Nonheme Iron-Mediated Amination of C(sp³)–H Bonds. Quinquepyridine-Supported Iron-Imide/Nitrene Intermediates by Experimental Studies and DFT Calculations

Abstract

The 7-coordinate complex [Fe(qpy)(MeCN)2](ClO4)2 (1, qpy = 2,2':6',2″:6″,2''':6''',2''''-quinquepyridine) is a highly active nonheme iron catalyst for intra- and intermolecular amination of C(sp(3))-H bonds. This complex effectively catalyzes the amination of limiting amounts of not only benzylic and allylic C(sp(3))-H bonds of hydrocarbons but also the C(sp(3))-H bonds of cyclic alkanes and cycloalkane/linear alkane moieties in sulfamate esters, such as those derived from menthane and steroids cholane and androstane, using PhI═NR or "PhI(OAc)2 + H2NR" [R = Ts (p-toluenesulfonyl), Ns (p-nitrobenzenesulfonyl)] as nitrogen source, with the amination products isolated in up to 93% yield. Iron imide/nitrene intermediates [Fe(qpy)(NR)(X)](n+) (CX, X = NR, solvent, or anion) are proposed in these amination reactions on the basis of experimental studies including ESI-MS analysis, crossover experiments, Hammett plots, and correlation with C-H bond dissociation energies and with support by DFT calculations. Species consistent with the formulations of [Fe(qpy)(NTs)2](2+) (CNTs) and [Fe(qpy)(NTs)](2+) (C) were detected by high-resolution ESI-MS analysis of the reaction mixture of 1 with PhI═NTs (4 equiv). DFT calculations revealed that the reaction barriers for H-atom abstraction of cyclohexane by the ground state of 7-coordinate CNTs and ground state of C are 15.3 and 14.2 kcal/mol, respectively, in line with the observed high activity of 1 in catalyzing the C-H amination of alkanes under mild conditions.

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

“…This bisnitrene species has a markedly different electronic structure than the previously detected mono-nitrene species and also differs from the electronic structure of the previously reported diamagnetic bis-imido Ru VI -porphyrin species (por)Ru VI ((= NR″) 2 ). 69–73 The work described herein further bears some similarity with the mono- and bis-nitrene species of non-heme iron complexes disclosed by Che and co-workers, 66 but again the electronic structures reported herein are entirely different.…”

A manganese catalyst for highly reactive yet chemoselective intramolecular C(sp3)–H amination

“…This ring-size selectivity has been noted with rhodium catalysis and is based on the geometric constraints imposed by the favored N—S—O bond angles of the sulfamate tether 1 . However, the ability to aminate effectively at 1° methyl groups with metallonitrenes is rare, and the reported examples with iron 12,14 and rhodium 31,32 are low yielding. A limitation to this trend can be observed when amination to form a six-membered heterocycle requires functionalization of an exceptionally strong γ C—H bond over a much weaker β C—H bond.…”

“…The unique generality of [Mn( t BuPc)] is partially attributed to its mechanistically distinct pathway for nitrene transfer that lies between the stepwise mechanism of iron and the concerted mechanism of rhodium. Discovery of an Earth-abundant base metal catalyst that is capable of aminating all types of C(sp 3 )—H bonds, including those challenging to access with precious noble metals, underscores the potential benefits in the continued development of these inexpensive, underexplored metals as catalysts for important synthetic reactions 5, 6, 11–14, 16–19 .…”

“…Conversely, base metal iron catalysts access mechanistically distinct single electron pathways for nitrene transfer, affording excellent chemoselectivity with diminished reactivity for stronger C—H bond types 11–14 . We hypothesized that a metal catalyst capable of transferring bound nitrenes to C(sp 3 )—H bonds via a stepwise mechanism with attenuated radical character of the metallonitrene oxidant relative to iron would achieve higher reactivity while maintaining chemoselectivity 20 .…”

Iron Catalysis in Organic Synthesis

“…930 In addition, examples for the insertion into tertiary and secondary aliphatic CÀH bonds have been presented as well. The aminating species was generated by reaction of the sulfamic acid esters with diacetoxyiodobenzene.…”