Newly synthesized furanoside-based NHC ligands for the arylation of aldehydes

Article abstract of DOI:10.3906/kim-1603-95

New furanoside-based NHC precursor salts (2) were synthesized using amino alcohols from the chloralose derivatives of glucose (a), galactose (b), and mannose (c). The novel compounds were fully characterized by ¹H NMR, ¹³C NMR, and elemental analyses. The catalytic activities of these salts were tested in the arylation of aldehydes as catalysts that were generated in situ from [RhCl(COD)]₂. In addition, 2a was converted to the rhodium complex 3a in order to compare the results obtained in situ. The newly synthesized compounds were very efficient in terms of yield; nevertheless they did not exhibit a chiral induction.

Full text of DOI:10.3906/kim-1603-95

Turk J Chem
2016) 40: 689 – 697
Turkish Journal of Chemistry
(
http://journals.tubitak.gov.tr/chem/
¨
˙
⃝
c TUBITAK
Research Article
doi:10.3906/kim-1603-95
Newly synthesized furanoside-based NHC ligands for the arylation of aldehydes
∗
∗
˙
˙
˙
Serpil DENIZALTI , Fatma C¸ ETIN TELLI , Selin YILDIRAN,
˙
Azize Ye ¸s im SALMAN, Bekir C¸ ETINKAYA
˙
Department of Chemistry, Ege University, Izmir, Turkey
Received: 22.03.2016
•
Accepted/Published Online: 14.06.2016
•
Final Version: 02.11.2016
Abstract: New furanoside-based NHC precursor salts (2) were synthesized using amino alcohols from the chloralose
1
derivatives of glucose (a), galactose (b), and mannose (c). The novel compounds were fully characterized by H NMR,
1
3
C NMR, and elemental analyses. The catalytic activities of these salts were tested in the arylation of aldehydes as
catalysts that were generated in situ from [RhCl(COD)] . In addition, 2a was converted to the rhodium complex 3a in
2
order to compare the results obtained in situ. The newly synthesized compounds were very efficient in terms of yield;
nevertheless they did not exhibit a chiral induction.
Key words: N-heterocyclic carbene, carbohydrates, arylation of aldehydes, rhodium
1. Introduction
Diarylmethanols are important structural motifs for the synthesis of biologically and pharmaceutically active
compounds.1
−4
In order to obtain these compounds, the addition of organometallic reagents to aldehydes has
been listed among the general methods. Organolithium, magnesium, zinc, and copper compounds are the most
widely used in arylation reactions. However, these reagents are toxic and sensitive to air and moisture.5
−10
In recent years, 1,2-addition of boronic acids to aldehydes catalyzed by rhodium complexes (especially N-
heterocyclic carbene (NHC) complexes) has become a very useful approach to prepare such compounds due
to their low toxicities, stabilities, and easy handling.8
,9,11−19
Moreover, NHCs have emerged as important
ligands in organometallic catalysis. In contrast to phosphine complexes, their strong affinities with metals
and better temperature, air, and moisture stabilities have increased their popularity as ligands. The steric and
electronic properties of these ligands can also be modified by altering the substituents at the nitrogen atoms and
heterocycle, which enable them to be used as ancillary ligands for the preparation of various complexes.2
0−25
In the literature, several carbohydrate-containing NHCs have been synthesized that include a C1 , C2 , C3 , or
C6 -pyranoside-scaffold.2
6−35
However, just a few of them were used as catalysts in Suzuki–Miyaura,31,32 olefin
metathesis,33 gold-catalyzed cyclopropanation reaction,34 or hydrosilylation of ketones.35 On the other hand,
to the best of our knowledge, there is no report concerning the synthesis and usage of furanoside-based NHCs
in the rhodium-catalyzed arylation of aldehydes.
