Designed To React: Terminal Copper Nitrenes and Their Application in Catalytic C?H Aminations

Article abstract of DOI:10.1002/anie.201713171

Heteroscorpionate ligands of the bis(pyrazolyl)methane family have been applied in the stabilisation of terminal copper tosyl nitrenes. These species are highly active intermediates in the copper-catalysed direct C?H amination and nitrene transfer. Novel perfluoroalkyl-pyrazolyl- and pyridinyl-containing ligands were synthesized to coordinate to a reactive copper nitrene centre. Four distinct copper tosyl nitrenes were prepared at low temperatures by the reaction with SO₂tBuPhINTs and copper(I) acetonitrile complexes. Their stoichiometric reactivity has been elucidated regarding the imination of phosphines and the aziridination of styrenes. The formation and thermal decay of the copper nitrenes were investigated by UV/Vis spectroscopy of the highly coloured species. Additionally, the compounds were studied by cryo-UHR-ESI mass spectrometry and DFT calculations. In addition, a mild catalytic procedure has been developed where the copper nitrene precursors enable the C?H amination of cyclohexane and toluene and the aziridination of styrenes.

Full text of DOI:10.1002/anie.201713171

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Accepted Article
Title: Designed to react: Terminal Copper Nitrenes and their
Application in Catalytic C-H Aminations
Authors: Sonja Herres-Pawlis, Julian Moegling, Alexander Hoffmann,
Fabian Thomas, Nicole Orth, Patricia Liebhäuser, Ulrich
Herber, Robert Rampmaier, Ivana Ivanovic-Burmazovic,
Gerhard Fink, and Julia Stanek
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201713171
Angew. Chem. 10.1002/ange.201713171
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10.1002/anie.201713171
Angewandte Chemie International Edition
FULL PAPER
Designed to react: Terminal Copper Nitrenes and their
Application in Catalytic C–H Aminations
Julian Moegling,[a] Alexander Hoffmann,[a] Fabian Thomas,[a] Nicole Orth,[b] Patricia Liebhäuser,[a] Ulrich
Herber,[a] Robert Rampmaier,[c] Julia Stanek,[a] Gerhard Fink,[a] Ivana Ivanović-Burmazović,[b] Sonja
Herres-Pawlis*[a]
Dedicated to Prof. Dietmar Stalke on the occasion of his 60th birthday
Abstract: Heteroscorpionate ligands of the bis(pyrazolyl)methane
family have been applied in the stabilisation of terminal copper tosyl
nitrenes. These species are highly active intermediates in the copper
catalysed direct C–H amination and nitrene transfer. Novel
perfluoroalkyl-pyrazolyl and pyridinyl containing ligands have been
synthesised, designed for the coordination to a reactive copper
nitrene centre. Four distinct copper tosyl nitrenes have been prepared
at low temperatures by the reaction with SO2tBuPhINTs and copper(I)
acetonitrile complexes. Their stoichiometric reactivity has been
elucidated regarding imination and aziridination of phosphines and
styrenes. The formation and thermal decay of the copper nitrenes
were investigated by UV/Vis spectroscopy of the highly coloured
species. Additionally, the compounds have been studied by cryo-
UHR-ESI mass spectrometry and DFT. A mild catalytic procedure has
been developed, where the copper nitrene precursors enable the C–
H amination of cyclohexane and the aziridination of styrenes.
long-known nitrene precursor PhINTs[10], the α-amination of
acylpyrazoles[11] and the formation of oxazoles.[8b]
Particularly, the presence of terminal copper nitrene species has
been evaluated as critical factor for reactivity.[6c,6g,7a,8-9] Taken
together, the majority of reports on this topic are considering these
discrete intermediates to mediate reactivity. So far, only few
stable copper nitrenes have been reported (Figure 1), whereby
none of them can be truly described as terminal copper nitrene.
Pioneering work has been accomplished by the Warren
group,[9b,12] who presented molecular structures of µ-nitrene
bridged dicopper complexes by employing NacNac ligands. They
inferred the catalytic reactivity in C–H aminations from the
dissociation of the dimetallic species into a terminal copper
nitrene. Although observed by Caulton et al.[6c] via ESI mass
spectrometry, a comprehensive investigation of a cationic copper
tosyl nitrene was achieved by the Ray group,[13] which was closely
followed by the report of a copper mesityl nitrene,[14] both featuring
a tridentate neutral amine ligand. Both systems were stabilised by
the aid of Lewis acids and the former was later reassessed in
terms of a tautomeric copper-imidyl form.[15] Bertrand et al.
