Unprecedented copper(I) bifluoride complexes: Synthesis, characterization and reactivity

Article abstract of DOI:10.1002/chem.201102655

To be or not to bifluoride: Two synthetic pathways to unprecedented N-heterocyclic carbene copper(I) bifluoride complexes have been developed (see scheme). Catalytic tests demonstrated that copper(I) bifluorides are very efficient catalysts, which do not

Full text of DOI:10.1002/chem.201102655

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DOI: 10.1002/chem.201102655
Unprecedented Copper(I) Bifluoride Complexes: Synthesis, Characterization
and Reactivity
Thomas Vergote,[a] Fady Nahra,[a] Alexandre Welle,[a] Michel Luhmer,[b]
Johan Wouters,[c] Nathalie Mager,[a] Olivier Riant,*[a] and Tom Leyssens*[a]
23]
Late-transition-metal complexes containing
a
fluoro
N-Heterocyclic carbene (NHC) ligands,[11c,
particularly
ligand are of considerable interest due to their high reactivi-
of the type developed by Arduengo and co-workers,[24] ap-
peared ideally suited to overcome the difficulties associated
with copper(I) fluoride complexes, that is, to stabilize cop-
per(I) fluoride and to afford solubility in aprotic organic sol-
vents. To our knowledge, only three NHC copper(I) fluoride
complexes have been reported in the literature, (IPr)Cu-F
(IPr=1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene),
(SICy)Cu-F (SICy=1,3-bis(cyclohexyl)imidazolin-2-ylidene)
and (IMes)Cu-F (IMes=1,3-bis(2,4,6-trimethylphenyl)imi-
dazol-2-ylidene), available upon treatment of (NHC)Cu-
OtBu with NEt3·3HF (Scheme 1).[11,12] Uniquely, these
[2]
ty,[1] relevance to metal-mediated C F bond cleavage, for-
À
mation[3] and potential use in catalysis.[4] Their rare abun-
dance compared to their analogous chloro, bromo, and iodo
complexes, is due to two main reasons. First, according to
the hard–soft acid–base theory,[5] the “hard” fluoride ion is
mismatched with the “soft” cations of late transition metals
in low oxidation states. Second, the p-donating ability of the
fluoro ligand can lead to destabilizing interactions with
filled d-orbitals.[6] The resulting metal–fluorine bond tends
to be labile and reactive.[7] Thus, despite the attention paid
to late-transition-metal fluoride complexes, Group 11 metals
have long been neglected, with only a small number of ex-
amples found in the literature.[8–12]
Recent studies have shown that fluoro complexes of Ni,[13]
Pd,[7a, 14] and Pt[15] are able to trap a molecule of HF when
treated with NEt3·3HF to generate a coordinated bifluoride
unit. Hydrogen(difluoride) (FHFÀ) is a unique anion as it
features the strongest known hydrogen bond.[16] This anion
is, furthermore, an anhydrous source of fluoride. These
metal complexes could turn out to be interesting tools in
catalysis. Since the report of [(Et3P)2Pt(Ph)FHF] by Coulson
almost 35 years ago,[17] only ten transition-metal species con-
taining a terminal bifluoride ligand have been isolated, and
a few more characterized, but none containing copper (M=
Rh,[18] Pd,[7a, 14] Ni,[13] Ru,[19] W,[20] Pt,[15] Mo,[21] and Mn[22]).
Scheme 1. Preparation of (NHC)–copper(I) fluoride and bifluoride com-
plexes.
(NHC)Cu-F complexes are soluble in a wide range of organ-
ic solvents and demonstrate a high catalytic activity towards
some catalytic transformations. The main drawback of these
complexes stems from the high atmospheric sensitivity of
the copper alkoxide intermediates and copper fluoride ad-
ducts that requires glove box and strictly controlled Schlenk
tube techniques for their preparation and handling. In our
attempts to extend this method of preparation to a wide
range of (NHC)Cu-F complexes for mechanistic investiga-
tions, we found that the reported procedures were highly de-
pendent on the reaction conditions and solvents used. Spe-
cific changes in the latter led to the formation of new com-
plexes bearing a bifluoride ligand.
