Chemoselective intramolecular allylic C–H amination versus CC aziridination through Co(ii)-based metalloradical catalysis

Abstract

Excellent chemoselectivity for intramolecular allylic C-H amination versus C]C aziridination was achieved through Co(II)-based metalloradical catalysis. Metalloradical catalyst [Co(P1)], the cobalt(II) complex of D 2h -symmetric porphyrin 3,5-Di t Bu-IbuPhyrin, was shown to be highly effective for selective intramolecular allylic C-H amination of both N-bishomoallylic and N-allylic sulfamoyl azides. The Co(II)-catalyzed intramolecular 1,6-C-H amination of these azides provides a general and efficient method to access the synthetically useful allylic 1,3-diamines under neutral and non-oxidative conditions, without complication from the competitive olefin aziridination. The origin of this remarkable chemoselectivity has direct relevance to the radical mechanism of Co(II)-based metalloradical amination.

Co(porphyrin)-catalysed amination of 1,2-dihydronaphthalene derivatives by aryl azides

“…Zhang and co-authors recently reported an excellent chemoselectivity of an intramolecular allylic CeH amination versus a C]C aziridination using a D 2h -symmetric cobalt porphyrin catalyst [22]. A similar trend was also observed by us [26] for the amination of cyclohexene catalysed by Co(TPP) (TPP ¼ dianion of tetraphenyl porphyrin).…”

Stereoselective Radical Amination of Electron-Deficient C(sp3)–H Bonds by Co(II)-Based Metalloradical Catalysis: Direct Synthesis of α-Amino Acid Derivatives via α-C–H Amination

“…6 These Co(II)-based metalloradical catalysts have proven to be unusually effective on the activation of various organic azides, including sulfonyl, 7 phosphoryl, 8 carbonyl 9 and aryl 10 azides, for amination of broad classes of C–H bonds under neutral and non-oxidative conditions. 11 Particularly, Co(II) complexes of D 2h -symmetric amidoporphyrins [Co( D 2h -Por)] have revealed uncommon catalytic capacity for efficient intramolecular amination of strong primary C–H bonds 7b,8 and have also displayed excellent chemoselectivity for intramolecular allylic C–H amination over the competitive C=C aziridination. 7c Several lines of experimental and computational evidence back the radical mechanism of Co(II)/azide-based C–H amination that involves an unusual Co(III)-nitrene radical intermediate undergoing a stepwise radical abstraction-substitution pathway.…”

“…11 Particularly, Co(II) complexes of D 2h -symmetric amidoporphyrins [Co( D 2h -Por)] have revealed uncommon catalytic capacity for efficient intramolecular amination of strong primary C–H bonds 7b,8 and have also displayed excellent chemoselectivity for intramolecular allylic C–H amination over the competitive C=C aziridination. 7c Several lines of experimental and computational evidence back the radical mechanism of Co(II)/azide-based C–H amination that involves an unusual Co(III)-nitrene radical intermediate undergoing a stepwise radical abstraction-substitution pathway. 7b,7c,12,13 Considering the non-electrophilic nature of this radical mechanism, which is fundamentally different from the electrophilic metallonitrene mechanism shared by the widely-studied Rh 2 - and other closed-shell systems, we envisaged the possibility of addressing the aforementioned challenge of intramolecular electron-deficient C–H amination through Co(II)-based metalloradical catalysis.…”

“…7c Several lines of experimental and computational evidence back the radical mechanism of Co(II)/azide-based C–H amination that involves an unusual Co(III)-nitrene radical intermediate undergoing a stepwise radical abstraction-substitution pathway. 7b,7c,12,13 Considering the non-electrophilic nature of this radical mechanism, which is fundamentally different from the electrophilic metallonitrene mechanism shared by the widely-studied Rh 2 - and other closed-shell systems, we envisaged the possibility of addressing the aforementioned challenge of intramolecular electron-deficient C–H amination through Co(II)-based metalloradical catalysis.…”

“…Although the yield was moderate (41%), the α-C–H amination was very clean without observation of β-C–H amination, indicative of its slow reaction rate. When the Co(II) complex of D 2h -symmetric amidoporphyrin 3,5-Di t BuIbuPhyin [Co( P1 )] was employed as the catalyst, 7b,7c,8 the amination rate was drastically enhanced to afford the desired amino acid derivative 2a in 98% yield in spite of a lower catalyst loading (2 mol %). This ligand-enhanced catalysis is presumably contributed to the cooperative hydrogen bonding interaction between the groups S=O of the substrate and N-H of the catalyst.…”