Functionalisation of unreactive C–H bonds plays a key role in the
atom economical synthesis of organic compounds.[1]
A principal approach is the insertion of metal-bound nitrenes into
C–H bonds[2], leading directly to the introduction of the N-group
motif into organic substrates, which is of major importance for the
fabrication of synthetically,[3] pharmaceutically[4] and biologically[5]
relevant molecules. Due to its low toxicity, cost and environmental
impact, copper compounds have been extensively studied and a
rich chemistry for catalytic direct C–H amination[6] and N-group
transfer[6g,7] has evolved. To date, tackling the full scope of organic
substrates for the manipulation of unfunctionalised C–H
compounds remains an important objective of chemical synthesis.
Copper nitrenes are proposed to be key intermediates in Cu-
catalysed C–H aminations and nitrene group transfers. This has
been concluded by theoretical investigations[6c,6g,7a,8] and
experiments.[9] Latest examples of their widespread reactivity
comprise the formation of sulfonamides and isothiazoles from the
reported copper nitrenes originating from
a
metal-free
phosphonitrene.[16] In solution, an equilibrium between a µ-nitrene
bridged dicopper nitrene, a terminal nitrene and a twofold nitrene-
coordinated copper(I) centre occurs. Attempts of isolating the
terminal copper nitrene leads to its dismutation into the other
isomers.[17] Most recently, Company, Ray and Ribas reported a
room temperature stable phenyl copper nitrene incorporated into
an aza-macrocycle.[18]
[a]
J. Moegling, Dr. A. Hoffmann, F. Thomas, P. Liebhäuser, Dr. U.
Herber, J. Stanek, Dr. G. Fink, Prof. Dr. S. Herres-Pawlis
Institut für Anorganische Chemie
RWTH Aachen University
Landoltweg 1, 52074 Aachen (Germany)
E-mail:sonja.herres-pawlis@ac.rwth-aachen.de
N. Orth, Prof. Dr. I. Ivanović-Burmazović
Department Chemie und Pharmazie
Friedrich-Alexander-Universität Erlangen-Nürnberg
Egerlandstraße 1, 91058 Erlangen (Germany)
R. Rampmaier
Department für Chemie und Pharmazie
Ludwig-Maximilians-Universität München
Butenandtstraße 5-13, 81377 München (Germany)
Figure 1. Synthetically and spectroscopically verified copper nitrenes.
[b]
[c]
The occurrence and intermediacy of discrete Cu nitrenes is a
fascinating object of research, but of the aforementioned
stabilised Cu nitrenes, solely Warren's copper nitrenes allowed
the development of an effective catalytic procedure.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201713171
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FULL PAPER
Table 1. Top row: molecular structures of the cationic copper(I) bis(pyrazolyl)methane complexes [Cu{HC(3-CF3Pz)2(Py)}MeCN]+ (C1), [Cu{HC(3-
tBuPz)2(Py)}MeCN]+ (C2), [Cu{HC(3,5-PhPz)2(Py)}MeCN]+ (C4) and [Cu{HC(3,5-CF3Pz)2(Py)}MeCN]+ (C5), which are used for the reaction with SPhINTs. Bottom
row: structures of the copper nitrenes in the singlet state and κ2-N,O binding mode.
ligand
HC(3-CF3Pz)2(Py)
HC(3-tBuPz)2(Py)
HC(3-tBuPz)2(1-MeIm)
HC(3,5-PhPz)2(Py)
HC(3,5-CF3Pz)2(Py)
molecular
structure
(X-ray)
no suitable crystals, but
same structural motif
(see Supp. Information
for NMR, EA and MS)
complex
C1
C2
C3
C4
C5
DFT
structure
does not form copper
nitrene
nitrene
N1
N2
N3
N4
This work contributes to the field of Cu nitrenes by introducing a
new family of well-defined stable nitrenes featuring versatile
bis(pyrazolyl)methane ligands. We present directly observed
terminal copper nitrenes and provide with a mild catalytic protocol
for the conversion of cyclohexane and toluene.
