C?N Bond Formation from a Masked High-Valent Copper Complex Stabilized by Redox Non-Innocent Ligands

Article abstract of DOI:10.1002/anie.201605132

The reactivity of a stable copper(II) complex bearing fully oxidized iminobenzoquinone redox ligands towards nucleophiles is described. In sharp contrast with its genuine low-valent counterpart bearing reduced ligands, this complex performs high-yielding C?N bond formations. Mechanistic studies suggest that this behavior could stem from a mechanism akin to reductive elimination occurring at the metal center but facilitated by the ligand: it is proposed that a masked high oxidation state of the metal can be stabilized as a lower copper(II) oxidation state by the redox ligands without forfeiting its ability to behave as a high-valent copper(III) center. These observations are substantiated by a combination of advanced EPR spectroscopy techniques with DFT studies. This work sheds light on the potential of redox ligands as promoters of unusual reactivities at metal centers and illustrates the concept of masked high-valent metallic species.

Full text of DOI:10.1002/anie.201605132

Angewandte
Communications
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International Edition: DOI: 10.1002/anie.201605132
German Edition: DOI: 10.1002/ange.201605132
Ligand Effects
À
C N Bond Formation from a Masked High-Valent Copper Complex
Stabilized by Redox Non-Innocent Ligands
Jꢀrꢀmy Jacquet, Pauline Chaumont, Geoffrey Gontard, Maylis Orio, Hervꢀ Vezin,
Sꢀbastien Blanchard, Marine Desage-El Murr,* and Louis Fensterbank*
Abstract: The reactivity of a stable copper(II) complex bearing
fully oxidized iminobenzoquinone redox ligands towards
nucleophiles is described. In sharp contrast with its genuine
low-valent counterpart bearing reduced ligands, this complex
So-called non-innocent or redox ligands[7] are molecular
scaffolds which are able to delocalize spin density in metal
complexes, and this specific behavior could lead to the
stabilization of formal higher metallic oxidation states
through electronic redistribution.[7d] Focusing on the well-
established family of amidophenolate and related ligands,[8]
we have recently reported[9] (Scheme 1) that oxidation of the
À
performs high-yielding C N bond formations. Mechanistic
studies suggest that this behavior could stem from a mechanism
akin to reductive elimination occurring at the metal center but
facilitated by the ligand: it is proposed that a masked high
oxidation state of the metal can be stabilized as a lower
copper(II) oxidation state by the redox ligands without
forfeiting its ability to behave as a high-valent copper(III)
center. These observations are substantiated by a combination
of advanced EPR spectroscopy techniques with DFT studies.
This work sheds light on the potential of redox ligands as
promoters of unusual reactivities at metal centers and illus-
trates the concept of masked high-valent metallic species.
T
he advent of catalytic manifolds relying on highly oxidized
organometallic species is currently an area of acute interest in
catalysis. The rich opportunities offered by such metallic
complexes have been showcased in the case of several metals
and among the most promising so far are copper(III),[1,2]
palladium(III) and palladium(IV),[1,3] as well as nickel(III)
and nickel(IV)[4] derivatives. These high-valent molecular
edifices display distinct selectivity from their lower valent
counterparts and can perform catalytically notoriously chal-
lenging events.[5] Mechanistic studies on these species are
difficult and stoichiometric studies of these high-valent
complexes can provide fundamental insights towards the
rationalization of their reactivity. High-valent species are
typically generated upon oxidation of a metal center with
strong oxidants such as hypervalent iodine and/or CF3+ or F+
sources.[4b,d,6]
Scheme 1. Distinct reactivity from two copper(II) centers. SQ: iminose-
miquinone ligand, BQ: iminobenzoquinone ligand. Tf=trifluorometha-
nesulfonyl.
