Effective Synthesis of Chiral N‐Fluoroaryl Aziridines through Enantioselective Aziridination of Alkenes with Fluoroaryl Azides

Abstract

The CoII complex of a D2‐symmetric chiral porphyrin ([Co(D2‐Por*)], see scheme) is a highly effective catalyst for the enantioselective aziridination of alkenes with fluoroaryl azides. The reaction can be performed at RT with low catalyst loading, and the olefin is the limiting reagent. Furthermore, the reaction is tolerant toward different combinations of aromatic olefins and fluoroaryl azides.

Characterization of Porphyrin-Co(III)-‘Nitrene Radical’ Species Relevant in Catalytic Nitrene Transfer Reactions

“…4 Given the ubiquity of nitrogen atoms in biologically active compounds, 8 nitrene transfer reactions also have very important applications in making molecules of interest. However, the applicability of metal-nitrenoids has so far been limited to alkene aziridination, 9,10 C–H amination 11,12 and amidation. 13 …”

“…Thus, well-studied complexes with redox active aminyl ligand radicals are limited to only a few well-characterized examples, 65 and related complexes bearing redox non-innocent nitrene ligands are even scarcer. 9,43,65–67 Given the importance of these radicaloid nitrene species in metal-catalyzed nitrene transfer and C–H functionalization reactions, 10,12,13,39–42,68 we gathered more experimental evidence for the formation of the previously reported mono-nitrene radical complexes upon reaction of cobalt(II) porphyrins with nitrene transfer reagents. 9,43 Furthermore, we here reveal the first example of a bis-nitrene species of 1 P1 upon reacting the stronger oxidizing nitrene transfer agent N -nosyl iminoiodane 4 Ns with 1 P1 .…”

Room temperature activation of aryloxysulfonyl azides by [Co(II)(TPP)] for selective radical aziridination of alkenes via metalloradical catalysis

“…1–2 Of various Co(II)-based metalloradical catalysts, Co(II) complexes supported by porphyrin ligands bearing amide functionalities [Co(AmidoPor)] have shown to be particularly effective in activating various organic azides, including phosphoryl azides, 3 sulfonyl azides, 4 and aryl azides, 5 for radical olefin aziridination, producing the corresponding aziridines in high yields under mild conditions. The high catalytic efficiency of [Co(AmidoPor)] metalloradical catalysts is attributed to the stabilization of the key α-Co(III)-aminyl radical (also known as Co(III)-nitrene radical) intermediate through hydrogen-bonding interaction with the amide group of the porphyrin ligand (Scheme 1: A1).…”

“…The high catalytic efficiency of [Co(AmidoPor)] metalloradical catalysts is attributed to the stabilization of the key α-Co(III)-aminyl radical (also known as Co(III)-nitrene radical) intermediate through hydrogen-bonding interaction with the amide group of the porphyrin ligand (Scheme 1: A1). 1–5 While the effectiveness of [Co(AmidoPor)] have been clearly demonstrated (especially in the case of enantioselective aziridination), it would be practically more desirable if the commercially available Co(II) complex of meso -tetraphenylporphyrin ([Co((TPP)]) could be applied effectively as metalloradical catalyst for radical olefin aziridination with azides when the concern of enantioselectivity is unnecessary. This practical desire is translated to the fundamental question of whether [Co((TPP)], which lacks of the hydrogen-bonding capability, can effectively activate azides to generate the corresponding α-Co(III)-aminyl radical (Scheme 1: A2) to serve as active intermediates for the radical aziridination process.…”

Selective radical amination of aldehydic C(sp2)–H bonds with fluoroaryl azides via Co(ii)-based metalloradical catalysis: synthesis of N-fluoroaryl amides from aldehydes under neutral and nonoxidative conditions

“…19–20 Recently, Co(II)-based metalloradical catalysis (MRC) was successfully applied for asymmetric olefin aziridination with fluoroaryl azides as effective nitrene sources. 21 The Co(II)-catalyzed aziridination was believed to proceed through Co(III)-nitrene radical A as the key intermediate (Figure 2), which undergoes radical addition to olefin, followed by 3-exo-tet radical cyclization. 21 As a further application of the concept of MRC, we anticipated the possibility of a Co(II)-based catalytic process for aldehydic C–H amination with fluoroaryl azides if the Co(III)-supported fluoroaryl nitrene radical A is capable of H-atom abstraction from aldehydes to generate Co(III)-amido complex B , followed by acyl radical substitution (Figure 2).…”