Enantioselective addition of dialkylzincs to aldehydes catalyzed by (-)-MITH

Article abstract of DOI:10.1002/asia.201200676

An effective catalytic system that imparts high enantioselectivity has been disclosed for the synthesis of optically active alcohols, which may undergo further chemical transformations. The enantioselective alkylation of aldehydes with dialkylzincs to afford the corresponding optically active alcohols with excellent enantioselectvities has been achieved in the presence of 0.1-0.5 mol % of the camphor-derived chiral ligand (-)-2-exo-morpholinoisobornane-10-thiol (MITH) (1) at room temperature or at 0 °C. I zinc so too: The enantioselective alkylation of aldehydes with dialkylzincs yielded the desired alcohol products with up to 99 % ee in the presence of 0.1-0.5 mol % of ligand (-)-2-exo-morpholinoisoborne-10-thiol (1) at room temperature or at 0°C. Copyright

Full text of DOI:10.1002/asia.201200676

DOI: 10.1002/asia.201200676
Enantioselective Addition of Dialkylzincs to Aldehydes Catalyzed by
(À)-MITH**
Ying-Ni Cheng, Hsyueh-Liang Wu, Ping-Yu Wu, Ying-Ying Shen, and Biing-Jiun Uang*[a]
Abstract: An effective catalytic system that imparts high enantioselectivity has
been disclosed for the synthesis of optically active alcohols, which may undergo
further chemical transformations. The enantioselective alkylation of aldehydes
with dialkylzincs to afford the corresponding optically active alcohols with excel-
lent enantioselectvities has been achieved in the presence of 0.1–0.5 mol% of the
camphor-derived chiral ligand (À)-2-exo-morpholinoisobornane-10-thiol (MITH)
(1) at room temperature or at 08C.
Keywords: alcohols · aldehydes ·
homogeneous catalysis organo-
zincs · enantioselective addition
·
Introduction
Results and Discussion
Enantioselective addition reactions of organozincs to car-
bonyl compounds or their imino derivatives,[1] in the pres-
ence of chiral catalysts to afford chiral alcohols or amines,
have attracted the attention of synthetic organic chemists
owing to their mild reaction conditions, high functional-
group tolerance, and low-toxicity of zinc metal. The con-
As part of our endeavors to develop camphor-derived chiral
ligands for catalytic enantioselective reactions,[9] (À)-2-exo-
morpholinoisobornane-10-thiol (MITH) (1) has been dem-
onstrated to be an effective promoter for the enantioselec-
tive addition of arylzinc,[9d] alkenylzinc,[9e] and alkynylzinc[9g]
to aldehydes, thereby affording the corresponding optically
active alcohols with excellent enantioselectvities (up to>
99% ee; Scheme 1). Ligand 1 was also employed as a chiral
À
comitant construction of a C C bond and a new chiral
center in the reaction course leads to its wide application in
the synthesis of optically active alcohols that will undergo
further elaboration to natural products and biologically
active components.[2] Although notable examples on the de-
velopment of a more effective ligand system for the addition
reaction of dialkylzinc,[3] vinylzinc,[4] arylzinc,[3i,m,5] and alky-
nylzinc substrates[6] to carbonyl compounds have been re-
ported, an efficient catalytic system for the highly enantiose-
lective addition of dimethylzinc to aldehydes is sparse.[7] The
development of a methodology for the enantioselective ad-
dition of dimethylzinc to carbonyl compounds remains chal-
lenging, because of the relatively lower reactivity of dime-
thylzinc. In this regard, chiral metal complexes have been
recently reported to effect this transformation.[8]
Scheme 1. Enantioselective addition of organozincs to carbonyl com-
pounds catalyzed by ligand 1.
promoter in the enantioselective addition of dimethylzinc to
a-ketoesters, thus providing the corresponding hydroxy
esters that bear quaternary chiral centers with good yields
and ee.[9f] Encouraged by these results, we anticipated that
ligand 1 could impart high catalytic activity and a high level
of enantioselectivity in the enantioselective addition of orga-
nozinc reagents to aldehydes. Herein, we describe our find-
ings in the catalytic enantioselective addition reactions of di-
alkylzincs to aldehydes.
[a] Y.-N. Cheng, Dr. H.-L. Wu, Dr. P.-Y. Wu, Y.-Y. Shen,
Prof. Dr. B.-J. Uang
Department of Chemistry
National Tsing Hua University
No. 101, Section 2, Kuang-Fu Rd, Hsinchu, 30043, ROC (Taiwan)
Fax : (+886)-3-5711082
E-mail: bjuang@mx.nthu.edu.tw
[**] MITH=(À)-2-exo-morpholinoisobornane-10-thiol
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201200676.
Chem. Asian J. 2012, 7, 2921 – 2924
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2921

