Alkylfluorenyl substituted N-heterocyclic carbenes in copper(i) catalysed hydrosilylation of aldehydes and ketones

Article abstract of DOI:10.1039/c5dt01888a

Copper(i) complexes featuring N-heterocyclic carbenes (NHCs) in which the nitrogen atoms are substituted by a 9-ethyl-9-fluorenyl group (EF) have been synthesised and tested in the hydrosylilation of functionalized and/or sterically demanding ketones and aldehydes. These reactions, carried out with triethylsilane as hydride source, were best achieved with the imidazolylidene copper complex 2d in which the EF substituents can freely rotate about the corresponding N-CEF bonds. The remarkable stability of the active species, which surpasses that of previously reported Cu-NHC catalysts is likely to rely on the ability of the NHC side arms to protect the copper centre during the catalytic cycle by forming sandwich-like intermediates, but also on its steric flexibility facilitating approach of encumbered substrates. TONs up to 1000 were reached.

Full text of DOI:10.1039/c5dt01888a

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Alkylfluorenyl substituted N-Heterocyclic Carbenes in Copper(I) catalysed
Hydrosilylation of Aldehydes and Ketones †
Matthieu Teci,a Nicolas Lentz,a Eric Brenner,*a Dominique Matt,*a and Loïc Toupetb
Table of contents
An alkylfluorenyl-substituted imidazolylidene copper complex efficiently catalyses the
hydrosilylation of functionalized and/or sterically demanding carbonyl compounds, using
triethylsilane as cost-effective hydride source; the catalyst displays remarkable stability as a
result of the steric properties of the NHC ligand.
rotating subunit
OSiEt3
O
N
N
Et3SiH (3 equiv.)
Cu Cl
[Cu] (0.1 mol%)
t-BuOK (5 mol%)
THF, 65°C, 70 h
TON : 1000
[Cu]

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ARTICLE
Alkylfluorenyl substituted N-Heterocyclic Carbenes in
Copper(I) catalysed Hydrosilylation of Aldehydes and
Ketones†
Cite this: DOI: 10.1039/x0xx00000x
Matthieu Teci,a Nicolas Lentz,a Eric Brenner,*a Dominique Matt,*a and Loïc
Toupetb
Received 00th February 2014,
Accepted 00th .... 2014
DOI: 10.1039/x0xx00000x
Copper(I) complexes featuring N-heterocyclic carbenes (NHCs) in which the nitrogen atoms are
substituted by a 9-ethyl-9-fluorenyl group (EF) have been synthesised and tested in the
hydrosylilation of functionalized and/or sterically demanding ketones and aldehydes. These
reactions, carried out with triethylsilane as hydride source, were best achieved with the
imidazolylidene copper complex 2 in which the EF substituents can freely rotate about the
corresponding N-CEF bonds. The remarkable stability of the active species, which surpasses
that of previously reported Cu-NHC catalysts is likely to rely on the ability of the NHC side arms
to protect the copper centre during the catalytic cycle by forming sandwich-like intermediates, but
also on its steric flexilbility facilitating approach of encumbered substrates. TONs up to 1000
were reached.
www.rsc.org/
atoms of the alkyl side groups.3 Ligands of this type, which
combine a strong coordination bond with two non covalent
Introduction
bonds have been termed bimodal pincers. In analogs in which
the benzannulated moiety has been replaced by smaller
imidazolinylidene or imidazolylidene rings, rotation of the AF
unit about the N–CAF bond may occur, this enabling the ligand
to adapt to the steric requirements of the metal environment. It
is worth mentioning that copper complexes containing other
bulky NHCs have recently been used in the hydrosylilation of
carbonyl compounds.4
Hydrosilylation of ketones and subsequent hydrolysis of the
resulting silyl ethers is a well-established method for the
preparation of a variety of alcohols, including chiral ones.1
Current research in this area focuses on the development of
new, cost-effective catalysts with the hope to move away from
the expensive Rh- or Ru-catalysts traditionally employed for
these reactions.2 Efforts to achieve this goal, logically, also
require the concomitant development of appropriate ligands.