copper(II) complex 1[10] with an electrophilic source of CF3
+
[S-(trifluoromethyl)-dibenzothiophenium triflate; TDTT]
affords the complex 2 in which the copper oxidation state
has been preserved as copper(II) while the 2eÀ oxidation is
sustained by the ligands (see the Supporting Information for
computational data). While originating from a CF3+ source, 2
undergoes internal transfer of CF3À in solution at electrophilic
sites, thus suggesting that the redox ligands can perform
a formal umpolung of the CF3 moiety. Furthermore, in the
presence of a radical acceptor, ligand-based single-electron
transfer (SET) in 1 allows the controlled generation of CF3C
radicals, thus enlarging the synthetic potential of this family of
complexes.[11]
We reasoned that the propensity of these ligands to
promote ligand-based (as opposed to metal-based) redox
events could be interfaced with the reactivity of metallic high-
valent oxidation states. Interestingly, redox non-innocent
ligands have already been used in a related but opposite
strategy aiming at preparing masked palladium(0) com-
plexes.[12] Metallic high-valent oxidation states are known to
behave as electrophiles and, upon investigating the reactiv-
ities of 1 and 2 towards nucleophiles, we found that while
being spectroscopically identified as a copper(II) complex, 2
behaves as a high-valent copper(III) species and performs
[*] J. Jacquet, P. Chaumont, G. Gontard, Dr. S. Blanchard,
Dr. M. Desage-El Murr, Prof. L. Fensterbank
Sorbonne Universitꢀs, UPMC, Universitꢀ Paris 06, UMR CNRS 8232
Institut Parisien de Chimie Molꢀculaire (France)
E-mail: marine.desage-el_murr@upmc.fr
louis.fensterbank@upmc.fr
Dr. M. Orio
Aix Marseille Universitꢀ, CNRS, Centrale Marseille, iSm2 UMR 7313
13397 Marseille cedex 20 (France)
Dr. H. Vezin
Laboratoire de Spectrochimie Infrarouge et Raman
Universitꢀ des Sciences et Technologies de Lille, UMR CNRS 8516,
59655 Villeneuve O’Ascq Cedex (France)
Supporting information and the ORCID identification numbers for
the authors of this article can be found under http://dx.doi.org/10.
1002/anie.201605132.
À
high-yielding C N bond formations with carbon nucleophiles
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to yield the N-arylated products 3. This reactivity is in stark
contrast with that of 1, which exhibits lower reactivity
(Scheme 1). The highly oxidized state of the ligands is
therefore key to the enhanced reactivity of 2.[8c,k]
tion revealed that the reaction is complete after 45 minutes
(entry 5, almost full conversion observed after 10 min).
Substituting PhBF3K with phenylboronic acid gave good
yield and fewer by-products (entry 7), and was therefore
selected for the rest of our study. Related complexes 1 [CuII-
(LSQ)2] and 6 [CuII(LBQ)2(OTf)2] (Scheme 2; see the Support-
ing Information for preparation) were found to give much
lower yields (Table S1, entries 8 and 9) for this transforma-
tion. This outcome is expected in the case of 1, which is not
electrophilic enough, but in the case of 6 it is an indication
that a high oxidation state of the ligands alone cannot account
for the observed reactivity, and underlines the importance of
the CF3 moiety in this reactivity.
Structural confirmation of the N-arylation products 3d,
3 f, 3g, and 3h (Table 1) was obtained through single-crystal
X-ray diffraction (Figure 1; see the Supporting Information).
However, switching to Csp3 partners (alkyl boronic acids) was
not successful. The influence of different electronic substitu-
tions on the redox ligand was investigated (Table 2) by
preparing analogues of 2 bearing diverse aryl substitutions on
the nitrogen center (7–10: Ar= 4-CH3(C6H4): 7; 4-CF3(C6H4):
8; 4-Cl(C6H4): 9; 3,5-(MeO)2(C6H3): 10). The resulting
N-arylated products (3c, 3e, 11, and 12) were isolated and
the best yields were observed for electron-deficient aryl
moieties (3c and 3e).
