Harnessing Electrophilic N-Aryl Catalytic Intermediates for Versatile C-N Bond Formation

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Abstract The pervasive presence of N-heterocycles in chemical probes, pharmaceuticals and materials that improve the quality of life and health of humans continues to spur the development of new reactions to simplify access to these scaffolds. Our research program addresses important unsolved problems in organic synthesis through the development of new reactions for the selective construction of C–NHAr, C–C and C–O bonds by harnessing the unique reactivity of N-aryl nitrogen catalytic and reactive intermediates generated in situ from nitroarenes and unactivated anilines. Despite the ubiquity of the C–NHAr bond in bioactive N-heterocycles, it cannot be formed using existing metal-catalyzed N-atom transfer reactions because these processes require a strong electron- withdrawing N-substituent. In contrast to the well-established chemistry of N-sulfonyl- or N-carbamoyl metal nitrenes, whose strong electron-withdrawing group is critical for their reactivity with C–H bonds and π-systems, the reactivity of N-aryl nitrenes and nitroarenes is poorly understood because of the difficulties in generating them and taming their reactivity. This lack of understanding has produced a gap in synthesis that obstructs access to complex, functionalized N-heterocyclic compounds and emphasizes the need for the development of methods to provide solutions to these problems. Our prior research efforts have established that N-aryl nitrenes can be formed from azides and that their reactivity is distinct to nitrenes bearing strong electron withdrawing groups. These efforts have provided the basis to support our future efforts in discovering new reactions of N-aryl nitrenes and nitrosoarenes that we will harness to simplify the synthesis of privileged N-heterocyclic scaffolds embedded in synthetic targets. Within this proposal, we have leveraged our understanding these N-aryl nitogen reactive intermediates to develop new transformations that create C–NHAr bonds. Towards this end, in Aim 1 we will develop new Fe(II)-catalyzed reductive cascade reactions that construct sp3-C–NHAr bonds intra- or intermolecularly from nitroarenes using silane reductants; in Aim 2 we will develop new single-electron transfer processes that generate N-aryl reactive intermediates with tunable oxygen transfer abilities for the synthesis of N-hydroxyindoles and oxindoles; and in Aim 3, we will develop oxidative methods for accessing electrophilic N- aryl nitrenoids from anilines and apply to the intra- and intermolecular synthesis of N-heterocycles. By establishing new strategies and tactics for the stereoselective formation of C–NAr, C–C and C–O bonds through harnessing the reactivity of N-aryl nitrogen intermediates, successful realization of these Aims will produce new tools to simplify the construction of novel and bioactive N-heterocycles.