Recently, a series of chiral Schiff base ligands were prepared using aminochloralose derivatives of glucose
and galactose by our group.36 These ligands were used as catalysts in the asymmetric Henry reaction in the
presence of Cu(II) ions giving yields of up to 95% and enantiomeric excesses up to 91%.36 In our previous study,
∗
Correspondence: serpil.denizalti@ege.edu.tr, fatma.cetin@ege.edu.tr
6
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DENIZALTI et al./Turk J Chem
the synthesis of alkoxy-substituted NHC-Rh(I) complexes and their applications in the arylation reaction were
also reported.19 The chiral center in these complexes was an α-carbon atom attached to nitrogen. Therefore, the
synthesis of a new family of C6 -furanoside-based NHCs from aminochloralose derivatives of glucose, galactose,
and mannose was reported instead of Schiff base ligands, and their catalytic activities for Rh-catalyzed addition
of phenylboronic acid to aldehydes were investigated. Furthermore, in this study, the effect of the substituent
on the β -carbon atom attached to nitrogen was examined.
2
2
. Results and discussion
.1. Synthesis and characterization of NHC precursors (2a–c) and rhodium complex (3a)
The synthesis of aminochloralose derivatives (a, b) was reported by our group in a previous study.36 In the
present study, a new aminochloralose derivative (c) from D-mannose was synthesized by selective tosylation of
the appropriate chloralose followed by azidation and reduction reactions (Figure 1).36,37 In the 1 H and 13
C
NMR spectra of the compound c, the H-1 and HC-CCl3 proton resonances were detected at 6.06 and 5.72 ppm
and 106.1 and 110.1 ppm, respectively, while the OCH3 protons were examined at 3.90 ppm.
Figure 1. Synthesis of aminochloralose c.
The imidazolium salts (2a–c) were prepared by a two-step procedure, using aminochloralose derivatives
as starting reagents (Figure 2). Firstly, 1-substituted imidazoles (1a–c) were obtained by the cyclocondensation
of glyoxal, ammonium acetate, formaldehyde, and amino alcohol, using the previously published procedure.38
The 1-substituted imidazoles (1a–c) were then readily converted into the desired imidazolium salts (2a–c),
which were obtained as light brown solids.
Figure 2. Reagents and conditions of reactions: (i) glyoxal, HCOOH, NH
CH Br, CH CN, reflux.
4 3 3
OAc, MeOH, reflux, 5 h; (ii) 2,4,6-(CH ) -
C
6
H
3
2
3
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DENIZALTI et al./Turk J Chem
The monosubstituted imidazoles (1a–c) and the imidazolium salts (2a–c) were characterized by spectro-
scopic methods. The 1 H and 13 C NMR spectra of the salts showed characteristic NCHN resonances between
9.33 and 9.65 ppm and at around 140.0 ppm, respectively. The signals of the imidazole ring resonances were
observed at 6.77–7.50 ppm. NHC Rh(I) complex (3a) was synthesized by transmetallation of an NHC-silver(I)
complex with [RhCl(COD)]2 (Figure 3).3
9−41
The synthesized complex 3a was fully characterized by 2D-NMR
(
COSY, HSQC, HMQC) technique. In the 1 H NMR spectra of 3a, the OH proton was not observed, which is
consistent with previous reports.19,42 It has also been demonstrated by Arnold’s group, who prepared a vari-
ety of silver alkoxide NHCs, that silver(I) oxide is sufficiently basic to deprotonate both the imidazolium and
the alcoholic functionalities.19,42 Moreover, the characteristic downfield signals for the C2 hydrogens were not
observed in the 1 H NMR spectra of the Rh(I) complexes. In the 13 C NMR spectra of 3a, Ccarbene resonance
was obtained at 180.1 ppm and the coupling constant J(103 Rh – 13 C) was calculated as 49.0 Hz.
Figure 3. Synthesis of the rhodium complex 3a.
