A rhodium(ii)/rhodium(iii) redox couple for C–H bond amination with alkylazides: a rhodium(iii)–nitrenoid intermediate with a tetradentate [14]-macrocyclic ligand

Abstract

The catalytic activity of a rhodium(II) dimer complex, [Rh^II(TMAA)]₂ (TMAA = tetramethyltetraaza[14]annulene), in C–H amination reactions with organic azides is explored. Organic azides (N₃–R) with an electron-withdrawing group such as a sulfonyl group (trisylazide; R = S(O)₂^iPr3C₆H₂ (Trs)) and a simple alkyl group (R = (CH₂)₄Ph, (CH₂)₂OCH₂Ph, CH₂Ph, or C₆H₄NO₂) are employed in intra- and intermolecular C–H bond amination reactions. The spectroscopic analysis using ESI-mass and EPR spectroscopy techniques on the reaction intermediate generated from [Rh^II(TMAA)]₂ and N₃–R reveals that a rhodium(III)–nitrenoid species is an active oxidant in the C–H bond amination reaction. DFT calculations suggest that the species can feature a radical localised nitrogen atom. The DFT calculation studies also indicate that the amination reaction involves hydrogen atom abstraction from the organic substrate R′–H by the NR moiety of 2^N˙R and successive rebound of the generated organic radical intermediate R′˙ to [Rh^III(NH–R)(TMAA)], giving [Rh^II(TMAA)] and R′–NH–R (amination product).