Chiral Iron Porphyrins Catalyze Enantioselective Intramolecular C(sp³)−H Bond Amination Upon Visible‐Light Irradiation

Abstract

Iron‐catalyzed asymmetric amination of C(sp3)−H bonds is appealing for synthetic applications due to the biocompatibility and high earth abundance of iron, but examples of such reactions are sparse. Herein we describe chiral iron complexes of meso‐ and β‐substituted‐porphyrins that can catalyze asymmetric intramolecular C(sp3)−H amination of aryl and arylsulfonyl azides to afford chiral indolines (29 examples) and benzofused cyclic sulfonamides (17 examples), respectively, with up to 93 % ee (yield: up to 99 %) using 410 nm light under mild conditions. Mechanistic studies, including DFT calculations, for the reactions of arylsulfonyl azides reveal that the Fe(NSO2Ar) intermediate generated in situ under photochemical conditions reacts with the C(sp3)−H bond through a stepwise hydrogen atom transfer/radical rebound mechanism, with enantioselectivity arising from cooperative noncovalent interactions between the Fe(NSO2Ar) unit and the peripheral substituents of the chiral porphyrin scaffold.

Tertiary Amides as Directing Groups for Enantioselective C−H Amination using Ion‐Paired Rhodium Complexes

“…One of the leading methods for enacting C−H amination in an enantioselective manner is to use metal nitrenoids [2] . A variety of metals have been explored for this purpose including ruthenium, [3] silver, [4] iridium, [5] cobalt, [6] manganese, [7] and iron [3f,8,9] . In recent years, enzymes have also been productively harnessed to enable asymmetric C−H amination [10] .…”

Recent Advances in Nonprecious Metal Catalysis

“…21 While biocatalysis has proved successful in enantioselective C(sp 3 )−H amination, 22 highly enantioselective protocols using small molecules have been dominated by precious metal catalysis (Ir, Rh, Ru), 23 and the few Fe-catalyzed variants have been limited in enantioselectivity and scope 20 until the recent publication from Dang, Liu, and Che. 24 Thanks to a newly developed panel of porphyrin ligands involving either or both sterically demanding octahydro-1,4:5,8-dimethanoanthracene-9-yl groups and amide hydrogen-bond donors, moderate to good enantioselectivities were obtained for a broad range of indolines (39−90% ee = enantiomeric excess) and sultams (30−93% ee, Scheme 5). A synergistic effect between the two types of substituents was found to occur in the most efficient catalyst, with steric distortion being the main factor in determining the selectivity.…”

Catalytic, asymmetric carbon–nitrogen bond formation using metal nitrenoids: from metal–ligand complexes via metalloporphyrins to enzymes

“…44b). 231 This novel and intriguing design combines the steric-blocking elements (in blue) from the D 4 -symmetric ligand discussed at the beginning of this section along with the hydrogen bond-donating capability (in red) of Zhang's D 2 -symmetric ligands. It was hypothesised that by combining both of these elements into one catalyst, a more selective nitrenoid transfer might be possible.…”