Chelating alkoxy NHC-Rh(I) complexes and their applications in the arylation of aldehydes

Article abstract of DOI:10.1016/j.tetlet.2014.05.125

In this study, new rhodium(I) complexes (5 and 6) have been prepared by the reaction of [RhCl(COD)]₂ with a series of imidazolium salts (3 and 4), which were obtained from a chiral amino alcohol. The catalytic activities of these complexes were tested in the arylation of aldehydes. It was found that the synthesized rhodium complexes were highly effective catalysts for the arylation of aldehydes in short reaction times (5 min, TOF = 1193 h ^-1). However, the obtained ee values (up to 32% ee) remained low. We have proposed a mechanism for the arylation reaction of aldehydes, which is confirmed via ¹⁹F NMR spectroscopy.

Full text of DOI:10.1016/j.tetlet.2014.05.125

Accepted Manuscript
Chelating alkoxy NHC-Rh(I) complexes and their applications in the arylation
of aldehydes
Serpil Denizaltı, Hayati Türkmen, Bekir Çetinkaya
PII:
S0040-4039(14)00958-7
DOI:
http://dx.doi.org/10.1016/j.tetlet.2014.05.125
TETL 44716
Reference:
Tetrahedron Letters
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Received Date:
Revised Date:
Accepted Date:
4 April 2014
10 May 2014
29 May 2014
Please cite this article as: Denizaltı, S., Türkmen, H., Çetinkaya, B., Chelating alkoxy NHC-Rh(I) complexes and
their applications in the arylation of aldehydes, Tetrahedron Letters (2014), doi: http://dx.doi.org/10.1016/j.tetlet.
2014.05.125
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Graphical Abstract
Chelating alkoxy NHC-Rh(I) complexes and
their applications in the arylation of
aldehydes
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Serpil Denizaltı, Hayati Türkmen, Bekir Çetinkaya

1
Tetrahedron Letters
journal homepage: www.els evier.com
Chelating alkoxy NHC-Rh(I) complexes and their applications in the
arylation of aldehydes
Serpil Denizaltı*, Hayati Türkmen, Bekir Çetinkaya
Ege University, Department of Chemistry, 35100 Bornova-Izmir, TURKEY
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
In this study, new rhodium(I) complexes (5 and 6) have been prepared by the reaction of
[RhCl(COD)]2 with a series of imidazolium salts (3 and 4), which were obtained from a chiral
amino alcohol. The catalytic activities of these complexes were tested in the arylation of
aldehydes. It was found that the synthesized rhodium complexes were highly effective catalysts
for the arylation of aldehydes in short reaction times (5 min, TOF = 1193 h-1). However, the
obtained ee values (up to 32% ee) remained low. We have proposed a mechanism for the
arylation reaction of aldehydes, which is confirmed via 19F NMR spectroscopy.
Available online
Keywords:
arylation of aldehydes
N-heterocyclic carbenes
rhodium complexes
2009 Elsevier Ltd. All rights reserved.
One of the most important methods for the preparation of
optically active compounds involves the use of transition metal
complexes bearing chiral ligands. In this context, recently
developed N-heterocyclic carbene ligands (NHCs) are very good
candidates due to their strong NHC-metal bonds, excellent σ-
donating ability and ready availability with various
functionalities.1
Figure 1. Chiral polydentate NHC ligands.
Initial studies on the NHC transition-metal-catalyzed
asymmetric reactions involved the use of monodentate ligands.2
However
such
complexes
exhibited
moderate
Chiral imidazolium salts 3 and 4 were prepared by two or
three-step procedures, starting from a commercially available
chiral amino alcohol (Scheme 1). 1-Substituted imidazoles 1 and
2 were obtained by the cyclocondensation of glyoxal, ammonium
acetate, formaldehyde and a chiral amino alcohol, according to
the previously published procedure.5 The hydroxyl group was
methylated using MeI in the presence of NaH as the base. The 1-
substituted imidazoles 1 and 2 were then readily converted into
the desired imidazolium salts 3 and 4, which were obtained as
colourless solids.
enantioselectivities. It became apparent that chiral polydentate
NHC ligands such as A-D provided better stabilities and
selectivities with moderate to good ee in various organic
transformations (Figure 1).3,4 The groups of Arnold and Mauduit
introduced several NHCs including C and D containing
stereogenic centers within the chelating N-alkoxy substituents.
They studied in situ generated copper complexes in the
asymmetric conjugate addition (ACA) reaction of diethylzinc to
2-cyclohexenone and observed enantioselectivities of up to 93%
ee.3 On the other hand, to the best of our knowledge, there is no
report concerning the use of chiral hydroxyalkyl imidazolium
salts in the rhodium-catalyzed asymmetric arylation of aldehydes.
Herein, we report the synthesis of a new family of alkoxy-
substituted NHC rhodium complexes and their applications in the
enantioselective Rh-catalyzed addition of phenylboronic acid to
aromatic aldehydes. These new ligand precursors as complexes
of Rh(I), as far as we aware, have not been examined in the
arylation of aldehydes.
The intermediate compounds and the imidazolium salts were
1
characterized by spectroscopic methods. The H and 13C NMR
spectra of the salts showed characteristic NCHN resonances
around 9.63-10.61 ppm and 137.2-139.9 ppm, respectively. The
signals due to the imidazole ring resonances were observed at
1
6.75-7.38 ppm. In the H NMR spectrum of 4, the OCH3 protons
were observed at 3.33 ppm.

