with inexpensive T-HYDRO catalyzed by dirhodium caprolactamate followed by Bronsted acid promoted cyclization. Substrate scope includes uses of L-tyrosine ester and dipeptide derivatives. Application of chiral phosphoric acids for asymmetric cyclization is discussed. Previously unreported peroxide cleavage with TiCl<sub>4</sub> enables a rapid synthesis of cleroindidin F. A general mechanism is proposed for transition metal catalyzed oxidative Mannich reactions of <italic>N</italic>-methylanilines with <italic>tert</italic>-butyl hydroperoxide (TBHP) as an oxidant. Kinetic data and isotope labeling of the <italic>N</italic>-methyl group suggest a rapid equilibrium between an iminum ion and solvent or TBHP preceding Mannich addition. Linear free-energy relationship (LFER) analysis, kinetic and product isotope effects, and utilization of radical clock substituents establish <italic>tert</italic>-butylperoxy radicals as an oxidant in dirhodium(II) caprolactamate [Rh<sub>2</sub>(cap)<sub>4</sub>] catalyzed oxidations of <italic>N,N</italic>-dialkylaniline by TBHP. Isotope effects measured for various transition metal salts reveal a general role of alternative catalysts as initiators of TBHP decomposition in <italic>N,N</italic>-dialkylaniline oxidations. New mechanistic insights led the development of the novel FeCl<sub>3</sub> catalyzed oxidative Mannich reaction.