A literature survey has shown methyl groups[6g,12b] and even tert-
butyl groups[22] in the vicinity of a metal nitrene centre to be
vulnerable towards intramolecular hydrogen atom abstraction.
The
dichloromethane-soluble
iminoiodinane
2-(tert-
butylsulfonyl)(p-toluenesulfonyliminoiodo)benzene (SPhINTs, see
the inset of Figure 2 for the structure) was utilised as nitrene
generating agent and exposed to the colourless Cu(I) complex
solutions at low temperatures. An immediate reaction occurred
and resulted in a colour change to green (N1, N3 and N4) and red
(N2). The distinct bands of the copper nitrenes were monitored by
low-temperature UV/Vis spectroscopy and allowed further
characterisation of the Cu nitrene reaction dynamics. The UV/Vis
spectra (Figure 2) show strong absorption bands for N1, N3 and
N4 centred around 400 nm (ε ~ 3000-4500 L mol-1cm-1) and
weaker features ranging from 480 to 650 nm (ε ~ 750 L mol-1cm-
1) for the four formed copper nitrenes.
Bis(pyrazolyl)methanes were already demonstrated to support
copper-peroxo cores,[19] representing the fastest Tyrosinase
models for the catalytic phenol hydroxylation. Five ligands have
been selected for the synthesis of tetrahedrally distorted copper(I)
-
-
complexes with PF6 or ClO4 as non-coordinating anions (Table
1). Ligands HC(3-tBuPz)2(Py)[20] and HC(3-tBuPz)2(1-MeIm)[19c]
have been reported previously and HC(3,5-PhPz)2(Py)
synthesised analogous to that procedure.[20-21] The ligands
HC(3,5-CF3Pz)2(Py) and HC(3-CF3Pz)2(Py) have been
synthesised for the purpose of providing inert, but sterically
demanding groups pointing towards the metal centre.
Similar absorption features have been observed by Ray et al. for
Sc3+ trapped copper tosylnitrenes.[13] Curiously, the complex C5
was not prone to undergo a reaction with SPhINTs, which we
relate to the inertness of this electron deficient Cu(I) complex
towards oxidation. Cyclic voltammetry of C1-C5 was performed
and the potential of the oxidative waves of the irreversible electron
transfer dissected (E versus Fc/Fc+, Table S2). The highest
oxidative peak potential was found for the electron deficient Cu(I)
complexes C1 (880 mV) and C5 (1000 mV) and the lowest for C4
(730 mV), thereby supporting the described trend in the copper
nitrene formation. To further elucidate the Cu/nitrene
stoichoimetry, we performed a titration, where SPhINTs was used
as titrant against the copper(I) complex (Figure S7 and S8). A
slightly sigmoidal curve was obtained with the absorption
maximum at the exact addition of 1.0 equivalents of titrant, which
serves as further indicator for the formulation of a terminal nitrene
with a 1:1 Cu/nitrene ratio. These nitrenes are almost indefinitely
stable at -90 °C in dichloromethane solutions, but upon warming
up above this temperature, thermal decay occurs (Figures S9-
S12). The half-life of the Cu nitrenes has been determined at -
42 °C to 10 min for N3 and 38 min for N2. The decay of N4 and
Figure 2. Top: UV/Vis spectra of copper tosyl nitrenes (3 mmol L-1 in
[Cu(I){L}MeCN]PF6, DCM). The molar absorption coefficient ε is corrected to the
estimated copper nitrene concentration derived from the ferrocene experiments
(Figures S3-S6). Inset: Structural formula of SPhINTs. Bottom scheme:
formation of copper nitrenes from bis(pyrazolyl)methane copper(I) complexes.
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N1 did not follow an exponential behaviour, with a rather too steep
initial decay, giving 50 % of the initial absorption after 14 and
5 min, resp. For all nitrenes we observed decay to species with
weaker absorption features, resulting in an almost complete loss
of colour of the solutions. The quantification of nitrenes N1-N4 and
their respective products of the thermal decay was conducted by
reduction, using an excess of ferrocene, and the detection of
ferrocenium by UV/Vis (Table S3). This allows for the
determination of the copper nitrene yield, assuming a single one-
electron transfer by ferrocene. Ray et al. found a two-electron
reduction process of a copper tosyl nitrene by ferrocene, whereby
a reduction of the decayed product by ferrocene was only
observed in the presence of a Lewis acid.[13] In our case the
ferrocenium yields for the nitrenes range from 71-95 % and 63-
75 % for the decayed products (except for the decayed product of
However in our case, such species could not be found in cryo-
UHR-ESI or ESI mass spectra of decayed samples and 1H NMR
spectra of worked-up mixtures.