2
Tetrahedron Letters
NHCs were prepared by Arnold’s group. They reported that
silver(I) oxide was sufficiently basic to deprotonate both the
imidazolium and the alcohol functionalities.8 The characteristic
downfield signals for the C2 hydrogens were not observed in the
1H NMR spectra of the Rh(I) complexes. The 13C NMR spectra
of Rh(I) complexes 5 and 6 showed Ccarbene resonances between
181.5-181.8 ppm and coupling constants J(103Rh–13C) were
observed to be between 49.5-50.3 Hz.
iPr
iPr
iPr
OH
O
N
N
N
N
H2N
i
ii
OH
1 (52%)
2 (70%)
iii
iii
1
The H NMR spectrum of complex 6 showed two sets of
signals (1.7:1 ratio) at room temperature. Notably, two species
were present in solution. These observations were in accordance
with the results published by Nanchen and Pfaltz.9 They
indicated that the assumption of two conformers of the Ir
complex arise from a flip of the chelate ring, being consistent
with the NOESY plot, which showed an NOE interaction
between the isopropyl group and the COD for one of the two
conformers. Their 13C NMR spectra also displayed two sets of
Ccarbene signals between 181.3 and 182.0 ppm and the coupling
constants J(103Rh–13C) were in the range of 50.3 Hz and 51.3 Hz.
iPr
iPr
O
N
OH
Br-
a : Ar = C6Me5
N
N
Br-
b : Ar = 2,4,6-Me3C6H2
N +
+
Ar
3a (89%)
3b (90%)
4 (95%)
iv
Chiral NHC-Rh(I) complexes 5 and 6 were tested in the 1,2-
addition of phenylboronic acid to aldehydes. We studied the
effects of the solvent, the catalyst loading, and temperature on the
arylation reaction between 4-isopropylbenzaldehyde and
phenylboronic acid using complex 5a. As solvent, we used
DMF/H2O, 1,4-dioxane/H2O and DME/H2O (Table 1, entries 1-
5). Evaluation of the data in Table 1 revealed several trends.
Firstly, DME was generally a better solvent for the arylation
reaction than DMF and 1,4-dioxane. Secondly, the performance
of catalyst 5a at lower temperatures resulted in a substantial drop
in activity (entries 4-9). We found that the reactions performed in
DME/H2O at 80 °C appeared to be best. As can be seen from
Table 1, the reaction catalyzed by 5a was complete within 15
minutes (TOF = 396 h-1) (Table 1, entry 5). However, no
enantioselectivity was observed under these conditions. Indeed,
lowering the reaction temperature, increasing the catalyst loading
and changing the solvent (DME/H2O) ratio10c did not improve the
enantioselectivity (Table 1, entries 6-13). Complex 6 was also
synthesized to investigate the influence of a methoxy group on
the arylation reaction. Duan et al. published the synthesis of
alkoxy/sulfonate substituted NHC ligands derived from
[2.2]paracyclophane and used these ligands in the arylation of
aldehydes. They also found that the methoxy substituent on the
ligand played an important role in the catalytic reaction.11 All of
the rhodium complexes showed excellent activity, resulting in
complete conversions in 15 minutes, but only optical induction of
32% ee was observed (TOF = 336 h-1) in the case of 6 and 4-
isopropylbenzaldehyde (Table 2, entry 2). Although we failed to
achieve high asymmetric induction, the yields were comparable
or even higher than those reported previously.10 With catalyst 5a
(0.3% catalyst loading) and using 2-methoxybenzaldehyde, 83%
conversion was obtained in only 15 minutes (TOF = 1107−1)
(Table 2, entry 6). To the best of our knowledge, the optimum
result for this reaction was obtained by Fürstner and Krause.
Their ligands were tested in the addition of phenylboronic acid to
4-methoxybenzaldehyde using different metal salts such as
[RhCl(COD)]2 and RhCl3.3H2O. They obtained the product in
<10% yield using [RhCl(COD)]2 (3 mol%) after 6.5 hours, and
93% yield using RhCl3.3H2O (1 mol%) within 12 minutes.12
Therefore, we tested ligand precursor 3a (1 mol%) with
[RhCl(COD)]2 (0.5 mol%) and 4-methoxybenzaldehyde, under
similiar circumstances, which provided the corresponding
product in 90% yield after 15 minutes using only 1 mol% of
rhodium (TOF = 360 h-1).
iv
+
iPr
iPr
N
iPr
Br-
O
N
Rh
N
O
O
N
N
Rh
Rh
N
6
5a
5b
(58%)*
(53%)*
(89%)*
Scheme 1. Reagents and conditions: (i) glyoxal, HCHO,
NH4OAc, MeOH, reflux, 5 h; (ii) NaH, MeI, DMF; (iii)
ArCH2Br, DMF, 25oC; (iv) LiOtBu, [RhCl(COD)]2, 70 oC; or
Ag2O, CH2Cl2, 25 oC and then [RhCl(COD)]2, CH2Cl2; or
[Rh(OMe)(COD)]2, DCE. * The quoted yields were obtained
using the first set of conditions listed in step (iv).
Chiral NHC Rh(I) complexes 5 and 6 could in principle be
synthesized in three different ways:6 i) deprotonation of the
imidazol(in)ium salt with a strong base such as LiOtBu and
subsequent reaction with [RhCl(COD)]2, ii) via a carbene-
transfer reaction of an in situ formed NHC-Ag species with
[RhCl(COD)]2 in dichloromethane at ambient temperature, iii)
using [RhOMe(COD)]2 which deprotonates and coordinates the
desired ligand in situ. However, it has been reported that
rhodium(I) complexes formed by deprotonation of an
imidazolinium salt with LiOtBu and subsequent reaction with
[RhCl(COD)]2 afford an achiral complex. This is assumed to be
due to the basic conditions which result in the loss of chirality
during the reaction.7 For this reason, the second approach was
attempted by transmetallation of an NHC-silver(I) complex with
[RhCl(COD)]2, which led to the corresponding rhodium(I)
complexes 5 and 6 with retention of chirality. Besides, other
methods were also employed to synthesize the rhodium
complexes. Both deprotonation of imidazolium salts with LiOtBu
and subsequent reaction with [RhCl(COD)]2 or the use of
[RhOMe(COD)]2 and the corresponding salts led to chiral NHC
alkoxide Rh(I) complexes.
The synthesized rhodium complexes were fully characterized
1
by standard 2D-NMR techniques. In the H NMR spectra of the
Rh complexes, the OH proton was not observed. This behaviour
was consistent with the literature.8 A variety of silver alkoxide

