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Electronically asynchronous transition states for C–N bond formation by electrophilic [Co iii (TAML)]-nitrene radical complexes involving substrate-to-ligand single-electron transfer and a cobalt-centered spin shuttle

DOI: 10.1021/acscatal.0c01343 DOI Help

Authors: Nicolaas P. Van Leest (University of Amsterdam) , Martijn A. Tepaske (University of Amsterdam) , Bas Venderbosch (University of Amsterdam) , Jean-pierre H. Oudsen (University of Amsterdam) , Moniek Tromp (University of Amsterdam) , Jarl Ivar Van Der Vlugt (University of Amsterdam) , Bas De Bruin (University of Amsterdam)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Acs Catalysis

State: Published (Approved)
Published: June 2020
Diamond Proposal Number(s): 22432

Abstract: The oxidation state of the redox non-innocent TAML (Tetra-Amido Macrocyclic Ligand) scaffold was recently shown to affect the formation of nitrene radical species on cobalt(III) upon reaction with PhI=NNs [J. Am. Chem. Soc. 2020, 142, 552–563]. For the neutral [CoIII(TAMLsq)] complex this leads to the doublet (S = ½) mono-nitrene radical species [CoIII(TAMLq)(N•Ns)(Y)] (bearing an unidentified sixth ligand Y in at least the frozen state), while a triplet (S = 1) bis-nitrene radical species [CoIII(TAMLq)(N•Ns)2]‒ is generated from the anionic [CoIII(TAMLred)]‒ complex. The one-electron reduced Fischer-type nitrene radicals (N•Ns‒) are formed through single (mono-nitrene) or double (bis-nitrene) ligand-to-substrate single-electron transfer (SET). In this work we describe the reactivity and mechanisms of these nitrene radical com-plexes in catalytic aziridination. We report that [CoIII(TAMLsq)] and [CoIII(TAMLred)]‒ are both effective catalysts for chemoselective (C=C versus C‒H bonds) and diastereoselective aziridination of styrene derivatives, cyclohexene and 1-hexene under mild and even aerobic (for [CoIII(TAMLred)]‒) conditions. Experimental (Hammett plots, [CoIII(TAML)]-nitrene radical formation and quantification under catalytic conditions, single-turnover experiments, and tests regarding catalyst decomposition, radical inhibition and radical trapping) in combination with computational (DFT, CASSCF) studies reveal that [CoIII(TAMLq)(N•Ns)(Y)], [CoIII(TAMLq)(N•Ns)2]‒ and [CoIII(TAMLsq)(N•Ns)]– are key electrophilic intermediates in the aziridination reactions. Surprisingly, the electrophilic one-electron reduced Fischer-type nitrene radicals do not react as would be expected for nitrene radicals (i.e. via radical addition and radical rebound). Instead, nitrene transfer proceeds through unusual electronically asynchronous transition states, in which (partial) styrene substrate to TAML ligand (single-) electron transfer precedes C−N coupling. The actual C−N bond formation processes are best described as involving a nucleophilic attack of the nitrene (radical) lone pair at the thus (partially) formed styrene radical cation. These processes are coupled to TAML-to-cobalt and cobalt-to-nitrene single-electron transfer, effectively leading to formation of an amido-γ-benzyl radical (NsN–CH2–•CH–Ph) bound to an intermediate spin (S = 1) cobalt(III) center. Hence, the TAML moiety can be regarded to act as a transient electron acceptor, the cobalt center behaves as a spin shuttle and the nitrene radical acts as a nucleophile. Such a mechanism was hitherto unknown for cobalt catalyzed hypovalent group transfer, and more general transition metal catalyzed nitrene transfer to alkenes, but is now shown to complement the known concerted and stepwise mechanisms for N-group transfer.

Journal Keywords: Aziridination; ligand-to-substrate single-electron transfer; substrate-to-ligand single-electron transfer; electronically asynchronous transition states; nitrene radical; spin shuttle

Subject Areas: Chemistry

Instruments: B18-Core EXAFS