Synthesis and characterization of [Cu(NHC)2]X Complexes: catalytic and mechanistic studies of hydrosilylation reactions

Article abstract of DOI:10.1002/chem.200701013

The preparation of two series of [Cu(NHC)₂]X complexes (NHC = N-heterocyclic carbene, X = PF₆ or BF₄) in high yields from readily available materials is reported. These complexes have been spectroscopically and structurall

Full text of DOI:10.1002/chem.200701013

DOI: 10.1002/chem.200701013
Synthesis and Characterization of [Cu(NHC)2]X Complexes: Catalytic and
A
Mechanistic Studies of Hydrosilylation Reactions
Silvia Díez-Gonzµlez,*[a, b] Edwin D. Stevens,[b] Natalie M. Scott,[b]
Jeffrey L. Petersen,[c] and Steven P. Nolan*[a, b]
Abstract: The preparation of two series
of [Cu(NHC)2]X complexes (NHC=N-
[Cu
(NHC)]-based systems, these cat-
isopropylphenyl)imidazol-2-ylidene
U
ionic complexes proved to be more ef-
ficient under similar reaction condi-
tions. The activation step of [Cu-
(IPr) ligands in [Cu(IPr)2]BF4 is dis-
N
heterocyclic carbene, X=PF6 or BF4)
in high yields from readily available
materials is reported. These complexes
have been spectroscopically and struc-
turally characterized. The activity of
these cationic bis-NHC complexes in
the hydrosilylation of ketones was ex-
amined, and both the ligand and the
counterion showed a significant influ-
ence on the catalytic performance.
Moreover, when compared with related
placed by tBuOÀ in the presence of
NaOtBu, producing the neutral [Cu-
(NHC)2]X precatalysts towards hydro-
A
N
silylation was investigated by means of
1H NMR spectroscopy. Notably, it was
shown that one of the N,N’-bis(2,6-di-
known to be a direct precursor of an
NHC–copper hydride, the actual active
species in this transformation. Further-
more, reagent loading and counterion
effects have been rationalized in light
of the species formed during the reac-
tion.
Keywords: copper · hydrosilylation ·
ketones · N-heterocyclic carbenes ·
reaction mechanisms
Introduction
Transition-metal catalysis has been successfully applied for
the reduction of olefins, alkynes, and many carbonyl com-
pounds by hydrogenation or hydrosilylation.[2] Hydrogena-
tion reactions often proceed in good yields, but only under
high pressure or elevated temperature. In contrast, since the
first report of a metal-catalyzed hydrosilylation of ketones
in the presence of the Wilkinsonꢀs catalyst,[3] smooth reac-
tion conditions can be employed, and in consequence, over-
reduced products are rarely detected. A sequence of hydro-
silylation/hydrolysis reactions leads to the formation of alco-
hols and amines. Alternatively, the silyl group can also be
retained as a protecting group, a process that is of great in-
terest in organic synthesis. Moreover, this approach replaces
the use of hazardous dihydrogen with easy-to-handle hydro-
silanes.
The reduction of carbonyl and pseudocarbonyl functions is
an essential transformation in organic synthesis.[1] Main-
group metal hydrides, in particular those of boron and alu-
minum, can accomplish this transformation, but they are re-
quired in stoichiometric amounts, which renders them unat-
tractive from both practical and economic points of view.
[a] Dr. S. Díez-Gonzµlez, Prof. Dr. S. P. Nolan
Institute of Chemical Research of Catalonia (ICIQ)
Av. Països Catalans, 16
43007 Tarragona (Spain)
Fax : (+34)977-920-244
E-mail: sdiez@iciq.es
snolan@iciq.es
Pioneering work by Stryker et al. showed that hexameric
[b] Dr. S. Díez-Gonzµlez, Prof. Dr. E. D. Stevens, Dr. N. M. Scott,
Prof. Dr. S. P. Nolan
[CuH(Ph3P)]6 was an efficient reagent in the conjugate re-
G
duction of a number of carbonyl derivatives, with high regio-
selectivity.[4] This stabilized form of copper hydride, which
was first reported by Osborn et al.,[5] put an end to the as-
Department of Chemistry
University of New Orleans
New Orleans, Louisiana 70148 (USA)
[c] Prof. Dr. J. L. Petersen
À
sumption that Cu H was too unstable to have any potential
C. Eugene Bennet Department of Chemistry
West Virginia University
Morgantown, West Virginia 26506-6045 (USA)
applications in organic chemistry.[6] The combination of the
Strykerꢀs catalyst and a hydrosilane as a hydride source al-
lowed the regioselective conjugate reduction of carbonyl
compounds under mild conditions.[7] This methodology has
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
158
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Chem. Eur. J. 2008, 14, 158 – 168

FULL PAPER
been expanded to different hydrosilanes and ancillary li-
gands,[8] and developed asymmetrically.[9]
N-Heterocyclic carbenes (NHCs)[10] are an interesting al-
ternative to phosphines for the copper-catalyzed hydrosilyla-
tion of carbonyl groups. [CuCl(IPr)] (IPr=N,N’-bis(2,6-dii-
A
sopropylphenyl)imidazol-2-ylidene) is an efficient catalyst
for the hydrosilylation of unhindered ketones[11] and for the
conjugate reduction of tri- and tetra-substituted a,b-unsatu-
rated ketones and esters.[12] More challenging ketones can
be hydrosilylated in the presence of [CuCl
ACHTREUNG
N,N’-bis(cyclohexyl)imidazol-2-ylidene) or [CuCl
AHCTREUNG
(SIMes=N,N’-bis(2,4,6-trimethylphenyl)-4,5-dihydroimida-
zol-2-ylidene).[13] In addition, we recently reported the prep-
aration and application of [Cu(IPr)2]X complexes (1a: X=
A
PF6, 1b: X=BF4) in catalysis.[14] Herein, we describe the
preparation and characterization of two series of homoleptic
bis-NHC complexes, and explore their catalytic activity in
the hydrosilylation of ketones. The structures of the NHC li-
gands employed in this study are shown in Figure 1.
Figure 1. Structures of the NHC ligands studied.
Results and Discussion
Synthesis of [Cu
(NHC)2]X complexes (2a–7a, 2b–7b) were synthesized in
very good yields from the corresponding tetrakis(acetoni-
trile)copper(I) and azolium salts in the presence of a strong
A
Table 1. Preparation of [Cu(NHC)2]X complexes.
