Communication pubs.acs.org/IC
Tripodal S‑Ligand Complexes of Copper(I) as Catalysts for Alkene Aziridination, Sulfide Sulfimidation, and C−H Amination Tsz Lung Lam,† Ken Chi-Hang Tso,† Bei Cao,† Chen Yang,† Daqing Chen,† Xiao-Yong Chang,† Jie-Sheng Huang,*,† and Chi-Ming Che†,‡ †
Department of Chemistry and State Key Laboratory of Synthetic Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, People’s Republic of China ‡ HKU Shenzhen Institute of Research and Innovation, Shenzhen 518053, People’s Republic of China S Supporting Information *
coordination mode, are among the well-documented strongly donating scorpionate ligands.9 There have been many studies on the coordination of R′TmR ligands with transition metals,10 but application of these metal complexes in catalysis remains sparse.11 In view of the extensive studies on Cu-catalyzed nitrene transfer reactions,1c−i,l−n,2 we turned attention to copper−R′TmR complexes.12−14 In this work, we report on the synthesis and characterization of several Cu(I) complexes [Cu(TmR)(PR″3)] and [Cu(R′TmR)(PR″3)]+, which are highly active and/or selective catalysts for nitrene transfer reactions, with product yields of up to 99%. The Cu(I) complexes employed in this work were prepared as depicted in Scheme 1. Reaction of K[TmPh]15 with CuCl and
ABSTRACT: Copper(I) complexes of tris(thioimidazolyl)borates (R′TmR), including [Cu(TmPh)(PR″3)] (R″ = Ph, Cu1; Cy, Cu2) and [Cu(R′TmPh)(PR″3)]+ (R′ = N-methylimidazole; R″ = Ph, Cy) were prepared and characterized by spectroscopic methods. The X-ray crystal structures of Cu1 and Cu2 feature a tripodal TmPh ligand coordinated in κ3-S,S,S mode. Using Cu2 as a catalyst (loading: 1 mol %), the aziridination of styrenes and sulfimidation of thioanisoles with PhINTs at RT for 3 and 0.5 h, respectively, both resulted in product yields of up to 99%. Cu2 also catalyzed intramolecular amination of the aryl C−H bond of vinyl azides with up to 98% yield. DFT calculations were performed to gain insight into the mechanism of the Cu2-catalyzed aziridination reaction.
Scheme 1. Synthesis of Cu1−Cu5 and X-ray Crystal Structure of Cu2 (C3-Symmetric, 30% Probability of Thermal Ellipsoids, Hydrogen Atoms Omitted)a
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etal-mediated nitrene transfer catalysis1,2 constitutes an appealing method of N−X (X = C, S, etc.) bond formation reactions. A wide variety of metal complexes have been reported to be active catalysts for nitrene transfer reactions, and the catalysts bearing N- and/or O-donor chelating ligands have received tremendous attention.1,2 Among the well-documented nitrene transfer catalysts bearing N-chelating ligands are metalloporphyrins,1d−g,i,j,l,m including engineered cytochrome P4503 bearing an iron porphyrin active site. Noticing that iron−sulfur clusters rank with iron− porphyrin-containing biological prosthetic groups in pervasive occurrence and multiplicity of function,4 together with the active site of aconitase which can be considered to contain a catalytically active metal center chelated by a cuboidal Fe3S4 cluster in κ3-S,S,S coordination mode,5 we paid attention to metal complexes of chelating S-ligands. In the literature, studies on nitrene transfer by metal catalysts bearing S-ligands are rare.6,7 Conry and co-workers6 reported copper-catalyzed aziridination of alkenes using a macrocyclic S3 ligand (1,4,7trithiacyclononane), which gave aziridine in ∼57% yield, substantially lower than the aziridine yield of up to 90% obtained using macrocyclic NS2 (1-aza-4,8-dithia-cyclodecanes) ligands. Sulfimidation of sulfides and amination of C−H bonds via nitrene transfer using S-ligated metal catalysts, to the best of our knowledge, have hitherto not been reported. Tris(thioimidazolyl)borates (R′TmR; for R′ = H: TmR), a family of tripodal S3 ligands which were first reported by Reglinski and co-workers in 19968 and primarily adopt κ3-S,S,S © XXXX American Chemical Society
a
[TmR]−1 (R = Ph,15 tBu12b) and Cu312b are known in the literature.