As part of a program aimed at exploring the catalytic
Results and discussion
aLaboratoire de Chimie Inorganique Moléculaire et Catalyse, Institut de
Chimie (UMR 7177 CNRS), Université de Strasbourg, 4 rue Blaise Pascal,
Synthesis of the complexes
67070
Strasbourg
Cedex,
France.
Eꢀmails:
dmatt@unistra.fr,
eric.brenner@unistra.fr
bInstitut de Physique de Rennes, UMR 6251 CNRS, Université de Rennes1,
Campus de Beaulieu, 35042, Rennes cedex, France.
The complexes used in this study (2a-d) were obtained by
reacting the appropriate azolium precursor (1a-d) with CuCl in
CH2Cl2 in the presence of K2CO3 (Scheme 1).
† Electronic supplementary information (ESI) available: Experimental details,
NMR spectra. For ESI or other electronic format see DOI: XXXXXX
properties of N-heterocyclic carbenes (NHCs) bearing
expanded alkylfluorenyl (AF) substituents, we have examined
the hydrosylilation of ketones and aldehydes with copper(I)
complexes based on such ligands. Previously, we had shown
that in their complexes AF-substituted NHCs derived from
benzimidazolium salts display a preorganised structure in
which the akyl groups are turned towards the metal centre, so as
to promote a terdentate, clamp-like bonding mode of the ligand,
involving the carbenic C atom and two weakly coordinating CH
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pbv of the corresponding carbenes must be regarded as a time-
dependent variable.
60%
58%
50%
40%
36%
30%
20%
10%
0%
N
N
N
N
Fig. 1 Percent buried volume (%Vbur) for the two extreme ligand conformations
of 2d calculated from previously reported X-ray studies. Parameters used for the
calculations: C-M distance: 2.1 Å; sphere radius: 3.5 Å
Catalysis
Scheme 1 Synthesis of copper complexes 2a-d
We began the catalytic investigations by examining the
properties of 2b
as the model substrate), using either THF or toluene as solvent,
and -BuOK or -BuONa as the base. The runs were carried out
applying a slight modification of a procedure described by
Nolan et al. (Table 1).8 Triethylsilane was chosen as a mild and
economical hydride source. Optimal conditions consisted in
carrying out 3h runs in toluene at room temperature, with 3
equivalents of silane, 3 mol% of copper catalyst, and 12 mol%
-d in the hydrosilylation of acetophenone (taken
Complex 2d has been briefly described in a previous report.5
As complex 2a underwent rapid oxidation in air, both in
solution and in the solid state (colour change from white to
blue), this complex was not considered for hydrosylilation tests.
In contrast, complexes 2b and 2c were found stable in air up to
t
t
ca. 150° over long periods. The most stable complex is 2d
which is fully air stable at temperatures up to 220°. The relative
stability of complexes 2a 2d is likely controlled by the ability
of the N-substituents to sterically protect or not the metal centre
vide infra).
In keeping with the freely rotating AF groups of 2c and 2d 6
,
-
of t-BuONa. Complex 2d proved to be the most efficient
catalyst, the corresponding runs affording 93% of the silyl ether
after 3h. In comparaison, under similar conditions the complex
[CuCl(IPr)], which is regarded as a very performing NHC-
copper catalyst for this reaction,8 resulted in 97% yield (Table 1
entries 6 and 7). Control experiments showed that no reaction
takes place in the absence of catalyst, nor in the presence of
ligand free copper(I) chloride (Table 1, entries 1 and 2).