2.2. Catalytic studies
The synthesized NHC ligands (2a–c) were tested in the 1,2-addition of phenylboronic acid to aldehydes. The
effects of the solvent and the precatalyst loading on the arylation reaction between 2-chlorobenzaldehyde
and phenylboronic acid were studied using 2a/[RhCl(COD)]2 . DMF/H2 O, 1,4-dioxane/H2 O, THF/H2 O,
t-amylalcohol/H2 O, DME/H2 O, and MeOH/DME (Table 1, entries 1–15) were used as solvents. MeOH and
DME were found to serve as better solvents for the arylation reaction than the others in terms of yield. When
MeOH, MeOH/DME, and DME/H2 O were used as solvents, the reaction catalyzed by 2a was found to complete
within 1 h (Table 1, Entries 1–3, 5). However, no enantioselectivity (or negligible ee) was observed under
these conditions. In addition, increasing the catalyst loading and changing the solvent did not improve the
enantioselectivity (Table 1, Entries 1–13). Comparing the data obtained here with our previous report19 in
terms of enantioselectivity revealed that the ee values could not be enhanced. However, the substituent on the
β -carbon atom attached to nitrogen reduced the enantioselectivity.
The rhodium complex (3a) was also synthesized and tested in the arylation reaction. The results are
comparable to those obtained in situ; therefore it was concluded that there is no need for the isolation and
purification of the rhodium complexes (Table 1, Entry 7).
Under the optimized reaction conditions, the scope of the reaction was extended using a variety of alde-
hydes (Table 2). It was found that aldehydes bearing both electron-donating and electron-drawing substituents
gave the diarylmethanols in good yields. However, moderate yields were obtained with 4-methoxybenzaldehyde
(
Table 2, Entry 6). In addition, the electron-poor aldehydes reacted with phenylboronic acid more easily than
the electron-rich ones (Table 2). 2-Furaldehyde was also used as a substrate in the addition reaction, which
afforded excellent yields (Table 2, Entry 8).
6
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DENIZALTI et al./Turk J Chem
Table 1. The addition of phenylboronic acid to 2-chlorobenzaldehyde using 2a–c.
−
Entry Ligand (% mol) Solvent
Yield (%) TOF (h 1
)
ee (%)
-
-
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
2a
2a
2a
2a
2a
2a
3a
2a
2a
2a
2a
2a
2a
MeOH
> 99
> 99
> 99
62
> 99
41
97
-
70
99
99
99
62
99
41
97
-
70
46
73
98
97
98
91
5
MeOH/DME (5/1)
MeOH/DME (3/1)
DME/H2O (3/1)
DME/H2O (5/1)
DME/H2O (5/1)
DME/H2O (5/1)
DMF/H2O (3/1)
THF/H2O (3/1)
-
3
2
2
2
-
-
-
3
4
-
a
0
1
2
3
4
5
6
tert-amyl alcohol/H2O (3/1) 46
Dioxane/H2O (3/1)
Dioxane/H2O (3/1)
Dioxane/H2O (3/1)
Dioxane/H2O (3/1)
Dioxane/H2O (3/1)
Dioxane/H2O (3/1)
73
98
97
98
91
5
b
c
2b
2c
-
-
-
-
t
◦
Reaction conditions: ligand (1% mol), [RhCl(COD)]
2
(0.5% mol), aldehyde (0.5 mmol), KO Bu (0.5 mmol), 80 C, 1
a
1
a
b
c
h. Determined by H NMR analysis (average of two runs).
30 min; 3 mol% catalyst; 5 mol% catalyst.
Table 2. The addition of phenylboronic acid to aldehydes.
Ligand t (h) Yield (%)a
Entry Aldehyde
2
2b
2
2
2b
2
2
2b
2
2
2b
2
2
2b
2
2
2b
a
1
1
1
1
1
1
1
1
1
1
1
1
4
4
4
4
4
4
4
4
4
4
4
4
> 99
> 99
> 99
71
> 99
> 99
91
> 99
> 99
> 99
93
> 99
> 99
> 99
> 99
30
52
66
98
92
> 99
> 99
> 99
> 99
1
2
3
4
5
6
7
8
2-Cl-PhCHO
c
a
4-Cl-PhCHO
c
a
4-NO2-PhCHO
4-Me-PhCHO
2-MeO-PhCHO
4-MeO-PhCHO
c
a
c
a
c
a
2
2
c
a
3,4,5-(MeO)3-PhCHO 2b
2
2
2b
2
c
a
2-Furaldehyde
c
Reaction conditions: ligand (1% mol), [RhCl(COD)]
2
(0.5% mol), aldehyde (0.5 mmol), MeOH/DME (1.5/0.5 mL),
t
◦
a
1
KO Bu (0.5 mmol), 80 C. Determined by H NMR analysis (average of two runs).