FULL PAPER
The enantioselective addition of dimethylzinc to benzal-
dehyde, catalyzed by 10 mol% of ligand 1, was initially
tested at 08C. The effect that the amount of dimethylzinc
used had on the enantioinduction was investigated and
using two equivalents of dimethylzinc gave good yields and
enantioselectivities (Table 1, entries 1–4). The enantioselec-
ligand decreased in the reaction, however, longer reaction
times or more dimethylzinc were required to give reasona-
ble yields for a lower loading of catalyst. The reaction rate
was enhanced as more dimethylzinc was used (Table 2, cf.
entry 1 vs 2; entry 6 vs 7). Given the efficiency and atom
economy of the reaction, the reaction conditions with three
equivalents of dimethylzinc in the presence of 0.5 mol% of
ligand 1 (Table 2, entry 5) were applied to study the scope
of the catalytic system with different aldehydes, and the re-
sults are summarized in Table 3. Generally, good yields and
Table 1. Optimization of the enantioselective addition of Me2Zn to ben-
zaldehyde.
Table 3. Enantioselective addition of Me2Zn to aldehydes catalyzed by
ligand 1.[a]
Entry
Me2Zn [equiv][a]
T [8C]
t [h]
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
1.5
2.0
3.0
5.0
2.0
2.0
2.0
2.0
0
0
0
0
25
10
0
72
48
20
18
6
48
96
48
58
88
84
89
82
82
80
70
89
94
95
95
93
94
94
89
Entry
R
1 [mol%]
t [h]
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
2-Tol
3-Tol
4-Tol
4-MeO-Ph
4-CF3-Ph
2-Cl-Ph
3-Cl-Ph
4-Cl-Ph
0.5
0.5
2.0
0.5
2.0
0.5
0.5
0.5
0.5
0.5
0.5
0.5
2.0
0.5
0.5
0.5
0.5
0.5
48
48
48
72
48
48
48
48
48
48
48
48
48
48
48
72
48
48
82 (2b)
80 (2c)
89 (2d)
82 (2e)
91 (2 f)
83 (2g)
86 (2h)
77 (2i)
70 (2j)
82 (2k)
77 (2l)
82 (2m)
90 (2n)
86 (2o)
95 (2p)
65 (2q)
83 (2r)
70 (2s)
90
90
96
85
78
84
90
93
89
92
78
83
93
92
64
83
42
86
7[d]
8[e]
0
[a] A 0.73m solution of dimethylzinc in hexanes was used, the concentra-
tion was titrated according to the literature procedure.[10] [b] Yield of iso-
lated product after column chromatography. [c] Determination by chiral
HPLC. [d] The reaction mixture was diluted to half the concentration
with hexanes. [e] A 1.0m solution of dimethylzinc in toluene was used.
9
4-Br-Ph
2-Thienyl
2-Furyl
10
11
12
13
14
15
16
17
18
3-Furyl
tive reaction conducted at room temperature furnished the
optically active alcohol 2a in good yield, with comparable
enantioselectivity, and was completed in a shorter reaction
time than that of a reaction at 08C (Table 1, entry 5). Reac-
tions that were carried out at lower temperature (Table 1,
entry 6), at lower concentration (Table 1, entry 7) or in tolu-
ene (Table 1, entry 8), afforded no improvement in yields
and enantioselectivities, respectively. The reaction condi-
tions specified (Table 1, entry 5) were applied for further
studies of the reaction parameters.
1-Naphthyl
2-Naphthyl
Cinnamyl
2-Me-Cinnamyl
hydrocinnamyl
cyclohexyl
[a] A 0.73m solution of dimethylzinc in hexanes was used.[10] [b] Yield of
isolated product. [c] Determined by chiral HPLC.
ee values were obtained with 0.5 mol% of chiral ligand 1,
except in the cases of para-CF3-benzaldehyde (Table 3,
entry 5), 2-furaldehyde and 3-furaldehyde (Table 3, en-
tries 11 and 12), and cinnamaldehydes (Table 3, entries 15
and 16). Furthermore, 2 mol% of ligand 1 is required to
provide good yields and enantioselectivities for 4-tolylben-
zaldehyde and 1-naphthaldehyde (Table 3, entries 3 and 13).
While the relationship of the enantiopurity of chiral ligand
and that of the product is usually assumed to be linear, the
convex deviation, positive nonlinear effect ((+)-NLE), also
known as asymmetric amplification, was observed from the
enantioselective addition of dimethylzinc to aldehyde in the
presence of 2 mol% of ligand 1 (Figure 1).[11] Such effects
arose from the reversible interchange of the coexisting ho-
mochiral and heterochiral zinc–amino thiolates. The homo-
chiral complex dissociated to form a monomeric enantiose-
lective zinc complex in this reaction, thus giving rise to the
high enantioinduction, whereas the heterochiral one re-
mained the same and inactive.
Subsequently, the effect that changing the amount of
ligand 1 had on the enantioselective reaction between dime-
thylzinc and benzaldehyde was investigated (Table 2). Inter-
estingly, the enantioselectivity did not vary as the amount of
Table 2. Effect of catalyst loading on the enantioselective addition of
Me2Zn to benzaldehyde.
Entry
1 [mol%]
Me2Zn [equiv][a]
t [h]
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
5.0
5.0
2.0
1.0
0.5
0.1
0.1
2.0
5.0
2.0
2.0
3.0
3.0
5.0
24
6
85
89
88
82
89
68
88
93
94
94
93
94
92
93
24
96
48
96
72
[a] A 0.73m solution of dimethylzinc in hexanes was used.[10] [b] Yield of
isolated product. [c] Determined by chiral HPLC.
After obtaining good results from the enantioselective ad-
dition of dimethylzinc to a variety of arylaldehydes, the
2922
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2012, 7, 2921 – 2924