[a] T. Vergote,+ F. Nahra,+ Dr. A. Welle, N. Mager, Prof. O. Riant,
Prof. T. Leyssens
Institute of Condensed Matter and Nanosciences
Molecules, Solids and Reactivity (IMCN/MOST)
Universitꢀ Catholique de Louvain
Place Louis Pasteur 1, 1348 Louvain-la-Neuve (Belgium)
Fax : (+32)10-474168
E-mail: tom.leyssens@uclouvain.be
olivier.riant@uclouvain.be
[b] Prof. M. Luhmer
Universitꢀ Libre de Bruxelles, RMN Haute Rꢀsolution
CP 160/08, Av. F.-D. Roosevelt 50, 1050 Brussels (Belgium)
Herein, we report the synthesis, characterization and reac-
tivity of unprecedented NHC–copper(I) bifluoride com-
plexes.
As stated above, the high sensitivity of (NHC)Cu-OtBu
complexes requires a very strict anaerobic environment and
various conditions were tested to generate and react these
intermediates without prior isolation (Scheme 2). Upon re-
acting (IPr)Cu-Cl with KOtBu followed by filtration under
[c] Prof. J. Wouters
Unitꢀ de Chimie Physique Thꢀorique et Structurale
Facultꢀs Universitaires Notre-Dame de la Paix
Rue de Bruxelles 61, 5000 Namur (Belgium)
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102655.
Chem. Eur. J. 2012, 18, 793 – 798
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separation (ca 2.243 ꢂ) is con-
siderably smaller than the cor-
responding distances in related
bifluoride complexes (except
for Ru complexes, ca. 2.276–
2.292 ꢂ[19]), but still larger than
twice the fluorine van der
Waals radius (1.4 ꢂ[25]). Fur-
thermore, crystal structures of
previously reported bifluoride
complexes show the bifluoride
ligand to exhibit a M-F-F angle
Scheme 2. Modified synthesis of (NHC)Cu-F, yielding (NHC)Cu-FHF
(method A).
Scheme 3.
tion.
ACHTUNGTREN(NUNG IPr)Cu-FHF ioniza-
an inert atmosphere to remove KCl and addition of
NEt3·3HF without removal of THF, a new copper complex
was isolated in 75% yield. This complex was almost identi-
cal to the copper fluoride complex reported in the literature,
except for the 19F NMR showing two peaks (at À220 and
À138 ppm) instead of one (at À240 ppm[11]). Furthermore,
1H NMR in CD3CN at room temperature showed an addi-
tional broad singlet at 14.1 ppm. To understand this unusual
result, single crystals were grown and X-ray analysis re-
vealed an unprecedented NHC–copper(I) bifluoride com-
plex; (IPr)Cu-FHF (1; Figure 1). Crystals of (IPr)Cu-F were
between 128 and 1568. The Cu-F-F angle found in 1, howev-
er, is a mere 108.878. In FHF salts, the F F distance is typi-
cally found between 2.24 and 2.28 ꢂ, with the proton sym-
À
À
metrically located between both fluorine atoms, and a F H
coupling constant of 120.5 Hz.[26] These data, therefore, sug-
gest that (IPr)Cu-FHF is most accurately described as a bi-
fluoride copper(I) salt[27] (Scheme 3).
The next step was to extend this new method to other
NHC copper complexes. Remarkably, for the IMes ligand,
similar reaction conditions led to the formation of an air-
stable bisACTHNUTRGNEUNG
(NHC)–copper(I) complex [(IMes)2Cu]+FHFÀ 2,
structurally characterized by X-ray diffraction (Figure 2).