692

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DENIZALTI et al./Turk J Chem
In conclusion, in this study, NHC precursors bearing a furanoside scaffold (2a–c) were synthesized,
characterized, and used in the arylation reaction of aldehydes for the first time. All of the ligand precursors
showed excellent activity, resulting in complete conversions. Although attempts to achieve asymmetric induction
failed, the yields of the diarylmethanols obtained are comparable or even higher than those reported in
alternative procedures.8
,9,16,17,43−46
3. Experimental
All manipulations were performed in air unless stated otherwise. All reagents were purchased from commercial
sources and used as received. 2,4,6-Trimethylbenzyl bromide was as previously described.47 Trichloroethylidene
acetal of D-mannose (I),4
8−51
aminochloralose derivatives (a–c),36,37 and 1-substituted imidazoles (1a–c)38
were prepared using a previously published procedure. The melting points were recorded with a Gallenkamp
electrothermal melting point apparatus. The FTIR spectra were recorded on a PerkinElmer Spectrum 100 series.
The 1 H NMR and 13 C NMR spectra were recorded with a Varian AS 400 Mercury instrument. Chemical shifts
(
δ) and coupling constants (J) were given in ppm and Hz, respectively. Optical rotations were recorded on a
Rudolph Research Analytical Autopol I automatic polarimeter with a wavelength of 589 nm. The concentration
c’ has units of g/100 mL. Elemental analyses were performed on a PerkinElmer PE 2400 elemental analyzer.
‘
3
.1. Synthesis of 6-amino-6-deoxy-3-O-methyl-1,2-O-(R)-trichloroethylidene- β -D-mannofuranose
c)36
Yield: 76%. [α]2
(
3.5
D
−1
(KBr); 3299 (–NH2 and –OH), 1591 (N–H), 1100
= –31.25 (c 0.23 in CH2 Cl2). IR cm
1
(
–OMe). Anal. Calc. C9 H14 Cl3 NO5 : C, 33.51; H, 4.37; N, 4.34. Found: C, 33.45; H, 4.35; N, 4.29%.
H
NMR (400 MHz, CDCl3): δ = 6.06 (d, J = 3.6 Hz, 1 H, H -1), 5.72 (s, 1 H, H C–CCl3), 5.05 (dd, J = 3.6 Hz,
1 H,H -2), 4.02–4.07 (m, 2 H, H -3,H -4), 3.92 (m, 1 H, H -5), 3.90 (s, 3 H, OCH3), 3.04, 2.81 (dd, J = 16.0,
4
.0 Hz, 2 H, H -6), 1.70 (br s, 3 H, –NH2 , –OH). 13 C NMR: 110.1, 106.1 (HC –CCl3 , C -1), 99.4 (HC–C Cl3),
80.8, 80.2, 80.1 (C -2, C -3, C -4), 70.6 (C -5), 59.2 (OC H3), 44.4 (C -6).
3.2. General procedure for the synthesis of 1-substituted imidazoles (1a–c)
Methanolic solution (6 mL) of glyoxal (40% w/v, 0.87 g, 6 mmol), ammonium acetate (0.46 g, 6 mmol),
formaldehyde (36% w/v, 0.50 g, 6 mmol), and aminochloralose derivative (3 mmol) was refluxed for 5 h. The
reaction mixture was concentrated by distillation. The residue was then treated with 2 M KOH solution (100
mL) and extracted with CH2 Cl2 (4 × 100 mL). The combined organic phases were dried over Na2 SO4 and
concentrated in a vacuum.
