Chiral amide from (1S,2R)-(+)-norephedrine alkaloid in the enantioselective addition of diethylzinc to aryl and heteroaryl aldehydes

Article abstract of DOI:10.1016/j.tetasy.2009.07.011

Chiral amide synthesized from (1S,2R)-(+)-norephedrine and furoic acid was found to catalyze the enantioselective ethylation of aromatic and heteroaromatic aldehydes to secondary alcohols with 99.8% enantioselectivity at 0 °C without the addition of a promoter such as titanium tetraisopropoxide.

Full text of DOI:10.1016/j.tetasy.2009.07.011

Tetrahedron: Asymmetry 20 (2009) 1731–1735
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Chiral amide from (1S,2R)-(+)-norephedrine alkaloid in the enantioselective
addition of diethylzinc to aryl and heteroaryl aldehydes
Nallamuthu Ananthi a, Umesh Balakrishnan a, Ajayan Vinu b, Katsuhiko Ariga b, Sivan Velmathi a,
*
a Organic & Polymer Synthesis Laboratory, Department of Chemistry, National Institute of Technology, Tiruchirappalli 620 015, India
b International Centre for Materials Nanoarchitectonics, National Institute for Material Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Chiral amide synthesized from (1S,2R)-(+)-norephedrine and furoic acid was found to catalyze the enan-
tioselective ethylation of aromatic and heteroaromatic aldehydes to secondary alcohols with 99.8%
enantioselectivity at 0 °C without the addition of a promoter such as titanium tetraisopropoxide.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 21 April 2009
Accepted 6 July 2009
Available online 5 August 2009
1. Introduction
Lewis acidic zinc species, which activates the aldehyde. Upon treat-
ment of an amino alcohol with an alkylzinc reagent, the nitrogen
Catalytic asymmetric carbon–carbon bond formation is one of
the most important research areas in organic synthesis.1 Among
the carbon–carbon bond formation reactions, the synthesis of opti-
cally active secondary alcohols via the addition of diethylzinc to
aldehydes using catalytic amounts of chiral catalysts has attracted
much attention because they are important intermediates in the
synthesis of many naturally occurring compounds, biologically ac-
tive intermediates, several synthetic drugs, and new materials with
interesting optical properties.2,3 The first highly enantioselective
addition of an organometallic reagent to an aldehyde was reported
by Mukaiyama et al. in 1979.4 Several other catalysts were also
used after this initial result, for example, DAIB by Noyori (ee upto
99%);5a quinine by Wynberg (ee upto 92%);5b methylephedrine by
Buono (ee upto 81%)5c; aminoalcohol by Oguni (ee upto 98%)5d and
diamide in combination with titanium tetraisopropoxide as an
additive by Ohno (ee upto 99%);5e diamines5f, and titanium alkox-
ides.5g Although many types of ligands can catalyze the dialkylzinc
addition to aldehydes, such as the derivatives of chiral amino alco-
hols or amino acids such as norephedrine,5d,6 proline,7 valine, bor-
neol,5a,8 terpene,9 and pyrrolidine10 the derivatives of chiral amino
alcohols were found to catalyse the reaction more efficiently.
Diethyl zinc does not give spontaneous addition to aldehydes
due to its non-polar character, originating from its linear sp-
hybridized geometry.11 A more polar (and reactive) bent com-
pound may be obtained upon coordination of an electronegative
group, or by reaction with a protic residue. Such a bent and acti-
vated complex can give addition to an aldehyde. If chiral activating
ligands are used, induction may take place upon reaction with an
aldehyde. The mechanism of diethylzinc addition to benzaldehyde
is well known with b-amino alcohols as ligands. The amino alcohol
acts as a Lewis base which activates the zinc reagent and forms a
and oxygen donor atoms of the amino alcohol coordinate to the
zinc atom, yielding a bifunctional catalyst which is capable of act-
ing as an alkyl donor. The zinc atom in the five-membered chelate
ring is a Lewis acid, which coordinates the aldehyde through the
oxygen non-bonding orbital, and hence the carbonyl carbon atom
is activated for nucleophilic attack.