Complex 2b displayed significantly lower activities than 2d
(Table 1, entry 8), seemingly because of gradual degradation of
the active species. This likely arises from a lower steric
protection of the metal by the carbene ligand of 2b than that
provided by the (time-averaged) sterically bulkier carbene
ligand of 2d, the fluorenylidene planes of which may be bent
towards the metal atom, so as to sandwich it. Interestingly,
(
,
the 2D ROESY spectra of these complexes, measured at room
temperature, showed strong through-space correlations between
the NCCH2 hydrogen atoms (termed hereafter
and the CH atoms of the NHC ring (see ESI. Such correlations
were not observed for 2b, as the corresponding -CH2 protons
remain here permanently3 directed towards the metal centre.7
We further noted that while the -CH2 protons of 2c appear at
α-CH2 protons)
α
α
nearly the same chemical shift as that of their precursor salt,
those of 2d have undergone a significant upfield shift (0.42
ppm) with respect to those of 1d. This could merely reflect a
shielding effect of the pi electrons of the NC=C double bond of
2d on the
backside of the NHC ring.
Palladium and silver complexes containing the carbene the corresponding carbenes do note differ by their steric
α
-CH2 protons, which may temporarely approach the complex 2d proved also to be more efficient than 2c in the
hydrosilylation of acetophenone (Table 1, entry 9). Note that
ligands derived from 1b-1d have been crystallographically properties, only by their electronic ones, the carbene ligand in
6
characterised in previous studies.3, These revealed that the 2d being a weaker donor. This finding is consistent with a
corresponding ligands may have the fluorenylidene planes recently reported theoretical study by Leyssens et al., who
either turned towards the metal or folded back to the CC bond showed that as a general trend Cu-catalysed hydroslylilation is
of the NHC ring, this leading to % buried volumes (pbv) of ca. facilitated with electronically poorer NHCs.9
58% for the former carbenes and of 36% for the latter (Fig. 1).
Thus, when AF groups can freely rotate (as in 1c and 1d), the
2 | J. Name., 2012, 00, 1-3
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For the reactions carried out at 65°C, the use of tꢀBuOK/THF
gave significantly better results than with tꢀBuONa-toluene
(Table 2, entries 8, 11, 13 and14).
Table 1 Catalytic hydrosilylation of acetophenone with Et3SiH using
complexes 2b-d or [CuCl(IPr)] : influence of the base and the solventa
The hydrosilylation of norcamphor, which was achieved at
room temperature, gave a 80:20 mixture of endo:exo products
(Table 2, entry 3). The observed diastereoselectivity is
consistent with a preferential attack of the hydride on the less
encumbered exo face. The hydrosilylation of 2-
methylcyclohexanone (Table 2, entry 4), which was best
achieved at 65°C, afforded the expected silylated alcohol as a
mixture of syn/anti diastereomers (45:55). Starting from the
bulkier menthone (Table 2, entry 5) did not increase the
selectivity (syn:anti ratio = 40:60; configuration with respect to
Entry
Complex
–
CuCl
2d
2d
2d
2d
IPrCuCl
2b
2c
Base
Solvent
THF
THF
THF
THF
toluene
toluene
toluene
toluene
toluene
Conv. (%)b
1
2
3
4
5
6
7
8
9
tꢀBuOK
tꢀBuOK
tꢀBuOK
tꢀBuONa
tꢀBuOK
tꢀBuONa
tꢀBuONa
tꢀBuONa
tꢀBuONa
0
0c
15
10
29
93
97
26
29
the C-O/C-iPr bonds).
Full conversion to the corresponding silylether was
achieved at room temperature with benzophenone as substrate
(Table
2,
entry
12).
Remarkably,
bromo-
and
chloroacetophenone derivatives were fully converted without
dehalogenation (Table 2, entries 7, 9 and 10). The deactivated
para-methoxyacetophenone was also readily hydrosilylated (at
65°C) (Table 2, entry 8). Other successful hydrosilylations
include that of acetylpyridine (using tꢀBuOK as base in THF;
(Table 2, entry 11), as well as those of the sterically demanding
ketones dicyclohexylketone and 2,2-dimethylpropiophenone
and (Table 2, entries 13 and 14).
Notably, para-cyano or para-nitro substituted acetophenone
(not drawn) could not be hydrosilylated under conditions
similar to the ones reported above. The electron-withdrawing
cyano and nitro groups supposed to inductively activate the the
conjugated ketone did in fact not result in the expected effect.