Enantioselective Addition of Dialkylzincs to Aldehydes
Table 5. Enantioselective addition of diethylzinc to aldehydes catalyzed
by ligand 1.[a]
Entry
R
t [h]
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
2-Tol
3-Tol
4-Tol
4-MeO-Ph
4-CF3-Ph
2-Cl-Ph
3-Cl-Ph
4-Cl-Ph
24
24
24
48
24
24
24
48
24
24
24
48
24
24
95 (3b)
94 (3c)
95 (3d)
94 (3e)
96 (3 f)
88 (3g)
92 (3h)
80 (3i)
85 (3j)
94 (3k)
95 (3l)
77 (3m)
83 (3n)
56 (3o)
94
94
97
93
97
99
96
86
96
95
71
90
64
82
Figure 1. Nonlinear effect of ligand 1 in the enantioselective addition of
dimethylzinc to benzaldehyde.[12]
9
4-Br-Ph
10
11
12
13
14
2-Naphthyl
Cinnamyl
2-Me-Cinnamyl
hydrocinnamyl
cyclohexyl
enantioselective addition of diethylzinc[3] to benzaldehyde
was investigated. Excellent enantioselectivities (91–97%)
and yields were obtained with 0.1–5 mol% of ligand
1 (Table 4). In the presence of only 0.5 mol% of ligand 1,
the reaction was completed in 30 min and furnished chiral
alcohol 3a in high ee at room temperature, whereas a longer
[a] A 1.0m solution of dimethylzinc in hexanes was used. [b] Yields of iso-
lated product. [c] Determined by chiral HPLC.
Table 4. Effect of catalyst loading on the enantioselective addition of di-
ethylzinc to benzaldehyde.
In contrast to our previous studies, alkylation of aromatic
aldehydes catalyzed by ligand 1 requires a much lower cata-
lyst loading. Presumably, the structure of the alkyl zincate is
different from that in previous studies. However, the real
reason remains to be understood. The mechanism of the
enantioselective addition of dialkylzinc to aldehydes in the
presence of a catalytic amount of b-amino alcohols[1b,13] and
b-amino thiols[14] has been reported. We have proposed
structure 4 as the major transition state in the catalytic addi-
tion reaction (Figure 2). Formation of a six-membered cyclic
Entry 1 [mol%] Et2Zn [equiv][a] T [8C] t [h] Yield [%][b] ee [%][c]
1
2
3
4
5
6
7
5.0
1.0
0.5
0.5
0.1
0.1
0.05
2.0
2.0
1.5
1.5
1.5
1.5
1.5
0
0
rt
0
rt
0
28
24
0.5 94
24
4
24
48
95
90
97
97
95
96
91
96
20
95
95
95
56
0
[a] A 1.0m solution of dimethylzinc in hexanes was used. [b] Yield of iso-
lated product. [c] Determined by chiral HPLC.
reaction time is necessary for the reaction conducted at 08C
(Table 4, entry 3 vs 4). Further studies on the reactions in
the presence of 0.1 mol% of ligand 1 indicated that higher
enantioinduction was observed at 08C (Table 4, entry 5 vs
6). The product 3a was obtained in very low ee (20%) when
the ethylation reaction was conducted with 0.05 mol% of
ligand 1 (Table 4, entry 7). Therefore, the reaction condi-
tions specified in entry 6 were applied to study the scope of
this reaction (Table 5).
Figure 2. Proposed reaction transition state of the enantioselective addi-
tion of Me2Zn to benzaldehyde catalyzed by ligand 1.
Excellent enatioselectivities (92–99%) of the correspond-
ing secondary alcohols were obtained from the enantioselec-
tive addition of diethylzinc to aromatic aldehydes that bear
various substituents (Table 5, entries 1–10), except in the
case of para-chlorobenzaldehyde (Table 5, entry 8). The
enantioselective reaction with cinnamaldehyde afforded
chiral alcohol 3l in good yield (95%) but with lower enan-
tioselectivity (71% ee; Table 5, entry 11) than that of its a-
substituted congener (77% yield and 90% ee; Table 5,
entry 12).
ring might occur by the coordination of sulfur and nitrogen
atoms with one zinc atom. In our case, the anti-cis structure
(4), rather than the anti-trans structure (5) that is the usual
transition state, has exhibited a minimized steric repulsion
between the phenyl ring and the bornane skeleton, and
therefore the transfer of the anti methyl group from the Re-
face of the benzaldehyde gives rise to the observed (R)-1-
phenyl-1-ethanol (2a).
Chem. Asian J. 2012, 7, 2921 – 2924
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
2923

