Mechanism-Guided Development of Transition-Metal-Catalyzed C-N Bond-Forming Reactions Using Dioxazolones as the Versatile Amidating Source.Acc Chem Res. 2021 06 01; 54(11):2683-2700.AC
Catalytic reactions that construct carbon-nitrogen bonds are one of central themes in both synthetic and medicinal chemistry since the obtainable nitrogen-containing motifs are commonly encountered in natural products and have also seen a growing prominence as key structural features in marketed drugs and preclinical candidates. Pd-catalyzed cross-couplings, such as Buchwald-Hartwig amination, are at the forefront of such synthetic methods in practical settings. However, they require prefunctionalized substrates such as (hetero)aryl halides that must be prepared independently, often by multiple operations. One emerging way to circumvent these preparatory steps and directly convert ubiquitous C-H bonds into valuable C-N bonds is catalytic C-H amination, which allows synthetic chemists to devise shorter and more efficient retrosynthetic schemes. The past two decades have witnessed considerable progress in expanding the repertoire of this strategy, especially by identifying effective amino group precursors. In this context, dioxazolones have experienced a dramatic resurgence in recent years as a versatile nitrogen source in combination with transition-metal catalyst systems that facilitate decarboxylation to access key metal-acylnitrenoid intermediates. In addition to their high robustness and easy accessibility from abundant carboxylic acids, the unique reactivity of the transient intermediates in the amido group transfer has led to a fruitful journey for mild and efficient C-H amidation reactions.This Account summarizes our recent contributions to the development of C-N bond-forming reactions using dioxazolones as effective nitrenoid precursors, which are categorized into two subsets according to their mechanistic differences: inner- versus outer-sphere pathways. The first section describes how we could unveil the synthetic potential of dioxazolones in the realm of the inner-sphere C-H amidation, where we demonstrated that dioxazolones serve not only as manageable alternatives to acyl azides but also as highly efficient reagents to significantly reduce the catalyst loading and temperature. Taking advantage of the mild conditions in combination with group 9 Cp*M complexes (M = Rh, Ir, Co) or isoelectronic Ru species, we have dramatically expanded the accessible synthetic scope. Mechanistic investigations revealed that the putative metal-nitrenoid species is involved as a key intermediate during catalysis, which leads to facile C-N bond formation. On the basis of the mechanistic underpinning, we have succeeded in developing novel catalytic platforms that harness the intermediacy of metal-nitrenoids to explore C-H insertion chemistry via an outer-sphere pathway. Indeed, the tailored catalysts were capable of suppressing the competitive Curtius-type decomposition, thus granting access to versatile lactam products. We have further repurposed the catalytic systems upon modification of chelating ligands and also the identity of the transition metal to achieve three goals: (i) addressing selectivity issues to control the regio-, chemo-, and enantioselectivities, (ii) developing sustainable catalysis by first-low metals, and (iii) navigating chemical space for (di)functionalization of alkenes/alkynes. Together with our own research efforts, highlighted herein are some important relevant advances by other groups. We finally conclude with a brief overview with an eye toward further developments.