The copper nitrenes N1-N4 have been theoretically investigated
by density functional theory (see SI Sections 9-10) using the
TPSSh functional with a def2-TZVP GD3BJ basis set (PCM
solvent-model for dichloromethane; empirical dispersion
correction with Becke-Johnson damping), which has successfully
described complicated copper systems with nearly identical
ligands.[24] For all copper nitrenes, singlet, triplet and broken-
symmetry states have been calculated and triplet ground states
in κ1-N,O binding mode are mostly stabilised. However, the
different conformers and states are close in energy such that we
assume conformational equilibria for the nitrenes such that the
DFT results should not be overinterpreted. The spin state of the
copper nitrenes N1-N4 was probed by NMR and thereby assigned
to a singlet by using the Evans' method[25] (Table S1). A distinct
µeff was measured expectedly for the products of thermal decay
of N1-N4, which are Cu(II)-amido complexes in the doublet state.
The UV/Vis features of N1-N4 were simulated with TD-DFT and
found to give the correct number of absorptions and matching
wavelength of the transitions for the singlet κ2-N,O conformers of
N1, N3 and N4 (Figures S21-S24).
N4), assuming
a one-electron reduction. Considering the
stoichiometry of the reaction, a two-electron transfer is discarded
based on the high yields of ferrocenium of the decayed products,
which would give yields of 150-180 % in this experiment when
starting from nitrenes N1-N3. While being aware, that the product
of one-electron reduction of N1-N4 differs to the decayed products
in terms of a proton (Scheme S4), the high amount of formation
of the copper nitrenes N1-N4 is also supported by the high yields
of the stoichiometric PPh3 imination by N1 and N3 and styrene
aziridination by N1 (described below).
The N-tosyl imination of PPh3 serves as proxy for the oxidative
ability of nitrido/nitrene complexes and has been monitored by
UV/Vis spectroscopy, showing pseudo first-order reaction kinetics
(Figure S13). The rate constants kobs were plotted against [PPh3]
(Figure 3, right), which allows for the extraction of the second-
order rate constants k2, revealing the nitrenes N3 and N1 to be
the fastest (k2 = 4.3 ± 0.8 L mol-1s-1 and 3.4 ± 0.5 L mol-1s-1)
compared to N2 (k2 = 0.6 ± 0.1 L mol-1s-1). The kinetics of N4
could not be investigated, due to the competitive thermal decay.
Taken together, N2 displays the highest thermal stability and the
slowest reaction rate with the reductant PPh3 (in comparison to
N1 and N3). Next, we investigated the TsNPPh3 formation, which
did not exceed 34 and 50 % for the sluggish oxidants N4 and N2,
but gave high yields for N3 and N1 (84 and 79 %), as determined
from 31P-NMR spectroscopy. ESI mass spectrometry shows
peaks for [TsNPPh3]+ and the complex-product adduct [C3–
TsNPPh3]+, which was further reproduced with the isotope
To further evidence the formation of copper nitrenes, cryo-UHR-
ESI mass spectrometry was used. The recorded spectra are given
in the Supporting Information and herein exemplary discussed for
N2 (Figure S14, 1st spectrum). They do not exactly resemble the
simulated one for the expected copper nitrene ion [N2]+ (2nd
spectrum). This can be explained by an overlay of [N2-H]+ (3rd
spectrum) and [N2+H]+ (4th spectrum) which accompany the
expected copper nitrene as minor contributors. These side-
products are the result of nitrene decay by intra- or intermolecular
hydrogen atom abstraction (for mechanism see Scheme S6) and
protonation by traces of moisture in the ESI setup. Stavropoulos
et al.[6g] also investigated copper nitrenes with cryo-spray ESI
mass spectrometry and found
a similar hydrogen atom
abstraction pathway. In this case, an elusive putative copper
nitrene was not stabilised, but found to completely decay to the
product of the intramolecular C–H amination. The product of the
decay was isolated on a preparative scale. In our case, such
