Copper-Catalyzed Aziridination with Redox-Active Ligands: Molecular Spin Catalysis

Article abstract of DOI:10.1002/chem.201705649

Small-molecule catalysts as mimics of biological systems illustrate the chemists’ attempts at emulating the tantalizing abilities displayed by nature's metalloenzymes. Among these innate behaviors, spin multistate reactivity is used by biological systems as it offers thermodynamic leverage towards challenging chemical reactivity but this concept is difficult to translate into the realm of synthetic organometallic catalysis. Here, we report a rare example of molecular spin catalysis involving multistate reactivity in a small-molecule biomimetic copper catalyst applied to aziridination. This behavior is supported by spin state flexibility enabled by the redox-active ligand.

Full text of DOI:10.1002/chem.201705649

A Journal of
Accepted Article
Title: Copper-catalyzed aziridination with redox-active ligands:
molecular spin catalysis
Authors: Yufeng REN, Khaled CHEAIB, Jeremy JACQUET, Hervé
VEZIN, Louis FENSTERBANK, Maylis ORIO, Sébastien
BLANCHARD, and Marine Desage-El Murr
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To be cited as: Chem. Eur. J. 10.1002/chem.201705649
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Copper-catalyzed aziridination with redox-active ligands:
molecular spin catalysis
Yufeng Ren, [a] Khaled Cheaib,[a] Jérémy Jacquet,[a] Hervé Vezin,[b] Louis Fensterbank,[a] Maylis Orio,[c]
Sébastien Blanchard[a] and Marine Desage-El Murr[a,d]
*
Abstract: Small-molecule catalysts as mimics of biological systems
illustrate the chemists’ attempts at emulating the tantalizing abilities
displayed by Nature’s metalloenzymes. Among these innate
behaviours, spin multistate reactivity is used by biological systems
as it offers thermodynamic leverage towards challenging chemical
reactivity but this concept is difficult to translate into the realm of
synthetic organometallic catalysis. Here we report a rare example of
molecular spin catalysis involving multistate reactivity in a small-
molecule biomimetic copper catalyst applied to aziridination. This
behaviour is supported by spin state flexibility enabled by the redox-
active ligand.
the latter favors a stepwise mechanism through radical
intermediates. Copper nitrene species are elusive[6] and the
need to isolate metal-nitrene adducts introduces a disconnect
between off-site spectroscopic interrogation of their electronic
structure and their actual involvement in the catalytic cycle.
Redox-active ligands have received sustained attention due to
their ability to perform biologically inspired transformations
implying ligand-based electronic participation akin to that
encountered in metalloenzymatic redox co-factors. This
research area stems from an early interest in bioinspired
catalytic systems and has morphed into broader synthetic
pursuits.[7] The use of cobalt (II)–porphyrin complexes for nitrene
transfer has been reported by Zhang and de Bruin and
mechanistic studies have provided information on key
porphyrin–Co(III)–nitrene intermediates involved in which the
porphyrin ligand acts as a redox-active ligand in a bis-nitrene
species.[8] A well-established family of redox-active ligands are
aminophenols[9] which can shuttle between three distinct redox
states: amidophenolate (AP), iminosemiquinone (SQ), and
Of all the tricks up Nature’s sleeve, multistate reactivity[1] as a
means to control reactivity within enzymatic active sites is
perhaps among the most astute and difficult behaviour to
emulate. Metalloenzyme-promoted mitigation of reactivity with
stability is handled through the occurrence of spin-flip events,
mostly metal-based, for example from a singlet to a triplet state.