À
Copper-catalyzed or copper-mediated formation of C O
À
and C N bonds involving an aryl electrophile and an amine is
known as the Ullmann coupling.[13] The mechanism of this
transformation is still a topic of investigation. This reaction
was shown to involve high-valent copper(III) species and
recent reports showed that a radical pathway is possible under
photoinduced conditions.[14] Copper(III) species have also
been postulated in the related Chan–Lam–Evans reaction, an
oxidative coupling which involves reverse electronics with
a carbon nucleophile and either an amine or alcohol.[15]
Although often invoked as intermediates in copper-catalyzed
À
transformations such as C H activation and cross-coupling,
transient copper(III) species are mostly elusive and difficult
to characterize.[2] However, detailed studies based on well-
defined stable macrocyclic aryl/copper(III) complexes have
provided a wealth of reactivity and mechanistic insights on
these copper-catalyzed transformations.[16] Notably, these
studies have shown that the reductive elimination step from
aryl/copper(III) complexes with a nucleophile occurs easily.
These results have unambiguously established the viability of
aryl/copper(III) intermediates in Ullmann reactions, and
suggest they could also be operative in the Chan–Lam–
Evans coupling, a reaction our transformation is reminiscent
of.
The reaction is complete in around 45 minutes. Investiga-
tion of the reaction medium at 708C by UV-vis studies (see
Figure S1) showed consumption of 2, followed by an intensity
increase around l = 270 nm, thus suggesting formation of new
Preparation of 2 was previously reported by our team by
oxidation of 1 with TDTT.[9] However, an optimized synthesis
was devised based on the use of non-commercially available
but easily synthesized 3-cyclopropyl-1-trifluoromethylbenzo-
thiophenium triflate (see the Supporting Information).[17]
Optimization studies for the N-arylation reaction were
conducted (Scheme 2; see Table S1 in the Supporting Infor-
Table 1: Scope of nucleophiles for the N-arylation reaction.
Scheme 2. Product distribution arising from N-arylation reaction.
1
Yields were determined by H NMR spectroscopy. Yields of isolated
mation) as the expected product 3a is obtained along with the
products 4 and 5, which were previously reported and arise
from internal CF3À transfer on either of the proximal carbon
atoms to the copper center.[9] The compound 5 is the most
stable and prolonged heating of 4 in acetonitrile results in full
conversion into 5. Initial attempts with phenyl potassium
trifluoroborate showed that the reaction occurs in either THF
or AcOEt (albeit in lower yield) but that acetonitrile inhibits
the formation of 3a (Table S1, entries 1–3). The reaction can
be carried out under either argon or air, and increasing the
amount of the nucleophile to 1.6 equivalents afforded
a higher yield (entry 4) while using 2 equivalents did not
produce significantly better results (entry 6). Time optimiza-
products given within parentheses.
Figure 1. X-ray structures of compounds 3d, 3 f, 3g, and 3h. Hydrogen
atoms are omitted for clarity.
2
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Table 2: Variation of the aryl substitution on the redox ligand and X-ray
structure of 12.
substrate-based radicals. Besides, 2 alone does not react with
TEMPO,[11] thus ruling out the possibility of an initial electron
transfer between TEMPO and 2. Mass spectrometry per-
formed on the reaction mixture allowed the observation of
a species corresponding to [(LBQ)2Cu(CF3)(Ph)] (see the
Supporting Information). The molecular formula was vali-
dated by high-resolution MS (HRMS, observed: m/z
799.34831, calculated: m/z 799.35062) and proves the incor-
poration of the nucleophilic aryl moiety in the complex, thus
providing strong evidence for an inner-sphere mechanism.
Competition experiments were performed with added exog-
enous ligands and addition of increasing amounts of triphe-
nylphosphine was found to inhibit the reaction. This obser-
vation suggests that a lack of available coordination sites on
the copper center shuts down the reaction and further
supports an inner-sphere mechanism.