G
ACHTREUNG
ACHTREUNG
base (Table 1).[14] An excess of base was generally used to
ensure total conversion of the carbene precursor. However,
these conditions were avoided in the case of complexes 5
and 7, bearing ICy and ItBu ligands, because partial decom-
position of the expected products was observed if an excess
of base was used. The pure white complexes isolated are
NHC
X
Complex
Base [equiv]
t [h]
Yield [%]
SIPr
SIPr
IMes
IMes
SIMes
SIMes
ICy
ICy
IAd
IAd
ItBu
ItBu
PF6
BF4
PF6
BF4
PF6
BF4
PF6
BF4
PF6
BF4
PF6
BF4
2a
2b
3a
3b
4a
4b
5a
5b
6a
6b
7a
7b
1.3
1.3
1.0
1.3
1.3
1.3
1.0
1.1
1.3
1.3
1.1
1.1
15
6
68
63
91
74
86
86
80
93
84
95
99
100
15
15
15
15
15
15
15
15
6
Abstract in Spanish: Se presenta la preparación de dos series
de complejos [Cu
A
6
co, X=PF6 o BF4) en altos rendimientos a partir de materia-
les de fµcil acceso. Estos complejos han sido caracterizados
espectróspica y estructuralmente. La actividad de estos com-
plejos catiónicos bis-NHC en la hidrosililación de cetonas ha
sido examinada y el ligando así como el contra-ión mostra-
ron una influencia significativa en la eficacia catalítica.
highly stable to air and moisture with the exceptions of 7a
and 7b.[15] These ItBu-containing complexes decomposed
slowly over several weeks, but this could be avoided by
simply flushing the storage vial with an inert atmosphere,
for example, argon.
Ademµs, al compararlos con otros sistemas [Cu(NHC)]rela-
A
cionados, estos complejos catiónicos mostraron ser mµs efi-
cientes bajo similares condiciones de reacción. La activación
1
The H NMR spectra of complexes 3a, 3b, 5–7a, and 5–
7b, which contain unsaturated NHC ligands, are character-
ized by a single resonance signal for the two imidazole pro-
tons at low field (d=7.34–7.64 ppm), as well as signals that
are characteristic of their corresponding side chains. The
13C NMR spectra give rise to characteristic low-field reso-
nance signals for the carbenic carbon at around d=
175 ppm. Saturated complexes 2a, 2b, 4a, and 4b result in
1H NMR spectra with resonances at around d=4.00 ppm for
the protons in the imidazoline ring, and the expected signals
for the mesityl and isopropylphenyl side chains. For these
complexes, the 13C NMR spectroscopy data show one signal
de los pre-catalizadores [Cu
ción ha sido investigada mediante H RMN. En particular, se
ha mostrado que uno de los ligandos IPr en [Cu(IPr)2]BF4
es desplazado por tBuOÀ en la presencia de NaOtBu, produ-
ciendo el complejo neutral [Cu(IPr)(OtBu)]. Este alcóxido
de cobre es conocido como un precursor directo de un hidru-
ro de NHC-cobre, la verdadera especie activa en esta trans-
formación. Asimismo, la influencia de la carga de reactivos y
del contra-ión han sido racionalizados segffln las especies for-
madas en la reacción catalítica.
A
1
Chem. Eur. J. 2008, 14, 158 – 168
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159

S. Díez-Gonzµlez, S. P. Nolan et al.
the metal center. For all com-
À
pounds, the Cu C bond lengths
lie in the range of 1.87 to
2.00 Š, comparable to the re-
À
ported Cu C bond lengths in
bis-NHC complexes.[16] An in-
À
crease in the Cu C distances as
a function of steric hindrance of
the N-aryl substituent was ob-
served. The bulkiness brought
by such substituents could also
be correlated to the torsion
angle between the ligands.
Complexes with the bulkiest li-
gands (IAd and ItBu; 6a, 6b,
À
and 7b) have the longest Cu C
bond lengths and
a
torsion
angle between the NHC ligands
of close to 908 (Table 3, last
entry). Complexes
4 and 5,
which have the less hindering li-
gands IMes and SIMes, have
À
shorter Cu C bond lengths and
smaller torsion angles. Complex
2a (Table 2), which has two
bulky SIPr ligands, has a long
À
Cu C bond length, but the li-
gands are only slightly twisted.
In this particular case, the pres-
ence of two sp3 carbon atoms in
the heterocyclic ring allowed
the formation of torsion angles
of 15.7 and 8.78 in the saturated
backbones of the SIPr ligands.
Interestingly, SIMes complexes
Figure 2. Ball and stick representations of [Cu
A
G
A
and [Cu(SIMes)2]BF4 (4b). Most hydrogen atoms have been omitted for clarity.
ACHTREUNG
for the carbenic carbon atoms at significantly lower field
(d=200 ppm) when compared with that of their unsaturated
analogues. No influence of the counterion in the NMR spec-
troscopic data was observed.
Crystallographic studies were carried out to unambiguous-
ly confirm the structures of these complexes. Crystals suit-
4a and 4b do not show such a feature and the imidazoline
rings are planar. We assume that in the case of 2a, the steric
congestion around the metal center owing to the isopropyl
groups is preferentially released by the torsion of the imida-
zoline rings rather than by ligand twisting.
In 3b, the unit cell contains two independent half-mole-
À
A
cules of the [Cu
A
3b, 4a, 4b, 6a, 6b, and 7b. Repeated attempts to crystallize
the other complexes were unsuccessful and generally led to
the formation of decomposition products and the corre-
sponding azolium salt. Ball and stick representations of the
complexes are given in Figure 2( 3a, 3b, 4a, and 4b) and
Figure 3 (6a, 6b, 2a, and 7b). A comparison of selected
bond lengths and angles is provided in Tables 2and 3 and
relevant crystallographic data for these complexes are sum-
marized in Table 4. All of the complexes have a two-coordi-
nate copper(I) atom in a near-linear environment with a C-
Cu-C angle very close to or equal to 1808. As seen for other
crystallographically characterized complexes that contain
NHC ligands with N-aryl substituents, the aryl groups are
twisted almost perpendicularly with respect to the imidazole
plane, which results in a favorable steric arrangement with
geometries of each pair of cations and anions are constrain-
ed by a crystallographic two-fold rotation axis passing
through the copper and boron atoms, respectively. In 4a, the
À
PF6 anion was disordered and the positions of the six inde-
pendent fluorine atoms were assigned fixed site occupancy
factors of 0.25 and refined by restraining the twelve F···F
distances along the edges of the octahedral array of fluorine
atoms to (2.24Æ0.02) Š. On the other hand, both Cu(1) and
P(1) lie at the intersection of three orthogonal two-fold ro-
tation axes and the carbenic carbon, labeled C(1), lies on
one of the crystallographic C2 axes that passes through
Cu(1). Similarly, unusually large thermal ellipsoids were ob-
served for the independent fluorine atom of the tetrafluoro-
borate anion in 7b, which reflects significant vibrational
À
motion normal to the B F bonds within this anion. Finally,
160
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Chem. Eur. J. 2008, 14, 158 – 168

Synthesis and Characterization of [Cu(NHC)2]X Complexes
A
FULL PAPER
short distances in space were
observed in several examples
between hydrogen atoms in the
NHC backbone and the fluo-
rine atoms in the counterion.