A classic example of such reactivity is found in epoxidation of
double bonds performed by cytochrome P450.[1a,2] Metal-
catalysed aziridination –which, unlike epoxidation, does not have
a biological counterpart– relies on the use of noble metals such
as Rh and Ru or cheaper first-row transition metals as Fe, Co,
Ni, Cu and Mn.[3] Recent work from the Arnold group on
aziridination performed by an engineered bacterial cP450
obtained through in vitro evolution casts a light on the productive
merger of bioinspired systems and bona fide synthetic
applications.[4] Because aziridination is electronically closely
related to epoxidation, for which metal-oxo complexes of first
row transition metals have proved highly efficient, chemists have
developed synthetic approaches relying on metals in conjunction
with nitrene precursors. While the mechanism remains a subject
of debate, a consensus revolves around the implication of metal-
nitrene species, whose electronic structure can be described as
either metal nitride M=N or metal-nitrene radical M-N!.[5] The
former is usually associated with a concerted mechanism while
iminobenzoquinone (BQ).[10] Complex 1 Cu(LSQ
) is a stable
2
three-spins system with a CuII centre and two biradical SQ
species[11] which has been shown to promote ligand-based
redox events.[12] Here, we report on the ability of this three-spins
system to carry out aziridination through molecular spin
catalysis,[13] wherein the redox-active ligands mediate electronic
transfer from the nitrene source to the metal complex.
Our initial studies began with the observation that complex 1 can
perform aziridination of 4-chlorostyrene. The influence of the
nitrene source was first investigated (Table 1). In the presence
of 5 mol% of complex 1, an excess of 4-chlorostyrene (4 equiv.)
in dichloromethane at room temperature, tosyliminoiodinane
(entry 3) gives access to aziridine 2 in 65 % yield, whereas
chloramine-T (entry 1) and sulfonyle azide (entry 2) are not
converted. An increase in substrate concentration, from 1 M to 5
M, enabled to obtain 78 % of aziridine 2 (entry 4). Similar yields
were also obtained when diminishing the catalytic load to 2
mol% (entry 5). Then, we turned our attention to the reaction
time: after 2 hours, iminoiodinane was totally converted and
aziridine 2 was obtained in 89 % yield with 1 mol% of catalyst
(entry 6), and in 90 % yield with 0.5 mol% of catalyst (entry 7),
thus demonstrating the rapid evolution of the reaction. The
reaction is not air-sensitive (entry 8). Switching dichloromethane
for acetonitrile afforded reduced reaction times as the reaction
was complete in 20 min with a quantitative yield of aziridination
substrate (entry 9). This could be due to degradation or side
reactivities of the aziridination product under prolonged reaction
conditions, although no other product was detected.
[a]
[b]
Y. Ren, Dr K. Cheaib, J. Jacquet, Prof. L. Fensterbank, Dr S.
Blanchard, Prof. M. Desage-El Murr
Sorbonne Universités, UPMC, Université Paris 06,UMR CNRS
8232, Institut Parisien de Chimie Moléculaire, France.
E-mail: marine.desage-el_murr@upmc.fr
Dr H. Vezin
Laboratoire de Spectrochimie Infrarouge et Raman, Université des
Sciences et Technologies de Lille, UMR CNRS 8516, 59655
Villeneuve d’Ascq Cedex, France.
[c]
[d]
Dr M. Orio
Aix Marseille Université, CNRS, Centrale Marseille, iSm2 UMR
7313, 13397 Marseille cedex 20, France.
Prof. M. Desage-El Murr
Institut de Chimie, Université de Strasbourg,
1 rue Blaise Pascal 67000 Strasbourg, France.
E-mail : desageelmurr@unistra.fr
complex 1 (x mol%)
nitrene source (1 equiv.)
NTs
Supporting information for this article is given via a link at the end of
the document.
CH2Cl2, rt
Cl
Cl
2
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Table 1. Optimization conditions for the aziridination of 4-chlorostyrene
catalyzed by complex 1 Cu(LSQ)2.
In UV-Vis-NIR, complex 1 [Cu(LSQ)2] displays a characteristic
intervalence charge transfer (IVCT) band at 795 nm (Fig. 1).
Upon addition of 4-chlorostyrene, no evolution is detected.
