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Oxidative nitrene transfer from azides to alkynes via Ti(ii)/Ti(iv) redox catalysis: formal [2+2+1] synthesis of pyrroles.

ABSTRACT: Catalytic oxidative nitrene transfer from azides with the early transition metals is rare, and has not been observed without the support of redox noninnocent spectator ligands. Here, we report the formal [2+2+1] coupling of azides and alkynes via TiII/TiIV redox catalysis from simple Ti halide imido precatalysts. These reactions yield polysubstituted N-alkyl pyrroles, including N-benzyl protected pyrroles and rare examples of very electron rich pentaalkyl pyrroles. Mechanistic analysis reveals that [2+2+1] reactions with bulky azides have different mechanistic features from previously-reported reactions using azobenzene as a nitrene source.

SUBMITTER: Pearce AJ

PROVIDER: S-EPMC6008247 | biostudies-literature | 2018 Jun

REPOSITORIES: biostudies-literature

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Oxidative nitrene transfer from azides to alkynes via Ti(ii)/Ti(iv) redox catalysis: formal [2+2+1] synthesis of pyrroles.

Pearce Adam J AJ See Xin Yi XY Tonks Ian A IA

Chemical communications (Cambridge, England) 20180601 50

Catalytic oxidative nitrene transfer from azides with the early transition metals is rare, and has not been observed without the support of redox noninnocent spectator ligands. Here, we report the formal [2+2+1] coupling of azides and alkynes via TiII/TiIV redox catalysis from simple Ti halide imido precatalysts. These reactions yield polysubstituted N-alkyl pyrroles, including N-benzyl protected pyrroles and rare examples of very electron rich pentaalkyl pyrroles. Mechanistic analysis reveals t ...[more]

PMID: 29796510

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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.

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Oxidative nitrene transfer from azides to alkynes via Ti(ii)/Ti(iv) redox catalysis: formal [2+2+1] synthesis of pyrroles.

Publications

Oxidative nitrene transfer from azides to alkynes via Ti(ii)/Ti(iv) redox catalysis: formal [2+2+1] synthesis of pyrroles.

Similar Datasets

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