New C2-symmetric six-membered carbene ligands for asymmetric diethylzinc addition of arylaldehydes

Article abstract of DOI:10.24820/ark.5550190.p010.410

A series of new six-membered NHC precursors were prepared by simply esterification of their parent compounds. Their applicability in asymmetric diethylzinc addition of arylaldehydes has been demonstrated and the corresponding secondary alcohol was obtained with good yields and moderate enantioselectivities. (Figure pressented)

Full text of DOI:10.24820/ark.5550190.p010.410

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Arkivoc 2018, part v, 0-0
New C2-symmetric six-membered carbene ligands for asymmetric diethylzinc
addition of arylaldehydes
Bihui Zhou,a,b Zijing Li,a Chen Yang,a Linlin Li,a Limei Fan,a Hongxia Huang,a and Jie Li*a
aDepartment of Pharmacy, School of Medicine, Zhejiang University City College, No. 48, Huzhou Road,
Hangzhou 310015, PR China
bCollege of Pharmaceutical Siences, Zhejiang University, Hangzhou 310058, PR China
Email: lijie@zucc.edu.cn
Received 11-25-2017
Accepted 02-17-2018
Published on line 05-28-2018
Abstract
A series of new six-membered NHC precursors were prepared by simply esterification of their parent
compounds. Their applicability in asymmetric diethylzinc addition of arylaldehydes has been demonstrated
and the corresponding secondary alcohol was obtained with good yields and moderate enantioselectivities.
Keywords: N-Heterocyclic carbene, tetrahydropyrimidinium, amino alcohols, asymmetric diethylzinc addition,
chiral secondary alcohols
DOI: https://doi.org/10.24820/ark.5550190.p010.410
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Introduction
Since the first isolation of free N-heterocyclic carbene (NHC) by Arduengo’s group in 1991,1 these types of
ligands have gathered considerable interests because of their attractive properties such as higher stability to
air, thermal and moisture than phosphane ligands, and NHCs are now ubiquitous in modern synthetic
chemistry.2-12 As an extension, the development of new chiral NHC precursors for catalysis has become an
important issue and an enormous number of new chiral carbenes emerged, some of which have shown
excellent enantioselectivity in asymmetric catalysis.13-18 However, among the chiral NHCs synthesized,
examples of chiral hydroxyalkyl NHC ligands are still rare. In 2004, Arnold and co-workers reported the
synthesis of salt 1 (Figure 1),19 the CuI complex of this ligand was used as a catalyst in diethylzinc conjugated
addition to cyclohexenone, affording the desired product in up to 51% ee. Almost at the same time, another
type of hydroxy-bearing NHC salt 2 derived from (L)-valine was prepared by Mauduit’s group and this
compound showed high efficiency in chiral molecular recognition.20 Furthermore, the same group designed
and synthesized a series of new bidentate NHC precursors 3 based on commercial available amino alcohols,21-
23
which were successfully applied in CuII-catalyzed asymmetric addition of diethylzinc to cyclohexanone, as
well as asymmetric allylic substitution of allyl phosphates with Grignard reagents. Moreover, the same ligand 3
was proved to be an excellent ligand in multicomponent catalytic enantioselective transformations.24 With
similar starting materials, Wilhelm’s group prepared several new tridentate NHC precursors 4 as ionic
liquids,25-26 and these salts were also used as catalyst in asymmetric diethylzinc addition to arylaldehydes,
giving the corresponding secondary alcohols in good yields and moderate ees (up to 66%).
Figure 1. Representative chiral hydroxyalkyl NHC ligands.
Results and Discussion
Very recently, we reported the synthesis of several enantiopure 3,4,5,6-tetrahydropyrimidinium salts 5
incorporating two hydroxyl groups as effective ligands in Pd-catalyzed deprotonative cross-coupling process
(DCCP).27 Next, we examined the activity of these ligands in asymmetric diethylzinc addition to aldehydes. The
yields are good but the ee values are rather low (Table 1, entries 1–6). Usually, the hydroxy group in the N-
substituent of the NHC ligand was introduced as a coordination group to block the rotation of N-substituent
which subsequently elevates the enantioselectivity in catalytic transformations. On the other hand, little
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attention was paid to modifications the OH group with a steric functional group, which may result in an
elevated chiral environment around the carbene center. With this expectation in mind, a series of derivatives
of salts 5 are therefore prepared in this paper with the aim to improve their performance in asymmetric
catalysis. As shown in Scheme 1, simple treatment of compounds 5a–f with 4-(tert-butyl)benzoyl chloride
provided esterification product 6a–f in good yields (76–92%). Luckily, single crystals of 6f were obtained from
CH2Cl2, and the ORTEP view of this compound was obtained (Figure 2).
