Enantioselective addition of diethylzinc to aldehydes catalyzed by d-glucosamine derivatives: Highly pronounced effect of trifluoromethylsulfonamide

Article abstract of DOI:10.1016/j.apcata.2010.01.009

We present the synthesis of β-hydroxy sulfonamides derived from d-glucosamine and their application as ligands in titanium tetraisopropoxide promoted enantioselective addition of diethylzinc to benzaldehyde and selected aromatic and aliphatic aldehydes. The N-trifluoromethylosulfonamido-d-glucosamine derivative is one of the most active ligands known and only 1 mol% of the ligand is sufficient for efficient catalysis of diethylzinc addition. The reaction is highly enantioselective for some aromatic aldehydes and enantiomeric excess up to 99% was obtained.

Full text of DOI:10.1016/j.apcata.2010.01.009

Applied Catalysis A: General 375 (2010) 247–251
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Enantioselective addition of diethylzinc to aldehydes catalyzed by -glucosamine
D
derivatives: Highly pronounced effect of trifluoromethylsulfonamide
*
´
Tomasz Bauer , Sławomir Smolinski
Department of Chemistry, Warsaw University, Pasteura 1, PL-02-093 Warsaw, Poland
A R T I C L E I N F O
A B S T R A C T
Article history:
We present the synthesis of b-hydroxy sulfonamides derived from D-glucosamine and their application
Received 31 August 2009
Received in revised form 5 January 2010
Accepted 6 January 2010
Available online 14 January 2010
as ligands in titanium tetraisopropoxide promoted enantioselective addition of diethylzinc to
benzaldehyde and selected aromatic and aliphatic aldehydes. The N-trifluoromethylosulfonamido-
D-
glucosamine derivative is one of the most active ligands known and only 1 mol% of the ligand is sufficient
for efficient catalysis of diethylzinc addition. The reaction is highly enantioselective for some aromatic
aldehydes and enantiomeric excess up to 99% was obtained.
Keywords:
ß 2010 Elsevier B.V. All rights reserved.
D
-Glucosamine
Diethylzinc
Titanium tetraisopropoxide
Enantioselective
Trifluoromethylsulfonamide
1. Introduction
used benzyl one in order to get better comparison of results [7]. The
third easily available site, the 4,6-O-acetal moiety is probably too
The asymmetric addition of dialkylzinc to aldehydes is one of the
most important methods for the synthesis of chiral secondary
alcohols. Since 1986, when DAIB was introduced by Noyori et al. [1],
the amount of very useful and efficient dialkylamino alcohols has
grown dramatically [2]. The new variant of diethylzinc addition in
the presence of titanium tetraisopropoxide and chiral (bis)sulfona-
mide as ligand was introduced by Yoshioka, Ohno and Kobayashi in
1989 [3,4], and subsequently was studied by numerous research
groups. Intensive studies have been conducted on (bis)sulfona-
far from the reaction center and should not have any influence on
the asymmetric induction [8].
2. Experimental
2.1. General
Melting points were determined using a Kofler hot stage
apparatus and are uncorrected. Specific rotations were recorded
using a PerkinElmer PE-241 polarimeter with a thermally jacketed
10-cm cell. 1H and 13C NMR spectra were recorded in CDCl3 using a
Varian 200 Unity Plus and Varian 500 Unity Plus spectrometers. All
chemical shifts are quoted in parts per million relative to tetra-
mides, diols and
b-hydroxy sulfonamides, which were proven as
very efficient ligands foradditions ofdialkylzincs[5]. Among various
amino alcohols used as ligands, derivatives of simple and easily
available carbohydrates were only rarely reported [6]. We have
previously reported the successful use of
D-glucosamine derived
methylsilane(d, 0.00 ppm). Massspectrawererecorded on aMariner
ligand 1 in titanium-mediated diethylzinc addition to aromatic and
aliphatic aldehydes [7]. The obvious advantage of this type of ligand
is its modular synthesis. Three sites (indicated by arrows in Fig. 1) of
our ligand can be easy altered during the synthesis, thus leading to
various ligands having the same chiral precursor.
In this work we present our study on the influence of the
sulfonamide moiety on the asymmetric induction. We have also
decided to confirm the importance of the configuration of the
instrument (Biosystem), Quatro LC Micromass and LCT Micromass
TOF HiRes apparatus. Reactions were carried out under argon using
Schlenk technique when necessary. Flash column chromatography
was made on silica gel (Kieselgel-60, Merck, 230–400 mesh). High
Performance Liquid Chromatography was conducted using Diacel
ChiracelOD-HandAD-Hchiralcolumns.Aminoalcohols5and6were
synthesized according to our earlier procedure [7].
anomeric center using methyl
b
-glycoside instead of previously
2.2. Methyl 4,6-O-benzylidene-2-deoxy-2-methylsulfonamido-a-D-
glucopyranoside (7)
To a solution of 5 (100 mg, 0.36 mmol) in anhydrous methylene
* Corresponding author. Tel.: +48 22 822 02 11x273; fax: +48 22 822 59 96.