Addition of increasing amounts of PhINTs (1, 2, 3 and 10 equiv.)
induces the disappearance of the IVCT band, which reaches its
minimum value upon addition of 2 equiv., followed by the growth
of a new broad band shifted to 720 nm. These results suggest
the consecutive formation of two distinct complexes, presumably
corresponding to mono- and bis-nitrene species,[8] upon addition
of increasing amounts of PhINTs. From TDDFT calculations, the
low-energy transition in the bis-nitrene spectrum can be
assigned to a interligand charge transfer rather than a IVCT
between the two nitrene as the donor state involves
predominantly the copper center and the nitrene ligands while
the acceptor state is distributed over the benzoquinone moieties
(see SI). Reaction between PhINTs and complex 1 [Cu(LSQ]2] is
very slow to reach its equilibrium. Spectra recorded for 0.5 and 1
equiv. mixtures show that these two species coexist under
substoichiometric conditions (See SI for detailed studies and
DFT).
entry[a]
[Styrene]
1 M
Nitrene source
time
18h
Catalyst
(x mol%)
Yield
(%)[b]
AcN
H
SO2N3
1
5
-
2
1 M
1 M
5 M
5 M
5 M
5 M
5 M
5 M
Chloramine-T
PhINTs
5
18h
18h
18h
18h
2h
-
3
5
65
78
80
89
90
87
99
4
PhINTs
5
5
PhINTs
2
6
PhINTs
1
7
PhINTs
0.5
1
2h
8[c]
9[d]
PhINTs
2h
PhINTs
1
20 min
[a]
[b]
4 equiv. styrene, NMR yields, calculated considering nitrene source as
limiting reactant, [c] Reaction performed under air, [d] Acetonitrile as solvent.
The scope of the reaction was assessed on a variety of
substrates and found to be quite large. Mono-substituted
aziridines were obtained in excellent to moderate yields
depending on the Csp2 or Csp3 hybridization of the substituting
group (substrates 2, 3, 10). Di-substituted aziridines could be
isolated with yields up to 86% (4, 5, 6, 7, 8, 11, 13, 14) and
several motifs can be accommodated (aldehyde, ester, ketone).
Also, the methodology could be extended to more challenging
gem-disubstituted (12) and tri-substituted (9, 15, 16) scaffolds.
Running the reaction on geranyl acetate, which possesses two
triply-substituted double bonds afforded an overall aziridination
yield of 67% divided between products 15 (15%) and 16 (52%),
the latter is preferentially formed presumably for steric reasons.
Figure 1. UV-Vis-NIR spectra of complex 1 and upon addition of increasing
amounts of PhINTs in DCM under argon atmosphere ([1]= 0.075 mM).
NTs
NTs
NTs
NTs
Cl
F
3, 97%
2, 99%
5, 86% 2h
4, 65% 2h
The X-band spectrum of complex 1 in solution at 293 K displays
a main feature around g=2, which corresponds to an apparent
S=1/2 ligand centered radical (fig S13). Evolution of this EPR
spectrum upon reaction with PhINTs was monitored for three
distinct substoichiometric nitrene to copper ratios (0.25:1, 0.5:1
and 0.75:1) (Fig. 2A). RT spectrum for the lowest concentration
displays a five lines pattern centered at g=2.0042, indicating a
species (presumably mono-nitrene 17) with two quasi equivalent
nitrogens coupled to an organic radical. Under stoichiometric
conditions (Fig. 2B), a drastic change in the EPR spectrum
occurs, indicating the formation of a new species, presumably
bis-nitrene 18. At the 1:1 ratio, the RT spectrum is typical for a
Cu(II) species. Increasing PhINTs (2 to 5 equiv.) results in an
increase of this signal. Under catalytic conditions (Fig. 2C), a
O
O
NTs
NTs
O
H
N
N
Ts
Ts
6, 86% 3h
7, 73% overnight
8, 23% overnight
9, 29% overnight
O
TsN
TsN
TsN
NTs
11, 12% overnight
13, 47% overnight
10, 36% overnight
12, 81% 5h
O
O
O
NTs
O
NTs
NTs
15, 15% overnight
16, 52% overnight
14, 44% overnight
copper pattern with Aiso
= 70 Gauss and a g factor of 2.13
Cu
Scheme 1. Scope of the aziridination performed under optimized conditions.
remain throughout the reaction, thus indicating no change in the
environment of the copper complex. After 20 hours, the copper
(II) signal disappears and the five lines pattern is observed,
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10.1002/chem.201705649
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consistent with regeneration of mono-nitrene 17, which seems to
be the resting state of the catalysis.