Among the ligands that have been synthesized and applied in
the diethylzinc addition reaction, ephedrine represents a class of
compound. The naturally occurring enantiomerically pure b-amino
alcohol ephedrine, has been shown to catalyze the addition of
diethylzinc to benzaldehyde by Chaloner.12 N-Alkylated deriva-
tives of norephedrine have been reported by Soai et al.13 to cata-
lyze the diethylzinc addition to aliphatic and aromatic aldehydes
with high asymmetric induction. It has been reported that the phe-
nolic unit is crucial for stable catalytic activity and enantioselectiv-
ity. Furthermore, the ortho-substituent in the prochiral aldehydes
causes the enantiomeric ratio to favor the (R)-enantiomer in con-
trast to the principal formation of the (S)-enantiomer.14 In addition
to amino alcohols, certain chiral hydroxyamides have recently
been proven to be efficient promoters for this valuable asymmetric
addition.15 Knochel16,2b and Seebach have opened up new routes to
functionalized dialkylzinc compounds, which favor the formation
of 1-phenylpropanol in high enantiomeric excess. The use of such
reagents opens up routes to more complex chiral alcohols in high
enantiomeric purity and thus broadens the scope of this process.
To the best of our knowledge there is no report of the diethylzinc
addition to aldehydes catalyzed by amide derived from (1S,2R)-
(+)-norephedrine alkaloid. There are some reports saying that
amide functional group is more active in the asymmetric diethyl-
zinc addition due to its rigidity.17 However, there are only few re-
ports on amide-based catalysts in asymmetric diethyl zinc addition
to aldehydes. Hence we became interested in synthesizing nor-
ephedrine-derived amides. Herein we report the synthesis of new
chiral amide synthesized from (1S,2R)-(+)-norephedrine and furoic
* Corresponding author.
E-mail address: svelmathi@hotmail.com (S. Velmathi).
0957-4166/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2009.07.011

1732
N. Ananthi et al. / Tetrahedron: Asymmetry 20 (2009) 1731–1735
acid and its catalytic application in the diethylzinc addition to
prochiral aldehydes. We hope that the present ligand system
may enhance the enantioselectivity by giving a more stable envi-
ronment to the prochiral aldehydes compared to amino alcohols.
OH
CHO
Et2Zn / Hexane
10 mol%, CL-1 / Toluene
R
0°C, 24 hrs.
R
2. Results and discussion
Scheme 2. Diethylzinc addition to different aldehydes using CL-1.
The chiral amide was synthesized from (1S,2R)-(+)-norephe-
drine and 2-furoic acid (Scheme 1) using the well known mixed
anhydride method with ethyl chloroformate as the activating re-
agent. Treatment of 2-furoic acid with ethylchloroformate in the
presence of triethylamine in THF gave a mixed anhydride, which
was subsequently reacted with (1S,2R)-(+)-norephedrine to give
chiral amide CL-1 in 80% yield.
Asymmetric addition of diethylzinc to benzaldehyde was car-
ried out using the CL-1 as catalyst. It has been previously reported
that the solvent plays an important role in controlling the activity
of the chiral ligand for the diethylzinc addition to aldehydes18,19
and toluene was found to be the best solvent. Thus the reaction
was carried out with 5 mol % of the CL-1 in toluene (Scheme 2).
In order to study the effect of temperature on the reaction, the
reaction was carried out at ꢀ77 °C, 0 °C, and 25 °C for 24 h. It
was found that the reaction temperature had a significant influ-
ence on the conversion and the enantioselectivity of the catalyst.
As the reaction temperature is increased from ꢀ77 °C to 25 °C,
the product yield increases from 90% to 98%. The CL-1 gave the
product with 50% enantioselectivity at 0 °C, which was decreased
to 40% at 25 °C. However, only the racemic product was obtained
at ꢀ77 °C.
In order to account for the preferential formation of the (R)-iso-
mer in the addition of diethylzinc to benzaldehyde using CL-1 as
catalyst, the possible transition state assemblies can be proposed
as shown in Figure 2. As in the transition state models, the phenyl
group present in the ligand exerts a steric effect with the phenyl
group in benzaldehyde, thus favoring the transition state I over
state II during the reaction. Due to the steric factor of the phenyl
groups, the ethyl group of the second coordinating diethyl zinc
molecule can only approach the re-face of the benzaldehyde.