Lipshutz et al. have already made similar observations for 3-
acetyl-benzonitrile.10 Possibly, the nitrile and nitro groups
coordinate strongly to the copper atom so as to prevent binding
of the keto groups.
a General conditions: complex (3 mol%), base (12 mol%), Et3SiH (6 mmol),
solvent (2 mL), acetophenone (2 mmol), 25°C, 3 h. b Conversion determined
by 1H NMR. c 9 mol% of CuCl.
In order to assess the scope of the reaction, tests were then run
with 14 other ketones (Table 2) applying conditions similar to
those described above. We observed that neither with tꢀBuOK
nor with tꢀBuONa were aldolisation products observed (Table
2, entries 1-13). As expected, the reaction rates were higher
with sterically non demanding as well as with activated
ketones. In these cases, hydrosilylation could be carried out
efficiently at room temperature within 2-3 h (Table 2, entries 1-
3, 6-7, 9 and 12). For sterically bulkier ketones and also with
non-activated ones, longer reaction times and/or higher
temperatures were required (Table 2, entries 4, 5, 8, 10 and 11).
Table 2 Hydrosilylation of ketones catalysed by complex 2da
Entry
1
Substrate
Base/Solvent
Temperature (°C)
25
Time (h)
2
Conv. (%)b
> 99
Yield (%)c
96
tꢀBuONa / toluene
2
3
tꢀBuONa / toluene
tꢀBuONa / toluene
25
25
2
2
> 99
> 99
94
94
(endo : exo / 80 : 20)
97
4
tꢀBuONa / toluene
65
2
> 99
(syn : anti / 45 : 55)
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96
5
6
7
8
9
tꢀBuONa / toluene
tꢀBuONa / toluene
tꢀBuONa / toluene
65
25
25
2
3
3
> 99
93
(syn : anti / 40 : 60)
90
97
98
tꢀBuONa / toluene
tꢀBuOK / THF
25
65
2
2
10
92
-
87
tꢀBuONa / toluene
tꢀBuONa / toluene
25
65
3
6
> 99
> 99
97
92
10
11
12
13
14
tꢀBuONa / toluene
tꢀBuOK / THF
65
65
18
4
> 99
> 99
-
95
tꢀBuONa / toluene
25
1
> 99
97
98
tꢀBuONa / toluene
tꢀBuOK / THF
65
65
2
4
4
> 99
tꢀBuONa / toluene
tꢀBuOK / THF
65
65
2
2
50
> 99
-
94
1
c
a General conditions: complex 2d (3 mol%), base (12 mol%), Et3SiH (6 mmol), solvent (2 mL), ketone (2 mmol). b Conversion determined by H NMR.
Isolated yields.
In order to test the endurance of 2d under catalytic were then repeated with only 0.1 mol% of catalyst. In this case,
conditions, additional hydrosylilation runs were carried out for no catalyst deactivation occurred and the reaction was
acetophenone over longer reaction periods (Table 3), these runs completed after 70 h (Table 3, entry 14), this corresponding to
being performed at 65°C with only 0.25 mol% of catalyst (a one of the highest TONs (TON = 1000) ever reported for NHC-
lower catalyst concentration being required for monitoring copper catalysts containing a five-membered diaminocarbene.
reaction progress). At the applied temperature, the best Low-charge hydrosylilation reactions using 0.25 or 0.5 mol%
conversions were obtained with the couple
Under these conditions, the conversion reached 97 % after 24 h above (Table 4).
(TON 388) (Table 3, entry 10). In comparison, for the same
t-BuOK/THF. of 2d was also successfully applied to all the ketones discussed
=
experiment, [CuCl(IPr)] resulted in a yield of only 35% (TON
= 140), this being indicative of a higher stability of the 2d
catalyst (Table 3, entry 9). Note that, consistent with the
observations made for the low-temperature experiments (see
above), catalysts 2b and 2c displayed lower activities at 65°C
then 2d and [CuCl(IPr)] (Table 3, entries 12 and 13). In order to
test catalyst 2d over a still longer period, the runs (at 65°C)
4 | J. Name., 2012, 00, 1-3
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2a
/2b, in which the AF groups do not rotate and where the
Table 3 Hydrosilylation of acetophenone catalysed by complexes 2b-d and
[IPrCuCl] (0.25 mol%) in the presence of Et3SiH: influence of the base and
the solventa
fluorenylidene planes are folded back away from the metal.