1
Yields were determined by H NMR spectroscopy. Yields of isolated
products given within parentheses. Hydrogen atoms are omitted for
clarity.
aromatic species. From a mechanistic point of view the
This reaction involves several paramagnetic species and
EPR spectroscopy can provide useful insights on the mech-
anisms at stake in homogeneous catalysis with paramagnetic
complexes bearing redox non-innocent ligands.[18] EPR stud-
ies were first performed on a solution of 2 alone, which
À
reaction of a carbon nucleophile to form a C N bond through
direct SN2 attack seems very unlikely as the nitrogen atom in 2
does not bear electrophilic reactivity. Other mechanistic
possibilities for this transformation include an outer-sphere
radical-type mechanism. However, when adding up to
5 equivalents of TEMPO (2,2,6,6-tetramethylpiperidine-N-
oxyl radical), the N-arylation product was isolated in 74%
yield, thus suggesting that the reaction does not involve
undergoes intramolecular CF3 transfer to afford
5
(Scheme 3).[9] Interestingly, small changes of its room-temper-
ature EPR signature are observed between dichloromethane
and THF solutions, thus indicating that coordination of an
Scheme 3. Proposed mechanisms rationalizing the observed masked high-valent reactivity of 2 in a) an intramolecular CF3 transfer (insets: EPR
and DFT studies on the intermediate 14 and X-ray structure of 5) and b) the N-arylation reaction (insets: HRMS, and DFT studies on the
intermediates 16 and 18, and X-ray structure of 3d. Hydrogen atoms are omitted for clarity.
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exogeneous ligand, here THF, on the copper center is
probable. Indeed, the main features of the THF spectrum
remain characteristic of a copper(II) species in solution, with
the typical four lines resulting from the hyperfine coupling of
S = 1/2 (d9 CuII) with its nuclear spin I = 3/2. A giso value of
2.11 and hyperfine coupling ACu = 68 G respectively are
measured. The spectral shape observed is due to the tumbling
effect.[19] Similar signals are measured in DCM solution.
Additionally, a weak quadruplet signal (intensity 1:3:3:1,
centered at g = 2.003) overlaps with the copper signal in THF.
Upon heating (608C), we can first observe a decrease of
copper(II) signal without any change in g and A values, thus
indicating that only redox reactions occur with no symmetry
change. Over the course of time at 608C (see Figure S2), we
can observe a growing signal (g = 2.010) resulting from the
formation of a ligand-based organic radical, while the
quadruplet signal disappears. The hyperfine structure of this
new ligand-based signal is more complex (insert), and its
the copper(I) species 18. DFT calculations also support this
copper(I)/radical SQ ligand electronic state in 18 rather than
a copper(II) center with a closed-shell BQ ligand species 17.
The intermediate 18 is characterized by the decoordination of
the amine bearing the phenyl and is 3.0 kcalmolÀ1 higher in
energy compared to 16. The alternative geometry in which the
phenyl would bind the oxygen center, 18*, has also been
considered but was found to be unfavorable. Final decoordi-
nation of the N-arylated amidophenolate occurs along with
the formation of an unidentified copper(II) species. Similarly,
transmetalation of a boronic acid occurring at a copper(III)
center followed by reductive elimination, was recently
reported by Grushin and co-workers.[20] In that case, the
À
geometry of the intermediate allows Ph CF3 reductive
elimination to occur, a step which is not possible in our
present system because of the relative trans positions of the
phenyl and CF3 moieties. Also, iminosemiquinone ligands
À
have been implicated in Ph Ph reductive elimination from
simulation [(14N) = 17.55 G, A(1H) = 38.94 G and A(1H)tBu
=
a zirconium (ZrIV) complex.[8i,21]
2.93 G; see Figure S3] is in agreement with an N-centered
iminosemiquinone radical species (14), presumably still
coordinated to the copper center, and resulting from the
In conclusion, we have shown that 2, spectroscopically
a copper(II) complex bearing fully oxidized iminobenzoqui-
none ligands, behaves as a copper(III) species towards carbon
À
À
intramolecular CF3 attack onto the other LBQ ligand
nucleophiles, thus performing C N bond formation. The
(Scheme 3a). DFT calculations confirmed that the electronic
structure of 14 is consistent with a copper(III) center bearing
an LSQ ligand and that this species is found to be 8.8 kcalmolÀ1
higher in energy with respect to 2. The structure of 5 was
confirmed by single-crystal X-ray diffraction.