E-mail address: tbauer@chem.uw.edu.pl (T. Bauer).
chloride (4 mL), triethylamine was added (0.15 mL, 1.2 mmol)
0926-860X/$ – see front matter ß 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.01.009

248
T. Bauer, S. Smolin´ski / Applied Catalysis A: General 375 (2010) 247–251
115.5, 102.0, 98.5, 81.1, 69.5, 68.8, 65.0, 62.2, 58.6, 55.4, 54.0,
45.53. [
a
]
D = À4.4 (c 0.97, CHCl3). HR MS [M++Na]: Calc. for
[C26H30N2NaO7S] – 537.1671. Found – 537.1649.
2.5. Methyl 4,6-O-benzylidene-2-deoxy-2-p-toluenesulfonamido-b-
D
-glucopyranoside (10)
To a solution of 6 (281 mg, 1.0 mmol) and Na2CO3 (170 mg,
1.55 mmol) in 1:1 water–acetone (5 mL) p-TsCl (190 mg,
1.0 mmol) was added, and the mixture was stirred for 3.5 h at
5 8C. Solvents were then evaporated in vacuo and the solid residue
was extracted with chloroform, organic phases were washed with
water, brine and dried over anhydrous MgSO4. After filtration and
evaporation crude 10 was purified by flash chromatography
(EtOAc–hexane 3:7, v/v) to give pure 10 (295 mg, 0.68 mmol, 68%
Fig. 1. Three sites of modular ligand easily altered during the synthesis.
followed by mesyl chloride (0.06 mL, 1.1 mmol) and DMAP (2 mg).
The mixture was stirred for 3 h at room temperature, washed with
water, brine and dried over anhydrous MgSO4. After filtration and
yield). Mp 182–184 8C. 1H NMR (200 MHz, CDCl3):
d 7.81–7.45 (m,
5H, PhCH3), 7.34–7.25 (m, 5H, Ph), 5,70 (d, 1H, NH), 5.49 (s, 1H,
CHPh), 4.31–4.23 (m, 1H, He-6), 4.11 (d, 1H, H-1), 3.92–3.21 (m, 6H,
Ha-6, H-2, H-4, H-3, H-5, OH), 3.05 (s, 3H, OCH3), 2.40 (s, 3H, CH3).
13C NMR (50 MHz, CDCl3):
d 143.5, 138.1, 137.2, 129.5, 129.3,
evaporation crude
(EtOAc–hexane 3:7, v/v) to give pure 7 (52 mg, 0.14 mmol, 40%
yield). Mp 205–207 8C. 1H NMR (200 MHz, CDCl3):
7.45–7.35 (m,
7 was purified by flash chromatography
d
5H, Ph), 5.56 (s, 1H, CHPh), 4.98–4.70 (m, 3H, H-1, NH, He-6), 4.36–
4.30 (m, 1H, Ha-6), 3.94–3.66 (m, 4H, H-2, H-4, H-3, H-5), 3.45 (s,
3H, OCH3), 3.14 (s, 1H, OH), 2.95 (s, 3H, CH3). 13C NMR (50 MHz,
128.5, 127.6, 126.5, 102.9, 101.9, 81.0, 71.7, 68.7, 66.2, 61.1, 57.0,
21.7. [
a
]
D
22 = À56.9 (c 0.56, CHCl3). HR MS [M++Na]: Calc. for
[C21H25NO7S+Na] – 458.1244. Found – 458.1234.
CDCl3):
d 136.4, 129.6, 128.5, 126.0, 102.1, 100.6, 79.4, 78.6, 77.1,
68.8, 62.7, 56.6, 55.9, 42.7, 38.9. [
a
]
D
22 = +60.0 (c 0.60, CHCl3). HR
2.6. Typical procedure for a diethylzinc addition
ESI-MS [M++Na]: Calc. for [C1H21NO7S+Na] – 382.0936. Found –
382.0937.
