Enantioselective addition of diethylzinc to aromatic aldehydes catalyzed by C2-symmetric chiral diols

Article abstract of DOI:10.1016/j.tetlet.2014.03.058

Optically pure C₂-symmetric diols have been synthesized with moderate yields in a straightforward manner, and are used as catalysts in the enantioselective alkylation of aromatic aldehydes with diethylzinc. The addition of diethylzinc to benzal

Full text of DOI:10.1016/j.tetlet.2014.03.058

Tetrahedron Letters 55 (2014) 2727–2729
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Enantioselective addition of diethylzinc to aromatic aldehydes
catalyzed by C2-symmetric chiral diols
⇑
Yaßsar Gök , Levent Kekeç
Department of Chemistry, Faculty of Arts and Sciences, Osmaniye Korkut Ata University, Karacaoglan Yerleskesi, 80000 Osmaniye, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
Optically pure C2-symmetric diols have been synthesized with moderate yields in a straightforward
manner, and are used as catalysts in the enantioselective alkylation of aromatic aldehydes with
diethylzinc. The addition of diethylzinc to benzaldehyde and sterically hindered 1-naphthaldehyde
was achieved with excellent enantioselectivities (97–99% ee) under catalysis with (1R,2R)-1,2-bis(3,5-
dibromophenyl)-ethane-1,2-diol and (1R,2R)-1,2-bis(3,5-diphenylphenyl)-ethane-1,2-diol.
Ó 2014 Elsevier Ltd. All rights reserved.
Received 2 January 2014
Revised 24 February 2014
Accepted 12 March 2014
Available online 20 March 2014
Keywords:
Enantioselectivity
Alkylation
Chiral diol
C2-Symmetry
Enantioselective carbonAcarbon bond forming reactions repre-
sent an interesting challenge in modern organic chemistry. In this
context, the alkylation of aldehydes with organometallic reagents
in the presence of a catalytic amount of a chiral molecule gives a
new CAC bond in an enantioselective manner. The addition of dial-
kylzinc to aldehydes catalyzed by b-amino alcohols is a very attrac-
tive method to obtain optically active secondary alcohols, which
are important building blocks in the synthesis of many biologically
and optically active compounds.1 Since the initial reports by Oguni,
Noyori, and Soai, many chiral b-amino alcohol type ligands have
been developed.1,2 Although some of these ligands can be obtained
through simple synthetic methods, most are not always easy to
prepare. A large number of chiral catalysts have been developed
and high selectivities have been achieved. In accordance with these
developments, significant research has been devoted toward the
synthesis of C2-symmetric chiral ligands such as diols,3 diamines
and disulfonamides,4 bisoxazolines,5 etc., as this symmetry ele-
ment eliminates the number of possible transition states.6 These
chiral ligands have been used with and without extra metals, such
as Ti(IV) for the enantioselective addition of organometallic re-
agents to aldehydes.7
ligands by our group.8 However, no example of its use as a ligand
for enantioselective catalysis has been described so far, to the best
of our knowledge. Replacing bromines with phenyl groups via
Suzuki cross-coupling reactions has provided a novel more bulky
C2-symmetric diol, (1R,2R)-1,2-bis(3,5-diphenylphenyl)-ethane-
1,2-diol (4).9 As part of our research on enantioselective catalysis,
we decided to evaluate these chiral diols in the addition of diethyl-
zinc to various aromatic aldehydes as a general catalytic bench-
mark reaction.
The C2-symmetric chiral ligands were prepared according to the
synthetic routes outlined in Scheme 1. Tetrabromo-substituted (E)-
stilbene 1 was not commercially available and needed to be
synthesized. This product was obtained in good overall yield via
a Horner–Wadsworth–Emmons reaction using a phosphonate es-
ter, according to an earlier reported method by our group.8 The de-
scribed method could be easily scaled-up. Diol 3 was synthesized
via a Sharpless asymmetric dihydroxylation (63%, >99% ee, 5 days)
using THF as a co-solvent in order to solve the solubility problem of
1.8a Next, using a Suzuki cross-coupling, four phenyl groups could
be introduced to afford diol 4 (70%) after 24 h. Although, diol 4 was
obtained in a short sequence, this method was not pursued due to
the poor overall yield and long reaction time. Therefore, a second
method was devised (Scheme 1). Thus, tetrabromo-substituted
(E)-stilbene 1 was treated with phenylboronic acid and K2CO3 in
the presence of [Pd(PPh3)4] in EtOH/H2O/toluene (1:2:4) at reflux
temperature, yielding tetraphenyl substituted (E)-stilbene 2 in
excellent yield (95%) in 22 h.10 Next, Sharpless asymmetric dihydr-
oxylation gave chiral C2-symmetric diol 4 (65%, >99% ee, 5 h).