reactivity exhibited by 2 can be rationalized through ligand-
based redox mechanisms implicating masked high-valent
intermediates and arising from initial oxidation of 1 with
a CF3+ source. The role of electrophilic fluorination sources as
by-standers in the oxidation of metallic centers and promot-
ing reductive elimination has been rationalized by Yu and co-
workers in the context of palladium and gold chemistry.[6] The
present work hints at a similar behavior as evidenced by EPR
measurements and DFT calculations. The complex 2 is a rare
example of a stable, masked, high-valent copper complex and
this unprecedented reactivity opens the way towards new
developments in high-oxidation-state copper chemistry.
Addition of PhB(OH)2 in the solution does not modify the
spectrum recorded at room temperature. However, upon
heating at 608C, evolutions are observed. In the first stage,
over about a 15 minute timespan, modification of the copper-
(II) species spectrum is observed, notably with the disappear-
ance of the copper signal at giso ꢀ 2.09 along with conforma-
tional changes and a giso ꢀ 2.15 at the final state (see Fig-
ure S4). No changes are measured for copper hyperfine
coupling. The weak quadruplet signal at g ꢀ 2.003 also quickly
disappears and no signature of organic radical is detected. The Experimental Section
CCDC 1471618 (3d), 1471619 (3 f), 1471620 (3g), 1471621 (3h),
remaining copper(II) signature then progressively diminishes
in intensity, and almost completely vanishes after an hour.
These data clearly show that upon reaction, the copper
species is modified and ultimately degraded. No transient
ligand-based radical copper species could be detected,
probably because of the faster reaction kinetics with the
boronic nucleophile.
1471622 (12), and 1472428 (5) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre.
Acknowledgments
Accordingly, a plausible pathway based on a coordination/
reductive elimination mechanistic manifold is proposed
(Scheme 3b). Initial transmetalation of 2 with phenylboronic
acid gives rise to a [(LBQ)2Cu(CF3)(Ph)] species where the
phenyl is bound to the copper center in a trans relative
position to the trifluoromethyl group. DFT studies performed
on the resulting [(LBQ)2Cu(CF3)(Ph)] species support this
putative geometry, which is formed with an energetic cost of
16.9 kcalmolÀ1 and leads to a copper(III) center with a radical
ligand (16) rather than a copper(II) center with closed-shell
iminobenzoquinone ligand (15). The intermediate 16 can be
best described as a fully delocalized p-radical which is equally
distributed on both sides of the ligand. This species then
We thank UPMC, CNRS, IR-RPE CNRS FR3443 RENARD
network (CW X-band EPR with Dr. J.-L. Cantin, INSP UMR
CNRS 7588, UPMC, and EPR in Lille). Patrick Herson and
Lise-Marie Chamoreau are acknowledged for XRD analysis
and Denis Lesage for MS and HRMS spectrometry. We
gratefully acknowledge the support of this work from the
COST Action 27 CM1305 ECOSTBio (Explicit Control Over
Spin-States in Technology and Biochemistry).
Keywords: copper · density functional calculations ·
ligand effects · reaction mechanisms · redox chemistry
À
undergoes C N bond-forming reductive elimination to yield
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Received: May 26, 2016
Revised: July 5, 2016
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Ligand Effects
J. Jacquet, P. Chaumont, G. Gontard,
M. Orio, H. Vezin, S. Blanchard,
M. Desage-El Murr,*
L. Fensterbank*
&&&& — &&&&
À
C N Bond Formation from a Masked
High-Valent Copper Complex Stabilized
by Redox Non-Innocent Ligands
CuIII in hiding: A stable copper(II) com-
plex bearing fully oxidized iminobenzo-
quinone redox ligands reacts as
nistic studies suggest that this behavior
could stem from a mechanism akin to
reductive elimination occurring at the
metal center but facilitated by the ligand.
a copper(III) species and performs high-
À
yielding C N bond formation. Mecha-
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!