products were also detected by mass spectrometry of mixtures of
the decayed nitrene N2. But, species of the type [N2-H] were
S
labelled PhI15NTs. To disprove the formation of TsNPPh3 from
the thermally decayed nitrenes, an experiment with the decayed
nitrene of N1, the N-tosyl amido complex [Cu{HC(3-
CF3Pz)2(Py)}HNTs]+, was carried out at room temperature,
showing only traces of the product. We propose the
conformational flexibility to be key to reactivity, as found already
earlier for Cu2O2 systems.[24]
1
neither found in worked-up samples nor by H NMR of mixtures
of the decayed nitrene N2. N1 could not be detected in the
experiments, but upon dilution with THF, the protonated species
[N1+H]+ was detected (Figure S18). The same species could be
produced by thermal decay. Every experiment was well
reproduced with the isotopically labelled SPhI15NTs and gave the
expected mass shift of 1 a.u. (Figures S14-S19). For N1 the
thermally decayed species is tentatively assigned to a copper
A more synthetically applicable reaction is the aziridination of
alkenes. The propensity of the investigated nitrenes to undergo
aziridination reactions is rather low, but nevertheless styrenes
containing electron withdrawing groups (EWG) in the para-
position were employed and shown by UV/Vis spectroscopy to
react with N1 three times faster than the thermal decay. With N1
being the fastest reactant in the stoichiometric aziridination, fair
yields could be achieved for p-EWG-styrenes and a poor
conversion for a styrene with p-electron donating group (p-OMe-
styrene), owing to the competing thermal decay (Figure 3, left). A
amido complex [Cu{HC(3-CF3Pz)2(Py)}HNTs]+ This is further
.
supported by EPR spectroscopy, featuring an axial Cu(II) EPR
spectrum (Figure S2).
Recently, Warren et al. reported the formation of masked copper
nitrenes from Cu(I)-MeCN complexes by intramolecular insertion
from the Cu nitrene moiety into the coordinated MeCN.[23]
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A remarkably interesting property is the instability of the nitrene
N1 with respect to ligand exchange. HC(3-CF3Pz)2(Py) is a much
weaker ligand than HC(3-tBuPz)2(Py) and can be replaced by the
latter. We exploited this attribute for an unusual experiment
inspired by the Cu2O2 core-capture experiment by Stack et al.[27]
Such a transformation is also observed at the addition of ligand
HC(3-tBuPz)2(Py) to copper nitrene N1 (Figure 4). The spectral
features of N1 completely disappear and N2 evolves,
accompanied by some decayed nitrene owing to the necessary
temperature increase to enable the exchange reaction. Thereby,
we present a novel Cu–NTs core-capture experiment, which is
also supported by the application of isodesmic equations from
DFT (Figure 4, inset, for the singlet κ2-N,O conformers). The
forward reaction was shown to be ca. 10 kcal mol-1 more favoured
compared to the back-reaction. A reversed reaction starting from
N2 is not observed experimentally. Hereby, N1 might serve as
nitrene precursor for metal complexes who are not capable to
form metal nitrenes from iminoiodinanes.
control experiment with the decayed nitrene N1, the N-tosyl amido
complex [Cu{HC(3-CF3Pz)2(Py)}HNTs]+ and p-CF3-styrene gave
only trace amounts of the corresponding aziridine. A further
evaluation of the reactivity in stoichiometric reactions towards
substrates for C–H amination reactions was impeded by the
competing thermal decay.
Figure 3. Left: yields of the stoichiometric reactions of N1 with ferrocene,
styrenes and PPh3 (the latter are corrected to the estimated copper nitrene yield).
Right: plot of pseudo first-order kinetic constants kobs versus the PPh3
concentration with respect to N1-N3.
But in the next step, we found that the presented systems are
primed for catalytic C–H aminations: with the aid of the Cu(I)
precursor and SPhINTs, we developed a catalytic procedure
allowing the conversion of toluene and cyclohexane (Scheme 1).