Scheme 1. Synthesis of NHC precursors 6a–f.
Figure 2. Crystal structure of 6f.
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Table 1. Comparison of NHC precursors
Entrya
Catalyst
Yield (%)b
ee (%)c
1
2
5a
5b
5c
5d
5e
5f
6a
6b
6c
6d
6e
6f
89
92
81
90
86
77
73
93
80
83
81
95
90
83
92
4
21
9
3
4
5
5
13
1
7
58
20
8
41
7
9
11
23
6
7
8
9
10
11
12
13
14
15
6g
6h
6i
a
Reaction condition: salt (10 mol %), KHMDS (30 mol %), Et2Zn (2 equiv), N2, xylene, rt, 24 h. b Isolated yield. c
Determined by chiral HPLC (CHIRALCEL OD Column) analysis.
The synthesized tetrahydropyrimidinium salts 6 were tested in asymmetric diethylzinc addition of 1-
naphthaldehyde (7a) according to our procedure in the same transformation with pyrimidone salts as
catalysts.28 As presented in Table 1, all derivatives 6a–f showed better enantioselectivities than their parent
compounds 5a–f, and 6b gave the best result (93% yield, 58% ee). We then tried various conditions of
different bases and solvents. Unfortunately, all combinations didn’t improve the enantioselectivity (see
Supporting information for details). Futhermore, three new salts 6g–6i (Figure 3), derived from the same
parent compound as 6b, were also prepared and tested in the same reaction, no improvement of ee value was
observed as well (entries 13–15).
Figure 3. The structures of 6g–6i.
Using 6b as catalyst, various arylaldehydes with different substituents were examined in this
transformation. As summarized in Table 2, the reaction proceeded well in most cases (73–96% yield).
Arylaldehydes bearing electron-donating (entries 3–7) and electron-withdrawing (entries 8–11) groups, as well
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as heterocyclic sustrates (entries 13–15), were all well-tolerated, giving the corresponding adducts 8b–8p in
good yields and moderate enantiomeric excesses. The best enantioselectivity was obtained starting from 2-
quinolinecarbaldehyde (66% ee, entry 15).
Table 2. Scope of methodology
Entrya
1
Ar
Product
8b
8c
Yield (%)b
83
ee (%)c
34
2-Naphthyl
Ph
2
91
37
3
2-MePh
3-MePh
3,4-diMePh
2-MeOPh
4-MeOPh
2-FPh
8d
8e
74
43
4
79
50
5
8f
86
44
6
8g
88
45
7
8h
8i
75
36
8
84
35
9
4-FPh
8j
80
45
10
11
12
13
14
15
4-BrPh
8k
90
36
4-CF3Ph
Cinnamyl
3-Pyridine
2-Thienyl
2-Quinolyl
8l
96
40
8m
8n
8o
8p
87
60
73
45
88
38
76
66
a Reaction condition: 6b (10 mol %), KHMDS (30 mol %), Et2Zn (2 equiv), N2, xylene, rt, 24 h.
b Isolated yields. c Determined by chiral HPLC (CHIRALCEL OD Column) analysis.
Conclusions
In summary, a series of new six-membered NHC precursors (6a–6i) have been prepared and the single-crystal
X-ray diffraction further confirmed the structure of compound 6f. The catalytical activity of these ligands in
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asymmetric diethylzinc addition to arylaldehydes was tested and the corresponding secondary alcohols were
obtained with excellent yields and moderate ees (up to 66%). Further work is currently underway to prepare
more chiral six-membered NHC ligands, as catalysts in other asymmetric transformation, by modification the
hydroxyl group in N-substituent.