H···F distances of around 3 Š
are present in 2a, 3a, 4a, 6a,
and 6b and the closest contacts
are 2.468, 2.695, or 2.314 Š in
3a, 6a, and 6b, respectively.
These values are close to or
smaller than the sum of the van
der Waals radii of hydrogen
and fluorine (2.55 Š), and
therefore, these data could
imply that there is some inter-
action. However, the distances
do not fall into the usual range
for hydrogen bonds.
Catalytic studies on the hydro-
silylation of ketones: We inves-
tigated the catalytic activity of
complexes 2–7 in the hydrosily-
lation of carbonyl compounds.
Cyclohexanone was used as a
model substrate under the reac-
tion conditions we previously
reported for complexes 1a and
2b.[14] The results are presented
in Table 5. Low conversions to
the silyl ether were obtained
with IMes-, SIMes-, and ItBu-
Figure 3. Ball and stick representations of [Cu
A
G
G
[Cu(ItBu)2]BF4 (7b). Most hydrogen atoms have been omitted for clarity.
ACHTREUNG
Table 2. Selected bond lengths [Š] and angles [8] for complex 2a.
À
Cu(1) C(1)
2.000(15)
1.910(15)
1.297(16)
1.390(16)
1.306(17)
1.366(16)
1.485(17)
1.488(16)
1.450(18)
1.519(18)
1.52(2)
C(1)-Cu(1)-C(4)
N(1)-C(1)-N(2)
N(3)-C(4)-N(4)
C(3)-C(2)-N(1)
C(6)-C(5)-N(3)
N(1)-C(1)-C(4)-N(3)
containing complexes; ICy-containing complexes 5a and 5b
and SIPr-containing complexes 2a and 2b led to good con-
version after relatively short reaction times. As often ob-
served, these screening results cannot be rationalized by
purely steric or electronic effects.[10f] The less bulky ligands,
IMes and SIMes, were not efficient in this transformation,
nor was the very bulky ItBu ligand. Furthermore, we ob-
À
Cu(1) C(4)
À
N(1) C(1)
À
N(3) C(4)
À
N(2) C(1)
À
N(4) C(4)
À
N(1) C(2)
À
N(3) C(5)
À
N(2) C(3)
À
À
N(4) C(6)
served a significant counterion effect and the BF4 com-
À
À
C(2) C(3)
pounds were systematically superior to their PF6 analogues.
À
C(5) C(6)
1.50(2)
8.338
The most important difference was observed with the bis-
IAd complexes: whereas reduction of cyclohexanone was
Cu(1)···P(1)
Table 3. Selected bond lengths [Š] and angles [8] for complexes 3a, 3b, 4a, 4b, 6a, 6b, and 7b.
3a
3b[a]
4a
4b
6a
6b
7b
À
Cu(1) C(1)
1.871(3)
1.353(3)
1.357(3)
1.382(4)
1.386(4)
1.337(4)
7.961
175.15(11)
104.2(2)
106.7(3)
67.05
1.884(2)
1.349(2)
1.349(2)
1.382(2)
1.383(2)
1.336(3)
7.586
179.88(9)
104.33(15)
106.34(17)
54.00
1.899(3)
1.328(2)
1.328(2)
1.483(3)
1.483(3)
1.531(5)
7.3327.151
180.0
1.896(4)
1.331(3)
1.331(3)
1.487(4)
1.487(4)
1.531(6)
1.938(5)
1.367(4)
1.367(4)
1.394(5)
1.394(5)
1.335(7)
1.933(5)
1.357(6)
1.368(6)
1.370(6)
1.388(6)
1.334(6)
1.909(2)
1.360(2)
1.360(2)
1.376(2)
1.376(2)
1.337(4)
À
N(1) C(1)
À
N(2) C(1)
À
N(1) C(2)
À
N(2) C(3)
À
C(2) C(3)
Cu(1)···P(1) or B(1)
C(1)-Cu(1)-C(1)’
N(1)-C(1)-N(2)
7.0927.746
7.475
180.0
180.0
177.97(19)
104.2(4)
107.9(4)
85.76
180.0
109.1(3)
102.3(1)
27.74
109.2(4)
102.36(16)
30.05
104.2(4)
107.1(2)
80.55
104.95(15)
107.31(9)
82.10
C(3)-C(2)-N(1)
N(1)-C(1)-C(1)’-N(1)’
[a] Two molecular units are present.
Chem. Eur. J. 2008, 14, 158 – 168
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
161

S. Díez-Gonzµlez, S. P. Nolan et al.
Table 4. Crystallographic data for 2a, 3a, 3b, 4a, 4b, 6a, 6b, and 7b.
2a 3a 3b
C54H76CuF6N4P C42H48CuF6N4P C42H48BCuF4N4 C42H52CuF6N4P C42H52BCuF4N4 C46H64CuF6N4P C49H70BCuF4N4 C22H40BCuF4N4
4a
4b
6a
6b
7b
formula
M [gmolÀ1
T [K]
crystalline
system
]
989.70
150(2)
monoclinic
817.35
150(2)
triclinic
759.19
295(2)
orthorhombic
821.39
223(2)
orthorhombic
763.23
150(2)
orthorhombic
881.52
150(2)
orthorhombic
881.44
150(2)
orthorhombic
510.93
295(2)
tetragonal
¯
space group Cc
P1
Pnna
Pnnn
Pnnn
I222
Pna21
P42/nbc
12.0350(5)
12.0350(5)
17.7379(10)
90.00
90.00
90.00
2569.2(2)
4
a [Š]
b [Š]
c [Š]
a [8]
b [8]
17.6928(8)
10.9435(5)
12.1449(6)
15.7673(7)
91.1810(10)
94.2690(10)
92.4800(10)
2087.23(17)
2
16.155(1)
31.477(2)
16.007(1)
90.00
90.00
90.00
8.2343(6)
12.1349(9)
20.6916(15)
90.00
90.00
90.00
8.1289(6)
11.7680(8)
20.4815(14)
90.00
90.00
90.00
11.4622(5)
12.3125(5)
14.1833(6)
90.00
90.00
90.00
40.1657(18)
19.3869(9)
11.3387(5)
90.00
90.00
90.00
17.7186(8)
16.7094(7)
90.00
99.4310(10)
90.00
g [8]
V [Š3]
Z
5167.5(4)
4
8139.6(10)
8
2067.6(3)
2
1959.3(2)
2
2001.67(15)
2
8829.3(7)
8
1calcd
[gcmÀ3
1.272
1.301
1.239
1.319
1.294
1.463
1.326
1.321
]
m [mmÀ1
]
0.514
0.622
860
5.88
804
6.28
0.611
0.654
0.554
8.95
F1(00400)
2
8523184
9323760
1080
crystal size
[mm]
0.60”0.25”0.15 0.50”0.30”0.20 0.32”0.40”0.50 0.12”0.28”0.38 0.60”0.30”0.15 0.70”0.50”0.30 0.80”0.40”0.20 0.15”0.30”0.40
q [8]
1.64–20.00
2.10–20.00
1.29–27.52
2.66–27.50
2.70–20.00
2.19–20.00
1.01–20.00
2.30–27.50
index range
h
k
Æ16
Æ17
Æ16
4788
689
Æ10
Æ11
Æ15
3887
488
À2 1 to 2 0
Æ40
À10 to 9
Æ15
Æ7
Æ11
Æ11
Æ13
921
Æ38
Æ18
Æ10
8215
À14 to 15
Æ15
Æ11
Æ19
920
l
À2 0 to 19
9309
Æ26
À2 3 to 2 2
1480
no. of data
no. of re-
straints
parameters
Gof on F2
2374
12110
0
146 03
121
612500
1.080
483
1.058
168
1.047
4
12 134
1.081
1086
1.115
79
1.118
1.086
1.053
R values (all R1 =0.0664
data) wR2 =0.1652
R1 =0.0311
wR2 =0.0689
R1 =0.0848
wR2 =0.1573
R1 =0.0570
wR2 =0.1271
R1 =0.0348
wR2 =0.0839
R1 =0.0272
wR2 =0.0721
R1 =0.0373
wR2 =0.0885
R1 =0.0606
wR2 =0.1506
Table 5. Catalyst screening for the hydrosilylation of cyclohexanone.