Table 4 Hydrosilylation of ketones using complex 2da
Temp.
(°C)
65
Conv.
(%)b
0
Conv.
(%)b
Yield
(%)c
Entry
Complex
Base / Solvent
Entry
1
Substrate
1
2
3
4
5
6
7
8
9
10
11
12
13
14
–
CuCl
IPrCuCl
2d
IPrCuCl
2d
IPrCuCl
2d
IPrCuCl
2d
2d
tꢀBuOK / THF
tꢀBuOK / THF
tꢀBuONa / toluene
tꢀBuONa / toluene
tꢀBuONa / toluene
tꢀBuONa / toluene
tꢀBuOK / THF
tꢀBuOK / THF
tꢀBuOK / THF
tꢀBuOK / THF
tꢀBuONa / THF
tꢀBuOK / THF
tꢀBuOK / THF
tꢀBuOK / THF
65
0c
> 99
> 99
97
93
RT
RT
65
10
23
23
41
5
11
35
97
2
3
65
RT
RT
65
65
65
65
65
65
> 99
> 99
96
94
23
12
2b
2c
2d
13
>99d
4
5
6
7
8
9
a General conditions: complex (0.25 mol%), base (5 mol%), Et3SiH (6 mmol),
acetophenone (2 mmol), solvent (2 mL), RT, 24 h. b Conversion determined
by 1H NMR. c 9 mol% of CuCl. dReaction performed with 0.1 mol% of 2d, 70
h.
> 99
97
94
93
95
96e
93
The considerable longer lifetime of catalyst 2d vs.
[CuCl(IPr)] is best explained by the steric differences between
the corresponding carbenes. In fact, the steric protection of the
Cu atom is significantly better in 2d than in[CuCl(IPr)]. This
can be seen, e.g., by comparing the % buried volumes of the
ligands, namely 37%-59% for the sterically flexible carbene
ligand of 2d (noted NHC2d), vs. 29% for that of [CuCl(IPr)].
The recently reported solid state structure of [AgCl(NHC2d)]
(Fig. 2) provides proof that both fluorenylidene planes of
NHC2d may be turned simultaneously towards the bound metal
centre, this resulting then in an efficient steric protection of the
metal (sandwich type structure) in possible Pd(0) intermediates.
A similar protection is not provided by ligands such as in
> 99
> 99
> 99
10
11
12
> 99
> 99
> 99
94e
96e
97d
Fig. 2 Solid state structure of of the silver analogue of 2d [AgCl(NHC2d)],6 showing
the flexible carbene ligand NHC2d when it displays its highest possible
encumbrance
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entries
5-7).
Note
that
hydrosilylation
of
p-
methoxybenzaldehyde required a reaction temperature of 65°C
(Table 5, entry 6), the other two isomers being readily
converted at 25°C.
In the presence of 0.25 mol% of catalyst and with tꢀ
BuONa/THF, the hydrosilylation of benzaldehyde derivatives
bearing electronwithdrawing groups, led again to mixtures of
silylether and homoester, the proportion of ester being here
higher than in the case of benzaldehyde. Obviously, the
competitive Tishchenko reaction occurred faster than in the
case of benzaldehyde. Thus, in order to favor the
13
14
> 99
> 99
95e
94
a General conditions: complex 2d (0.25 mol%), tꢀBuOK (5 mol%), Et3SiH (6
b
mmol), ketone (2 mmol), THF (2 mL), 65°C, 24 h. Conversion determined
by H NMR. c Isolated yields. Reaction performed at RT, 18 h. e Reaction
1
d
hydrosilylation process over the Tishchenko reaction, the 2d:t-
performed with 0.5 mol% of 2d.