a: Yield: 90%. [α]2
3.2
D
= –8.70 (c 0.23 in CH2 Cl2). Anal. Calc. C12 H15 Cl3 N2 O5 : C, 38.58; H,
1
4
6
.05; N, 7.50. Found: C, 38.50; H, 6.79; N, 6.62%. 1 H NMR (400 MHz, CDCl3): δ = 7.40 (s, 1 H, NCH N),
.90, 6.80 (s, 2 H, im–CH), 6.07 (d, J = 4.0 Hz, 1 H, H -1), 5.31 (s, 1 H, H C–CCl3), 4.725 (d, J = 4.0 Hz, 1
H,H -2), 4.32 (dd, J = 4.0, 8.0 Hz, 1 H,H -4), 4.19–4.23 (m, 1 H, H -3), 4.07–4.14 (m, 2 H, H -6), 3.95–4.01 (m,
1
1
5
H, H -5), 3.47 (s, 3 H, OCH3). 13 C NMR (100 MHz, CDCl3): δ = 137.8 (NC HN), 128.1, 120.3 (im–C H),
07.1, 105.9 (HC –CCl3 , C -1), 96.7 (HC–C Cl3), 83.9, 82.8, 81.9 (C -2, C -3, C -4), 67.3 (C -5), 58.5 (OC H3),
1.3 (C -6).
b: Yield: 95%. [α]2
3.3
D
= –15.38 (c 0.26 in CH2 Cl2). Anal. Calc. C12 H15 Cl3 N2 O5 : C, 38.58; H,
1
6
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DENIZALTI et al./Turk J Chem
4
6
.05; N, 7.50. Found: C, 38.49; H, 6.82; N, 6.63%. 1 H NMR (400 MHz, CDCl3): δ = 7.45 (s, 1 H, NCH N),
.99, 6.96 (s, 2 H, im-CH), 6.21 (d, J = 4.0 Hz, 1 H, H -1), 5.71 (s, 1 H, H C–CCl3), 5.29 (s, 1 H,H -4), 4.95
(
d, J = 4.0 Hz, 1 H,H -2), 4.08–4.10 (m, 1 H, H -3), 3.93–3.96 (m, 3 H, H -5, H -6), 3.41 (s, 3 H, OCH3). 13
NMR (100 MHz, CDCl3): δ = 138.2 (NC HN), 128.8, 119.8 (im–C H), 109.5, 107.0 (HC –CCl3 , C -1), 99.1
C
(
HC–C Cl3), 86.8, 86.7, 85.4 (C -2, C -3, C -4), 70.5 (C -5), 57.7 (OC H3), 50.4 (C -6).
c: Yield: 97%. [α]2
3.1
D
= –25.00 (c 0.24 in CH2 Cl2). Anal. Calc. C12 H15 Cl3 N2 O5 : C, 38.58; H,
1
4
7
.05; N, 7.50. Found: C, 38.51; H, 6.80; N, 6.62%. 1 H NMR (400 MHz, CDCl3): δ = 7.35 (s, 1 H, NCH N),
.00, 6.98 (s, 2 H, im–CH), 6.10 (d, J = 4.0 Hz, 1 H, H -1), 5.66 (s, 1 H, H C–CCl3), 5.06 (t, J = 4.0 Hz, 1
H,H -2), 4.22–4.27 (m, 1 H, H -3), 4.08–4.15 (m, 2 H, H -6), 3.93–3.99 (m, 2 H, H -5), 4.02 (t, J = 4.0 Hz, 1
H,H -4), 3.70–3.74 (m, 1 H, H -5), 3.55 (s, 3 H, OCH3). 13 C NMR (100 MHz, CDCl3): δ =138.3 (NC HN),
1
28.6, 120.4 (im–C H), 110.2, 106.3 (HC –CCl3 , C -1), 99.2 (HC–C Cl3), 80.3, 80.2, 78.9 (C -2, C -3, C -4), 69.2
C -5), 59.3 (OC H3), 49.5 (C -6).