tion in the reaction with the model substrate were then
screened for the hydrosilylation of 2-methylcyclohexanone.
As shown in Table 6, no significant differences were ob-
served, which indicates that NHC ligands are moderately di-
astereo-directing for this particular reaction. The reactivity
trend depending on the counterion was also confirmed here,
although there was no influence on the diastereoselectivity.
For the sake of comparison, the reaction was also carried
Complex Conversion[a] [%]
Conversion[a] [%] Conversion[a] [%]
t=0.5 h
t=2 h
t=2 4 h
1a[b]
1b[b]
2a
2b
3a
3b
4a
4b
5a
5b
6a
6b
7a
30
99
22
64
<5
<5
<5
6
7
9
<5
17
<5
<5
99
–
89
97 (1 h)
<5
<5
<5
17
–
–
97 (2.5 h)
–
<5
<5
11
Table 6. Influence of copper catalyst on the diastereoselectivity of the re-
action.
52
30
41
5
97 (8 h)
94 (4 h)
12
Complex
t [h]
Yield[a] [%]
cis/trans[b]
5298 (4 h)
<5
12
2
4
A
G
1
6
3
95
9210:90
90
4
4
98
99
95
20:80
1a[c]
1b[c]
2a
2b
5a
7b
49
15:85
10:90
10:90
15:85
20:80
25:75
[a] Determined by GC. [b] Values from reference [14].
[d]
2
2
50
60
[d]
10
6
24
completed in 4 h in the presence of 6b, only 12% of the ex-
pected silyl ether was formed with 6a after 24 h.
5b
6b
We were also interested in the possible influence of the
NHC ligand and/or counterion on the diastereoselectivity of
the reaction. Cationic catalysts capable of reaching comple-
[a] Isolated yields are the average of two runs. [b] Determined by
1H NMR spectroscopy. [c] Values from reference [14]. [d] Conversion de-
termined by H NMR spectroscopy.
1
162
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Chem. Eur. J. 2008, 14, 158 – 168

Synthesis and Characterization of [Cu(NHC)2]X Complexes
A
FULL PAPER
out with [CuCl
(IPr)] as the catalyst,[13] again with no effect
A
Table 8. [Cu(ICy)2]BF4-catalyzed hydrosilylation of hindered ketones.
N
on the diastereoisomeric ratio.
Another trend can be observed from the results presented
in Table 6. The methyl group in the a position to the car-
bonyl group considerably prolonged the required reaction
times. This effect was especially significant for SIPr-contain-
ing complex 2b: whereas cyclohexanone was reduced in 1 h,
only 60% conversion was obtained for 2-methylcyclohexa-
none after 24 h. However, ICy-based complexes showed the
smallest influence of the steric hindrance around the carbon-
yl group. These observations are consistent with our previ-
ous results concerning hydrosilylation reactions in the pres-
Entry
1
Product
t [h]
Yield[a] [%]
99
0.25
2
3
4
5
6
7
8
9
5.0
1.5
0.5
1.0
2.5
0.25
2.5
4.0
96[b]
98
95
97
94
95
92
82
ence of [CuCl
(NHC)] complexes.[13]
A
To verify this trend, the more hindered ketone, dicyclo-
hexyl, was used as a model substrate for comparing our dif-
ferent catalytic systems (Table 7). When comparing the ac-
Table 7. Comparison of catalysts.
Entry
Complex
Et3SiH [equiv] Solvent T [8C] t [h] Yield[a] [%]
1
2
3
4
5
6
5b
1b
1b
2THF
2THF
3
3
3
55
55
1.5 98
9.0 40
55
[b]
THF
toluene 80
toluene 55
3.0 98
0.5 99
1.5 98
G
A
G
ACHTREUNG
C
G
2toluene 55
1.5 50
[a] Isolated yields are the average of two runs. [b] Determined by GC.
tivity of 5b with 1b, half the reaction time was required for
reaching comparable yields in the presence of the ICy-based
catalyst (Table 7, entries 1 and 3). Furthermore, for this par-
ticular case, three equivalents of silane were required with
1b to ensure good conversion. However, two equivalents
were generally enough for other substrates.[14] On the other
hand, an even faster conversion was obtained with [CuCl-
[a] Isolated yields are the average of two runs. [b] meso trans–cis/cis–
trans/meso cis–cis=39:9:52.
Mechanistic studies: In our previous report, some insights
into the mechanism of this hydrosilylation reaction were
provided. Study of the hydrosilylation of cyclohexanone in
A
However, when comparable reaction conditions were used
(T=558C, 2equiv of hydride source), 5b was the optimal
catalyst (Table 7, entries 5 and 6). We then further explored
the presence of [Cu(CH3CN)4]BF4 (3 mol%) and IPr under
R
different conditions indicated that a monocarbene copper
compound rather than a bis-carbene complex would be the
actual active species.[14] When complexes 1b, 5b, or 6a were
heated in THF at reflux, no NHC decoordination or decom-
position was observed after 24 h. Furthermore, displacement
the activity of [Cu(ICy)2]BF4 5b with a number of ketones
E
with varying steric congestion around the carbonyl bond
under our hydrosilylation conditions. Under optimized con-
ditions ([Cu] (3 mol%), base (12mol%), silane (2equiv) in
THF at 558C), the expected silyl ethers were formed in ex-
cellent yields after short reaction times (Table 8). Acyclic,
cyclic, and aromatic ketones were screened with comparable
good results and the presence of functional groups, such as
halogen or CF3, were also compatible with our catalytic
system (Table 8, entries 8 and 9).