Experimental Section
General. MS spectra were measured on a Finnigan LCQDECA XP instrument and a Agilent Q-TOF 1290 LC/6224
1
MS system; H and 13C NMR spectra were obtained on Bruker AVANCE III 500 MHz and 600 MHz
spectrometers (Bruker Co., Switzerland) with TMS as the internal standard; silica gel GF254 and H (10−40 mm,
Qingdao Marine Chemical Factory, China) were used for TLC and CC. Unless otherwise noted, all reactions
were carried out under an atmosphere of argon or nitrogen.
General procedure for the synthesis of compounds 6. To a mixture of salt 5 (2 mmol) and Et3N (9.6 mmol) in
o
dry dichloromethane (10 mL) was added 4-tert-butylbenzoyl chloride (8 mmol) at 0 C. After stiring at room
temperature for 12 h, the mixture was poured into water (25 mL) and extracted with dichloromethane (3 × 10
mL). The organic fractions were combined, washed with brine and dried over Na2SO4. The solvent was then
removed under reduced pressure and the residue was purified by column chromatography (silica gel,
CH2Cl2/MeOH) to afford the corresponding products 6.
6a. White powder; yield 1.31 g (89%); 1H NMR (500 MHz, CDCl3) δ 9.22 (s, 1H), 8.19 (d, J 8.6 Hz, 4H), 7.60 (d, J
8.6 Hz, 4H), 7.29 (d, J 7.5 Hz, 2H), 7.13 (t, J 7.8 Hz, 4H), 6.99 (d, J 7.5 Hz, 4H), 5.61 (dd, J 10.7, 3.5 Hz, 2H), 5.46 –
5.35 (m, 2H), 4.47 (m, 2H), 3.35 – 3.21 (m, 2H), 2.89 – 2.76 (m, 2H), 1.80 – 1.73 (m, 2H), 1.38 (s, 18H); 13C NMR
(125 MHz, CDCl3) δ 166.44, 157.70, 131.59, 131.58, 130.14, 129.48, 129.32, 127.57, 125.95, 125.85, 65.75,
60.10, 38.75, 35.27, 31.16, 18.65; ESIMS m/z 645.3.
6b. 81% yield; 1H NMR (500 MHz, CDCl3) δ 8.53 (s, 1H), 7.99 (d, J 8.6 Hz, 4H), 7.46 (d, J 8.6 Hz, 4H), 4.78 (dd, J
12.4, 10.4 Hz, 2H), 4.27 (dd, J 12.4, 3.4 Hz, 2H), 3.95 (m, 2H), 3.47 – 3.37 (m, 2H), 3.20 – 3.09 (m, 2H), 2.03 –
1.94 (m, 2H), 1.82 (dd, J 6.5, 4.0 Hz, 2H), 1.33 (s, 18H), 1.02 (d, J 6.6 Hz, 6H), 0.52 (d, J 6.6 Hz, 6H); 13C NMR
(125 MHz, CDCl3) δ 166.35, 157.56, 155.43, 129.90, 125.88, 125.71, 70.31, 60.73, 38.82, 35.17, 31.06, 26.80,
+
19.10, 18.98; HR-ESIMS: m/z 578.4090 [M–BF4+H]+ (calcd for C36H54N2O4 , 578.4078).
6c. 84% yield; 1H NMR (500 MHz, CDCl3) δ 8.53 (s, 1H), 8.02 (d, J 8.6 Hz, 4H), 7.49 (d, J 8.7 Hz, 4H), 4.80 (dd, J
12.3, 10.5 Hz, 2H), 4.25 (dd, J 12.4, 3.4 Hz, 2H), 4.03 (m, 2H), 3.43 (dd, J 12.7, 6.2 Hz, 2H), 3.16 – 3.07 (m, 2H),
2.02 – 1.98 (m, 2H), 1.57 – 1.52 (m, 2H), 1.33 (s, 18H), 0.97 (d, J 6.6 Hz, 6H), 0.87 – 0.81 (m, 2H), 0.75 (m, 2H),
0.50 (t, J 7.4 Hz, 6H); 13C NMR (125 MHz, CDCl3) δ 166.37, 157.42, 155.34, 129.82, 126.01, 125.72, 69.17,
61.05, 39.06, 35.14, 32.99, 31.05, 25.10, 18.74, 14.98, 10.69; HR-ESIMS: m/z 605.4395 [M–BF4]+ (calcd for
+
C38H57N2O4 , 605.4313).
