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Cobalt-Porphyrin-Catalysed Intramolecular Ring-Closing C-H Amination of Aliphatic Azides: A Nitrene-Radical Approach to Saturated Heterocycles.

ABSTRACT: Cobalt-porphyrin-catalysed intramolecular ring-closing C-H bond amination enables direct synthesis of various N-heterocycles from aliphatic azides. Pyrrolidines, oxazolidines, imidazolidines, isoindolines and tetrahydroisoquinoline can be obtained in good to excellent yields in a single reaction step with an air- and moisture-stable catalyst. Kinetic studies of the reaction in combination with DFT calculations reveal a metallo-radical-type mechanism involving rate-limiting azide activation to form the key cobalt(III)-nitrene radical intermediate. A subsequent low barrier intramolecular hydrogen-atom transfer from a benzylic C-H bond to the nitrene-radical intermediate followed by a radical rebound step leads to formation of the desired N-heterocyclic ring products. Kinetic isotope competition experiments are in agreement with a radical-type C-H bond-activation step (intramolecular KIE=7), which occurs after the rate-limiting azide activation step. The use of di-tert-butyldicarbonate (Boc₂ O) significantly enhances the reaction rate by preventing competitive binding of the formed amine product. Under these conditions, the reaction shows clean first-order kinetics in both the [catalyst] and the [azide substrate], and is zero-order in [Boc₂ O]. Modest enantioselectivities (29-46?% ee in the temperature range of 100-80?°C) could be achieved in the ring closure of (4-azidobutyl)benzene using a new chiral cobalt-porphyrin catalyst equipped with four (1S)-(-)-camphanic-ester groups.

SUBMITTER: Kuijpers PF

PROVIDER: S-EPMC5488222 | biostudies-literature | 2017 Jun

REPOSITORIES: biostudies-literature

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Cobalt-Porphyrin-Catalysed Intramolecular Ring-Closing C-H Amination of Aliphatic Azides: A Nitrene-Radical Approach to Saturated Heterocycles.

Kuijpers Petrus F PF Tiekink Martijn J MJ Breukelaar Willem B WB Broere Daniël L J DLJ van Leest Nicolaas P NP van der Vlugt Jarl Ivar JI Reek Joost N H JNH de Bruin Bas B

Chemistry (Weinheim an der Bergstrasse, Germany) 20170502 33

Cobalt-porphyrin-catalysed intramolecular ring-closing C-H bond amination enables direct synthesis of various N-heterocycles from aliphatic azides. Pyrrolidines, oxazolidines, imidazolidines, isoindolines and tetrahydroisoquinoline can be obtained in good to excellent yields in a single reaction step with an air- and moisture-stable catalyst. Kinetic studies of the reaction in combination with DFT calculations reveal a metallo-radical-type mechanism involving rate-limiting azide activation to fo ...[more]

PMID: 28332743

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Project description:To fully characterize the Co(III)-'nitrene radical' species that are proposed as intermediates in nitrene transfer reactions mediated by cobalt(II) porphyrins, different combinations of cobalt(II) complexes of porphyrins and nitrene transfer reagents were combined, and the generated species were studied using EPR, UV-vis, IR, VCD, UHR-ESI-MS, and XANES/XAFS measurements. Reactions of cobalt(II) porphyrins 1(P1) (P1 = meso-tetraphenylporphyrin (TPP)) and 1(P2) (P2 = 3,5-Di(t)Bu-ChenPhyrin) with organic azides 2(Ns) (NsN3), 2(Ts) (TsN3), and 2(Troc) (TrocN3) led to the formation of mono-nitrene species 3(P1)(Ns), 3(P2)(Ts), and 3(P2)(Troc), respectively, which are best described as [Co(III)(por)(NR″(•-))] nitrene radicals (imidyl radicals) resulting from single electron transfer from the cobalt(II) porphyrin to the 'nitrene' moiety (Ns: R″ = -SO2-p-C6H5NO2; Ts: R″ = -SO2C6H6; Troc: R″ = -C(O)OCH2CCl3). Remarkably, the reaction of 1(P1) with N-nosyl iminoiodane (PhI═NNs) 4(Ns) led to the formation of a bis-nitrene species 5(P1)(Ns). This species is best described as a triple-radical complex [(por(•-))Co(III)(NR″(•-))2] containing three ligand-centered unpaired electrons: two nitrene radicals (NR″(•-)) and one oxidized porphyrin radical (por(•-)). Thus, the formation of the second nitrene radical involves another intramolecular one-electron transfer to the "nitrene" moiety, but now from the porphyrin ring instead of the metal center. Interestingly, this bis-nitrene species is observed only on reacting 4(Ns) with 1(P1). Reaction of the more bulky 1(P2) with 4(Ns) results again in formation of mainly mono-nitrene species 3(P2)(Ns) according to EPR and ESI-MS spectroscopic studies. The mono- and bis-nitrene species were initially expected to be five- and six-coordinate species, respectively, but XANES data revealed that both mono- and bis-nitrene species are six-coordinate O(h) species. The nature of the sixth ligand bound to cobalt(III) in the mono-nitrene case remains elusive, but some plausible candidates are NH3, NH2(-), NsNH(-), and OH(-); NsNH(-) being the most plausible. Conversion of mono-nitrene species 3(P1)(Ns) into bis-nitrene species 5(P1)(Ns) upon reaction with 4(Ns) was demonstrated. Solutions containing 3(P1)(Ns) and 5(P1)(Ns) proved to be still active in catalytic aziridination of styrene, consistent with their proposed key involvement in nitrene transfer reactions mediated by cobalt(II) porphyrins.

Dataset Information

Cobalt-Porphyrin-Catalysed Intramolecular Ring-Closing C-H Amination of Aliphatic Azides: A Nitrene-Radical Approach to Saturated Heterocycles.

Publications

Cobalt-Porphyrin-Catalysed Intramolecular Ring-Closing C-H Amination of Aliphatic Azides: A Nitrene-Radical Approach to Saturated Heterocycles.

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