It is important to note that for all of the substrates tested,
ICy-containing complex 5b was more efficient (i.e., shorter
reaction times) than its IPr analogue 1b, the greatest differ-
ence in reaction time being observed for aromatic ketones.
of an IPr moiety in [Cu(IPr)2]BF4 (1b) by a phosphine
N
ligand was unsuccessful.[17] Treatment of 1b with PPh3 or
PCy3 at temperatures ranging from 70 to 1108C only led to
the recovery of the starting complex.[18] No reaction was ob-
served after heating at reflux in [D5]pyridine either. All of
À
the above results seem to indicate a fairly strong NHC
copper bond.
Next, we carried out several experiments, monitored by
1H NMR spectroscopy, to clarify the nature of the postulat-
ed active monocarbene species. Complex 1b was treated
Chem. Eur. J. 2008, 14, 158 – 168
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163

S. Díez-Gonzµlez, S. P. Nolan et al.
with NaOtBu in [D8]THF under our catalytic conditions
(base (4 equiv), [Cu] (0.06m), RT). The appearance of two
the isolation and characterization of such species.[20] Relying
on this report and on our own observations presented
above, we propose a mechanism for this copper-catalyzed
hydrosilylation of ketones, which is depicted in Scheme 1.
À
new septuplet resonances (due to CH
A
newly formed species) was observed. These signals could
either imply the formation of two new compounds or the
formation of one species in which both IPr ligands on the
copper center are inequivalent.[19] Integration of the
1H NMR spectrum showed that both of the new resonance
signals had an equimolecular proportion and that around
10% of the starting complex 1b still remained. Interestingly,
these proportions remained unchanged after longer reaction
times (from 10 min to 19 h), higher reaction temperature
(608C instead of RT), higher base loading (6 equiv of base
instead of 4), or higher concentration (up to 0.9m). Howev-
er, a minimum of four equivalents of base were required to
ensure optimal conversion of the starting complex.
First, formation of [Cu
A
G
N
and NaOtBu occurs. Then, the active catalyst, an NHC–
copper hydride species, would be formed by s-bond meta-
thesis between [Cu
N
N
dition of hydride to the carbonyl carbon would result in a
copper alkoxide that would undergo another s-bond meta-
thesis with the hydrosilane to form the expected silyl ether
and regenerate the active catalyst.[21] This proposed mecha-
nism is also in agreement with the experimental evidence
for the phosphine–copper catalyst systems.[22]
Moreover, in our previous catalytic system based on
[CuCl(NHC)] complexes, we proposed that the excess base
G
We attempted to isolate the newly formed species by car-
rying out the same reaction on a larger scale. After stirring
for two hours, the reaction mixture was filtered inside a
glove box over a plug of Celite and the resulting solution
was mixed with pentane. The white powder that formed was
filtered and identified as a mixture of 1b with some uniden-
tified byproducts (around 15% of the mass balance). The
filtrate was concentrated under vacuum and showed the sig-
nals previously discussed (Figures 4 and 5). By comparison
generally required would interact with the hydrosilane and
facilitate the second s-bond metathesis. With the present
system, such excess is required for the efficient formation of
[Cu
N
N
mol%), released from the initial bis-NHC complex, could
also play such a role.[23] As the hydrosilane loading is lower
in this case (2instead of 3 equiv), IPr would activate the hy-
dride source more efficiently towards s-bond metathesis.
Hence, the better catalytic activity in the hydrosilylation of
1
with known samples, we could assign the signals in the H
carbonyls of the cationic compounds [Cu
A
and 13C NMR spectra to an equimolar mixture of IPr ligand
compared to that of [CuCl(NHC)] can be tentatively ration-
ACHTREUNG
and [Cu
13C NMR spectra could be assigned in [D8]THF and C6D6.[19]
Formation of a copper hydride from [Cu(NHC)(OtBu)]
A
G
alized. We therefore postulate that the favorable displace-
ment of a neutral NHC instead of an anionic chloride from
the copper center by a tert-butoxide adds up to the activat-
ing effect of the free NHC towards the hydrosilane in in-
creasing the reaction rate.
A
ACHTREUNG
upon treatment with a hydrosilane has previously been
evoked for the hydrosilylation of ketones and confirmed by
Figure 4. 1H NMR assignment of the isolated products in [D8]THF.
164
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Chem. Eur. J. 2008, 14, 158 – 168

Synthesis and Characterization of [Cu(NHC)2]X Complexes
A
FULL PAPER
Figure 5. 13C NMR assignment of the isolated products in C6D6.
Scheme 2.
Conclusion
The straightforward preparation of homoleptic bis-carbene–
copper complexes from readily available materials has been
demonstrated. These air- and moisture-stable complexes are
efficient precatalysts for the hydrosilylation of ketones.
When compared with their [CuCl(NHC)] analogues, the cat-
U
ionic species proved to be more efficient under similar reac-
tion conditions. 1H NMR spectroscopic studies clearly
Scheme 1. Proposed catalytic cycle.
showed the activation of [Cu
A
NaOtBu to form [Cu(NHC)(OtBu)], which is the direct pre-
A
ACHTREUNG
cursor of an NHC–copper hydride that is the actual active
species in this transformation. Moreover, the observed coun-
terion effect in the catalytic studies could be rationalized as
a consequence of the difference in solubility of the inorganic
salts formed. Applications to other carbonyl or pseudocar-
bonyl containing substrates and further activity studies are
currently ongoing in our laboratories.
Finally, to clarify the counterion effect observed during
the catalytic studies, complex 1a was treated with NaOtBu
in [D8]THF. Under the same conditions as those for 1b,
which yielded 85% of the alkoxide species (see above), only
30% of the complex was converted into [Cu
A
E
and IPr, even after 24 h. This difference in reactivity could
be rationalized by the different solubility of the inorganic
salts formed in THF. Qualitative experiments showed that
NaPF6 is more soluble in THF than NaBF4.[24] The easier
precipitation of NaBF4 in the reaction mixture would act as
a driving force in this transformation to shift the reaction
Experimental Section
General considerations: All ketones were used as received. Solid re-
agents were stored under argon in a glove box that contained less than
1 ppm of oxygen. Tetrakis(acetonitrile)copper(I) hexafluorophosphate
and tetrafluoroborate,[25] imidazolium, and imidazolinium salts and IPr
equilibrium towards the formation of [Cu
N
N
(Scheme 2).
Chem. Eur. J. 2008, 14, 158 – 168
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165

S. Díez-Gonzµlez, S. P. Nolan et al.
were synthesized according to literature procedures.[26] Solvents were dis-
tilled over appropriate drying agents. 1H and 13C NMR spectra were re-
corded on a 400 MHz spectrometer at room temperature. Chemical shifts
(d) are reported in ppm with respect to tetramethylsilane as the internal
standard. For 31P NMR spectroscopy, experiments were calibrated with
H3PO4 as external standard. For 11B NMR spectroscopy, experiments
were calibrated with Et2O·BF3 as external standard. Elemental analyses
were performed by Robertson Microlit Laboratories (Madison, NJ,
USA). All reported yields are isolated yields and are the average of at
least two runs.