BuONa ratio was increased to 1:5 instead of 0.25:5. In that case
no ester formed (Table 6, entry 2-4). It should also be
mentioned that with the chloro- and bromo- substrates, the
reaction took place without dehalogenation.
The reactivity of aldehydes was also examined (Table 6).
Benzaldehyde was first studied (Table 5). Surprisingly,
independently of the solvent used, hydrosilylation with 2d
proceeded with concomitant formation of benzyl benzoate (A)
Table 6 Hydrosilylation of aromatic aldehydes catalysed by complex 2d.
(Table 5, entries 1-4). In fact, the formation of this homoester
arose from a competitive Tishchenko-type reaction, known to
operate without copper.11 The amount of Tischenko product
could be kept at his lowest level when using tꢀBuONa instead
of tꢀBuOK (Table 5, entries 1-4). Of further advantage was to
use THF as the solvent rather than toluene (Table 5, entry 3).
It is noteworthy that when using tꢀBuONa/THF with Et3SiH
in the absence of any copper catalyst, no reaction took place
(Table 5, entry 6). Repeating this reaction with tꢀBuOK instead
of tꢀBuONa, led selectively and quantitatively to the homoester
O
2d (0.25 - 1 mol%), t-BuONa (5 mol%)
OSiEt3
H
Et3SiH (3 equiv.)
H
THF, rt, 2 h
Conv.
Yield
(%)c
Entry
1
Substrate
(%)b
> 99e
> 99e
95
A
within 2 h at room temperature (Table 5, entry 5). The
superiority of the potassium vs. the sodium salt is consistent
with observations made by Berke.11 Furthermore, in the
absence of silane, no Tishchenko reaction occurred,
demonstrating that the silane may function as a hydride transfer
catalyst.
2
94
Table 5 Hydrosilylation of benzaldehyde catalysed by 2d using Et3SiH:
influence of the base and the solventa
3
4
5
6
7
> 99e
> 99e
> 99
> 99d
> 99
92f
94
96
91
91
Conv.
Entry
Complex
Base / Solvent
A:B
(%)b
98
1
2
3
4
5
6
7
2d
2d
2d
2d
-
tꢀBuOK / THF
tꢀBuOK / toluene
tꢀBuONa / THF
tꢀBuONa / toluene
tꢀBuOK / THF
50:50
55:45
7:93
15:85
100:0
-
95
88
100
100
0
-
tꢀBuONa / THF
tꢀBuONa / THF
CuCl
0c
-
a
General conditions: 2d (0.25 mol%), base (5 mol%), Et3SiH (6 mmol),
benzaldehyde (2 mmol), solvent (2 mL), RT, 2 h. b Conversions determined
by 1H NMR. c 9 mol% of CuCl.
Overall, effective hydrosylilation of benzaldehyde was best
performed with 2d (0.25 mol%) in THF at room temperature
a
General conditions: Activation of complex 2d (0.25 mol%) with tꢀBuONa
(5 mol%) and Et3SiH (6 mmol) in THF (2 mL) at room temperature, then
addition of the aldehyde (2 mmol). Reaction mixture stirred at room
with
tꢀBuONa
as
base,
this
resulting
in
a
PhCH2OSiEt3:PhC(O)OCH2Ph ratio of 93:7 after 2h reaction
time (Table 5, entry 3). No Tishchenko products were observed
with benzaldehyde derivatives containing electron donating
b
temperature for 2 h. Conversion to product based on starting material
determined by 1H NMR. c Isolated yields. d Reaction performed at 65°C for 1
h. e Reaction performed with 1 mol% of 2d. f Reaction mixture stirred at room
temperature for 4 h.
groups,
notably
pꢀmethylbenzaldehyde,
p-
methoxybenzaldehyde and
o-methoxybenzaldehyde (Table 5,
6 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C5DT01888A
(NCBu), 43.4 (CH2CH2CH2CH3), 25.9 (CH2CH2CH2CH3), 22.5
(CH2CH2CH2CH3), 14.1 (CH2CH2CH2CH3), carbene signal not
detected. Found C, 74.57; H, 5.79; N, 4.12. Calc. for C41H38ClCuN2
(Mr = 656.20): C, 74.87; H, 5.82; N, 4.26%.
