Observation of a Photogenerated Rh₂ Nitrenoid Intermediate in C–H Amination

Abstract

Rh-catalyzed C-H amination is a powerful method for nitrogenating organic molecules. While Rh nitrenoids are often invoked as reactive intermediates in these reactions, the exquisite reactivity and fleeting lifetime of these species has precluded their observation. Here, we report the photogeneration of a transient Rh nitrenoid that participates in C-H amination. The developed approach to Rh nitrenoids, based on photochemical cleavage of N-Cl bonds in N-chloroamido ligands, has enabled characterization of a reactive Rh nitrenoid by mass spectrometry and transient absorption spectroscopy. We anticipate that photogeneration of metal nitrenoids will contribute to the development of C-H amination catalysis by providing tools to directly study the structures of these critical intermediates.

Photochemical C−H Amination of Ethers and Geminal Difunctionalization Reactions in One Pot

“…The deuterium kinetic isotope effect (KIE) was determined from parallel reactions of THF and [D 8 ]‐THF with 1 a under identical conditions, resulting in a KIE k THF / k DTHF of 1.3 for the C−H insertion reaction (see the Supporting Information). This KIE is in line with the value of 1.48 that was obtained from the photolytic reaction of tosyl azide with THF . This low value is not in agreement with a concerted amination mechanism by nitrene 12 (Scheme ), since typically KIEs of larger than 2.5 have been observed for concerted C−H insertions .…”

“…This KIE is in line with the value of 1.48 that was obtained from the photolytic reaction of tosyl azide with THF . This low value is not in agreement with a concerted amination mechanism by nitrene 12 (Scheme ), since typically KIEs of larger than 2.5 have been observed for concerted C−H insertions . The observed KIE is rather in line with a radical process, in which hydrogen transfer is not the rate‐determining step, or a single‐electron‐transfer process.…”

Terminal Imido Complexes of the Groups 9–11: Electronic Structure and Developments in the Last Decade

“…Moving down the group to tetracoordinated 4 d rhodium(III) complex 18 , which is isostructural with 12 Tol , reveals a larger Rh−N distance of 1.780(2) Å [72] . This effect is enhanced for the hexacoordinated nitrene complex 19 (2.303(4) Å) [78,79] . Moving to the 5 d element iridium reveals arguably similar bond lengths as found for rhodium.…”

“…Until 2018, only a desorption ionization mass spectrometry experiment [77] and computational studies gave indication for their intermediacy [78] . Powers and colleagues presented spectroscopic proof [79] for the formation of a rhodium imido complex in 2018 and managed to crystallize the adamantyl imido compound 19 one year later (Figure 20). [80] …”

Characterization of a Reactive Rh2 Nitrenoid by Crystalline Matrix Isolation

“…The critical Rh2 nitrenoid intermediate proposed to be responsible for C-H amination and olefin aziridination has been detected by mass spectrometry 21 and time-resolved spectroscopic methods, 22 but due to the fleeting lifetime of these species, structural data of these transient species has been unavailable. Critical issues, such as the preferred electronic configuration of Rh2 nitrenoids, have not been resolved despite extensive theoretical interest.…”