Bis[1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene]copper(I) tetrafluoro-
G
borate (3b): Tetrakis(acetonitrile)copper(I) tetrafluoroborate (0.157 g,
0.5 mmol), IMes·HCl (0.340 g, 1 mmol), and NaOtBu (0.127 g, 1.3 mmol)
were added to a vial fitted with a screw cap inside a glove box, and
stirred in dry THF (12mL) outside the glove box for 15 h. After filtering
the reaction mixture through a plug of Celite (CH2Cl2), the filtrate was
mixed with pentane to form a precipitate. A second filtration led to the
isolation of 3b as a white solid (0.280 g, 74%). Crystals suitable for X-
ray diffraction were grown by slow diffusion of hexane from a saturated
solution of 3b in CH2Cl2. 1H NMR (400 MHz, [D6]acetone, 258C, TMS:
d=7.46 (s, 4H; NCH), 7.05 (s, 8H; HAr), 2.45 (s, 12H; CH3), 1.75 ppm (s,
Synthesis of [Cu
A
24H; CH ); 13C NMR (100 MHz, [D6]acetone, 258C, TMS): d=178.8 (C,
3
Bis[1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene]cop-
ACHTREUNG
NCN), 140.7 (C, CAr), 136.3 (C, CAr), 135.9 (C, CAr), 130.5 (CH, CAr),
124.5 (CH, NCH), 21.6 (CH3), 17.7 ppm (CH3); 11B NMR (128 MHz,
[D6]acetone, 258C): d=À0.97 ppm (s); 19F (376 MHz, [D6]acetone,
258C): d=À152.0 ppm (s); elemental analysis calcd (%) for
C42H48BCuF4N4: C 66.44, H 6.37, N 7.38; found: C 66.25, H 6.50, N 7.28.
per(I) hexafluorophosphate (2a): Tetrakis(acetonitrile)copper(I) hexa-
fluorophosphate (0.186 g, 0.5 mmol), SIPr·HBF4 (0.478 g, 1 mmol), and
NaOtBu (0.127 g, 1.3 mmol) were added to a vial fitted with a screw cap
inside a glove box, and stirred in dry THF (12mL) outside the glove box
for 15 h. After filtering the reaction mixture through a plug of Celite
(CH2Cl2), the filtrate was mixed with pentane to form a precipitate. A
second filtration led to the isolation 2a as a white solid (0.337 g, 68%).
Crystals suitable for X-ray diffraction were grown by slow diffusion of
hexane from a saturated solution of 2a in CH2Cl2. 1H NMR (400 MHz,
[D6]acetone, 258C, TMS): d=7.42(t, J=7.5 Hz, 4H; HAr), 7.18 (d, J=
7.5 Hz, 8H; HAr), 3.96 (s, 4H; CH2CH2), 2.94 (septuplet, J=6.8 Hz, CH-
Bis[1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene]copper(I)
T
hexafluorophosphate (4a): Tetrakis(acetonitrile)copper(I) hexafluoro-
phosphate (0.186 g, 0.5 mmol), SIMes·HBF4 (0.393 g, 1 mmol), and
NaOtBu (0.127 g, 1.3 mmol) were added to a vial fitted with a screw cap
inside a glove box, and stirred in dry THF (12mL) outside the glove box
for 15 h. After filtering the reaction mixture through a plug of Celite
(CH2Cl2), the filtrate was mixed with pentane to form a precipitate. A
second filtration led to the isolation of 4a as a white solid (0.354 g,
86%). Crystals suitable for X-ray diffraction were grown by slow diffu-
sion of methyl tert-butyl ether from a saturated solution of 4a in CH2Cl2.
1H NMR (400 MHz, CDCl3, 2 58C, TMS): d=6.86 (s, 8H; HAr), 3.85 (s,
8H; CH2CH2), 2.39 (s, 12H; CH3), 1.84 ppm (s, 24H; CH3); 13C NMR
(100 MHz, [D6]acetone, 258C, TMS): d=201.4 (C, NCN), 138.6 (C, CAr),
135.5 (CH, CAr), 134.7 (C, CAr), 129.4 (CH, CAr), 51.0 (CH2,
NCH2CH2N), 21.1 (CH3), 17.2ppm (CH 3); 31P NMR (162MHz,
[D6]acetone, 258C): d=À141.1 ppm (m); 19F (376 MHz[D6]acetone,
258C): d=À72.8 ppm (d, JFP =706 Hz); elemental analysis calcd (%) for
C42H52CuF6N4P: C 61.41, H 6.38, N 6.82; found: C 61.53, H 6.33, N 6.71.
(CH3)2), 1.16 (d, J=6.8 Hz, 24H; CH3), 0.92ppm (d, J=6.8 Hz, 24H;
CH3); 13C NMR (100 MHz, CDCl3, 2 58C, TMS): d=199.8 (C, NCN),
146.1 (CH, CAr), 134.8 (C, CAr), 129.9 (CH, CAr), 125.0 (CH, CAr), 54.5
(CH2, NCH2), 28.6 (CH, CH(CH3)2), 25.1 (CH3), 24.3 ppm (CH3);
U
31P NMR (162MHz, [D 6]acetone, 258C): d=À141.1 ppm (m); 19F
(376 MHz, [D6]acetone, 258C): d=À72.8 ppm (d, JFP =706 Hz); elemen-
tal analysis calcd (%) for C54H76CuF6N4P: C 65.53, H 7.77, N 5.66; found:
C 65.28, H 7.72, N 5.56.
Bis[1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene]cop-
G
per(I) tetrafluoroborate (2b): Tetrakis(acetonitrile)copper(I) tetrafluoro-
borate (0.157 g, 0.5 mmol), SIPr·HBF4 (0.478 g, 1 mmol), and NaOtBu
(0.127 g, 1.3 mmol) were added to a vial fitted with a screw cap inside a
glove box, and stirred in dry THF (12mL) outside the glove box for 6 h.
After filtering the reaction mixture through a plug of Celite (CH2Cl2),
the filtrate was mixed with pentane to form a precipitate. A second filtra-
tion led to the isolation of 2b as a white solid (0.295 g, 63%). 1H NMR
(400 MHz, [D6]acetone, 258C, TMS): d=7.45 (t, J=7.9 Hz, 4H; HAr),
7.26 (d, J=7.9 Hz, 8H; HAr), 4.12(s, 8H; C H2CH2), 3.08 (septuplet, J=
Bis[1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene]copper(I)
C
tetrafluoroborate (4b). Tetrakis(acetonitrile)copper(I) tetrafluoroborate
(0.157 g, 0.5 mmol), SIMes·HBF4 (0.393 g, 1 mmol), and NaOtBu
(0.127 g, 1.3 mmol) were added to a vial fitted with a screw cap inside a
glove box, and stirred in dry THF (12mL) outside the glove box for 15 h.