6d. 92% yield; 1H NMR (500 MHz, CDCl3) δ 8.53 (s, 1H), 8.02 (d, J 8.6 Hz, 4H), 7.49 (d, J 8.6 Hz, 4H), 4.75 (m,
2H), 4.35 (dd, J 9.3, 4.6 Hz, 2H), 4.02 (dd, J 12.4, 3.4 Hz, 2H), 3.53 – 3.42 (m, 2H), 3.27 – 3.16 (m, 2H), 2.05 (dd, J
7.3, 4.2 Hz, 2H), 1.43 (m, 2H), 1.33 (s, 18H), 1.31 – 1.27 (m, 2H), 1.17 – 1.10 (m, 2H), 0.72 (d, J 6.6 Hz, 6H), 0.63
(d, J 6.5 Hz, 6H); 13C NMR (125 MHz, CDCl3) δ 166.46, 157.52, 154.79, 129.92, 125.89, 125.77, 62.60, 62.31,
38.47, 36.24, 35.18, 31.08, 29.70, 24.61, 22.82, 21.46; HR-ESIMS: m/z 605.4410 [M–BF4]+ (calcd for
+
C38H57N2O4 , 605.4313).
6e. 85% yield; 1H NMR (500 MHz, CDCl3) δ 8.60 (s, 1H), 7.96 – 7.89 (m, 4H), 7.46 – 7.41 (m, 4H), 7.25 – 7.13 (m,
10H), 4.68 (dd, m, 2H), 4.54 (d, J 9.8 Hz, 2H), 4.24 (dd, J 12.4, 3.5 Hz, 2H), 3.14 (m, 4H), 2.99 (dd, J m, 2H), 2.79
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(m, 2H), 1.74 – 1.66 (m, 2H), 1.25 (s, 18H); 13C NMR (125 MHz, CDCl3) δ 166.04, 157.52, 154.32, 134.83,
129.71, 129.11, 128.88, 127.42, 125.99, 125.72, 65.24, 62.84, 41.03, 35.11, 34.99, 30.98, 18.66; HR-ESIMS: m/z
+
673.4094 [M–BF4]+ (calcd for C44H53N2O4 , 673.4000).
1
6f. 76% yield; H NMR (500 MHz, CDCl3) δ 8.58 (s, 1H), 7.99 (d, J 7.3 Hz, 4H), 7.45 (d, J 8.1 Hz, 4H), 4.98 (t, J
11.5 Hz, 2H), 4.23 (m, 4H), 3.50 (dd, J 12.8, 6.1 Hz, 2H), 3.35 – 3.26 (m, 2H), 2.01 – 1.93 (m, 2H), 1.33 (s, 18H),
1.26 (s, 18H); 13C NMR (125 MHz, CDCl3) δ 166.32, 157.36, 129.81, 126.03, 125.56, 72.41, 58.69, 40.65, 35.11,
+
33.98, 31.05, 29.67, 27.37, 18.81; HR-ESIMS: m/z 605.4412 [M–BF4]+ (calcd for C38H57N2O4 , 605.4313).
6g. 84% yield; 1H NMR (500 MHz, CDCl3) δ 8.40 (s, 1H), 6.83 (s, 4H), 4.52 (m, 4H), 3.89 – 3.80 (m, 2H), 3.38 (dd,
J 12.8, 6.5 Hz, 2H), 3.28 (dd, J 12.7, 6.6 Hz, 2H), 2.28 (s, 6H), 2.26 (s, 12H), 2.04 – 1.98 (m, 2H), 1.96 – 1.88 (m,
2H), 1.04 (d, J 6.6 Hz, 6H), 0.70 (d, J 6.7 Hz, 6H); 13C NMR (125 MHz, CDCl3) δ 169.16, 154.83, 139.75, 135.27,
129.79, 128.52, 69.83, 62.17, 39.85, 29.68, 27.04, 21.07, 19.93, 18.96, 18.88; HR-ESIMS: m/z 549.3754 [M–
+
BF4]+ (calcd for C34H49N2O4 , 549.3687).