Figure 1. X-ray structure of 1. Selected bond lengths (ꢂ), angles and tor-
À
À
sional angles (8): Cu F(1) 1.872, F(1) F(2) 2.243, F(2)-F(1)-Cu 108.87,
F(2)-F(1)-Cu-C 48.74 (Hydrogen atoms are omitted for clarity).[35]
Figure 2. X-ray structure of 2. Selected bond lengths (ꢂ), angles and tor-
À
À
À
sional angles (8): Cu C 2.182, F(1) F(2) 2.250, Cu F(1) 6.490, C-Cu-C’
180.00, N(1)-C-C’-N(2) 60.04 (Hydrogen atoms are omitted for clarity).[35]
also grown,[11d] and the structures of both compounds were
unambiguously shown to be different.
Complex 1 was fully characterized and unlike the corre-
sponding IPrCu-F complex, proved to be highly air stable in
the solid state and moderately stable in solution, with degra-
dation occurring after two days. As mentioned above, the
19F NMR spectrum exhibits two different peaks, one at
À138 ppm, corresponding to the distal fluorine atom (Cu-F-
H-F), and one at À220 ppm, referring to the proximal fluo-
BisACTHNUGRTENUNG(NHC)–copper(I) complexes are well known with classi-
À
À [28]
cal non-coordinating counterions such as PF6 and BF4 ,
but to our knowledge no such complex has ever been de-
scribed containing a FHFÀ counterion. Complex 2 shows a
bis-coordinated copper(I) atom in a linear environment with
a C-Cu-C angle of 1808. The aryl groups are twisted almost
perpendicularly with respect to the imidazole plane, result-
1
rine. The broad singlet in the H NMR spectrum refers to
À
the acidic proton. These results are in agreement with those
found for other transition-metal bifluorides described in the
aforementioned literature.
The acidic proton resonance of complex 1 appears as a
broad singlet at room temperature at d=14.1 ppm (in
CD3CN). At low temperature (193 K), this singlet resolves
into a triplet with a coupling constant of 120 Hz. (IPr)Cu-
ing in a favorable steric arrangement. The Cu C bond
length of 2.182 ꢂ is comparable, even slightly longer, to re-
À
ported Cu C bond lengths in bis
(NHC) complexes. The tor-
sion angle between the two NHC ligands of 60.048 is also in
agreement with those found in other [(IMes)2Cu]X (X=
PF6 , BF4 ) complexes. Short distances are observed be-
tween hydrogen atoms of the NHC backbone and the fluo-
À
À
À
À
FHF shows an elongated Cu F bond (1.872 ꢂ) as compared
to the 1.820 ꢂ bond length found in (IPr)Cu-F. The F—F
rine atoms of the counterion. The closest H F contact is
2.461 ꢂ, which is smaller than the sum of the hydrogen and
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Asymmetric Catalysis
COMMUNICATION
fluorine van der Waals radii (2.55 ꢂ), which could imply
some interaction between fluorine atoms and ring protons.
The acidic proton resonance is observed at 13.6 ppm (in
CD2Cl2), while the 19F NMR exhibits a singlet at À170 ppm,
29]
characteristic of a symmetric FHFÀ anion.[26,
19F NMR
spectroscopy thus appears to be a direct and practical
method to determine whether the obtained complex is
either of the (NHC)Cu-FHF or the (NHC)2Cu-FHF type.
Encouraged by these results we sought to develop a more
practical and convenient way of synthesizing these new
NHC–copper(I) bifluoride complexes. After numerous at-
tempts, a general reaction pathway was established. As rep-
resented in Scheme 4, reaction occurs directly between
Scheme 4. New synthetic route towards (NHC)xCu-FHF (method B).
Figure 3. Structure and acronyms of NHC ligands.