Herein, we report the synthesis of optically pure C2-symmetric
diols with moderate yields in a straightforward manner. One of
these chiral C2-symmetric diols, (1R,2R)-1,2-bis(3,5-dibromophe-
nyl)-ethane-1,2-diol (3) has been previously reported as an inter-
mediate in the synthesis of bisphosphane and bisoxazoline
⇑
Corresponding author. Tel.: +90 328 8271000/2541; fax: +90 328 8250097.
E-mail address: yasargok@osmaniye.edu.tr (Y. Gök).
http://dx.doi.org/10.1016/j.tetlet.2014.03.058
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

2728
Y. Gök, L. Kekeç / Tetrahedron Letters 55 (2014) 2727–2729
Table 1
Br
Enantioselective addition of diethlyzinc to aromatic aldehydes catalyzed by chiral
diols 3 or 4
Br
OH
O
Br
ligand (10 mol%)
solvent, 24 h
+
Et2Zn
Br
Ar
1
Ar
H
Entry Ar
Ligand Solvent
Temperature
(°C)
Yielda
(%)
eeb
(%)
Pd(PPh3)4, PhB(OH)2
K2CO3, reflux
EtOH/H2O/toluene
95%, 22 h
AD-mix- , MeSO NH
β
2
t-BuOH/H2O/THF, 0 o2
C
1
2
3
4
5
6
7
8
9
Ph
Ph
Ph
Ph
3
3
4
4
3
3
4
4
3
3
3
3
4
4
4
3
3
3
4
4
3
3
4
4
3
3
3
3
4
4
4
4
CH2Cl2
25
43
49
34
38
50
59
47
53
59
61
26
41
54
24
26
40
35
43
45
32
44
41
30
38
43
45
35
37
17
16
16
15
99
99
77
79
81
89
97
99
65
70
52
53
59
42
47
93
31
29
78
71
59
61
60
60
rac
rac
10
34
rac
rac
32
9
Toluene 25
CH2Cl2 25
Toluene 25
CH2Cl2 25
Toluene 25
CH2Cl2 25
Toluene 25
CH2Cl2
CH2Cl2
Toluene 25
Toluene
CH2Cl2
Toluene 25
Toluene
CH2Cl2
Toluene 25
63%, 5 d
1-Naphth
1-Naphth
1-Naphth
1-Naphth
2-ClC6H4
2-ClC6H4
2-ClC6H4
2-ClC6H4
2-ClC6H4
2-ClC6H4
2-ClC6H4
3-ClC6H4
3-ClC6H4
3-ClC6H4
3-ClC6H4
3-ClC6H4
2-MeOC6H4
2-MeOC6H4
2-MeOC6H4
2-MeOC6H4
3-MeOC6H4
3-MeOC6H4
3-MeOC6H4
3-MeOC6H4
3-MeOC6H4
3-MeOC6H4
3-MeOC6H4
3-MeOC6H4
HO
OH
Br
Br
25
0
Br
Br
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
2
3
>99% ee
Pd(PPh3)4, PhB(OH)2
K2CO3, reflux
0
25
AD-mix- , MeSO NH
β
2
2
C
t-BuOH/H2O/THF, 0 o
EtOH/H2O/toluene
65%, 5 h
0
25
70%, 24 h
Toluene
CH2Cl2
0
25
HO
OH
Toluene 25
CH2Cl2 25
Toluene 25
CH2Cl2 25
Toluene 25
CH2Cl2
CH2Cl2
Toluene 25
Toluene
CH2Cl2
CH2Cl2
Toluene 25
Toluene
25
0
4
>99% ee
0
25
0
Scheme 1. Synthesis of optically pure C2-symmetric diols 3 and 4.