T0
T=1h30
T=3H
A)
0,25_1
0,25_1
0,5_1
B)
C)
0,5_1
0,75_1
0,75_1
1_1
1_1
2.5_1
2.5_1
5_1
5_1
3200
3300
3400
3500
3600
3700
2700 3000 3300 3600 3900 4200
2500
3000
3500
4000
Magnetic Field [G]
Magnetic Field [G]
Magnetic Field [G]
D)
35Cl coupling
observed
Cl
insertion product
Ts
Ph
N
tBu
II
tBu
O
N
N
Cu
O
tBu
tBu
Ph
19
Figure 2. X-band EPR studies of A) formation of the mono-nitrene species 17 under substoichiometric conditions, B) formation of bis-nitrene species 18, C)
aziridination reaction under operando conditions, D) 2D HYSCORE spectra recorded at 5K. Conditions: 5 mM [Cu] in CH2Cl2.
While catalysis occurs at room temperature, HYSCORE
experiments were performed at 5K in order to get structural
insights on the various species involved. For 1 alone, a weak
coupling signal with proton and carbon is observed in the
quadrant (+,+) (Fig. S10). Additionally, a 14N Aiso = 3.2 MHz is
measured in the (-,+) quadrant. At low PhINTs concentration, a
15 MHz coupling due to nitrogen is observed in both quadrants
(Fig. S11). For 5 equiv., a new nitrogen correlation peak appears
in the (-,+) quadrant, indicating that a second nitrene has been
incorporated (Fig. S12). Additionally, a coupling with 1H is
observed with a maximum value of 11 MHz arising from the tosyl
group. The spectra of Figures S11 and S12 are thus
representative of mono-nitrene 17 and bis-nitrene 18,
respectively. A HYSCORE spectrum recorded at 5K on frozen
reaction aliquots displays a new ligand-centered S=1/2 signal
and only weak 13C and 1H couplings in the (+,+) quadrant.
Interestingly, in the low frequency region, we can observe two
peaks centered at 1.4 and 2.8 MHz respectively, arising from a
coupling with the chlorine atom of 4-chlorostyrene (Fig. 2D). This
spectrum is in agreement with the formation of intermediate 19,
as proposed in the catalytic cycle (Fig. 3), resulting from the
reaction of the nitrene radical in 17 with the alkene.
DFT calculations were undertaken to gain insight into the
species observed during catalysis. The structures of the putative
species were subjected to geometry optimization. Their
electronic properties were investigated using Broken-Symmetry
calculations to evaluate the ground spin and first excited states
of each intermediate. Complex 1 displays a quasi square planar
geometry (Fig. S16) and the spin density plot (Fig. S17) is
compatible with a Cu(II) center bound to two redox-active
iminosemiquinone (SQ) ligands. In agreement with previous
studies[11,12b] the system is characterized by a doublet S=1/2
ground spin state due to strong antiferromagnetic coupling
between the SQ moieties (Table S1). A quartet S=3/2 state is
close enough in energy to be thermally accessible, as
experimentally observed by SQUID technique.[11] The theoretical
structure of complex 17 is trigonal bipyramidal (Fig. S18) and its
electronic structure shows a Cu(II) center interacting with an SQ
moiety and a NTs radical (Fig. S19). The computed geometry for
17 does not allow for an IVCT band neither between the NTs
radical and the SQ ligand nor between the SQ and BQ ligands in
agreement with UV-vis data.
A ferromagnetic interaction
dominates in 17 resulting in a quartet S=3/2 ground spin state
and the energetic separation with the first doublet S =1/2 state is
consistent with its observation at RT (Table S1). Complex 18
displays an octahedral geometry (Fig. S20) and the spin density
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plot (Fig. S21) is compatible with a Cu(II) center bound to two
NTs radicals. The system is characterized by a doublet S=1/2
ground spin state due to antiferromagnetic coupling between the
NTs moieties (Table S1). The theoretical structure of complex 19
is also trigonal bipyramidal (Fig. S22) and its electronic structure
shows a Cu(II) center interacting with an SQ moiety and a
styrene-based radical (Fig. S23). An antiferromagnetic
interaction dominates in 19 resulting in a doublet S=1/2 ground
spin state in agreement with HYSCORE measurements (Table
S1).