(
3
1
.3. General procedure for the synthesis of imidazolium salts (2a–c)
a–c (5.7 mmol) was dissolved in CH3 CN and 2,4,6-trimethylbenzyl bromide (5.7 mmol) was added. The
mixture was refluxed overnight. The solvent was concentrated and Et2 O was added. The solid separated out
was filtered and recrystallized from CH2 Cl2 /Et2 O.
a: Yield: 71%. [α]2
3.3
D
= –9.09 (c 0.22 in CH2 Cl2). Anal. Calc. C22 H28 BrCl3 N2 O5 : C, 45.04;
2
1
H, 4.81; N, 4.77. Found: C, 44.99; H, 4.79; N, 4.75%. H NMR (400 MHz, CDCl3): δ = 9.65 (s, 1 H,
NCH N), 7.30, 6.77 (s, 2 H, im–CH), 6.93 (s, 2 H, NCH2 C6H2 (CH3)3), 6.02 (d, J = 4.0 Hz, 1 H, H -1),
5.47 (s, 2 H, NCH2 C6 H2 (CH3)3), 5.25 (s, 1 H, H C–CCl3), 4.87–4.90 (m, 1 H,H -5), 4.70 (d, J = 4.0 Hz,
1 H,H -2), 4.36–4.40 (m, 2 H, H -6), 4.19 (dd, 1 H,J = 4.0, 12.0 Hz, H -4), 4.02 (d, J = 4.0 Hz, 1 H, H -3),
3
.48 (s, 3 H, OCH3). 13 C NMR (100 MHz, CDCl3): δ = 140.1 (NC HN), 138.1, 130.0, 129.9, 125.2, (Ar–C),
123.4, 119.9 (im–C H), 107.2, 105.9 (HC –CCl3 , C -1), 96.9 (HC–C Cl3), 84.6, 82.4, 81.3 (C -2, C -3, C -4), 65.2
(
NC H2 C6 H2 (CH3)3), 59.1 (C -5), 53.1 (C -6), 48.3 (OC H3), 21.0, 19.9 (NCH2 C6 H2(C H3)3).
b: Yield: 82%. [α]2
3.1
D
= –18.18 (c 0.22 in CH2 Cl2). Anal. Calc. C22 H28 BrCl3 N2 O5 : C, 45.04; H,
2
4
7
.81; N, 4.77. Found: C, 45.01; H, 4.78; N, 4.74%. 1 H NMR (400 MHz, CDCl3): δ = 9.33 (s, 1 H, NCH N),
.46, 6.89 (s, 2 H, im–CH), 6.94 (s, 2 H, NCH2 C6H2 (CH3)3), 6.07 (d, J = 4.0 Hz, 1 H, H -1), 5.76 (s, 1 H,
H C–CCl3), 5.46 (s, 2 H, NCH2 C6 H2 (CH3)3), 4.56-4.60 (m, 1 H, H -5), 4.37-4.45 (m, 2 H, H -6), 4.24 (d, J
1
3
=
4.0 Hz, 1 H,H -4), 4.16 (t, J = 4.0 Hz, 1 H, H -3), 3.43 (s, 3 H, OCH3).
δ = 140.1 (NC HN), 138.3, 136.4, 130.0, 125.0 (Ar–C), 123.7, 120.3 (im–C H), 109.5, 107.3 (HC –CCl3 , C -1),
9.4 (HC–C Cl3), 87.7, 87.1, 85.5 (C -2, C -3, C -4), 68.5 (NC H2 C6 H2 (CH3)3), 57.8 (C -5), 53.1 (C -6), 48.1
C NMR (100 MHz, CDCl3):
9
(
OC H3), 21.0, 19.9 (NCH2 C6 H2(C H3)3).
c: Yield: 81%. [α]2
3.1
D
= –19.05 (c 0.21 in CH2 Cl2). Anal. Calc. C22 H28 BrCl3 N2 O5 : C, 45.04; H,
2
4
7
.81; N, 4.77. Found: C, 45.02; H, 4.78; N, 4.76%. 1 H NMR (400 MHz, CDCl3): δ = 9.41 (s, 1 H, NCH N),
.50, 6.95 (s, 2 H, im–CH), 6.92 (s, 2 H, NCH2 C6H2 (CH3)3), 5.90 (d, J = 4.0 Hz, 1 H, H -1), 5.69 (s, 1
H, H C–CCl3), 5.51 (s, 2 H, NCH2 C6 H2 (CH3)3), 5.09 (t, J = 4.0 Hz, 1 H, H -2), 4.54–4.61 (m, 2 H, H -6),
.40–4.43 (m, 1 H, H -5), 4.03–4.05 (m, 2 H,H -3, H-4), 3.56 (s, 3 H, OCH3). 13 C NMR (100 MHz, CDCl3):
δ = 140.0 (NC HN), 138.2, 136.9, 129.9, 125.2 (Ar–C), 123.2, 120.7 (im–C H), 110.0, 105.0 (HC –CCl3 , C -1),
4
694

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DENIZALTI et al./Turk J Chem
98.4 (HC–C Cl3), 82.6, 81.4, 78.8 (C -2, C -3, C -4), 67.0 (NC H2 C6 H2 (CH3)3), 60.4 (C -5), 53.0 (C -6), 48.2
(
OC H3), 21.0, 19.9 (NCH2 C6 H2(C H3)3).