Initially, the product alkoxide was formed as the alkylzinc alk-
oxide with the transfer of alkyl group to the re-face of the alde-
hyde, and the formation of a stable tetramer a is the driving
force for the reconstitution of the catalyst which is believed to be
monomeric b.23 Finally, The main product, 1-phenyl propanol
was obtained from the alkoxide after the acid work-up.
As generally observed in most of the asymmetric diethylzinc
additions, by increasing the temperature, the less stereoselective
pathway was more favorable.21 This may be due to the free rota-
tion possible when increasing the temperature, which leads to an
unstable transition state during the reaction. This unstable transi-
tion state is responsible for the low enantiomeric excess of the
product in such a manner that the addition of ethyl group to the
carbonyl functional group of the aldehyde is not that specific. In
our case, increasing the temperature from 0 °C to room tempera-
ture also caused the enantiomeric excess to decrease from 50% to
40%.
As discussed in the Introduction, the common additive that is
employed to help facilitate the asymmetric addition of diorgano-
zinc reagents to carbonyl substrates, is titanium tetraisopropoxide
Ti(O-iPr)4. This reagent has become nearly ubiquitous in its appli-
cation in this process, as it has been proven to be a powerful pro-
moter of the diethylzinc addition reaction.24–27 Walsh et al.
demonstrated that there is a reaction between the Ti(O-iPr)4 and
diethylzinc to give a reactive intermediate EtTi(O-iPr)3 that is likely
to be the alkylating agent. An interesting aspect of Ti(O-iPr)4 in the
diorganozinc addition reaction is the impact on the stereochemical
outcome of the enantioselective addition. In rare cases of the dieth-
ylzinc addition to aldehydes, a reversal in enantioselectivity of the
product can be observed in the presence of Ti(O-iPr)4.28 This is due
to the formation of different transition states mediated by either
zinc or titanium.
In order to study the effect of promoter, the diethylzinc addition
to benzaldehyde was carried out, in the presence of Ti(O-iPr)4. (R)-
1-Phenyl propanol was formed in 99.8% ee with 90% yield, which is
almost similar to the results obtained using CL-1 without the use of
promoter. This result indicates that our catalyst CL-1 has the capa-
bility of controlling the catalytic activity with excellent enantiose-
lectivity, without the addition of any promoters. Thus we decided
to study the diethylzinc addition to other substituted benzalde-
hydes, salicylaldehydes, and heterocyclic aldehydes without the
The effect of the catalyst concentration on the product yield and
the enantioselectivity of CL-1 was also studied at 0 °C for 24 h.
Interestingly, the CL-1 registered 99.8% enantioselectivity with
90% yield with 10 mol % of the CL-1. These results indicate that
at 0 °C, 10 mol % of the CL-1 in toluene, and a reaction time of
24 h are the best conditions to obtain the highest enantioselectiv-
ity and as a result they have been chosen for further studies. The
optimization of reaction condition is given in Table 1.
Ligands with multidentate donors favor the formation of orga-
nometallic complexes, which possess well-organized spatial
arrangements to give rise to good asymmetric induction in the cat-
alytic process.20 It is expected that a tridentate chiral ligand should
confer more rigidity to the reactive zinc-chelate catalyst than a
bidentate one, and thus may enhance the enantioselectivity of
the addition reaction.21 The ligand reported here is also tridentate
in nature, capable of forming well-organized organometallic com-
plexes with diethylzinc for the reduction of prochiral aldehydes.
The catalytic cycle for the addition of diethylzinc to benzalde-
hyde catalyzed by CL-1 is shown in Figure 1. In the first step, the
CL-1 reacts with diethylzinc to yield monomeric alkylzinc complex
a. This alkoxide can subsequently form mono-alkoxide diethylzinc
complex b by reaction with another equivalent of diethylzinc.
Transition state c clearly indicates that the aldehyde is attacked
at the re-face to yield the chiral alkoxide d. On work-up, this alkox-
ide is converted into (R)-1-phenyl propanol e.
