Development of Nonclassical Photoprecursors for Rh Nitrenes.

Characterization of reactive intermediates in C-H functionalization is challenging due to the fleeting lifetimes of these species. Synthetic photochemistry provides a strategy to generate post-turnover-limiting-step intermediates in catalysis under cryogenic conditions that enable characterization. We have a long-standing interest in the structure and reactivity of Rh nitrene intermediates, which are implicated as transient intermediates in Rh-catalyzed C-H amination. Previously, we demonstrated that Rh complexes bearing organic azide ligands can serve as solid-state and photoprecursors in the synthesis of transient Rh nitrenoids. Complementary solution-phase experiments have not been available due to the weak binding of most organic azides to Rh complexes. Furthermore, the volatility of the N that is evolved during nitrene synthesis from these precursors has prevented the observation of C-H functionalization from lattice-confined nitrenes. Motivated by these challenges, here we describe the synthesis and photochemistry of nonclassical nitrene precursors based on sulfilimine ligands. Sulfilimines bind to Rh carboxylate complexes more tightly than the corresponding azides, which has enabled the full solid-state and solution-phase characterization of these new complexes. The higher binding affinity of sulfilimine ligands as compared with organic azides has enabled both solution-phase and solid-state nitrene photochemistry. Cryogenic photochemical studies of Rh sulfilimine complexes confined within polystyrene thin films demonstrate that sulfilimine photochemistry can be accomplished at low temperature but that C-H amination is rapid at temperatures compatible with N═S photoactivation. The potential of these structures to serve as platforms for multistep cascades is discussed.