(NHC)Cu-Cl and AgHF2. THF is the solvent of choice for
method used. Steric congestion around the metal center is
presumed to be the predominant factor.
the mono
ACHTUNGTRENNUNG
3,4,5,6-tetrahydro-2
G
With these complexes readily prepared and characterized,
they were expected to show interesting catalytic activity due
to the unique nature of the bifluoride counterion and the
recent advances in (NHC)–copper(I)-catalyzed reactions.[23]
For these reasons, the reactivity of these complexes was in-
vestigated in previously described copper(I)-catalyzed reac-
tions.
The first reaction to be investigated was the reduction of
ketones to the corresponding alcohol. Since the pioneering
work by Stryker et al., copper(I) hydrides have emerged as
useful and easy tools for this reaction by means of hydrosily-
lation.[31] In their work, Nolan et al. established both
the bisACHTUNGTRENNUNG(NHC) complexes, due to its ability to solvate cation-
ic metallic species.[30]
With these two methods in hand, six new (NHC)x–cop-
per(I) bifluoride complexes (1–6) were successfully synthe-
sized (Table 1, Figure 3), two of these being chiral NHC–
copper(I) bifluorides ((IBP)Cu-FHF (5) and (IBP*)Cu-
FHF) (6)) based on the chiral NHC ligand family recently
reported by Tomioka et al.[23e]
Table 1. Preparation of (NHC)xCu-FHF complexes.
(NHC)–copper(I), as well as bisACTHNUGRTNEUNG(NHC)–copper(I) complexes
to be successful in reducing a variety of ketones.[28c,23f] How-
ever, the copper(I) complexes used required activation by a
base (NaOtBu).
When investigating this reaction using the bifluoride com-
plexes described here, the reaction was shown to occur in-
stantaneously without the requirement of any activating sub-
stances (Table 2). Reduction of benzylacetone proceeds
smoothly under very mild conditions to generate the desired
alcohol, upon hydrolysis.
(NHC)CuCl Method[a] Solvent[b]
G
Yield [%][d]
IPr
A (or B) THF
75 (83)[e]
60
IMes
SIPr
SIMes
B
DMPU
A (or B) THF
A (or B) DMPU
60 (50)[e]
A
B
B
THF
THF
64
quant.
R
[a] Method A was run in THF, method B was run in THF or in DMPU
depending on the type of copper(I) bifluoride formed. [b] Solvent used in
Table 2. Reduction of benzylacetone by (NHC)xCu-FHF complexes.
method
B : THF for (NHC)Cu-FHF complexes and DMPU for
(NHC)2Cu-FHF complexes. [c] The type of (NHC)xCuFHF was deter-
mined by 19F NMR spectroscopy in accordance with the data of com-
plexes 1 and 2. [d] Isolated yields. [e] Isolated yields for method B.
Entry
Catalyst
t [h]
Yield [%][a]
1
2
3
1
2
3
1
0.25
0.5
>99
>99
>99
Interestingly, the formation of either a (NHC)2Cu-FHF or
a (NHC)Cu-FHF bifluoride complex depends entirely on
the nature of the NHCs R group, independent of the
1
[a] Yields determined by GC and H NMR spectroscopy.
Chem. Eur. J. 2012, 18, 793 – 798
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795

O. Riant, T. Leyssens et al.
After these encouraging results, the 1,4-conjugate b-bory-
lation and b-silylation reactions were investigated. These
type of reactions provide access to intermediates useful in
the construction of complex products. Recent studies have
shown that (NHC)–copper(I) complexes can successfully
catalyze these reactions with high yield and selectivity. Once
more an activating agent was required.[32] Bifluoride com-
plexes, once again were shown to be active in both reactions,
generating good yields, and did not require an activating
agent (Table 3, entries 1 and 2). Unfortunately, the chiral
(R)-N-tert-butanesulfinyl imines.[34] By varying the reaction
conditions, their method allowed the achievement of oppo-
site stereocontrol by giving access to the two diastereoiso-
mers. Intrigued by these results and the remarkable reactivi-
ty these bifluoride copper(I) complexes have shown towards
silane activation, we hoped to achieve similar results react-
ing allylsilanes with chiral (R)-N-tert-butanesulfinyl imines.
To this end, different reaction conditions were screened and
IPrCu-FHF was shown to promptly catalyze this reaction
with high yield and excellent stereoselectivity (Table 4).
Table 3. 1,4-conjugate b-borylation and b-silylation reactions.
Table 4. Cu-catalyzed diastereoselective allylation of (R)-N-tert-butane-
sulfinyl aldimines.
Entry
R[a]
Product
Yield [%][b]
d.r.[c]
Entry
Catalyst
Product
Yield [%][a]
ee [%][b]
1
2
3
4
5
C6H5
11a
11b
11c
11d
11e
81
90
91
83
69
95:5
92:8
95:5
96:4
93:7
4-MeOC6H4
2-MeOC6H4
2-MeC6H4
cyclohexyl
1
2
3
3
3
6
8a
83
–
–
31
8b[c]
8b[c]
99
58[d]
[a] Yield of isolated product. [b] Determined by HPLC after oxidation to
the alcohol by NaBO3/H2O. [c] B(Pin)=pinacolborane. [d] Determined
[a] Starting material prepared according to literature procedures (see
Supporting Information). [b] Isolated yields. [c] Determined by 1H NMR
spectroscopy, the configuration was assigned by comparison with litera-
ture data.
AHCTUNGTRENNUNG
1
by H NMR spectroscopy.
complexes studied here did not lead to a high enantioselec-
tivity (Table 3, entry 3).
These reaction conditions lead to the difficult to obtain (R)-
amine diastereoisomer, whereas the reaction conditions de-
scribed by Lin et al. to obtain this diastereoisomer require
the use of HMPA (HMPA=hexamethylphosphoramide), a
human carcinogen, rendering the method somewhat imprac-
tical on a larger scale. To our knowledge, this is the first Cu-
catalyzed allylation reaction of chiral aldimines.
In conclusion, we have developed two synthetic pathways
to unprecedented (NHC)–copper(I) bifluoride complexes.
These complexes were fully characterized. Catalytic tests
demonstrated the copper(I) bifluorides to be very efficient
catalysts that do not require any additional activating agent.
We also established the first Cu-catalyzed diastereoselective
allylation of (R)-N-tert-butanesulfinyl aldimines. The
method enables efficient, simple and general synthesis of
enantiomerically enriched homoallylic amines at room tem-
perature in high yields. Further studies are being conducted
to better understand the mechanistic details of the reactions
described above.
To further investigate the possibility of inducing enantio-
selectivity with these chiral (NHC)Cu-FHF complexes, Cu-
catalyzed allylation reaction of aldehydes were investigated.
Recent reports showed that (NHC)Cu-F complexes are
good catalysts for this reaction but require the addition of
fluorosilanes to achieve efficient yields and productive re-
sults.[11d, 33] The bifluoride complexes described here do not
require any additives and readily catalyze the allylation re-
action of aldehydes (Scheme 5). High yields are achieved
Scheme 5. Allylation of aldehydes using (NHC)Cu-FHF complexes.
under mild conditions and remarkably, a moderate enantio-
selectivity has been obtained using complexes 5 and 6 (46%
with 6, see Supporting Information).
Recently, another allylation-type reaction became a sub-
ject of high interest in the literature: the allylation of aldi-
mines. Lin et al. reported a practical synthesis of chiral ho-
moallylic amines based on Zn-mediated allylation of chiral
Acknowledgements
Financial support from the Universitꢀ Catholique de Louvain is grateful-
ly acknowledged.
Keywords: allylation · asymmetric catalysis · bifluoride ·
copper · N-heterocyclic carbenes
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COMMUNICATION
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Received: August 25, 2011
Published online: December 14, 2011
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