These substrates bear inert C–H bonds and a copper-catalysed
C–H amination at room temperature has been rarely
observed.[6g,6h]
While
some
protocols
require
high
temperatures,[6b,12b] elevated temperatures were not beneficial in
our case. C1 and C4 excel in comparison to C2, C3 and C5
(Tables S5 and S6). The optimised protocol consists of PhCl as
solvent and uses SPhINTs. The overall modest yields of the
challenging substrate cyclohexane are the consequence of the
competing Cu(I)-catalysed cleavage of SPhINTs into SPhI and
H2NTs, where the latter was found quantitatively beside the
product after stirring for 48 h. This Cu(I)-catalysed cleavage of an
iminoiodiane has already been observed before and is a common
side-reaction.[26] Additionally, the solubility of the starting
materials is reduced by cyclohexane, denying the use of a vast
excess of substrate. Besides the catalytic C–H amination, the
catalytic aziridination of styrenes is easily accomplished by using
PhINTs in dichloromethane (Table S4, entries 1-6). In the case of
p-OMe-styrene the catalyst C4 shows high product yields.
Although C5 does not act as suitable precursor for the
stabilisation of a copper nitrene at low temperatures, a catalytic
activity in C–H amination reactions and aziridination reactions at
room temperature was observed (Tables S4-S6).
Figure 4. Cu–NTs core-capture starting from N1. The temperature is raised
from -78 °C to -42 °C to initiate the reaction and form N2. See supporting
information for details.
In summary, four distinct copper nitrenes, featuring tailored
bis(pyrazolyl)methane ligands, were characterised regarding their
stability, spectroscopy and reactivity. The herein reported
unprecedented copper N-tosyl nitrenes exhibit a high thermal
stability for terminal copper tosyl nitrenes, which exceeds that of
the Sc3+ stabilised system of Ray et al.[13] and preserve the
monometallic structure. The compounds have been investigated
by means of UV/Vis, cryo-UHR-ESI mass spectrometry and DFT
and thereby assigned as terminal copper nitrenes. A mild catalytic
procedure for the C–H amination of cyclohexane and toluene has
been developed, underpinning the C–H activating capability of the
investigated copper nitrenes. The latest discoveries on the copper
tosyl nitrene mediated formation of sulfonamides and isothiazoles
further corroborate the importance of this copper nitrene subclass.
All in all, we were able to fill the gap for the long-standing search
of terminal copper nitrenes and place bis(pyrazolyl)methane-
supported copper nitrenes at the forefront of direct C–H
aminations.
Scheme 1. Catalytic direct amination of cyclohexane and toluene. Reaction
conditions: catalyst C1 or C4 (0.01 mmol); SPhINTs (0.1 mmol); PhCl (0.15-
1 mL); molecular sieves (3 Å). The yield was determined by 1H NMR
spectroscopy, using an internal standard. See supporting information for details.
Acknowledgements
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10.1002/anie.201713171
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Financial support by the Deutsche Forschungsgemeinschaft in
the framework of the SFB 985 (Project A1), the SFB749 (Project
B10), the RWTH Aachen in form of a Seed fund and the "Solar
Technologies Go Hybrid" intiative by the State of Bavaria is
gratefully acknowledged.
Keywords: terminal copper nitrenes• catalysis • C–H insertion •
kinetics • N donor ligands • amination
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10.1002/anie.201713171
Angewandte Chemie International Edition
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Abstract: Four distinct terminal copper
tosyl nitrenes have been stabilised at low
temperatures from heteroscorpionate
ligands of the bis(pyrazolyl)methane
family. They were investigated by UV/Vis
Julian Moegling, Alexander Hoffmann,
Fabian Thomas, Nicole Orth, Patricia
Liebhäuser, Ulrich Herber, Robert
Rampmaier, Julia Stanek, Gerhard Fink,
Ivana Ivanović-Burmazović, Sonja
Herres-Pawlis
spectroscopy,
cryo-UHR-ESI
mass
spectrometry and DFT. A mild catalytic
procedure has been developed, enabling
the C–H amination of cyclohexane and
toluene as well as the aziridination of
styrenes.
((Insert TOC Graphic here: max.
width: 5.5 cm; max. height: 5.0 cm))
Page No. – Page No.
Designed to react: Terminal Copper
Nitrenes and their Application in
Catalytic C–H Aminations
This article is protected by copyright. All rights reserved.