3
O
OH
5a
(1 mol%)
H
+ PhB(OH)2
iPr
KOtBu, solvent/H2O (3/1)
iPr
Table 1 The addition of phenylboronic acid to 4-isopropylbenzaldehyde catalyzed by 5a
Entry
1
Solvent
DMF
Temp.(oC)
Time
15 min
5 min
15 min
5 min
15 min
15 min
30 min
45 min
48 h
Yield (%)
-
TOF (h-1)
-
80
80
80
80
80
60
60
60
40
40
40
40
40
2
1,4-dioxane
1,4-dioxane
DME
30
361
396
651
396
224
178
132
0.77
3.5
3
>99
54
4
5
DME
>99
56
6
DME
7
DME
89
8
DME
>99
37
9
DME
10
11
12
13
DME
8 h
84a
DME
10 h
>99a
89a,b
>99a,b
3.3
DME
8 h
3.7
DME
10 h
3.3
a 3 mol% catalyst
b DME/H2O (5/1)
OH
Ph
cat. (1 mol%)
KOtBu, DME/H2O (3/1)
Ar-CHO + PhB(OH)2
Ar
Table 2 The addition of phenylboronic acid to aldehydes catalyzed by complexes 5 and 6
Entry
1
Catalyst
5b
6
Ar-CHO
Time (min)
Yield (%)
96
ee (%)
TOF (h-1)
384
4-iPr-C6H4CHO
15
15
15
15
15
15
5
4.5b
2
4-iPr-C6H4CHO
84
32.1b
336
3
5a
5a
5a
5aa
5b
6
4-MeO-C6H4CHO
4-Cl-C6H4CHO
90
n.d
360
4
>99
88
n.d
396
5
2-MeO-C6H4CHO
2-MeO-C6H4CHO
2-MeO-C6H4CHO
2-MeO-C6H4CHO
2-Cl-C6H4CHO
7.7c
352
6
83
6.9c
1107
1084
988
7
90
-
8
5
82
-
9
5a
5b
6
5
>99
>99
>99
31
7.2d
1193
1193
1193
374
10
11
12
13
14
15
16
17
18
19
2-Cl-C6H4CHO
5
-
2-Cl-C6H4CHO
5
2.1d
5a
5a
5b
5b
6
2,4,6-Me3-C6H2CHO
2,4,6-Me3-C6H2CHO
2,4,6-Me3-C6H2CHO
2,4,6-Me3-C6H2CHO
2,4,6-Me3-C6H2CHO
2,4,6-Me3-C6H2CHO
3-MeO-C6H4CHO
3-MeO-C6H4CHO
5
-
-
-
-
-
-
-
-
15
5
>99
28
396
337
15
5
>99
35
396
422
6
15
5
>99
84
396
5a
5a
1012
396
15
>99