0
We subsequently focused our attention on the enantioselective
alkylation of various aromatic aldehydes catalyzed by newly syn-
thesized chiral C2-symmetric diols 3 and 4, in order to determine
the efficiency of the ligands (Table 1). This reaction is one of the
most versatile methods for the formation of carbon–carbon
bonds.1e–j Reactions were carried out by using 2 equiv of diethyl-
zinc (1 M solution in hexane) at 0 °C or rt for 24 h in the presence
of the ligands (0.1 equiv) in different solvents.11 The initial test
substrate was benzaldehyde, which is regarded as a standard sub-
strate for evaluating newly synthesized enantioselective catalysts.
Benzaldehyde gave the corresponding secondary alcohol with
excellent enantioselectivities (99% ee) and moderate yields in both
dichloromethane and toluene (Table 1, entries 1 and 2) under
catalysis by the diol 3. When the reaction was performed with diol
4 in dichloromethane, we observed a decrease in the enantioselec-
tivity and yield (Table 1, entry 3). Changing the solvent from
dichloromethane to toluene resulted in both slightly higher enanti-
oselectivity and yield (Table 1, entry 4). We next examined the
influence of these two solvents on the addition of diethlyzinc to
various aromatic aldehydes catalyzed by diols 3 and 4. Excellent
enantioselectivity, but a moderate yield was obtained on ethyla-
tion of sterically hindered 1-naphthaldehyde using diol 4 as the
catalyst in toluene (Table 1, entry 8). Using dichloromethane as
the solvent led to a slight decrease in both the enantioselectivity
and yield (Table 1, entry 7). Changing the catalyst to diol 3 resulted
in a parallel outcome. A higher enantioselectivity and yield were
observed in toluene than in dichloromethane (Table 1, entries 5
and 6). The electronic nature of the substituent on the phenyl ring
seems to have a variable effect on the enantioselectivity in the
solvents employed. Thus, aldehydes with electron-withdrawing
a
Isolated yield.
b
Determined by HPLC analysis with a chiral stationary phase column (Chiralcel
OD-H or Chiralcel OB).
chlorine at the 2- or 3-positions gave better enantioselectivities
in dichloromethane than in toluene (Table 1, entries 9–20), cata-
lyzed by either diol 3 or 4. The ethylation of 2-chlorobenzaldehyde
using diol 3 resulted in better selectivity in dichloromethane than
in toluene (Table 1, entries 9 and 11). These reactions were also run
at 0 °C in dichloromethane and toluene (Table 1, entries 10 and 12),
but significant differences were achieved. Similarly, diol 4 cata-
lyzed the reaction more efficiently in dichloromethane than in
toluene (Table 1, entries 13 and 14). The use of 3-chlorobenzalde-
hyde gave excellent enantioselectivity in dichloromethane with
diol 3 (Table 1, entry 16). However, poor selectivity was obtained
in toluene (Table 1, entry 17). Performing the reaction at 0 °C in tol-
uene did not result in any improvements (Table 1, entry 18). With
diol 4, we observed similar results; the selectivity was better in
dichloromethane than in toluene (Table 1, entries 19 and 20). We
also investigated the effect of a strong electron-releasing substitu-
ent on the phenyl ring, as in 2- and 3-methoxybenzaldehydes (Ta-
ble 1, entries 21–32). It was found that changing the solvent made
no difference to the enantioselective addition to 2-methoxybenzal-
dehyde using diols 3 (Table 1, entries 21 and 22) and 4 (Table 1, en-
tries 23 and 24). We observed no enantioselectivities but moderate
yields when we alkylated 3-methoxybenzaldehyde in dichloro-
methane (Table 1, entries 25, 26, and 29–30) with diols 3 and 4.
Changing the solvent to toluene resulted in a slightly higher

Y. Gök, L. Kekeç / Tetrahedron Letters 55 (2014) 2727–2729
2729
Table 2
References and notes
Enantioselective addition of diethlyzinc to aromatic aldehydes in the presence of
Ti(OiPr)4 and chiral diols 3 or 4
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Matsoda, Y. J. Organomet. Chem. 1990, 382, 19; (d) Soai, K.; Yokoyama, S.;
Hayasaka, T. J. Org. Chem. 1991, 56, 4264; (e) Noyori, R.; Kitamura, M. Angew.