PPh3 in MeOH for 12 h at RT afforded [Cu(TmPh)(PPh3)] (Cu1) in 72% isolated yield. A similar reaction using PCy3, instead of PPh3, led to the isolation of [Cu(TmPh)(PCy3)] (Cu2) in a yield of 48%. [Cu(TmtBu)(PPh3)] (Cu3)12b and other analogues, [Cu(TmEt)(PPh3)],12a [Cu(TmMe)(PR″3)] (R″ = Ph, Cy, m-Tol, p-Tol),12c and [Cu(TmMe)(PR″Ph2)] (R″ = Me, Et),12c have been reported previously. [Ag(TmPh)(PPh3)] (Ag1) and [Au(TmPh)(PPh3)] (Au1) were isolated in ∼80% yields by treatment of “AgNO3 + PPh3” and [Au(PPh3)Received: January 25, 2017
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DOI: 10.1021/acs.inorgchem.7b00226 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry Cl], respectively, with K[TmPh]. Copper complexes with R′TmR (R′ ≠ H) are relatively few; the known examples [Cu(PhTmMe)(PR″3)] (R″ = Cy,14a Ph14b) bear monoanionic PhTmMe ligand. Neutral R′TmR ligands, such as (MeIm)TmMe (MeIm = N-methylimidazole) and (MeIm)TmTol, have been seen in Ru(II or III), Rh(III), or Mn(I) complexes.16 We synthesized (MeIm)TmPh and isolated the PF6− salts of cationic complexes [Cu{(MeIm)TmPh}(PPh3)]+ (Cu4, 63% yield) and [Cu{(MeIm)TmPh}(PCy3)]+ (Cu5, 45% yield) from reactions of PPh3 and PCy3, respectively, with (MeIm)TmPh and [Cu(MeCN)4]PF6 in CH2Cl2. All of Cu1−Cu5, Ag1, and Au1 exhibit diamagnetic 1H NMR spectra. A typical spectrum, for Cu2, in CDCl3 is depicted in Figure S1 (Supporting Information), which shows only a single set of signals for the three thioimidazolyl groups, in accordance with the κ3-S,S,S coordination mode of TmPh in Cu2. For TmR complexes Cu1, Cu2, Ag1, and Au2, their FABMS spectra display parent-ion peaks, and their IR spectra show ν(B−H) bands at 2372−2376 cm−1 comparable to those of Cu312b and other [Cu(TmR)(PR″3)]12a,c complexes (ν(B−H) 2358−2435 cm−1). Cu5 and Cu6 exhibited peaks of [Cu(R′TmR)(PR″3)]+ in their FAB-MS spectra. Attempts to obtain diffraction-quality crystals have been successful for Cu1 and Cu2 (Table S1 in Supporting Information); their X-ray crystal structures are depicted in Figure S2 (Supporting Information) and Scheme 1, respectively, both featuring a distorted tetrahedral structure. Ag1 is likely to adopt a structure analogous to that of Cu1 in view of similar tetrahedral structures of [Ag(TmtBu)(PPh3)],12b [Ag(PhTmMe)(PCy3)],14a and [Ag(TmMe)(PiBu3)],17 whereas Au1 adopts a linear structure (determined by X-ray crystallography albeit with a relatively large R value of 0.13, see Figure S3, Table S2) like [Au(PhTmMe)(PEt3)].14b In the literature, copper-catalyzed nitrene transfer reactions (catalyst loading: 5−10 mol % in most cases) involve N-ligands (e.g., bis(oxazolines) and diimines) and C-ligands (e.g., Nheterocyclic carbenes);1c−i,l−n,2 tripodal N-ligands18,19 were also employed, especially the Cu(I)-tris(pyrazolyl)borates (TpR) systems reported by Pérez and co-workers.18 In this work, we initially used Cu1 as the catalyst (loading: 1 mol %) for the reaction of iminoiodane PhINTs with styrene (10 equiv) in dichloromethane. After reaction at room temperature for 3 h, the aziridination product was formed in 92% yield (entry 1, Table S3) as determined by 1H NMR. Changing the catalyst to Ag1 reduced the product yield to
