Enantioselective diethylzinc addition to aromatic aldehydes catalyzed by novel Ti(IV) complex of three-dentate chiral sulfonamide ligands

Article abstract of DOI:10.1080/00397910902900688

The synthesis of several novel three-dentate sulfonamide alcohol ligands is described, starting from camphorsulfonyl chloride. The influence of temperature and ligand structure on the asymmetric addition of dietylzinc to aromatic aldehydes has been studied. Enantioselectivities up to 76% have been obtained.

Full text of DOI:10.1080/00397910902900688

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Enantioselective Diethylzinc
Addition to Aromatic Aldehydes
Catalyzed by Novel Ti(IV)
Complex of Three-Dentate
Chiral Sulfonamide Ligands
Minoo Dabiri a , Peyman Salehi b , Seddigheh Heydari
a & Gholamreza Kozehgary a
a Department of Chemistry , Faculty of Science,
Shahid Beheshti University , G. C., Tehran, Iran
b Department of Phytochemistry , Medicinal Plants
and Drugs Research Institute, Shahid Beheshti
University , G. C., Tehran, Iran
Published online: 12 Nov 2009.
To cite this article: Minoo Dabiri , Peyman Salehi , Seddigheh Heydari & Gholamreza
Kozehgary (2009) Enantioselective Diethylzinc Addition to Aromatic Aldehydes
Catalyzed by Novel Ti(IV) Complex of Three-Dentate Chiral Sulfonamide Ligands,
Synthetic Communications: An International Journal for Rapid Communication of
Synthetic Organic Chemistry, 39:24, 4350-4361, DOI: 10.1080/00397910902900688
To link to this article: http://dx.doi.org/10.1080/00397910902900688
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Synthetic Communications1, 39: 4350–4361, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910902900688
Enantioselective Diethylzinc Addition to
Aromatic Aldehydes Catalyzed by Novel Ti(IV)
Complex of Three-Dentate Chiral Sulfonamide
Ligands
Minoo Dabiri,1 Peyman Salehi,2 Seddigheh Heydari,1 and
Gholamreza Kozehgary1
1Department of Chemistry, Faculty of Science, Shahid Beheshti University,
G. C., Tehran, Iran
2Department of Phytochemistry, Medicinal Plants and Drugs Research
Institute, Shahid Beheshti University, G. C., Tehran, Iran
Abstract: The synthesis of several novel three-dentate sulfonamide alcohol ligands
is described, starting from camphorsulfonyl chloride. The influence of temperature
and ligand structure on the asymmetric addition of dietylzinc to aromatic aldehydes
has been studied. Enantioselectivities up to 76% have been obtained.
Keywords: Aldehydes, diethylzinc, enantioselectivity, sulfonamide alcohols
INTRODUCTION
The development of stereoselective asymmetric reactions using organoca-
talysts has become a research area of great importance, and a number of
new organocatalysts have been devised for this purpose.[1] In recent years,
there has been great interest in the synthesis of chiral sulfonamide ligands
both as auxiliaries and catalysts for a wide variety of asymmetric
transformations.[2] An easy-to-perform and well-documented reaction
for testing the catalytic reactivity and enantio-differentiating ability of
Received January 5, 2009.
Address correspondence to Minoo Dabiri, Department of Chemistry, Faculty
of Science, Shahid Beheshti University, G. C., Evin, Tehran, Iran. E-mail:
m-dabiri@sbu.ac.ir
4350