After filtering the reaction mixture through a plug of Celite (CH2Cl2),
the filtrate was mixed with pentane to form a precipitate. A second filtra-
tion led to the isolation of 4b as a white solid (0.329 g, 86%). Crystals
suitable for X-ray diffraction were grown by slow diffusion of methyl
tert-butyl ether from a saturated solution of 4b in CH2Cl2. 1H NMR
(400 MHz, CDCl3, 2 85C, TMS): d=6.83 (s, 8H; HAr), 3.84 (s, 8H;
CH2CH2), 2.37 (s, 12H; CH3), 1.81 ppm (s, 24H; CH3); 13C NMR
(100 MHz, CDCl3, 2 58C, TMS): d=201.3 (C, NCN), 138.7 (C, CAr), 135.8
(CH, CAr), 134.7 (C, CAr), 129.6 (CH, CAr), 51.2(CH 2, NCH2CH2N), 21.3
(CH3), 17.5 ppm (CH3); 11B NMR (128 MHz, [D6]acetone, 258C): d=
À0.97 ppm (s); 19F (376 MHz, [D6]acetone, 258C): d=À152.1 ppm (s); el-
emental analysis calcd (%) for C42H52BCuF4N4: C 66.09, H 6.87, N 7.34;
found: C 65.97, H 6.71, H 7.09.
6.8 Hz, 8H; CH(CH3)2), 1.26 (d, J=6.8 Hz, 24H; CH3), 1.10 ppm (d, J=
E
6.8 Hz, 24H; CH3); 13C NMR (100 MHz, CDCl3, 2 58C, TMS): d=201.4
(C, NCN), 146.5 (CH, CAr), 134.3 (C, CAr), 129.7 (C, CAr), 124.4 (CH,
CAr), 54.0 (CH2, NCH2CH2N), 28.5 (CH, CH
(CH3)2), 24.3 (CH3),
G
23.6 ppm (CH3); 11B NMR (128 MHz, [D6]acetone, 258C): d=À0.97 ppm
(s); 19F (376 MHz, [D6]acetone, 258C): d=À153.7 ppm (s); elemental
analysis calcd (%) for C54H76BCuF4N4: C 69.62, H 8.22, N 6.01; found: C
69.88, H 8.45, N 5.82.
Bis[1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene]copper(I) hexafluoro-
N
phosphate (3a): Tetrakis(acetonitrile)copper(I) hexafluorophosphate
(0.186 g, 0.5 mmol), IMes·HCl (0.340 g, 1 mmol), and NaOtBu (0.098 g,
1.0 mmol) were added to a vial fitted with a screw cap inside a glove box,
and stirred in dry THF (12mL) outside the glove box for 15 h. After fil-
tering the reaction mixture through a plug of Celite (CH2Cl2), the filtrate
was mixed with pentane to form a precipitate. A second filtration led to
the isolation of 3a as a white solid (0.372g, 91%). Crystals suitable for
X-ray diffraction were grown by slow diffusion of hexane from a saturat-
ed solution of 3a in CH2Cl2. 1H NMR (400 MHz, [D6]acetone, 258C,
TMS): d=7.46 (s, 4H; NCH), 7.05 (s, 8H; HAr), 2.45 (s, 12H; CH3),
1.74 ppm (s, 24H; CH3); 13C NMR (100 MHz, [D6]acetone, 258C, TMS):
d=178.8 (C, NCN), 140.8 (C, CAr), 136.3 (C, CAr), 136.0 (C, CAr), 130.5
(CH, CAr), 124.5 (CH, NCH), 21.7 (CH3), 17.7 ppm (CH3); 31P NMR
(162MHz, [D 6]acetone, 258C): d=À141.1 ppm (m); 19F (376 MHz,
[D6]acetone, 258C): d=À72.7 ppm (d, JFP =706 Hz); elemental analysis
calcd (%) for C42H48CuF6N4P: C 61.72, H 5.92, N 6.85; found: C 61.45, H
5.64, N 6.52.
Bis[1,3-bis(cyclohexyl)imidazol-2-ylidene]copper(I) hexafluorophosphate
N
(5a): Tetrakis(acetonitrile)copper(I) hexafluorophosphate (0.186 g,
0.5 mmol), ICy·HBF4 (0.320 g, 1 mmol), and NaOtBu (0.096 g, 1.0 mmol)
were added to a vial fitted with a screw cap inside a glove box, and
stirred in dry THF (12mL) outside the glove box for 15 h. After filtering
the reaction mixture through a plug of Celite (THF), the filtrate was
mixed with pentane to form a precipitate. A second filtration led to the
isolation of 5a as a white solid (0.267 g, 80%). 1H NMR (400 MHz,
[D6]acetone, 258C, TMS): d=7.51 (s, 4H; NCH imidazole), 4.54–4.32(m,
4H; NCH cyclohexyl), 2.28–2.11 (m, 8H; CH2), 2.09–1.84 (m, 16H;
CH2), 1.80–1.67 (m, 4H; CH2), 1.57–1.39 (m, 8H; CH2), 1.36–1.19 ppm
(m, 4H; CH2); 13C NMR (100 MHz, [D6]acetone, 258C): d=174.2(C,
NCN), 120.0 (CH, NCH imidazole), 62.7 (CH, NCH cyclohexyl), 36.1
(CH2), 26.7 (CH2), 26.2 ppm (CH2); 31P NMR (162MHz, [D 6]acetone,
258C): d=À141.1 ppm (m); 19F (376 MHz, [D6]acetone, 258C): d=
166
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Chem. Eur. J. 2008, 14, 158 – 168

Synthesis and Characterization of [Cu(NHC)2]X Complexes
A
FULL PAPER
À72.7 ppm (d,
C30H48CuF6N4P: C 53.52, H 7.19, N 8.32; found: C 53.58, H 7.18, N 8.25.
Bis[1,3-bis(cyclohexyl)imidazol-2-ylidene]copper(I) tetrafluoroborate
(5b): Tetrakis(acetonitrile)copper(I) tetrafluoroborate (0.157 g,
J
FP =706 Hz); elemental analysis calcd (%) for
258C): d=À72.7 ppm (d, JFP =707 Hz); elemental analysis calcd (%) for
C22H40CuF6N4P: C 46.43, H 7.08, N 9.84; found: C 46.24, H 6.96, N 9.84.