Biing-Jiun Uang et al.
FULL PAPER
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Conclusions
In conclusion, the preparation of highly enantioenriched
chiral secondary alcohols through the enantioselective addi-
tion of dialkylzinc to a family of aromatic aldehydes cata-
lyzed by ligand 1 has been demonstrated. The presented cat-
alytic system shows high catalytic activity and enantioselec-
tivity, and, thus a low catalyst loading is required. For meth-
ylation, 0.5 mol% of ligand 1 was required, whereas for eth-
ylation, as low as 0.1 mol% of ligand 1 was required.
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A stock solution of ligand 1 was prepared by mixing ligand 1 (12.8 mg,
0.05 mmol) and a dimethylzinc solution (5.0 mL, 3.7 mmol, 0.73m in hex-
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Acknowledgements
We thank the National Science Council, Republic of China (Taiwan) for
the financial support (NSC 99-2113M-007-010-MY3).
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117, 6327–6335; b) T. Rasmussen, P.-O. Norrby, J. Am. Chem. Soc.
2001, 123, 2464–2465.
[14] For an example: C. L. Gibson, Tetrahedron: Asymmetry 1999, 10,
1551–1561.
Received: July 25, 2012
Published online: October 5, 2012
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Chem. Asian J. 2012, 7, 2921 – 2924