Enantioselective Diethylzinc Addition
4351
Figure 1. Sulfonamide alcohols used as ligands in the enantioselective addition of
dialkylzinc to aldehydes.
certain catalytic systems is the addition of diethylzinc to aromatic alde-
hydes.[3] In particular, many chiral sulfonamide ligands have been tested
in this addition with varying success (Fig. 1).[4]
Chiral bidentate and tetradentate camphorsulfonamide ligands[4]
have been used in the catalytic enantioselective addition of dialkylzinc
to aldehydes, but to the best of our knowledge, few reports have emerged
about tridentate camphor sulfonamide derivatives for this reaction.[5]
In this article, the enantioselective alkylation of aromatic aldehydes
by diethylzinc in the presence of catalytic amounts of novel chiral
sulfonamide ligands is reported.
RESULTS AND DISCUSSION
A series of novel chiral ligands were synthesized by the reaction of
camphorsulfonyl chloride (1) with the appropriate amines and reduction
of the produced ketones, 2a–c, with sodium borohydride[6] (Scheme 1).
After purification, rotating frame Overhauser effect spectroscopy
(ROESY) spectroscopic study showed that the exo-alcohol was the major
product (3a–c).
To evaluate the catalytic activity of chiral sulfonamide ligands, we
examined camphor-based tridentate sulfonamides (3a–c) and their ketone
precursors (2a–c) as catalysts in the presence of titanium tetraisopropox-
ide (Scheme 2) for the enantioselective addition of diethylzinc to benzal-
dehyde. The results are summarized in Table 1.
First, we started with reactions catalyzed by ligands 2a–c. Because of
lack of other strongly binding sites, we could expect that only sulfonamide
nitrogen atoms and the carbonyl group of the ketone chelated the metal cen-
ter. Not surprisingly, almost racemic products for ligands 2b,c were obtained,
and ligand 2a gave very modest enantioselectivitiy (Table 1, entries 1–3).
Then we turned our attention to the synthesized tridentate ligands
3a–c, which had the ability to coordinate to the central metal atom by

4352
M. Dabiri et al.
Scheme 1. The synthesis of tridentate chiral sulfonamide ligands.
an extra hydroxy group manipulated in the camphor ring. Increasing the
enantioselectivity indicated that the hydroxy groups on isoborneol-
10-sulfonamide moieties had a great impact on the enantioselectivity of
the reaction (Table 1, entries 7, 11, and 12).
To evaluate the asymmetric induction by camphor moiety, we also
synthesized and examined b-sulfonamide alcohols 5a,b as catalysts in
the presence of titanium tetraisopropoxide (Scheme 3) (Table 1, entries
15–20).
The comparison of ligands 3a and 5a,b again showed that the
camphor moiety had an important role in the enantioselectivity of the
reaction (Table 1, entries 7, 15, and 18).
Addition of catalytic amounts (10 mol%) of benzoic acid in the
reaction of diethylzinc to benzaldehyde with 3a led to a moderate
increase in the enantiomeric excess (ee) and yield of the reaction
(Table 1, entry 10, and Table 2, entry 2).
Scheme 2. Enantioselective addition of diethylzinc to benzaldehyde in the pre-
sence of chiral ligands.

Enantioselective Diethylzinc Addition
4353
Table 1. Enantioselective addition of diethylzinc to benzaldehyde catalyzed by
chiral ligands (2, 3, and 5)
Ligand
Entry (mol %) T (ꢀC) Time (h) Yield (%)a Ee (%)b Configurationc
1
2
3
4
5
6
7
8
9d
10e
11
12
13
14
15
16
17d
18
19
20d
2a (20)
2b (20)
2c (12)
3a (5)
ꢁ20
ꢁ20
ꢁ20
ꢁ20
ꢁ20
ꢁ20
ꢁ20
þ25
ꢁ20
ꢁ20
ꢁ20
ꢁ20
þ25
ꢁ20
ꢁ20
þ10
þ10
ꢁ20
þ10
þ10
24
24
24
40
20
40
20
40
20
20
20
24
40
40
24
40
40
24
40
40
52
32
57
41
79
88
80
100
65
87
80
41
100
58
62
84
64
62
80
53
17
9
3
(S)
(R)
(R)
(S)
(S)
(S)
(S)
(S)
(R)
(S)
(S)
(S)
(S)
(S)
(S)
(S)
(R)
(S)
(S)
(R)
11
54
57
57
36
31
59
36
23
8
8
8
9
20
9
3a (10)
3a (15)
3a (20)
3a (20)
3a (20)
3a (20)
3b (20)
3c (20)
3c (20)
3c (20)
5b (10)
5b (10)
5b (10)
5a (20)
5a (10)
5a (10)
7
9
aGC yield on the mixture of the two enantiomers.
bEe was determined by GC using a chiral capillary column (HP-chiral).
cAbsolute configuration of the major enantiomer was determined by compari-
son with an authentic sample.
dWithout any Ti(OPri)4.
eWith 10 mol% of benzoic acid.
Scheme 3. The synthesis of bidentate chiral sulfonamide ligands.