To the ligand 8 (4.3 mg, 0.01 mmol) dissolved in methylene
chloride (5 mL)titanium tetraisopropoxide(0.42 mL, 1.4 mmol) was
added. The mixture was stirred for 1 h at room temperature, cooled
to À0 8C, and diethylzinc (2.7 mL of 1.1 M toluene solution, 3 mmol)
was added. Stirring was continued at this temperature, and freshly
distilled benzaldehyde (0.1 mL, 1 mmol) was added. The mixture
was allowed to warm up to room temperature and stirred for the
time indicated in Table 1. The reaction was quenched with 1N HCl
(10 mL), and insolubles were filtered off. The organic layer was
separated, and the aqueous layer was extracted 3 times with 5 mL of
ethyl acetate. Combined organic extracts were washed with brine,
dried over anhydrous MgSO4 and purified by flash chromatography
(hexane–ethyl acetate 5:1, v/v) to give (R)-1-phenylpropanol. Yield:
135 mg; 99%. This product was subjected to HPLC analysis using a
Chiralcel OD-H column (3% 2-propanol in hexane).
2.3. Methyl 4,6-O-benzylidene-2-deoxy-2-
trifluoromethylsulfonamido-a-D-glucopyranoside (8)
To a solution of 5 (100 mg, 0.36 mmol) in anhydrous methylene
chloride (5 mL), triethylamine was added (0.15 mL, 1.2 mmol)
followed by triflic anhydride (0.07 mL, 1.1 mmol) and DMAP
(2 mg). The mixture was stirred for 1 h at room temperature,
washed with water, brine and dried over anhydrous MgSO4. After
filtration and evaporation crude 8 was purified by flash chroma-
tography (EtOAc–hexane 3:7, v/v) to give pure
0.34 mmol, 93% yield). Mp 164–168 8C. 1H NMR (200 MHz, CDCl3):
7.51–7.367 (m, 4H, PhCH3), 5.55 (s, 1H, CHPh), 4.78 (d, 1H, NH),
4.33–4.27 (m, 1H, H-1), 3.97–3.49 (m, 7H, He-6, Ha-6, H-2, H-4, H-3,
H-5, OH), 3.45 (s, 3H, OCH3). 13C NMR (50 MHz, CDCl3):
136.8,
129.4, 128.4, 126.3, 102.0, 99.1, 81.5, 68.9, 68.7, 62.4, 58.8, 55.7.
22 = +68.0 (c 0.95, CHCl3). HR ESI-MS [M++Na]: Calc. for
8 (138 mg,
d
d
3. Results and discussion
[a]
D
[C15H18NO7F3S+Na] – 436.0648. Found – 436.0641.
Methyl a- and b-glycosides 5 and 6 were synthesized according
to previously published procedure (Scheme 1) [7].
2.4. Methyl 4,6-O-benzylidene-2-deoxy-2-(5-
The synthesis of sulfonamide derivatives was accomplished in
respectable yields using Schotten–Baumann procedure (tosyl and
dansyl derivatives) or DMAP catalyzed reaction with sulfonyl
chloride or anhydride in the presence of triethylamine (mesyl and
triflic derivatives, respectively) (Scheme 2).
The catalytic activity of these ligands was tested using model
reaction with benzaldehyde under standard conditions (methy-
lene chloride as solvent, 1.4 equiv. of titanium tetraisopropoxide
and 3 equiv. of diethylzinc, room temperature). Results are
presented in Table 1.
dimethylamino)naphthalene-1-sulfonamido-a-D-glucopyranoside
(9)
To a solution of 5 (100 mg, 0.36 mmol) and Na2CO3 (53 mg,
0.5 mmol) in 1:1 water–acetone (5 mL) dansyl chloride (135 mg,
0.5 mmol) was added, and the mixture was stirred for 3.5 h at 5 8C.
Solvents were then evaporated in vacuo and the solid residue was
extracted with chloroform, organic phases were washed with
water, brine and dried over anhydrous MgSO4. After filtration and
evaporation crude
(EtOAc–hexane 3:7, v/v) to give pure 9 (162 mg, 0.32 mmol, 88%
yield). Mp 169–172 8C. 1H NMR (500 MHz, CDCl3):
8.56 (d, 1H,
9
was purified by flash chromatography
The reaction of methyl
39%, thus confirming importance of
b
-glycoside 10 gave very low ee, only
-configuration at the
a
d
anomeric center (entry 2 vs. entry 1). Not surprisingly, for
sterically not demanding methylsulfonamide 7 we observed very
poor selectivity, only 30% ee (entry 4). Dansyl derivative 9, which
could exhibit more pronounced shielding effect, gave results
comparable to previously investigated chiral ligand 1 (94% ee,
entry 6).