Together with DFT calculations, variable temperature EPR
experiments (see SI) indicate that all the species (1, 17, 18, 19)
involved in the reaction have a complex electronic structure due
to the interaction of two ligand radicals (S=1/2) and one metal
centre (S=1/2), leading to a mixture of populated spin states at
room temperature. Small changes in the coordination sphere
have been shown to have an impact on the spin states of related
complexes.[14] Interestingly, while starting complex 1, bis-nitrene
18 and intermediate 19 present a doublet ground state, mono-
nitrene 17 has a quadruplet ground state. In a classical
approach, reaction between [Cu(LSQ)2] and PhINTs to form
mono-nitrene 17 should be spin forbidden, and thus extremely
slow. However, due to its unique electronic structure, the excited
state S=1/2 of 17 becomes accessible, as attested by the
observed radical centered S=1/2 signature at RT (Fig. S14).
Hence, through multistate configuration, the reaction becomes
spin allowed, ensuring fast kinetics for each step. The
implication of redox-active ligands in spin-forbidden processes
has been discussed by de Bruin.[15] We thus propose the
following mechanism in which each intermediate has been
spectroscopically identified, and the electronic structures
confirmed by DFT calculations (Fig. 3). Two-state reactivity of
copper-nitrenes involving two spins systems has been studied
by Pérez,[16] Norrby[17] and Ray in aziridination,[18] and by Ray in
alkane hydroxylation.[19] Norrby has proposed a CuI/CuIII redox
cycle in aziridination. Strikingly, in our system, a CuII oxidation
prevails throughout the catalytic cycle and this metal redox
homeostasis is enabled by the fact that redox variations are all
ligand-based.
In conclusion, we report here a rare example of molecular spin
catalysis in copper-catalyzed aziridination performed by
a
copper complex bearing two redox-active iminosemiquinone
ligands. This unique behaviour is enabled by the spin fluxionality
exhibited by the starting complex and the intermediates involved
in the reaction, which is allowed by the redox-active nature of
the ligands. This work showcases the ability of redox-active
catalytic manifolds to perform otherwise well-known reactions
through distinct and unconventional mechanisms and opens
wide perspectives in redox and biomimetic catalysis.
Figure 3. Mechanistic proposal for the aziridination reaction outlining the spin configuration of the involved species with DFT inserts. SQ: iminosemiquinone, BQ:
iminobenzoquinone.
Ph
tBu
rt
tBu
complex 1
O
N
N
O
CuII
.
.
.
.
SQ
II SQ
Cu
tBu
SQ CuII SQ
4K
tBu
Ph
PhI NTs
PhI
Ts
N
Ts
Ph
I
NTs
PhI
Ph
Ph
tBu
tBu
N
II
Cu
tBu
O
N
N
II
tBu
O
N
N
Cu
O
tBu
O
tBu
N
tBu
Ph
tBu
Ph
Ts
mono-nitrene 17
bis-nitrene 18
NTs
NTs
NTs
CuII
SQ
BQ
BQ
BQ
NTs
CuII
Cl
Cl
PhI NTs
Cl
Ts
Ph
N
tBu
II
tBu
O
N
N
O
Cu
tBu
tBu
Ph
inserted substrate 19
Ar
CuII
SQ
BQ
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Acknowledgements
The authors thank UPMC, CNRS, IR-RPE CNRS FR3443
RENARD network (CW X-band EPR with Dr. J.-L. Cantin, INSP
UMR CNRS 7588, UPMC and X-band EPR in Lille). Sorbonne
Universités is acknowledged for an Emergence Sorbonne
Universités grant (MDEM and KC) and CSC for a PhD fellowship
(YR). The authors gratefully acknowledge the COST Action 27
CM1305 ECOSTBio (Explicit Control Over Spin-States in
Technology and Biochemistry).
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Keywords: redox-active ligand • copper catalysis • aziridination
• spin catalysis • multi-state reactivity
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Yufeng Ren, Khaled Cheaib, Jérémy
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