4354
M. Dabiri et al.
Table 2. Enantioselective addition of diethylzinc to aromatic aldehydes catalyzed
by chiral ligand 3aa
Entry
Aldehyde
Yield (%)b
Ee (%)c,d
1
2e
3
4
5
6
7
8
p-FC6H4CHO
p-FC6H4CHO
p-ClC6H4CHO
p-BrC6H4CHO
p-MeC6H4CHO
p-MeOC6H4CHO
o-MeOC6H4CHO
o-ClC6H4CHO
94
93
97
98
89
87
96
95
56
58
52
56
59
54
76
73
aCondition: ꢁ20ꢀC, 40 h, and 15 mol% of 3a.
bMeasured as conversion percentage by GC.
cDetermined by capillary chiral GC analysis using the chiral column
(HP-chiral).
dAbsolute configuration was determined by comparing the sign of specific
rotation.[7–10] The major enantiomer in all cases had the S configuration.
eWith 10 mol% of benzoic acid.
Increasing the ratio of ligand 3a to benzaldehyde increased the
enantioselectivity for production of (S)-1-phenyl-1-propanol, and the
best enantioselectivity (57% ee) was obtained when 15 mol% of 3a was
used (Table 1, entries 4–7).
We observed that lower temperatures favored the enantioselectivity
of the reaction; thus, when the reactions were carried out at þ25ꢀC with
20 mol% of 3a, lower ee values were obtained than when ꢁ20ꢀC was used
(Table 1, entries 7 and 8). When we considered the mechanism reported
by Yus et al.,[5a,11] we introduced an additional sp2 nitrogen binding site
in the ligand, which retarded the interaction of aldehyde with titanium
and reduced the Lewis acidity of titanium, reflected in reduction of ee
and yield of the reactions (Table 1, entries 11–14) (Fig. 2b). On the other
hand, introduction of an additional oxygen in ligand 3a resulted in a
more rigid transition state and therefore improved the ee of the reaction
(Table 1, entries 4–8) (Fig. 2a). For ligand 3a, in absence of titanium
tetraisopropoxide, lower ee values and also product with opposite
configuration were obtained (Table 1, entry 9).
Once the optimal ligand 3a was found, different aromatic aldehydes
were tested as substrates in the enantioselective addition of diethylzinc
under the same conditions described in entry 6 (Table 1). The results
are summarized in Table 2. For substituted aromatic aldehydes,
moderate to good enantioselectivities were obtained.

Enantioselective Diethylzinc Addition
4355
Figure 2. Proposed mechanism of (a) 3a and (b) 3c.
Substituents at the ortho position of the substrate had a favorable
effect on the enantioselectivity. Both 2-methoxy- and 2-chlorobenzalde-
hyde gave good enantioselectivities (Table 2, entries 7 and 8). The best
enantioselectivity, up to 76%, was obtained for 2-methoxybenzaldehyde
(Table 2, entry 7).
EXPERIMENTAL
General
Melting points were obtained in open capillary tubes and measured on an
electrothermal 9200 apparatus. Mass spectra (MS) were recorded on a
Finnigan-Mat 8430 mass spectrometer operating at an ionization potential
of 70eV. Infrared (IR) spectra were recorded as KBr pellets on a Nicolet
Impact 400D spectrophotometer. Conversions were determined with a
Hewlett-Packard HP-5890 gas chromatography (GC) instrument equipped
with a flame ionization detector and a 30-m HP-1 capillary column, using
nitrogen (2mL=min) as carrier gas. The enantiomeric ratio were determined
with the aforementioned apparatus using a 30-m WCOT fused silica capil-
1
lary column (HP-chiral). H and 13C NMR spectra were determined on a
Bruker 300 DRX Avance instrument at 300.13 and 75.47 MHz, respectively.
Synthesis of Ligands 2a–c and 5a,b
A solution of amine (10 mmol) and triethylamine (12 mmol) in dry
CH2Cl2 (10 ml) at 0ꢀC was slowly added to a solution of camphorsulfonyl
chloride (2.5 g, 10 mmol) in dry CH2Cl2 (10 ml) over 0.5 h. The resulting
mixture was stirred at room temperature for another 15 h. When the