Experimental
All commercial reagents were used as supplied, except liquid
aldehydes, which were treated with a NaHCO3(aq.)/CH2Cl2
mixture prior to use. The syntheses and catalytic tests were
performed in Schlenk-type flasks under dry nitrogen. Solvents
were used as received except for tetrahydrofuran (THF)
(distilled from sodium/benzophenone) and toluene (distilled
from sodium). Routine 1H and 13C{1H} NMR spectra were
recorded on a FT Bruker AVANCE 300 instrument (1H: 300.1
(1,3-bis(9-ethyl-9-fluoren-9-yl)imidazolin-2-ylidene)copper(I)
chloride (2c): A suspension of imidazolium salt (1c) (0.204 g, 0.42
mmol), finely crushed K2CO3 (0.773 g, 5.85 mmol) and CuCl (0.068
g, 0.69 mmol) in CH2Cl2 (5 mL) was stirred at room temperature
overnight. The mixture was filtered through a pad of silica gel and
the pad was washed with CH2Cl2 (ca. 3 × 20 mL). The filtrate was
concentrated in vacuum to ca. 1 mL. Pentane (6 mL) was added and
the resulting white precipitate was decanted. After removal of the
supernatant, the solid was washed with pentane (3 × 20 mL) to
afford pure complex 2c as a white solid (0.161 g, 69%); m.p. > 220
MHz, 13C: 75.5 MHz) at 25°C. H NMR spectral data were
1
referenced to residual protonated solvents (CHCl3,
chemical shifts are reported relative to deuterated solvents
(CDCl3, 77.16). Data are represented in the following order:
δ
7.26), 13C
δ
chemical shift, integration, multiplicity (s = singlet, d = doublet,
t = triplet, q = quartet, m = multiplet, br = broad), coupling
1
3
°C. H NMR (CDCl3, 500 MHz), δ 7.68 (4H, d, J = 7.3 Hz, ArH),
3
3
3
7.48 (4H, d, J = 7.2 Hz, ArH), 7.42 (4H, dd, J = J’ = 7.3 Hz,
ArH), 7.35 (4H, dd, J = J’ = 7.4 Hz, ArH), 3.10 (4H, q, J = 6.2
constant (J) in Hz, and assignment. In the NMR data given
3
3
3
hereafter, Cq denotes a quaternary carbon atom. Flash
chromatography was performed as described by Still et al.
employing Geduran SI (E. Merck, 0.040-0.063 mm) silica.12
Routine thin-layer chromatography analyses were carried out
by using plates coated with Merck Kieselgel 60 GF254.
Elemental analyses were performed by the Service de
Microanalyse, Institut de Chimie (UMR 7177 CNRS),
Strasbourg. Melting points were determined with a Büchi 535
3
Hz, CH2CH3), 3.04 (4H, s, NCH2), 0.44 (6H, t, J = 6.2 Hz, CH3).
13C{1H} NMR (CDCl3, 125 MHz), δ 196.4 (NCN), 145.4 (arom.
Cq), 140.6 (arom. Cq), 129.3 (arom. CH), 128.4 (arom. CH), 123.6
(arom. CH), 120.5 (arom. CH), 73.2 (NCEt), 46.2 (NCH2), 33.5
(CH2CH3), 8.4 (CH2CH3). Found C, 71.56; H, 5.44; N, 4.94. Calc.
for C33H30ClCuN2 (Mr = 552.14) C, 71.60; H, 5.46; N, 5.06%.
capillary melting-point apparatus and are uncorrected. Complex Conclusions
2d5 and IPrCuCl8 were synthetised according to known
procedures.