6h. 73% yield; 1H NMR (500 MHz, CDCl3) δ 8.92 (d, J 8.7 Hz, 2H), 8.62 (s, 1H), 8.38 (dd, J 7.3, 1.1 Hz, 2H), 8.03
(d, J 8.2 Hz, 2H), 7.88 (d, J 8.1 Hz, 2H), 7.64 (m, 2H), 7.58 – 7.51 (m, 4H), 4.75 (dd, J 12.5, 10.4 Hz, 2H), 4.39 (dd,
J 12.5, 3.4 Hz, 2H), 4.00 (m, 2H), 3.49 – 3.40 (m, 2H), 3.16 – 3.09 (m, 2H), 2.02 – 1.95 (m, 2H), 1.83 – 1.74 (m,
2H), 1.00 (d, J 6.6 Hz, 6H), 0.40 (d, J 6.6 Hz, 6H); 13C NMR (125 MHz, CDCl3) δ 166.64, 155.07, 134.12, 133.77,
131.43, 128.68, 128.11, 126.27, 125.39, 125.11, 124.97, 124.54, 70.28, 61.16, 39.07, 29.68, 19.03, 18.88; HR-
+
ESIMS: m/z 565.3186 [M–BF4]+ (calcd for C36H41N2O4 , 565.3061).
6i. 86% yield; 1H NMR (500 MHz, CDCl3) δ 8.73 (s, 2H), 8.67 (s, 1H), 8.08 (d, J 8.0 Hz, 2H), 8.02 (dd, J 8.6, 1.6 Hz,
2H), 7.86 (dd, J 8.3, 3.1 Hz, 4H), 7.64 – 7.54 (m, 4H), 4.84 (dd, J 12.3, 10.6 Hz, 2H), 4.30 (dd, J 12.4, 3.4 Hz, 2H),
4.03 (m, 2H), 3.50 – 3.39 (m, 2H), 3.21 – 3.09 (m, 2H), 2.05 – 1.97 (m, 2H), 1.86 – 1.72 (m, 2H), 0.96 (d, J 6.6 Hz,
6H), 0.42 (d, J 6.6 Hz, 6H); 13C NMR (125 MHz, CDCl3) δ 166.33, 155.02, 135.70, 132.48, 131.77, 129.73, 128.60,
128.37, 127.58, 126.79, 125.98, 124.96, 70.38, 61.26, 39.21, 26.79, 19.03, 18.98; HR-ESIMS: m/z 565.3191 [M–
+
BF4]+ (calcd for C36H41N2O4 , 565.3061).
Representative procedure for the asymmetric addition of diethylzinc to aldehyde. Under argon atmosphere,
a mixture of salt 6b (0.01 mmol) and KHMDS (0.03 mmol) in xylene (1 mL) was stirred for 5 min at room
temperature. Then diethylzinc (0.2 mmol) was added dropwise, followed by addition of aldehdye 7 (0.1
mmol). Upon stirring for 24 h at room temperature, the reaction was quenched by HCl (1 M, 1.0 mL), and
extracted with Et2O (3 × 2 mL). The combined organic phases were washed with water and dried over Na2SO4
and concentrated under vacuum. The residue was further purified by column chromatography (silica gel,
hexane/AcOEt) to give product 8.
8a. 93% yield, 58% ee ; the spectral data were comparable to those reported.29 The ee was determined by
HPLC analysis with Daicel Chiralcel OD-H (hexane/iPrOH = 90/10, flow rate = 0.5 mL/min, tr (minor) = 15.7 min,
tr (major) = 28.6 min).