4356
M. Dabiri et al.
reaction was completed, the mixture was poured into a 0.5 M HCl
solution (50 ml), and the obtained mixture was extracted with ethyl
acetate (3 ꢂ 50 ml). The organic layer was washed with HCl solution
and water and dried over anhydrous Na2SO4. The desired products were
obtained after concentration under reduced pressure. In some cases, the
residue was purified by flash-column chromatography. The desired
camphorsulfonamides were obtained in 85–95% yields.
Synthesis of Ligands 3a–c
The sulfonamides (9 mmol) were dissolved in methanol (30 ml) at 0ꢀC.
Sodium borohydride (30 mmol) was added, with vigorous stirring, to this
solution over 120 min. The resulting mixture was stirred for 5 min to 1 h
until thin-layer chromatography (TLC) showed that the ketone was
consumed completely. Methanol was removed under reduced pressure,
and the residue was added to water (25–30 ml) and extracted with ethyl
acetate (3 ꢂ 40 ml). The organic layer was dried over anhydrous Na2SO4,
and the solvent was removed on a rotary evaporator. The crude product
was purified by flash-column chromatography to afford the expected
exo-borneol derivatives in 85–91% yields.
Data
(1S,2R,4S)-N-(R)-1-Hydroxybutan-2-yl-(7,7-dimethyl-2-
oxobicyclo[2.2.1]heptane-1-methanesulfonamide) (2a)
25
Yield 95% as colorless oil; [a]D ¼ þ24.5 (c ¼ 1.02, CHCl3); IR (KBr)
1
3506, 3299, 2966, 1740, 1328, 1146 cmꢁ1; H NMR (CDCl3, 300 MHz,
d, ppm) 0.70–0.88 (m, 9H), 1.25–1.40 (m, 4H), 1.70–192 (m, 6H), 2.16
(m, 2H), 2.38–2.41 (m, 1H), 2.88 (d, J ¼ 15.3Hz 1H), 3.2 (bs, 1H), 3.34
(d, J ¼ 15.3Hz, 1H), 3.47 (m, 1H); 13C NMR (CDCl3, 75 MHz, d, ppm)
10.3, 19.5, 19.6, 24.9, 25.6 (2C), 26.7, 42.5, 45.7, 48.0, 57.1, 58.7, 64.0,
215.9; MS (EI) 304 (Mþ þ 1, 28), 286 (25), 272 (23), 215 (31), 151 (22),
109 (63), 58 (100), 41 (72).
(1S,2R,4S)-N-(R)-1-Hydroxybutan-2-yl-(2-hydroxy-7,7-
dimethylbicyclo[2.2.1]heptane-1-methanesulfonamide) (3a)
25
Yield 87%; mp 89–91ꢀC; [a]D ¼ ꢁ29.4 (c ¼ 1.0, CHCl3); IR (KBr) 3506,
3464, 3177, 2937, 1305, 1130 cmꢁ1; 1H NMR (CDCl3, 300 MHz, d, ppm)