Bis
Tetrakis(acetonitrile)copper(I) tetrafluoroborate (0.157 g, 0.5 mmol), I-
tBu·HBF4 (0.268 g, 1 mmol), and NaOtBu (0.107 g, 1.1 mmol) were
[1,3-bis(tert-butyl)imidazol-2-ylidene]copper(I) tetrafluoroborate (7b):
ACHTREUNG
AHCTREUNG
0.5 mmol), ICy·HBF4 (0.320 g, 1 mmol) and NaOtBu (0.107 g, 1.1 mmol)
were added to a vial fitted with a screw cap inside a glove box, and
stirred in dry THF (12mL) outside the glove box for 15 h. After filtering
the reaction mixture through a plug of Celite (THF), the filtrate was
mixed with pentane to form a precipitate. A second filtration led to the
isolation of 5b (0.284 g, 93%). 1H NMR (400 MHz, [D6]acetone, 258C,
TMS): d=7.51 (s, 4H; NCH imidazole), 4.55–4.33 (m, 4H; NCH cyclo-
hexyl), 2.25–2.10 (m, 8H; CH2), 2.01–1.85 (m, 16H; CH2), 1.79–1.67 (m,
4H; CH2), 1.57–1.39 (m, 8H; CH2), 1.36–1.19 ppm (m, 4H; CH2);
13C NMR (100 MHz, [D6]acetone, 258C): d=173.8 (C, NCN), 120.0 (CH,
NCH imidazole), 62.4 (CH, NCH cyclohexyl), 35.8 (CH2), 26.5 (CH2),
25.9 ppm (CH2); 11B NMR (128 MHz, [D6]acetone, 258C): d=À0.97 ppm
(s); 19F (376 MHz, [D6]acetone, 258C): d=À152.0 ppm (s); elemental
analysis calcd (%) for C30H48BCuF4N4: C 58.58, H 7.87, N 9.11; found: C
58.44, H 7.81, N 9.00.
added to a vial fitted with a screw cap inside a glove box, and stirred in
dry THF (12mL) outside the glove box for 6 h. After filtering the reac-
tion mixture through a plug of Celite (acetone), the filtrate was mixed
with pentane to form a precipitate. A second filtration led to the isolation
of 7b (0.255 g, 100%). Crystals suitable for X-ray diffraction were grown
by slow diffusion of methyl tert-butyl ether from a saturated solution of
7b in THF. 1H NMR (400 MHz, [D6]acetone, 258C, TMS): d=7.64 (s,
4H; NCH), 1.84 ppm (s, 36H; CH3); 13C NMR (100 MHz, [D6]acetone,
258C): d=171.2(C, NCN), 119.8 (CH, NCH), 58.8 (C,
C(CH3)3),
C
General procedure for the hydrosilylation of ketones: Complex 5b
(18 mg, 0.03 mmol, 3 mol%) and sodium tert-butoxide (12mg, 12mol%)
were added to a vial fitted with a septum screw cap inside a glove box,
and stirred in dry THF (2mL, 55 8C, 10 min) outside the glove box
before adding triethylsilane (0.33 mL, 2mmol, 2equiv) through the
septum by means of a syringe. After stirring for a further 10 min, the
ketone (1 mmol) was added. When the starting material was a solid, it
was added as a solution in THF. The reaction was monitored by GC.
After consumption of the starting material or when no further conversion
occurred, the reaction mixture was opened to air and filtered through a
plug of active charcoal and Celite (ethyl acetate). The organic phase was
concentrated in vacuo and the purity of the residue established by GC
and 1H NMR spectroscopy. Flash chromatography was then performed
unless the crude product was estimated to be greater than 95% pure.
Bis[1,3-bis(adamantyl)imidazol-2-ylidene]copper(I) hexafluorophosphate
G
(6a): Tetrakis(acetonitrile)copper(I) hexafluorophosphate (0.186 g,
0.5 mmol), IAd·HBF4 (0.424 g, 1 mmol), and NaOtBu (0.127 g, 1.3 mmol)
were added to a vial fitted with a screw cap inside a glove box, and
stirred in dry THF (12mL) outside the glove box for 15 h. After filtering
the reaction mixture through a plug of Celite (CH2Cl2), the filtrate was
mixed with pentane to form a precipitate. A second filtration led to the
isolation of 6a as a white solid (0.369 g, 84%). Crystals suitable for X-ray
diffraction were grown by slow diffusion of methyl tert-butyl ether from a
saturated solution of 6a in CH2Cl2. 1H NMR (400 MHz, CDCl3, 2 58C,
TMS): d=7.34 (s, 4H; NCH), 2.38 (s, 24H; CH), 2.23 (s, 12H; CH2),
1.87–1.72(m, 1H2 ; CH), 1.7–21.50 ppm (m, 1H2 ; CH);
13C NMR
(100 MHz, CDCl3, 2 58C, TMS): d=169.8 (C, NCN), 117.3 (CH, NCH),
58.0 (C, NCCH2), 44.9 (CH), 35.7 (CH), 29.7 ppm (CH2); 31P NMR
(162MHz, [D 6]acetone, 258C): d=À141.1 ppm (m); 19F (376 MHz,
[D6]acetone, 258C): d=À72.8 ppm (d, JFP =706 Hz); elemental analysis
calcd (%) for C46H64CuF6N4P: C 62.67, H 7.32, N 6.36; found: C 62.39, H
7.21, N 6.23.
X-ray crystallography: CCDC 652351, 652352, 652353, 652354, 652355,
652356, 652357, and 652358 contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/data_
request/cif.
Bis
(6b):
A
tetrafluoroborate
(0.157 g,
0.5 mmol), IAd·HBF4 (0.424 g, 1 mmol), and NaOtBu (0.127 g, 1.3 mmol)
were added to a vial fitted with a screw cap inside a glove box, and
stirred in dry THF (12mL) outside the glove box for 15 h. After filtering
the reaction mixture through a plug of Celite (CH2Cl2), the filtrate was
mixed with pentane to form a precipitate. A second filtration led to the
isolation of 6b as a white solid (0.389 g, 95%). Crystals suitable for X-
ray diffraction were grown by slow cooling of a saturated solution of 6b
in acetone. One molecule of acetone crystallized with 6b. 1H NMR
(400 MHz, CDCl3, 2 58C, TMS): d=7.36 (s, 4H; NCH), 2.39 (s, 24H;
CH), 2.23 (s, 12H; CH2), 1.85–1.72(m, 12H; CH), 1.72–1.59 ppm (m,
12H; CH); 13C NMR (400 MHz, CDCl3, 2 85C, TMS): d=169.9 (C,
NCN), 117.3 (CH, NCH), 58.0 (C, NC), 44.9 (CH), 35.7 (CH), 29.8 ppm
(CH2); 11B NMR (128 MHz, [D6]acetone, 258C): d=À0.90 ppm (s); 19F
(376 MHz, [D6]acetone, 258C): d=À152.0 ppm (s); elemental analysis
calcd (%) for C46H64BCuF4N4: C 67.10, H 7.83, N 6.80; found: C 67.23, H
7.82, N 6.99.
Acknowledgements
The ICIQ Foundation and the Petroleum Research Fund administered
by the ACS are gratefully acknowledged for their financial support of
this work. S.P.N. is an ICREA Research Professor.
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Received: July 3, 2007
Published online: November 12, 2007
168
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Chem. Eur. J. 2008, 14, 158 – 168