We have shown that the AF-substituted imidazolylidene copper
complex 2d efficiently catalyses the hydrosilylation of
functionalized and/or sterically demanding carbonyl
compounds, using triethylsilane as cost-effective hydride
source. Its intrinsic activity compares with that of [CuCl(IPr)],
presently considered as the most performing copper-based
NHC complex based on a five-membered NHC. Remarkably,
the activity of 2d was not altered after 70 h reaction time
(TONs up to 1000), thereby contrasting with its less stable IPr
analogue. The high stability of complex 2d is likely to arise
from the capacity of its imidazolylidene ligand to sterically
protect, when necessary, the metal centre by forming
intermediates with a sandwiched metal atom, this being not
possible with benzimidazolylidene analogues. On the other
hand, the steric flexibility of the ligand related to rotational
freedom of the ethylfluorenyl groups facilitates conversion of
encumbered substrates. Overall, the reported results constitute a
new illustration of the catalytic potential of NHCs displaying
variable steric encumbrance.
General procedure for copper-catalysed hydrosilylation
reactions :
In a Schlenk tube under nitrogen were introduced the copper
complex (3ꢀ
10-2–10-5 mmol) and the appropriate base (0.24
mmol) followed by the solvent (2 mL). The mixture was stirred
at room temperature for 10 min. Triethylsilane (6 mmol) was
then added and the mixture stirred at room temperature for 10
min. After activation, the carbonyl compound (2 mmol) was
added and the reaction mixture was stirred at room temperature
for a period of time indicated in the corresponding Table. The
mixture was filtered through a short pad of Celite using CH2Cl2
as solvent. The residue was then purified by flash
chromatography (SiO2; AcOEt–petroleum ether) to afford the
hydrosilylated product as a colourless oil.
(1,3-bis(9-n-butyl-9H-fluoren-9-yl)benzimidazol-2-ylidene)cop
per(I) chloride (2b) A suspension of benzimidazolium salt 1b
(0.400 g, 0.67 mmol), finely crushed K2CO3 (1.29 g, 9.77 mmol),
and CuCl (0.098 g, 0.99 mmol) in CH2Cl2 (10 mL) was stirred at
room temperature overnight. The mixture was filtered through a pad
of silica gel and the pad was washed with CH2Cl2 (ca. 3 × 30 mL).
The filtrate was concentrated in vacuo to ca. 1 mL. Pentane (10 mL)
was added and the resulting white precipitate was decanted. After
removal of the supernatant, the solid was washed with pentane (3 ×
10 mL) to afford pure complex 2b as a white solid (0.411 g, 94%);
m.p. decomp. 200 °C. 1H NMR (CDCl3, 500 MHz), δ 7.78 (4H, d, 3J
Acknowledgements
This work was supported by the Ministère de l’Enseignement
Supérieur et de la Recherche for a grant to M.T. We gratefully
acknowledge the CNRS and the University of Strasbourg for
their financial support.
1. B. Marciniec, in B. Cornils, W. A. Herrmann (eds.), Applied
Hogeneous Catalysis with Organometallic Compounds, VCH,
Weinheim, 1996, Chapter 2.6.
3
3
= 7.3 Hz, ArH of fluorenylidene), 7.42 (4H, dd, J = J’ = 7.3 Hz,
ArH of fluorenylidene), 7.32-7.27 (4H, m, ArH of fluorenylidene),
6.48 (2H, br signal, NCCCH of benzimidazolylidene unit), 5.98 (2H,
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3
CH2CH2CH2CH3), 1.38 (4H, m, CH2CH2CH2CH3), 0.77 (6H, t, J =
7.3 Hz, CH2CH2CH2CH3), 0.67 (4H, m, CH2CH2CH2CH3). 13C{1H}
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(arom. Cq), 129.6 (arom. CH), 128.8 (arom. CH), 123.5 (arom. CH),
123.2 (arom. CH), 120.7 (arom. CH), 113.9 (arom. CH), 73.7
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This journal is © The Royal Society of Chemistry 2012
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ARTICLE
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