Enantioselective Diethylzinc Addition
4357
0.81 (s, 3H), 0.88–1.09 (m, 6H), 1.47–1.58 (m, 4H), 1.70–1.76 (m, 5H),
2.15 (s, 1H), 2.89 (d, J ¼ 13.7 Hz, 1H), 3.40 (bs, 1H), 3.50 (d, J ¼ 13.7 Hz,
Hz, 1H), 3.68 (m, 1H), 4.06 (bs, 1H), 5.33 (bs, 1H); 13C NMR (CDCl3,
75 MHz, d, ppm) 10.56, 19.8, 20.5, 25.2, 27.3, 30.4, 30.9, 38.9, 44.3,
48.6, 50.4, 53.5, 57.5, 64.6; MS (EI) 306 (Mþ þ 1, 5), 288 (27), 274 (5),
256 (72), 58 (100), 41 (32).
(1S,2R,4S)-N-Quinolin-8-yl-(7,7-dimethyl-2-oxobicyclo[2.2.1]
heptane-1-methanesulfonamide) (2b)
25
Yield 96%; mp 120–121ꢀC; [a]D ¼ þ25.5 (c ¼ 0.98, CHCl3); IR (KBr)
1
3246, 2958, 1739, 1503, 1372, 1152 cmꢁ1; H NMR (CDCl3, 300 MHz,
d, ppm) 0.82 (s, 3H), 1.10 (s, 3H), 1.42–1.46 (m, 1H), 1.81–1.94 (m,
4 H), 2.30–237 (m, 1H), 2.56 (m, 1H), 3.05 (d, J ¼ 14.85 Hz, 1H),
3.65 (d, J ¼ 14.85 Hz, 1H), 7.27–7.55 (m, 3H), 7.94 (t, J ¼ 4.26Hz,
1H), 8.15 (dd, J ¼ 8.29 Hz, 1H), 8.80 (m, 1H), 9.11 (bs, 1H); 13C
NMR (CDCl3, 75 MHz, d, ppm) 19.7, 19.9, 25.2, 26.9, 42.5, 42.8,
48.0, 48.6, 58.5, 114.4, 122.0, 122.0, 127.1, 128.3, 134.3, 136.3, 138.4,
148.7; MS (EI) 358 (Mþ, 27), 277 (26), 144 (100), 116 (22), 81 (24),
41 (40).
(1S,2R,4S)-N-Quinolin-8-yl-(2-hydroxy-7,7-
dimethylbicyclo[2.2.1]heptane-1-methanesulfonamide) (3b)
25
Yield 88%; mp 98–99ꢀC; [a]D ¼ ꢁ23 (c ¼ 1.0, CHCl3); IR (KBr) 3543,
3296, 1952, 1505, 1369, 1141 cmꢁ1 1H NMR (CDCl3, 300 MHz, d,
;
ppm) 0.69 (s, 3H), 0.97 (s, 3H), 1.09–1.26 (m, 2H), 1.63–1.81 (m, 5 H),
3.03 (d, J ¼ 13.8 Hz, 1H), 3.23 (bs, 1H), 3.57 (d, J ¼ 13.8 Hz, 1H),
4.09–4.20 (m, 1H), 7.46–7.55 (m, 3H), 7.81 (d, J ¼ 6.80 Hz, 1H), 8.16
(d, J ¼ 8.29 Hz, 1H), 8.82 (m, 1H), 9.00 (bs, 1H); 13C NMR (CDCl3,
75 MHz, d, ppm) 19.8, 20.4, 27.3, 30.3, 39.1, 44.3, 48.8, 50.4, 51.1, 60.3,
114.1, 122.2, 122.2, 127.0, 128.4, 134.0, 136.4, 138.3, 148.8; MS (EI)
360 (Mþ, 25), 342 (2), 190 (73), 144 (100), 116 (25), 43 (26), 55 (25),
31 (75).
(1S,2R,4S)-N-Pyridin-2-ylmethyl-(7,7-dimethyl-2-
oxobicyclo[2.2.1]heptane-1-methanesulfonamide) (2c)
25
Yield 87% as colorless oil; [a]D ¼ þ14 (c ¼ 1.02, CHCl3); IR (KBr) 3290,
2958, 1740, 1595, 1328, 1148 cmꢁ1; 1H NMR (CDCl3, 300 MHz, d, ppm)
0.71 (s, 3H), 0.9 (s, 3H), 1.29 (m, 1H), 1.74–2.26 (m, 6H), 2.80 (d,