4
Tetrahedron Letters
20
21
22
23
5b
5b
6
3-MeO-C6H4CHO
3-MeO-C6H4CHO
3-MeO-C6H4CHO
3-MeO-C6H4CHO
5
82
>99
64
-
-
-
-
988
396
771
396
15
5
6
15
>99
a 0.3 mol% catalyst
bChiracel OD-H column, hexane/IPA (95/5), flow rate 0.5 mL/min, 254 nm (for 4-isopropylbenzaldehyde).13
cChiracel OD-H column, hexane/IPA (90/10), flow rate 1.0 mL/min, 254 nm (for 2-methoxybenzaldehyde). 14
dKromasil 5-cellucoat column, heptane/IPA (90/10), flow rate 0.7 mL/min, 254 nm (for 2-chlorobenzaldehyde).
A
plausible mechanism for the NHC-Rh(I)-catalyzed
arylation reaction of aldehydes is proposed in Scheme 2.15 In the
first step, transmetalation of phenylboronic acid with the rhodium
catalyst, which contains a basic oxygen and thus can accept a
proton, produces complex I (a phenylrhodium intermediate). To
confirm the formation of I the initial intermediate, complex 5a
o
was treated with 4-fluorophenylboronic acid in CDCl3 at 30 C.
(a)
(b)
When the 19F NMR spectrum of 4-F-C6H4B(OH)2 and that of the
in situ formed complex were compared, it was possible to
distinguish the chemical shifts of the F atom. The 19F NMR
spectrum of 4-F-C6H4B(OH)2 showed a characteristic resonance
at -106.25 ppm, whereas the coordinated fluorobenzene
resonance was at -111.03 ppm (Figure 2). The chelating oxygen-
containing arm in this complex can temporarily dissociate,
creating unsaturation. The coordination of the aryl-rhodium
species with aldehydes provides intermediate II, which
undergoes insertion of the C-O double bond of the aldehyde into
the C-Rh bond of II to form alkoxy-rhodium complex III. The
last step results in the release of the diarylmethanol.
Figure 2. 19F NMR (376 MHz) spectra of (a) 4-F-C6H4B(OH)2; (b) 5a + 4-F-
C6H4B(OH)2 in CDCl3 at 30 oC.
Acknowledgments
We thank Ege University Research Fund (2009/Fen/029) for
financial support of this work and the Technological and Science
Council of Türkiye (TÜBİTAK) and TUBA for a grant. We also
thank Aylin Atik and Nobel Farma/FARGEM for HPLC
analyses.
References and notes
In conclusion, chiral NHC-Rh(I) complexes 5 and 6 were
prepared and characterized by spectroscopic techniques. Their
catalytic activities were evaluated for the arylation of aldehydes.
Complexes 5a and 5b showed excellent catalytic activities for the
arylation of aldehydes. Attempts to obtain enantioselective
products by lowering the temperature, increasing the catalyst
loading or changing the ratio of the solvent system, did not
improve the ee values. In this study, high conversions were
obtained in the arylation reactions (TOF values up to 1193 h-1).
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