dansyl-H), 8.34 (dd, 1H, dansyl-H), 8.27 (d, 1H, dansyl-H), 7.60 (q,
1H, dansyl-H), 7.54 (q, 1H, dansyl-H), 7.41 (m, 2H, Ph), 7.31 (m, 3H,
Ph), 7.19 (d, 1H, dansyl-H), 5.44 (s, 1H, CHPh), 5.38 (d, 1H, NH), 4.15
3.93 (s, 1H, H-1), 3.77 (m, 1H, Ha-6), 3.65–3.59 (m, 2H, H-2, H-4),
3.48–3.41(m, 2H, H-3, H-5), 3.26 (m, 1H, OH), 2.94 (s, 3H, OCH3),
2.90 (s, 6H, N(CH3)2). 13C NMR (125 MHz, CDCl3):
131.2, 130.2, 130.0, 129.5, 129.3, 128.9, 128.4, 126.4, 123.5, 118.3,
d
137.0, 134.8,
The trifluoromethyl group exerts one of the strongest induc-
tively electron-withdrawing effects so far recognized and SO2CF3

T. Bauer, S. Smolin´ski / Applied Catalysis A: General 375 (2010) 247–251
249
Scheme 1. Synthesis of 1,2-amino alcohols. Reagents and conditions: (i) ClCO2Me, Na2CO3, H2O–acetone; (ii) 4% HCl/MeOHanh, reflux; (iii) PhCH(OMe)2, p-TsOHcat, DMF; and
(iv) 4N KOH/EtOH, MeOCH2CH2OH;.
Scheme 2. Synthesis of
b-hydroxysulfonamide ligands. Reagents and conditions: (i) p-TsCl, Na2CO3, acetone–H2O; (ii) MsCl, Et3N, CH2Cl2, DMAP; (iii) Tf2O, Et3N, CH2Cl2,
DMAP; and (iv) DNSC, Na2CO3, acetone–H2O.

250
T. Bauer, S. Smolin´ski / Applied Catalysis A: General 375 (2010) 247–251
Table 1
ligand 8 reaction with benzaldehyde was completed in less than
1.5 h and afforded the product in 94% ee (entry 4). Ligand 8 was
equally efficient when used in a smaller amount and even in the
presence of 0.01 equiv. of 8 enantioselectivity remains on the same
level, albeit the chemical yield slightly decreased (entry 5).
Taking into account the results presented in Table 1, we carried
out diethylzinc additions to some other representative aromatic
and aliphatic aldehydes in the presence of chiral ligand 8 (Table 2).
All reactions were performed at ambient temperature in the
presence of 0.01 equiv. of chiral ligand 8 using the standard
procedure as described for benzaldehyde. Reactions of all halogen
and methoxy substituted aromatic aldehydes derivatives pro-
ceeded with good to excellent yield and, in most cases, with good
to excellent enantioselectivity (Table 2, entries 1–12). All nitro
derivatives as well as 4-N,N-dimethylaminobenzaldehyde
appeared to be completely unsuitable for this reaction and drastic
decreases of enantiomeric excess and chemical yield of products
were observed (Table 2, entries 13–16).
Enantioselective addition of diethylzinc to benzaldehyde in the presence of ligands
1 and 7–10.
Entry
Ligand (equiv.)
Time (h)
Yield (%)
eea (%) (config)b
1
2
3
4
5
6
1 (0.1)
10 (0.1)
7 (0.1)
8 (0.1)
8 (0.01)
9 (0.1)
3
2
99
88
96
92
83
98
97 (R)c
39 (R)
30 (R)
94 (R)
93 (R)
94 (R)
20
1.5
20
12
a
Determined by HPLC on Daicel Chiralcel OD-H chiral column.
Determined by comparison to the literature data.
Ref. [7].
b
c
moiety is one of the most acidifying groups [9]. At the same time, it
is a relatively bulky substituent, comparable to the isopropyl group
[10]. On the basis of literature data [5g] we expected that ligand 8
should be much more active than 1, while its inductive properties
would remain on at least the same level. Results of experiment
fully confirmed our expectations. In the presence of 0.1 equiv. of
The enantioselectivity using chiral ligand 8 depends strongly on
the nature of the substituents of the aromatic aldehyde, varying
from the excellent 98% and >99% ee for 2-chloro- and 3-methoxy-
Fig. 2. Stereochemical models for an ethylation of benzaldehyde (sphere depicts Ar or R group of sulfonamide moiety).