4358
M. Dabiri et al.
J ¼ 13.7 Hz, 1H), 3.34 (d, J ¼ 13.7 Hz, 1H), 4.38 (m, 2H), 6.52 (t,
J ¼ 5.13Hz, 1H), 7.08 (t, J ¼ 5.88 Hz, 1H), 7.30 (d, J ¼ 7.94 Hz, 1H),
7.57 (t, J ¼ 7.63Hz, 1H), 8.40 (m, 1H); 13C NMR (CDCl3, 75 MHz, d,
ppm) 19.4, 19.7, 25.9, 26.8, 42.5, 42.6, 48.2, 48.3, 49.8, 58.8, 122.1,
122.5, 136.9, 149.0, 156.4, 216.0; MS (EI) 323 (Mþ þ 1, 15), 171 (76),
155 (73), 107 (100), 79 (26), 41 (36).
(1S,2R,4S)-N-Pyridin-2-ylmethyl-(2-hydroxy-7,7-
dimethylbicyclo[2.2.1]heptane-1-methanesulfonamide) (3c)
25
Yield 85%; mp 59–61ꢀC; [a]D ¼ ꢁ28 (c ¼ 1.0, CHCl3); IR (KBr) 3529,
3299, 2933, 1594, 1322, 1141 cmꢁ1 1H NMR (CDCl3, 300 MHz, d,
;
ppm) 0.62 (s, 3H), 0.87 (s, 3H), 1.12 (m, 1H), 1.34 (m, 1H),
1.55–1.65 (m, 5 H), 2.67 (d, J ¼ 15 Hz, 1H), 3.28 (d, J ¼ 15 Hz, 1H),
3.87–3.99 (m, 2H), 4.33 (s, 1H), 6.56 (bs, 1H), 7.11 (t, J ¼ 6.3 Hz,
1H), 7.24 (d, J ¼ 7.47 Hz, 1H), 7.58 (t, J ¼ 706Hz, 1H), 8.42 (d,
J ¼ 4.43Hz, 1H); 13C NMR (CDCl3, 75 MHz, d, ppm) 19.7, 20.4,
27.2, 30.1, 39.2, 44.2, 47.8, 48.5, 50.2, 52.2, 75.8, 122.4, 122.8, 137.2,
149.1, 156.1; MS (EI) 325 (Mþ þ 1, 24), 306 (8), 172 (45), 155 (23),
108 (100), 79 (50), 41 (53).
(R)-2-(2-Naphthalensulfonylamino)-2-ethyl-1-ethanol (5b)
25
Yield 96%; mp 118–121ꢀC; [a]D ¼ þ10.4 (c ¼ 0.97, CHCl3); IR (KBr)
3514, 3281, 1427, 1311, 1156 cmꢁ1 1H NMR (CDCl3, 300 MHz, d,
;
ppm) 0.61 (t, J ¼ 7.29 Hz, 3H), 1.16–1.25 (m, 1H), 1.48–1.56 (m,
1H), 3.01–3.28 (m, 2H), 3.36 (bs, 1H), 4.63 (bs, 1H), 7.63–7.72 (m,
2H), 7.84–8.16 (m, 4 H); 13C NMR (CDCl3, 75 MHz, d, ppm) 10.3,
24.3, 57.1, 63.6, 122.9, 127.3, 127.9, 128.2, 128.9, 129.6 (2C), 134.4,
139.4; MS (EI) 280 (Mþ þ 1, 12), 262 (12), 248 (24), 191 (22), 127
(100), 31 (21).
(R)-2-(2-Benzensulfonylamino)-2-ethyl-1-ethanol (5a)
25
Yield 92%; mp 57ꢀC; [a]D ¼ þ19.6 (c ¼ 1.02, CHCl3); IR (KBr) 3514,
1
3281, 1447, 1321, 1161 cmꢁ1; HNMR (CDCl3, 300 MHz, d, ppm) 0.69
(t, J ¼ 7.47 Hz, 3H), 1.35–1.50 (2H, m), 2.84 (bs, 1H), 3.17 (m, 1H),
3.47–3.56 (m, 2H), 5.53 (bs, 1H), 7.27–7.59 (m, 3H), 7.90–7.92 (m, 2H);
13C NMR (CDCl3, 75 MHz, d, ppm) 10.1, 24.5, 57.2, 64.3, 127.0 (2C),
129.0 (2C), 132.6, 140.6; MS (EI) 230 (Mþ þ 1, 23), 212 (19), 198 (64),
170 (14), 141 (43), 125 (17), 77 (100), 51 (33), 31 (26).

Enantioselective Diethylzinc Addition
4359
General Procedure for the Enantioselective Addition of Diethylzinc
to Benzaldehyde
The ligand (0.11 mmol) was placed in a test tube and dissolved in dry
toluene (2 mL). The solution was stirred for 5 min. A 1.0 M solution of
diethylzinc in n-hexane (2.2 mmol, 2.2 mL) was then added, and after
the mixture was stirred for 5 min, a solution of benzaldehyde (1.11 mmol)
in dry toluene (1 mL) was added by a syringe. The mixture was stirred at
the appropriate temperature for the time reported in the Table 1. Satu-
rated aqueous NH4Cl was added (10 mL), and the mixture was extracted
with ethyl acetate (3 ꢂ 20 mL). The collected organic phases were washed
with water, dried over Na2SO4, and analyzed by GC, after suitable
dilution.
CONCLUSION
In conclusion, we have shown that three-dentate sulfonamide alcohol
ligand 3a, which is easily prepared from (R)-2-amino-1-butanol and
camphorsulfonyl chloride, can be successfully used as a ligand in the
enantioselective addition of diethylzinc to aryl aldehydes.
ACKNOWLEDGMENTS
Financial support from the Research Council of Shahid Beheshti
University and the Catalyst Center of Excellence (CCE) is gratefully
acknowledged.
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