Chem., Int. Ed. 1991, 30, 49; (f) Soai, K.; Niwa, S. Chem. Rev. 1992, 92, 833; (g)
Bolm, C.; Muñiz-Fernández, K.; Seger, A.; Raabe, G.; Günther, K. J. Org. Chem.
1998, 63, 7860; (h) Soai, K.; Shibata, T. In Comprehensive Asymmetric Catalysis;
Jacobsen, E. N., Pfaltz, A., Yamamoto, H., Eds.; Springer Verlag: Berlin, 1999; Vol.
2, pp 911–922; (i) Zhao, G.; Li, X.-G.; Wang, R. Tetrahedron: Asymmetry 2001, 12,
399; (j) Pu, L.; Yu, H.-B. Chem. Rev. 2001, 101, 757.
2. (a) Watanabe, M.; Soai, K. J. Chem. Soc., Perkin Trans. 1 1994, 3125; (b) Hanyu,
N.; Aoki, T.; Mino, T.; Sakamoto, T.; Fujita, T. Tetrahedron: Asymmetry 2000, 11,
2971; (c) Superchi, S.; Mecca, T.; Giorgio, E.; Rosini, C. Tetrahedron: Asymmetry
2001, 12, 1235; (d) Lu, J.; Xu, X.; Wang, S.; Wang, C.; Hu, Y.; Hu, H. J. Chem. Soc.,
Perkin Trans. 1 2002, 2900; (e) Wolf, C.; Francis, C. J.; Hawes, P. A.; Shah, M.
Tetrahedron: Asymmetry 2002, 13, 1733; (f) Panda, M.; Phuan, P.; Kozlowski, M.
C. J. Org. Chem. 2003, 68, 564; (g) Kang, S. W.; Ko, D. H.; Kim, K. H.; Ha, D.-C. Org.
Lett. 2003, 5, 4517; (h) Bauer, T.; Gajewiak, J. Tetrahedron 2004, 60, 9163; (i)
García-Delgado, N.; Fontes, M.; Pericas, M. A.; Riera, A.; Verdaguer, X.
Tetrahedron: Asymmetry 2004, 15, 2085; (j) Blay, G.; Fernandez, I.; Marco-
Aleixandre, A.; Pedro, J. R. Tetrahedron: Asymmetry 2005, 16, 1207; (k) Hsieh, S.-
S.; Gau, H.-M. Chirality 2006, 18, 569; (l) Szakonyi, Z.; Balazs, A.; Martinek, T. A.;
Fülöp, F. Tetrahedron: Asymmetry 2006, 17, 199; (m) Bisai, A.; Singh, P. K.; Singh,
V. K. Tetrahedron 2007, 63, 598; (n) Martins, J. E. D.; Wills, M. Tetrahedron:
Asymmetry 2008, 19, 1250; (o) Mao, J. Synth. Commun. 2009, 39, 3710; (p)
Zacharia, J. T.; Tanaka, T.; Uesaka, Y.; Hayashi, M. Synthesis 2012, 44, 1625.
3. (a) Seebach, D. Angew. Chem., Int. Ed. Engl. 1990, 29, 1320; (b) von dem Bussche-
Hunnefeld, J. L.; Seebach, D. Tetrahedron 1992, 48, 5719; (c) Seebach, D.; Beck,
A. K.; Schmidt, B.; Wang, Y. M. Tetrahedron 1994, 50, 4363; (d) Prasad, K. R. K.;
Joshi, N. N. Tetrahedron: Asymmetry 1996, 7, 1957; (e) Mori, M.; Nakai, T.
Tetrahedron Lett. 1997, 38, 6233; (f) Zhang, F.-Y.; Yip, C.-W.; Cao, R.; Chan, A. S.
C. Tetrahedron: Asymmetry 1997, 8, 585; (g) Balsells, J.; Davis, J. T.; Carroll, P.;
Walsh, P. J. J. Am. Chem. Soc. 2002, 124, 10336; (h) Baker-Salisbury, M. G.;
Starkman, B. S.; Frisenda, G. M.; Roteta, L. A.; Tanski, J. M. Inorg. Chim. Acta
2014, 409, 394.
