N-functionalized aziridines, which are both useful intermediates and important synthetic targets, can be envisioned as arising from the addition of nitrenes (i.e., NR fragments) to olefinic substrates. The exceptional reactivity of most nitrenes, in particular with respect to unimolecular decomposition, prevents general application of nitrene-transfer to the synthesis of N-functionalized aziridines. Here we demonstrate N-aryl aziridine synthesis via 1) olefin aziridination with N-aminopyridinium reagents to afford N-pyridinium aziridines followed by 2) Ni-catalyzed C–N cross-coupling of the N-pyridinium aziridines with aryl boronic acids. The N-pyridinium aziridine intermediates also participate in ring-opening chemistry with a variety of nucleophiles to afford 1,2-aminofunctionalization products. Mechanistic investigations indicate aziridine cross-coupling proceeds via a noncanonical mechanism involving initial aziridine opening promoted by the bromide counterion of the Ni catalyst, C–N cross-coupling, and finally aziridine reclosure. Together, these results provide new opportunities to achieve selective incorporation of generic aryl nitrene equivalents in organic molecules.
The combination of an organic photocatalyst [4CzIPN (1,2,3,5‐tetrakis(carbazol‐9‐yl)‐4,6 dicyanobenzene) or 5MeOCzBN (2,3,4,5,6‐pentakis(3,6‐dimethoxy‐9 H‐carbazol‐9‐yl)benzonitrile)], quinuclidine, and tetra‐n‐butylammonium phosphate (hydrogen‐bonding catalyst) was employed for amide bond formations. The hydrogen‐bonded OH group activated the adjacent C−H bond of alcohols towards hydrogen atom transfer (HAT) by a radical species. The quinuclidinium radical cation, generated through single‐electron oxidation of quinuclidine by the photocatalyst, employed to abstract a hydrogen atom from the α‐C−H bond of alcohols selectively due to a polarity effect‐produced α‐hydroxyalkyl radical, which subsequently converted to the corresponding aldehyde under aerobic conditions. Then the coupling of the aldehyde and an amine formed a hemiaminal intermediate that upon photocatalytic oxidation produced the amide.
This review summarizes the synthesis and reactivity of N-aminopyridinium salts, discusses applications in organic synthesis, and highlights the potential for these reagents to enable novel synthetic disconnections and innovations.
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