T. Bauer, S. Smolin´ski / Applied Catalysis A: General 375 (2010) 247–251
251
Table 2
titanium to one of the sulfonamide oxygen, as demonstrated by
Walsh et al. [12] cannot be also excluded, as shown in model
transition state II. The isopropoxy group takes position in the least
crowded environment and benzaldehyde co-ordinates in-be-
tween. In consequence, the si-face of the aldehyde carbonyl group
is shielded by the sulfonamide substituent, and high enantios-
electivity is observed for bulky groups as trifluoromethyl one,
Enantioselective addition of diethylzinc to various aldehydes in the presence of
ligand 8a.
Entry
Aldehyde
Yield (%)
ee (%)b (config)c
1
2
2-Methoxybenzaldehyde
3-Methoxybenzaldehyde
4-Methoxybenzaldehyde
2-Fluorobenzaldehyde
3-Fluorobenzaldehyde
4-Fluorobenzaldehyde
2-Chlorobenzaldehyde
3-Chlorobenzaldehyde
4-Chlorobenzaldehyde
2-Bromobenzaldehyde
3-Bromobenzaldehyde
4-Bromobenzaldehyde
2-Nitrobenzaldehyde
3-Nitrobenzaldehyde
4-Nitrobenzaldehyde
4-N,N-Dimethylaminobenzaldehyde
trans-Cinnamaldehyde
Cyclohexanecarboxaldehyde
n-Hexanal
83
74
98
84
58
73
94
74
78
91
98
74
15
23
38
22
82
97
84
75 (R)
>99 (R)
77 (R)
94 (R)d
58 (R)d
78 (R)
98 (R)
34 (R)
88 (R)
76 (R)
58 (R)
81 (R)
36 (R)
20 (R)
16 (R)
10 (R)
87 (R)
85 (R)e
88 (R)e
3
4
much lower for the simple methyl group. Additionally,
p-stacking
5
effect of phenyl ring of aromatic aldehyde and aromatic rings of
tosyl or dansyl group can further rigidify the transition state I.
Chiral ligand 10 at the anomeric center has an equatorial methoxy
substituent, which is located in space far away from reacting
center. It allows for at least four possible transition states III–VI
and seems to be the main cause of the low asymmetric induction
observed for ligand 9.
6
7
8
9
10
11
12
13
14
15
16
17
18
19
4. Conclusions
In summary, we have presented synthesis and application of
efficient new ligand. We have shown that addition of aromatic,
a
aliphatic and
a-, b-unsaturated aldehydes proceeds with high
Reaction conditions: CH2Cl2, rt, ZnEt2 (3 equiv.), Ti(OiPr)4 (1.4 equiv.), ligand 8
(0.01 equiv.).
yield and enantioselectivity even with 0.01 equiv. of ligand 8.
b
Determined by HPLC on Daicel Chiralcel OD-H chiral column.
Determined by comparison to the literature data.
c
d
e
Acknowledgment
Determined by HPLC on Daicel Chiralcel AD-H chiral column.
Determined as benzoates by HPLC on Daicel Chiralcel OD-H chiral column.
Financial support from the Warsaw University (501/64-BST-
143101) is gratefully acknowledged.
substituted benzaldehydes (entries 2 and 7), respectively, to
inferior 34% ee for 3-chlorobenzaldehyde (entry 8). High stereo-
selectivity is observed for methoxy-substituent at the position 3,
such substituents at positions 2 and 4 are less favored (entries 1
and 3). Oppositely, halogen substituents at position 3 are less
favored for all tested aldehydes, while enantioselectivity for 2- and
4-subtituted aldehydes depends on the nature of the halogen.
Stereoselectivity for a-, b-unsaturated and aliphatic aldehydes
was also good (entries 17–19), at the same level despite the nature
of the aldehyde.
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stacking interactions between aldehyde and p-toluenesulfonamide
phenyl groups. High level of stereoselectivity obtained here for
trifluoromethylsulfonamide 8 clearly indicates the importance of
the steric influence of the bulky sulfonamide group on the
asymmetric induction, albeit
p-stacking effect for tosyl and dansyl
derivatives cannot be excluded. The exact mechanism of the
titanium promoted diethylzinc addition to aldehydes is so far not
certain and can be different for various types of ligands. Recent
structural and mechanistic investigations showed that the active
catalytic intermediate could be an ethyltitanium species derived
from the transfer of an ethyl group from zinc to titanium [11]. Also,
in our case the catalytic species can be formed in the way proposed
by Paquette et al. [5g], wherein the final step of the formation of the
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migration of ethyl substituent to an ‘‘apical’’ position. Thus, the
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reaction can be based on model transition states I–VI presented
in Fig. 2.
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In chiral ligands 1 and 7–9 the axial methoxy substituent
‘‘pushes’’ sulfonamide group toward titanium. The coordination of