In some cases where norephedrine-derived ligands were em-
ployed as a catalyst, it was found that the absolute configuration
of the products is mainly controlled by the stereogenic centers of
norephedrine on chiral ligands.22
O
OH
OH
NH
NH2
EtCOOCl, Et3N, THF
Rt, 24hrs.
+
COOH
CH3
O
O
CH3
CL-1
Scheme 1. Synthesis of chiral ligand (CL)-1 from (1S,2R)-(+)-norephedrine alkaloid.

N. Ananthi et al. / Tetrahedron: Asymmetry 20 (2009) 1731–1735
1733
Table 1
Diethylzinc addition to benzaldehyde using CL-1 as the catalysta
S. no.
Ligand
Cat. amount
Temp (°C)
Yieldb (%)
eec (%)
Absolute configurationd
1
2
3
4
5
CL-1
CL-1
CL-1
CL-1
CL-1
5 mol %
5 mol %
5 mol %
10 mol %
10 mol %
ꢀ77 °C
0 °C
rt
0 °C
0 °C
90
98
91
90
90
0
50
40
99.8
99.8
—
(R)
(R)
(R)
(R)e
a
The reaction was carried out in toluene, for 24 h.
Isolated yield by column chromatography.
Determined by HPLC analysis using chiralcel OD–H column.
The absolute configuration was assigned by comparison of the sign of the specific rotation to the literature data.
The reaction was carried out with Ti(O-iPr)4.
b
c
d
e
O
H3C
Ph
Et2Zn
H3C
N
O
OZnEt
Zn
O
Et
d
a
H+
Ph
H3C
O
H3C
O
H3C
N
OH
N
O
e
Zn
O
Et
O
Zn
O
Zn
Et
b
Et
Zn
Et
c
PhCHO
H
O
Figure 1. Catalytic cycle for the addition of diethylzinc to benzaldehyde catalyzed by CL-1.
O
H3C
N
O
H3C
N
especially in ee value (Table 2, entries 5, 10, and 12) which could be
attributed to the additional coordination of the oxygen atom of the
substituent with diethylzinc. Similarly, heterocyclic aldehydes (Ta-
ble 2, entries 11 and 12) are reduced with less ee. This may be due
to the binding of the ring heteroatom (N and O) in the substrate
with the second zinc atom. Therefore, the reactivity and enantiose-
lectivity of the chiral ligand were weakened to a certain extend.
Unlike the other aromatic aldehydes, 4-hydroxy, 3-methoxy
benzaldehyde alone gave the (S)-isomer in which case the addition
of diethylzinc takes place in the si-face of the aldehyde. This may
be due to the steric hindrance caused by the presence of the hydro-
xy and methoxy groups at the 4- and 3-position, respectively (Ta-
ble 2, entry 10).
O
O
Zn
O
Zn
O
Et
Et
Zn
Et
Zn
Et
H
H
O
O
Figure 2. Transition state models I and II for CL-1 as a catalyst.
addition of any promoters such as Ti(O-iPr)4, the results are given
in Table 2.
The electronic effect of the substituents present in the benzal-
dehyde showed the same effect on the enantioselectivity which
is observed generally. Aldehydes with electron-withdrawing
groups (Table 2, entries 3, 4, 7, 8, and 9) are reduced with higher
ee than aldehydes having electron-donating groups (Table 2, en-
tries 5, 6, and 10). Among the nitro-substituted benzaldehyde, o-
substituted aldehydes were reduced with less ee when compared
to the m-substituted aldehydes, which in turn were reduced with
less ee when compared to the p-substituted benzaldehydes (Table
2, entries 2, 3, and 4).18 This may be due to the steric repulsion of
the substituent and the ethyl group. As a result, the ethyl nucleo-
phile can hardly approach the carbon atom of the carbonyl group
of the aldehyde.