3.4. Synthesis of rhodium complex 3a
Yield: 82%. Anal. Calc. C30 H38 Cl3 N2 O5 Rh: C, 50.33; H, 5.35; N, 3.91. Found: C, 49.27; H, 5.50; N, 3.89%.
1
H NMR (400 MHz, CDCl3): δ = 6.92 (s, 2 H, NCH2 C6H2 (CH3)3), 6.90, 6.77 (s, 2 H, im–CH), 6.15 (dd, J
=
4.0, 19.6 Hz, 1 H, H -1), 5.73, 5.58 (d, J = 4.0, 14.4 Hz, 2 H, NCH2 C6 H2 (CH3)3), 5.29 (d,J = 8.0 Hz, 1 H,
H C–CCl3), 4.93–5.07 (m, 2 H, COD–CH), 4.89–3.99 (m, 1 H,H -5), 4.73–4.85 (m, 1 H, H -6), 4.74 (d, J = 4.0
Hz, 1 H,H -2), 4.40–4.47 (m, 1 H, H -6), 4.33 (m, 1 H, H -4), 4.11 (d, J = 4.0 Hz, 1 H, H -3), 3.42–3.58 (m, 2
H, COD–CH), 3.55 (s, 3 H, OCH3), 2.26, 2.30 (s, 9 H, NCH2 C6 H2 (CH3)3), 2.34–2.52 (m, 4 H, COD–CH2),
1
3
1
1
.89–2.03 (m, 4 H, COD–CH2).
C NMR (100 MHz, CDCl3): δ = 180.1 (d, JRh,C = 49.0 Hz, Ccarbene),
38.5, 129.4, 128.6, 127.9, 123.2, 118.4 (Ar–C , im–C), 107.2, 106.2 (HC –CCl3 , C -1), 97.8, 98.4 (d, J = 6.6
Hz, COD–C H), 96.8 (HC–C Cl3), 84.6, 83.4, 82.9 (C -2, C -3, C -4), 68.4, 67.8 (d, J = 15.0 Hz, COD–C H),
6
6.5 (NC H2 C6 H2 (CH3)3), 59.3 (C -6), 54.8 (C -5), 49.3 (OC H3), 33.3, 32.4, 28.8, 28.0 COD–C H2), 21.0,
0.0 (NCH2 C6 H2(C H3)3).
2
3
.5. General procedure for the arylation of aldehydes
Phenylboronic acid (1 mmol), ligand (or complex 3a) (0.01 mmol), [RhCl(COD)]2 (0.005 mmol), and KO Bu
0.5 mmol) were successively added to a two-necked flask. The vessel was evacuated and flushed with argon
t
(
three times. MeOH (1.5 mL) and DME (0.5 mL) were syringed, and then the aldehyde (1 mmol) was added to
◦
the mixture. The mixture was heated to 80 C for 1 or 4 h. After cooling to ambient temperature, the reaction
mixture was diluted with ethyl acetate (3 mL) and washed with water (2 mL). The organic phase was dried
(
Na2 SO4) and evaporated in a vacuum. Yields were determined by 1 H NMR. The enantiomeric excesses were
determined by HPLC using a chiral OJ-H column.
Acknowledgments
The authors acknowledge Ege University. We also thank Dr Levent Artok and Dr M Emin G u¨ nay for HPLC
analyses.
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