8b. 83% yield, 34% ee; The spectral data were comparable to those reported.30 The ee was determined by
HPLC analysis with Daicel Chiralcel OD-H (hexane/iPrOH = 90/10, flow rate = 0.5 mL/min, tr (minor) = 19.2 min,
tr (major) = 22.5 min). 8c. 91% yield, 37% ee; The spectral data were comparable to those reported.31 The ee
was determined by HPLC analysis with Daicel Chiralcel OD-H (hexane/iPrOH = 90/10, flow rate = 0.5 mL/min, tr
(major) = 10.6 min, tr (minor) = 12.2 min). 8d. 74% yield, 43% ee; The spectral data were comparable to those
reported.31 The ee was determined by HPLC analysis with Daicel Chiralcel OD-H (hexane/iPrOH = 90/10, flow
rate = 0.5 mL/min, tr (major) = 12.7 min, tr (minor) = 15.4 min). 8e. 79% yield, 50% ee; The spectral data were
comparable to those reported.29 The ee was determined by HPLC analysis with Daicel Chiralcel OD-H
(hexane/iPrOH = 90/10, flow rate = 0.5 mL/min, tr (minor) = 18.3 min, tr (major) = 20.5 min). 8f. 92% yield, 20%
ee; The spectral data were comparable to those reported.28 The ee was determined by HPLC analysis with
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Daicel Chiralcel OD-H (hexane/iPrOH = 90/10, flow rate = 0.4 mL/min, tr (minor) = 8.9 min, tr (major) = 9.7
min).
8g. 63% yield, 24% ee; The spectral data were comparable to those reported.29 The ee was determined by
HPLC analysis with Daicel Chiralcel OD-H (hexane/iPrOH = 90/10, flow rate = 0.5 mL/min, tr (minor) = 18.5 min,
tr (major) = 21.9 min).
8h. 75% yield, 36% ee; The spectral data were comparable to those reported.30 The ee was determined by
HPLC analysis with Daicel Chiralcel OD-H (hexane/iPrOH = 90/10, flow rate = 0.5 mL/min, tr (major) = 10.6 min,
tr (minor) = 12.2 min).
8i. 84% yield, 35% ee; The spectral data were comparable to those reported.32 The ee was determined by
HPLC analysis with Daicel Chiralcel OD-H (hexane/iPrOH = 90/10, flow rate = 0.5 mL/min, tr (major) = 11.9 min,
tr (minor) = 15.3 min).
8j. 80% yield, 45% ee; The spectral data were comparable to those reported.29 The ee was determined by
HPLC analysis with Daicel Chiralcel OD-H (hexane/iPrOH = 93/7, flow rate = 0.5 mL/min, tr (major) = 11.6 min, tr
(minor) = 13.6 min).
8k. 90% yield, 36% ee; The spectral data were comparable to those reported.32 The ee was determined by
HPLC analysis with Daicel Chiralcel OD-H (hexane/iPrOH = 93/7, flow rate = 0.5 mL/min, tr (minor) = 11.9 min, tr
(major) = 12.8 min).
8l. 83% yield, 28% ee; The spectral data were comparable to those reported.28 The ee was determined by
HPLC analysis with Daicel Chiralcel OD-H (hexane/iPrOH = 93/7, flow rate = 0.5 mL/min, tr (major) = 11.2 min, tr
(minor) = 13.1 min).
8m. 87% yield, 60% ee; The spectral data were comparable to those reported.29 The ee was determined by
HPLC analysis with Daicel Chiralcel OD-H (hexane/iPrOH = 93/7, flow rate = 0.5 mL/min, tr (minor) = 14.1 min, tr
(major) = 15.8 min).
8n. 73% yield, 45% ee; The spectral data were comparable to those reported.33 The ee was determined by
HPLC analysis with Daicel Chiralcel OD-H (hexane/iPrOH = 93/7, flow rate = 0.5 mL/min, tr (minor) = 13.6 min, tr
(major) = 14.6 min).
8o. 88% yield, 38% ee; The spectral data were comparable to those reported.30 The ee was determined by
HPLC analysis with Daicel Chiralcel OD-H (hexane/iPrOH = 93/7, flow rate = 0.5 mL/min, tr (major) = 9.4 min, tr
(minor) = 10.7 min).
8p. 76% yield, 66% ee; The spectral data were comparable to those reported.34 The ee was determined by
HPLC analysis with Daicel Chiralcel OD-H (hexane/iPrOH = 90/10, flow rate = 0.5 mL/min, tr (minor) = 14.1 min,
tr (major) = 30.2 min.
Acknowledgements
We are grateful to the National Natural Science Foundation of China (31670357) and Hangzhou Science and
Technology Information Institute of China (20150633B45).
Supplementary Material
1H and 13C NMR spectra of new compounds and crystallographic data of 6f.
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