3. Conclusion
In conclusion, the norephedrine-derived catalyst CL-1 was
synthesized and found to alkylate prochiral aldehydes in the
diethylzinc addition to give the (R)-isomer specifically (except for
4-hydroxy 3-methoxy benzaldehyde) in up to 99.8% ee. The stereo-
chemical outcome of this diethylzinc addition to aldehydes can be
explained by taking the model of the transition state formed
during the reaction. This catalyst is advantageous in such a way
that it can be synthesized in one step. Both enantiomers of nor-
ephedrine are available. We have also demonstrated that the CL-
1 is also capable of alkylating salicylaldehyde and heterocyclic
aldehydes to produce the corresponding optically active heterocy-
clic alcohols, which are the essential building blocks of many
It was noted that aldehydes that contain an electron-donating
oxygen atom have a significant negative influence on this reaction,

1734
N. Ananthi et al. / Tetrahedron: Asymmetry 20 (2009) 1731–1735
Table 2
Diethylzinc addition to different aldehydes using CL-1a
S. no.
Aldehyde
% Yieldb
% eec
Absolute configurationd
1
2
3
4
5
6
7
8
9
Benzaldehyde
90
70
82
92
90
90
95
23
46
99.8
68
95
85
30
(R)
(R)
(R)
(R)
(R)
(R)
(R)
(R)
(R)
(S)
(R)
(R)
o-Nitro benzaldehyde
m-Nitro benzaldehyde
p-Nitro benzaldehyde
p-Methoxy benzaldehyde
p-Methyl benzaldehyde
p-Chloro benzaldehyde
Salicylaldehyde
76
99.8
99.8
99
20
53
5-Chloro salicylaldehyde
10
11
12
4-Hydroxy 3-methoxy benzaldehyde
Pyrrole-2-carboxaldehyde
Furfural
20
99
96
10
a
The reaction was carried out in toluene, for 24 h at 0 °C.
Isolated yield by column chromatography.
Determined by HPLC analysis using chiralcel OD-H column.
b
c
d
The absolute configuration was assigned by comparison of the sign of the specific rotation to the literature data.
biologically active compounds. In addition, the novel catalyst is an
additive which is capable of alkylating the prochiral aldehydes in
excellent enantioselectivities of up to 99.8% with 90% yield without
any promoters.
3J = 3.6 Hz, 1H, CH), 7.32–7.37 (m, 5H, phenyl), 7.44 (s, 1H, NH);
13C NMR, (400 MHz, CDCl3, 25 °C), d (ppm): 14.21, 50.65, 76.2,
112.16, 114.57, 126.24, 127.53, 128.20, 140.64, 144.04, 147.67,
158.64; MS(EI) 245 (M+, 55%), 227 (100%), 197 (9%), 158 (20%).
4.3. Typical procedure for the asymmetric addition of Et2Zn to
benzaldehyde
4. Experimental
4.1. General remarks
To a solution of the CL-1 (24.5 mg, 0.1 mmol) dissolved in dry
toluene (1.5 ml), Et2Zn (2 ml, 1 M solution in hexane) was added
dropwise at 0 °C. After the mixture was stirred for 1 h, benzalde-
hyde (0.1 ml, 1 mmol) was added, and the reaction mixture was
again stirred for 24 h at 0 °C. Then the reaction mixture was
quenched with 2 M HCl and extracted with chloroform. The opti-
cally active 1-phenyl propan-1-ol was obtained in 90% yield after
purification by column chromatography with silicagel as adsorbent
using 98:2 hexane/ethyl acetate as eluent.
1H and 13C NMR spectra were recorded in CDCl3 with a BRUKER
AMX-400 MHz instrument using TMS as an internal standard.
Commercial Precoated Silica Gel (Merck 60F-254) plates were used
for TLC. 60–120 mesh Silica Gel was used for Column Chromatog-
raphy. Specific rotations were recorded with a Rudolph Autopol
IV polarimeter. Enantiomeric excess was determined with a Shima-
dzu 2010A HPLC instrument (Chiral column:Chiral Cel OD–H, Mo-
bile Phase: 98:2 Hexane/i-PrOH, flow rate:0.5 min ml, UV detector
k = 254 nm). FTIR spectra were recorded with a Perkin Elmer–DXB
spectrometer. Melting points were determined with a Kherea dig-
ital melting point apparatus and are uncorrected.
4.4. