Neutral Dinuclear Copper(I)-NHC Complexes: Synthesis and Application in the Hydrosilylation of Ketones

Article abstract of DOI:10.1021/acscatal.6b02723

The synthesis of a class of highly stable neutral dinuclear Cu(I)-NHC complexes using 1,2,4-triazole as a bridging ligand is described. Various NHCs were used to generate a library of [Cu(μ-trz)(NHC)]₂ complexes. Interestingly, [Cu(μ-trz)(IPr)]₂ was found to be highly active in the hydrosilylation of ketones, without the need for an external base or any other additive. A wide range of aryl and alkyl ketones, as well as sterically hindered ketones, was successfully reduced to alcohols using the lowest catalyst loading reported to date.

Full text of DOI:10.1021/acscatal.6b02723

Research Article
pubs.acs.org/acscatalysis
Neutral Dinuclear Copper(I)-NHC Complexes: Synthesis and
Application in the Hydrosilylation of Ketones
†
†
†
‡
†
Michael Trose, Faïma Lazreg, Tao Chang, Fady Nahra, David B. Cordes,
†
,†,‡
Alexandra M. Z. Slawin, and Catherine S. J. Cazin*
†
EaStCHEM School of Chemistry, University of St Andrews, St Andrews KY16 9ST, U.K.
‡
Ghent University, Department of Inorganic and Physical Chemistry, Krijgslaan 281, S-3, B-9000 Ghent, Belgium
S Supporting Information
*
ABSTRACT: The synthesis of a class of highly stable neutral dinuclear Cu(I)-NHC
complexes using 1,2,4-triazole as a bridging ligand is described. Various NHCs were used
to generate a library of [Cu(μ-trz)(NHC)] complexes. Interestingly, [Cu(μ-trz)(IPr)]2
2
was found to be highly active in the hydrosilylation of ketones, without the need for an
external base or any other additive. A wide range of aryl and alkyl ketones, as well as
sterically hindered ketones, was successfully reduced to alcohols using the lowest catalyst
loading reported to date.
KEYWORDS: N-heterocyclic carbenes, copper(I), hydrosilylation, ketones, dinuclear
5
a
n the past decade, copper-NHC complexes have been
heavily studied and employed in catalysis. In this synthetic
Sadighi and co-workers. The latter proved highly unstable
when conventional five-membered NHCs were used; however,
recently, increased stability of related complexes was achieved
1
I
exploration effort, mostly mononuclear complexes, both neutral
and cationic, have been reported. Dinuclear complexes with
bridging or chelating ligands are also found in the literature,
although in significantly smaller numbers. Among those that
have been reported, cationic complexes are usually formed by
two copper(I) NHC fragments bridged by an anionic ligand
6
by using expanded-ring NHCs or highly donating cyclic
alkylaminocarbene (CAAC) ligands. Examples of polynuclear
complexes (presenting three or more copper centers) are rare8
and have been obtained using multidentate NHC ligands.
Despite recent advances in the use of nonconventional NHC
7
9
2
3
4a
4b
ligands, five-membered NHCs based on the imidazolium
such as hydride, fluoride, alkenyl, and boryl with the
general formula [Cu (μ-X)(NHC) ]Y (Figure 1). Neutral
core are still the most widely studied and are the most readily
available. Noting the significant difference in catalytic effi-
ciency between the mononuclear cationic copper complexes,
2
2
5
species are more scarce with only two known examples, one
of which is the highly unstable copper hydride, reported by
10
[
Cu(NHC) ]Y, and its neutral relatives, [Cu(X)(NHC)], we
2
sought to synthesize stable neutral dinuclear copper-NHC
complexes bearing conventional five-membered NHC ligands
and to evaluate their reactivity in known copper-catalyzed
transformations (Figure 1). In order to achieve the isolation of
neutral dinuclear complexes, 1,2,4-triazole 1 (Htrz) was selected
for its ability to coordinate two adjacent copper centers. Htrz and
its corresponding triazolate anion (trz) have been widely used for
11
this bridging task. However, only one example of a well-defined
neutral dinuclear copper(I) complex, bearing triphenylphosphine
12
as a ligand, has been reported to date. It should be noted that
these authors did not report their use in catalysis but simply their
synthesis.
Received: September 23, 2016
Revised: November 17, 2016
Figure 1. Previous work describing mono-, di-, and trinuclear copper-
NHC complexes and details of the present report on dinuclear
complexes.
©
XXXX American Chemical Society
238
DOI: 10.1021/acscatal.6b02723
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ACS Catalysis
Research Article
In order to synthesize the desired complexes, [Cu(Cl)-
1
3
(
IPr)] (IPr = N,N′-bis(2,6-diisopropylphenyl)imidazol-2-
ylidene) was used as a benchmark reagent in initial synthetic
efforts (Scheme 1). When this complex was reacted with 1 at
Scheme 1. Synthesis of 2a from Various Copper-NHC
Precursors
Figure 2. Two views of the molecular structure of 2a. Solvent
molecules and H atoms are omitted for clarity. Thermal ellipsoids are
1
6
shown at 50% probability. Selected bond lengths (Å), angles (deg),
1
room temperature in THF, no reaction was observed, whereas
the addition of CsOH leads to complete formation of the
and torsion angles (deg): Cu1−C1 = 1.896(2); Cu1−N32 =
1
1
.964(2); Cu1−N31 = 2.001(2); N32 −Cu1−C1 = 134.57(10);
1
14
N31−Cu1−C1 = 119.15(10); N31−Cu1−N32 = 106.17(9);
desired [Cu(μ-trz)(IPr)] (2a). [Cu(OH)(IPr)] was also
2
1
1
C1−Cu1−Cu1 = 170.22(7); N31−Cu1−C1−N2 = −76.7(2); N32 −
reacted with 1, in the absence of CsOH, to yield 2a as expected,
revealing that the copper-NHC hydroxide species might be an
intermediate in the initial reaction.
Considering [Cu(Cl)(NHC)] complexes are easily accessed
and isolated, with regard to the corresponding [Cu(OH)-
1
Cu1−C1−N2 = 98.9(2); N31−Cu1−C1−N5 = 99.3(2); N32 −Cu1−
1
C1−N5 = −85.0(2); Cu1−N31−N32−Cu1 = 5.4(2); N31−Cu1−
1
1
1
16
N32 −N31 = 4.37(15); N32 −Cu1−N31−N32 = −4.59(16).
(
NHC)] analogues, we opted, for the sake of simplicity, to
bond distances are found in copper-NHC pyridyl-azolate com-
plexes, despite the latter showing a more distorted trigonal
target the in situ generation of the hydroxides. Gratifyingly,
various [Cu(μ-trz)(NHC)]2 complexes were obtained in
excellent yields using this strategy (Table 1). Additionally, the
17
geometry in comparison to 2a. A distinctive feature in 2a is
the six-membered ring formed by the four nitrogens of the two
triazolate bridging ligands and the two copper atoms, the latter
being separated by a distance of 3.7687(11) Å. Analyzing the
torsion angles of this ring, an almost flat conformation was
found; moreover, the ring is practically perpendicular to the
NHC−Cu−Cu−NHC plane, most likely to minimize steric
repulsions. These characteristics were found in all other com-
a
Table 1. Synthesis of [Cu(μ-trz)(NHC)] Complexes
2
plexes (2b−e) with a different degree of distortion caused by
b
18
Entry
NHC
solvent
[Cu(μ-trz)(NHC)]2
NHC = IPr (2a)
yield (%)
the steric demand of the NHC ligand (%V ), mainly
Bur
c
1
1
2
3
4
5
IPr
THF
90 (92)
93 (97)
92 (95)
91
evidenced by the deviation in linearity of the Cu1 −Cu1−C1
c
c
1
IMes
SIMes
ItBu
ICy
THF
NHC = IMes (2b)
NHC = SIMes (2c)
torsional angle such as in 2d (C1−Cu1−Cu1 = 161.76(4)°;
CH Cl
see Figure S6 in the Supporting Information).
2
2
t
CH Cl
NHC = I Bu (2d)
Interested in the applications of these dimers in catalysis,
we sought to compare their performance with that of the
corresponding mononuclear copper-NHC complexes in a well-
studied reaction. We hoped that the bridging triazolate could act
as an internal base, thus avoiding the use of any external additives.
Among the various transformations catalyzed by copper-NHC
2
2
CH Cl
NHC = ICy (2e)
89
2
2
a
Reaction conditions: [Cu(Cl)(NHC)] (100 mg), 1 (1.1 equiv),
CsOH (2.0 equiv), solvent (2 mL), 25 °C, 4 h, under argon. Isolated
yield. Large-scale (1 g) isolated yield given in parentheses. IMes =
b
c
N,N′-bis-[2,4,6-(trimethyl)phenyl]imidazol-2-ylidene, SIMes = N,N′-
t
19
bis[2,4,6-(trimethyl)phenyl]imidazolin-2-ylidene, I Bu = N,N′-(di-tert-
species, in which a base is typically needed, the hydrosilylation
butyl)imidazol-2-ylidene, ICy = N,N′-(dicyclohexyl)imidazol-2-ylidene.
of ketones was selected as a model reaction (Scheme 2).
Nolan and co-workers reported that the neutral complexes
[Cu(Cl)(ICy)] and [Cu(Cl)(SIMes)] and the cationic
complexes [Cu(ICy) ]BF and [Cu(IPr) ]BF were able to
purification of the product proved straightforward, since excess
Htrz and CsOH can be removed by simple filtration through
Celite. Both aromatic (Table 1, entries 1−3) and aliphatic
substituents (Table 1, entries 4 and 5) on the nitrogens of the
NHC are well tolerated, giving the desired complexes in high
yields. Whereas 2a,b are soluble in THF, 2c,d proved insoluble
in this solvent and CH Cl had to be used instead. In addition,
2
4
2
4
promote this transformation using 3 mol % of catalyst with a
t
20
substoichiometric amount of base (3−12 mol % of NaO Bu).
The authors proposed an initial activation of the copper
precatalyst by the base, forming in situ the corresponding
[Cu(O Bu)(NHC)] species, which, upon reacting with the
t
2
2
the scaling up of 2a−c to 1 g was achieved without any issue.
silane, generated the [Cu(H)(NHC)] intermediate involved in
15
20a,c
These complexes are air stable in the solid state.
the hydrosilylation.
Because both the chloride and the
X-ray analysis of 2a confirmed the expected structure, as
shown in Figure 2. All other complexes were fully characterized,
and their structures were confirmed by X-ray diffraction on
cationic NHC complexes were not able to react directly with
the silane to give the intermediate copper hydride species, the
use of a tert-butoxide base proved necessary. Later on, using
both kinetic and computational studies, it was shown that the
base activates both precatalyst and silane, forming a hypervalent
silicon species. The latter permitted a better hydride transfer
to copper, generating the copper hydride needed for the
16
single crystals (see the Supporting Information).
a displays a distorted-trigonal geometry around each copper
2
atom, displaying two different bond distances with the two
1
nitrogens N31 and N32 . The same geometry and comparable
2
39
DOI: 10.1021/acscatal.6b02723
ACS Catal. 2017, 7, 238−242

ACS Catalysis
Research Article
Scheme 2. Cu-NHC Catalysts for the Hydrosilylation of
Ketones
substituents 2d,e were not active under these conditions
(Table 2, entries 4 and 5). Among these catalysts, 2a proved
the most active and was chosen for further optimization.
Surprisingly, reducing the catalyst loading to 0.05 mol % did
not affect the catalytic activity of 2a, converting 3a in 2 h into
the corresponding silyl ether (Table 2, entry 7). Under these
conditions, 2a is the most efficient catalyst of all the copper-
NHC catalysts reported to date for this transformation (TON =
20−23
2
000). Next, the amount of silane was reduced to 1.1 equiv
without any loss in efficiency (Table 2, entry 9). Using these
optimized conditions followed by deprotection with TBAF, 5a
was isolated in excellent yield (Scheme 3, method A).
Scheme 3. Optimized Procedures for the Hydrosilylation of
3a and Its Subsequent Deprotection To Yield 5a
21
hydrosilylation to proceed. Recently, other methodologies
have been reported using different [Cu(Cl)(NHC)] complexes,
exclusively based on the fine tuning of the NHC ligand, but
without overcoming the need for a significant amount of base.
To date, the only copper(I)-NHC precatalysts able to perform
this reaction without the use of any external base are the copper
Despite these satisfactory results, we still wanted to use a less
expensive and a more environmentally friendly silane in lieu of
22
Me(OEt) SiH; for these reasons, the use of polymethylhy-
2
drosiloxane (PMHS) was investigated next. When PMHS
was tested under the optimal conditions, no conversion was
observed. However, when the catalyst loading was increased to
bifluoride complexes [Cu(HF )(NHC)], used exclusively for
2
23
−
the hydrosilylation of benzyl acetone. In this case, F acts as
an internal base. It should be mentioned that Htrz possesses
0
.25 mol %, complete consumption of 3a was achieved,
showing the feasibility of using this environmentally friendly
and safe hydride source. At this point, the deprotection and
purification steps were altered; TBAF and flash chromatog-
raphy were replaced by NaOH in methanol, followed by a
simple extraction to remove siloxanes formed during the
deprotection step (Scheme 3, method B). Interestingly, when
the scope of the reaction was investigated using 0.05 mol % of
acidity (pK = 14.8, in DMSO) comparable to that of HF
a
(
pK = 15, in DMSO), which suggests that the triazolate anion
a
can also act as an internal base. For these reasons, the dinuclear
copper complexes [Cu(μ-trz)(NHC)] were tested in this
2
reaction, using the hindered dicyclohexyl ketone (3a) as a
model substrate (Table 2). All complexes bearing aromatic
substituents on the nitrogens of the NHC, 2a−c, showed high
activity (Table 2, entries 1−3), whereas those bearing aliphatic
2
a and PMHS, a wide range of ketones were successfully
converted into the corresponding alcohols after simple
deprotection (Scheme 4). Acetophenone derivatives bearing,
in the para position, both electron-withdrawing (EWG; 5c−e)
and electron-donating (EDG; 5f) groups were well tolerated.
The same tolerance was noted for the ortho-substituted 5g.
Following this trend in reactivity, a test using 4,4′-dimethox-
ybenzophenone, as a challenging substrate (electronically),
showed that a longer reaction time was needed. Nevertheless,
full conversion into 5k was achieved. Moreover, branched
aliphatic substitutions on the α-carbon of acetophenone (R′ in
Scheme 4), such as a tert-butyl moiety, did not affect the yield
a
Table 2. Synthesis of [Cu(μ-trz)(NHC)] Complexes
2
b
silane
(equiv)
time
(h)
conversn
(%)
entry catalyst loading (mol %)
1
2
3
4
5
6
7
8
9
1
2a
2b
2c
2d
2e
2a
2a
2a
2a
2a
1
2
0.75
0.75
0.75
0.75
0.75
0.75
2
>99
95
84
0
1
2
(
5i). When highly hindered aliphatic ketones were considered,
1
2
acyclic 5-nonanone (5o), adamantone (5q), and 1-adamantyl
methyl ketone (5r) were all successfully converted to the desired
alcohols. Encouraged by these results, we tested the method-
ology on a more complex molecule: i.e., trans-androsterone.
Gratifyingly, this steroid was fully converted to the correspond-
ing alcohol 5s in excellent yield, further demonstrating the
robustness of this methodology. It should be mentioned that
only one diastereoisomer of the product was observed in the
last entry, suggesting that this method might be used in a
diastereoselective manner.
1
2
1
2
5
0.05
0.05
0.01
0.05
0.05
1.5
1.5
1.5
1.1
1.1
85
>99
0
2
2
>99
72
c
0
4
a
Reaction conditions unless specified otherwise: 3a (0.5 mmol),
Me(OEt) SiH (0.55−1 mmol), [Cu(μ-trz)(NHC)] (0.01−0.5 mol %),
2
2
b
THF (0.75 mL), 55 °C, under argon. Yield determined by GC, on the
basis of 3a; average of two reactions. 35 °C.
In conclusion, a novel and highly stable family of neutral
dinuclear Cu(I)-NHC complexes was synthesized and fully
c
2
40
DOI: 10.1021/acscatal.6b02723
ACS Catal. 2017, 7, 238−242

ACS Catalysis
Research Article
a
Scheme 4. Hydrosilylation of Ketones using 2a and PMHS
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(
The authors gratefully acknowledge the Royal Society
University Research Fellowship to C.S.J.C.) for funding.
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.6b02723.
■
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AUTHOR INFORMATION
Corresponding Author
E-mail for C.S.J.C.: Catherine.Cazin@UGent.be.
ORCID
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ACS Catalysis
Research Article
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(
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(
2
2
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DOI: 10.1021/acscatal.6b02723
ACS Catal. 2017, 7, 238−242