Iron-Catalyzed Alkyne Carboamination via an Isolable Iron Imide Complex

Abstract

Transition metal imide-mediated C–N bond formation is a powerful strategy for the introduction of nitrogen into organic compounds. We have discovered that the reaction of N-mesityl­(β-diketiminato)­iron imide complex tBuLFeNMes ( tBuL = 3,5-bis­(2,6-diisopropylphenylimino)-2,2,6,6-tetramethylheptyl and Mes = 2,4,6-trimethylphenyl) with a terminal alkyne substrate gives a β-alkynyl enamine product by a novel alkyne carboamination process. Stoichiometric experiments revealed a catalyst deactivation pathway involving generation of the acetylide complex, tBuLFeCCPh, and mesityl amine (MesNH2) from the acetylene complex, tBuLFe­(HCCPh), and mesityl azide (MesN3). This reactivity is suppressed in the presence of coordinating additive 4-tert-butylpyridine ( t BuPy), likely through formation of the four-coordinate complex tBuLFe­(HCCPh)­( t BuPy). These insights were instrumental in identifying reaction conditions that allow for turnover of the iron catalyst.

Stabilization of a high-spin three-coordinate Fe(iii) imidyl complex by radical delocalization

“…This distance is similar to those of the four-coordinate Fe(III) imidyl complexes reported by Betley, ( Ar L)FeCl(]NC 6 H 4p-t Bu) (1.768(2) Å) 39 and ( tBu L)FeCl(]NDipp) (1.768(4) Å), 40 and that of the imido-Fe 4 S 4 cluster complex reported by Suess and co-workers, (IMes) 3 Fe 4 S 4 ]NDipp (1.763(2) Å, IMes ¼ 1,3bis(2,4,6-trimethylphenyl)imidazol-2-ylidene). 58 In addition, the short N(3)-C (33) distance and the elongation of the C(33)-C (34) and C(33)-C(38) distances of the mesityl ring (Fig. 2) indicated radical delocalization.…”

“…4,5 In addition, high-spin states are rare among these compounds, with most of them being in low- and intermediate-spin states. 3,6–35 Many of the terminal imido complexes of Fe characterized to date contain traditional closed-shell imido ligands, represented as NR 2− (R = alkyl, aryl). 3 It has been shown that inducing a weak ligand field around the metal center enables the isolation of the open-shell imidyl ligand with a radical on its N, represented as ˙NR − .…”

Isolation and Reactivity of an Iron Azametallacyclobutene Complex

“…We recently reported the iron-catalyzed coupling of organic azides with aryl acetylenes to afford β-alkynyl enamines 1 (Scheme 2, top). 40 Our mechanistic evaluation of this carboamination process led us to consider two pathways for the initial interaction of the alkyne substrate and the proposed iron imide intermediate (Scheme 2, bottom). In pathway A, the basic imide nitrogen 41 deprotonates the relatively acidic terminal proton of the aryl acetylene to give an alkynyl amide complex.…”

Carbene-Like Reactivity in an Iron Azametallacyclobutene Complex: Insights from Electronic Structure

“…We recently reported the isolation and characterization of the first monometallic iron azametallacyclobutene complex (1; Scheme 1b) 15 that was discovered during our investigation into the mechanism of iron-catalyzed alkyne carboamination. 16 Details from the molecular structure of 1 hinted at a delocalized π system within the metallacycle (vide infra). Herein we report our investigation of the electronic structure of 1 as well as our exploration into its reactivity prompted by observations from this computational study.…”