OH
O
ligand (10 mol%)
+
Et2Zn
a
Ti(OiPr)4
CH2Cl2
rt, 24 h
Ar
Ar
H
Entry
Ar
Ligand
Yieldb (%)
eec (%)
1
2
3
4
5
6
7
8
9
Ph
Ph
3
4
3
4
3
4
3
4
3
4
3
4
56
45
61
60
73
65
54
57
56
42
53
26
32
27
62
34
12
7
47
9
19
16
rac
rac
1-Naphth
1-Naphth
2-ClC6H4
2-ClC6H4
3-ClC6H4
3-ClC6H4
2-MeOC6H4
2-MeOC6H4
3-MeOC6H4
3-MeOC6H4
10
11
12
a Reactions were carried out with Ti(OiPr)4 (0.94 equiv).
Isolated yield.
b
c
Determined by HPLC analysis with a chiral stationary phase column (Chiralcel
OD-H or Chiralcel OB).
enantioselectivity, but lower yields (Table 1, entries 27, 28, 31, and
32). For this substrate, the highest enantioselectivity was obtained
from the reaction catalyzed by diol 3 in toluene at 0 °C (Table 1, en-
try 28).
4. (a) Takahashi, H.; Kawakita, T.; Yoshioka, M.; Kobayashi, S.; Ohno, M.
Tetrahedron Lett. 1989, 30, 7095; (b) Takahashi, H.; Kawakita, T.; Ohno, M.;
Yoshioka, M.; Kobayashi, S. Tetrahedron 1992, 48, 5691; (c) Noel, T.; Vandayck,
K.; Robeyns, K.; Van Meervelt, L.; Van der Eycken, J. Tetrahedron 2009, 65, 8879.
5. (a) Falorni, M.; Collu, C.; Conti, S.; Giacomelli, G. Tetrahedron: Asymmetry 1996,
7, 293; (b) Hwang, C.-D.; Uang, B.-J. Tetrahedron: Asymmetry 1998, 9, 3979; (c)
Seebach, D.; Pichota, A.; Beck, A. K.; Pikerton, A. B.; Litz, T.; Karjalainen, J.;
Gramlich, V. Org. Lett. 1999, 1, 55.
6. (a) Whitesell, J. Chem. Rev. 1989, 89, 1581; (b) Halm, C.; Kurth, M. J. Angew.
Chem., Int. Ed. 1998, 37, 510.
7. (a) Sarvary, I.; Wan, Y.; Frejd, T. J. Chem. Soc., Perkin Trans. 1 2002, 645; (b)
Harada, T.; Handa, K.; Hiraoka, Y.; Marutani, Y.; Nakatsugawa, M. Tetrahedron:
Asymmetry 2004, 15, 3879; (c) Michigami, K.; Hayashi, M. Tetrahedron 2013, 69,
4221.
We also explored the reactivity and selectivity of the addition of
diethylzinc to aromatic aldehydes in the presence of titanium iso-
propoxide and chiral diols 3 and 4 (Table 2). The process using
titanium-based catalysts tends to be more efficient and selec-
tive.3a,12 According to the literature, we used chiral diols in combi-
nation with Ti(OiPr)4. Correspondingly, we examined the effect of
Ti(OiPr)4 on the reaction of aromatic aldehydes with diethylzinc.
Dichloromethane was chosen as the solvent since it gave a better
enantioselectivity in the absence of an additive. Under our reaction
conditions, an aldehyde/Et2Zn (1 M solution in hexane)/Ti(OiPr)4/
ligand ratio of 1:2:0.94:0.1 was used at room temperature for
24 h. We found that the addition of the titanium species led to sig-
nificantly decreased enantioselectivities in all cases, but the con-
versions were noteworthy (Table 2, up to 73%).
8. (a) Gök, Y.; Noel, T.; Van der Eycken, J. Tetrahedron: Asymmetry 2010, 21, 2768;
(b) Gök, Y.; Van der Eycken, J. Helv. Chim. Acta 2012, 95, 831.
