Tripodal S-Ligand Complexes of Copper(I) as Catalysts for Alkene Aziridination, Sulfide Sulfimidation, and C-H Amination

Article abstract of DOI:10.1021/acs.inorgchem.7b00226

Copper(I) complexes of tris(thioimidazolyl)borates (R′Tm^R), including [Cu(Tm^Ph)(PR″₃)] (R″ = Ph, Cu1; Cy, Cu2) and [Cu(R′Tm^Ph)(PR″₃)]⁺ (R′ = N-methylimidazole; R″ = Ph, Cy) were prepared an

Full text of DOI:10.1021/acs.inorgchem.7b00226

Communication
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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
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 PhINTs 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.
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
Scheme 1. Synthesis of Cu1−Cu5 and X-ray Crystal
Structure of Cu2 (C3-Symmetric, 30% Probability of
a
Thermal Ellipsoids, Hydrogen Atoms Omitted)
etal-mediated nitrene transfer catalysis1,2 constitutes an
M
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 (1,4,7-
trithiacyclononane), 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.
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)-
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
Received: January 25, 2017
© XXXX American Chemical Society
A
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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.
Table 1. Aziridination of Alkenes Catalyzed by Cu(I)
Complexes
a
1
All of Cu1−Cu5, Ag1, and Au1 exhibit diamagnetic H
a
Reaction conditions: catalyst (4 μmol), substrate (4 mmol), PhI
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 FAB-
MS 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
b
NTs (0.4 mmol), MeCN (3 mL), 3 h, RT, N2. Determined by NMR
with 1,1-diphenylethene as internal standard.
Copper-catalyzed sulfimidation reactions have received a
considerable attention,2a though much less studied than the
aziridination of alkenes.1c−i,l−n The use of N-donor mono- or
bidentate oxazoline auxiliary ligands in copper-catalyzed
sulfimidation of sulfides is well documented.20 Although
metal catalysts with tripodal ligands for aziridination of alkenes
have been reported,18,19 the behavior of tripodal auxiliary
ligands toward the metal-catalyzed sulfimidation reaction has
yet to be examined.2
In view of the highest efficiency of Cu2 among the other
complexes in this work in catalyzing alkene aziridination, we
studied the reaction of PhINTs with a number of sulfides (1
equiv), including thioanisole and its para-substituted deriva-
tives, in the presence of Cu2 (1 mol %) in acetonitrile at room
temperature. The reaction for thioanisole afforded the
sulfimidation product in 91% yield within 30 min (entry 1,
Table 2). Under similar conditions, a panel of para-substituted
a
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., N-
heterocyclic carbenes);1c−i,l−n,2 tripodal N-ligands18,19 were
also employed, especially the Cu(I)-tris(pyrazolyl)borates
Table 2. Sulfimidation of Sulfides Catalyzed by Cu2
́
(TpR) systems reported by Perez and co-workers.18
In this work, we initially used Cu1 as the catalyst (loading: 1
mol %) for the reaction of iminoiodane PhINTs with styrene
(10 equiv) in dichloromethane. After reaction at room
temperature for 3 h, the aziridination product was formed in
a
Reaction conditions: catalyst Cu2 (4 μmol), substrate (0.4 mmol),
b
1
PhINTs (0.4 mmol), MeCN (3 mL), 30 min, RT, N2. Determined
92% yield (entry 1, Table S3) as determined by H NMR.
by NMR with 1,1-diphenylethene as internal standard.
Changing the catalyst to Ag1 reduced the product yield to
<10%; no aziridination product was detected with Au1 as a
catalyst (Table S3). When the solvent was changed to
acetonitrile, the Cu1-catalyzed styrene aziridination led to a
higher product yield of 96% (entry 1, Table 1). We then
examined the catalytic properties of Cu2, Cu3, Cu4, and Cu5
toward the styrene aziridination under the same conditions,
which afforded the aziridine product in 99%, 66%, 98%, and
93% yield, respectively (entries 2−5, Table 1). Thus, change of
the TmR ligand of the catalyst from TmPh to TmtBu markedly
reduced the aziridination activity (probably owing to larger
steric hindrance of TmtBu), and Cu2 was found to be the
catalyst of choice for the aziridination reaction. With Cu2 (1
mol %) as the catalyst, the reactions of a series of para-
substituted styrenes p-X-C6H4CHCH2 (X = F, Cl, Br, CN,
Me, OMe) and 2-vinyl naphthalene with PhINTs in
acetonitrile at room temperature for 3 h gave the corresponding
aziridination products in 87−99% yields (entries 6−12, Table
1).
thioanisoles were converted into the sulfimides in 95−99%
yields (entries 2−5, Table 2). When the methyl group of
thioanisole was replaced by a benzyl group, a 99% yield of the
sulfimide product was achieved (entry 6, Table 2).
We have also examined the catalytic behavior of Cu2 toward
the C−H amination reaction, and intramolecular amination of
the aryl C−H bond was achieved for the reaction of several
vinyl azides in refluxing 1,2-dichloroethane (DCE) for 48 h,
giving the amination products in isolated yield of up to 98%
(Scheme 2). This reaction complements the rare examples of
Cu-catalyzed intermolecular nitrene transfer with organic
azides,21 despite previous examples of intramolecular aryl C−
H amination of vinyl azides catalyzed by complexes of other
transition metals.22
We propose that the Cu2-catalyzed nitrene transfer reactions
possibly involve Cu-nitrene species, like related Cu-catalyzed
systems.18a,19,23 We performed density functional theory
B
DOI: 10.1021/acs.inorgchem.7b00226
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Communication
Scheme 2. Cu2-Catalyzed Intramolecular C−H Amination
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b00226.
Experimental procedures, computational details, Tables
S1−S3, Figures S1−S5, Scheme S1 (PDF)
X-ray crystal structures of Cu1, Cu2, Au1 (CIF)
(DFT) calculations on the aziridination of styrene and p-
fluorostyrene by [Cu(TmPh)(NTs)] (IntA; Figure 1, Figures
AUTHOR INFORMATION
Corresponding Author
*E-mail: jshuang@hku.hk.
■
ORCID
Jie-Sheng Huang: 0000-0003-1724-4362
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Hong Kong Research Grants
Council (HKU 702312P), Basic Research Program of
Shenzhen (No. JCYJ20160229123546997), and The University
of Hong Kong (Seeds Funding for Basic Research).
REFERENCES
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