9. Experimental data for compound 4: white solid (>99% ee, 65% yield). Mp : 226–
227 °C. Anal. Calcd for C38H30O2: C, 88.00; H, 5.83%. Found: C: 87.81; H: 5.89%.
IR (KBr disc) m
max/cmꢀ1: 3530 (–OH), 3467 (–OH), 3081, 3056, 3034 (CHAr),
2860 (CHaliph), 1949, 1887, 1764, 1689, 1594, 1575, 1496, 1453, 1407, 1340,
1264, 1170, 1076, 1040, 1028, 879, 753, 695, 632, 538. 1H NMR (400 MHz,
CDCl3) d: 7.72 (t, J = 1.75 Hz, 2H, ArH), 7.55–7.52 (m, 8H, ArH), 7.45–7.35 (m,
16H, ArH), 4.97 (s, 2H, CH), 3.03 (s, 2H, OH). 13C NMR (100 MHz, CDCl3) d:
141.81, 140.90, 128.76, 127.50, 127.27, 125.81, 124.91, 79.44. MS (MALDI-TOF)
In conclusion, we have reported the synthesis of C2-symmetric
chiral diols 3 and 4 in good yields via short synthetic sequences.
These ligands were able to catalyze the enantioselective addition
of diethylzinc to various aromatic aldehydes. In general, the best
results were obtained with benzaldehyde and 1-naphthaldehyde.
The use of aromatic aldehydes with substituents on the aryl ring re-
sulted in lower enantioselectivities, however, the yields were very
similar. The presence of electron-releasing or electron-withdraw-
ing substituents resulted in similar enantioselectivities, with the
exception of 3-methoxy-substituted benzaldehyde, which gave
racemic alkylated products and poor yields. These ligands could
also catalyze the alkylation reaction in the presence of titanium iso-
propoxide with lower enantioselectivities, but with higher yields.
m/z: 501.10 [MꢀOH]+, 541.23 [M+Na]+.½a 2D0
ꢀ98.4 (c, 1.0, EtOH). Conditions for
ꢁ
HPLC: Chiralcel OD-H column, solvent: n-hexane/EtOH (90:10), flow
rate = 1 mL/min, T = 35 °C, retention times: 30.5 min for (1R,2R)-4, 37.4 min
for (1S,2S)-4.
10. Imai, M.; Ikegami, M.; Momotake, A.; Nagahata, R.; Arai, T. Photochem.
Photobiol. Sci. 2003, 2, 1181.
11. General procedure for the enantioselective diethylzinc addition to aromatic
aldehydes: Under an argon atmosphere, chiral ligand 3 or 4 (0.025 mmol)
was dissolved in dry solvent and Ti(OiPr)4 (0.23 mmol) was added and the
mixture stirred for 1 h. Et2Zn (0.5 mmol, 1 M in hexane) was added and the
resulting yellow solution was stirred for 20 min at room temperature. The
mixture was cooled to 0 °C and the aromatic aldehyde was added (0.25 mmol).
The mixture was stirred for another 24 h. After quenching with saturated
NH4Cl solution (1 mL), H2O (25 mL) was added and the mixture extracted with
EtOAc (3 ꢂ 25 mL). The combined organic phases were dried over Na2SO4 and
evaporated in vacuo. The crude product was purified by flash chromatography
to give the corresponding alcohol. Enantiomeric excesses were determined by
Acknowledgements
This study was supported by the Scientific & Technological Re-
search Council of Turkey (TUBITAK) (Project No: TBAG-112T017)
and Research Fund of Osmaniye Korkut Ata University (Project
No: OKUBAP-2013-PT3-005). Dr. Halil Zeki Gök (Department of
Chemistry, Osmaniye Korkut Ata University) is kindly acknowl-
edged for his technical assistance.
HPLC analysis with
Chiralcel OB).
a chiral stationary phase column (Chiralcel OD-H or
12. (a) Duthaler, R. O.; Hafner, A. Chem. Rev. 1992, 92, 807; (b) Lake, F.; Moberg, C.
Tetrahedron: Asymmetry 2001, 12, 755; (c) Xu, Q.; Wang, H.; Pan, X.; Chan, A. S.
C.; Yang, T. Tetrahedron Lett. 2001, 42, 6171.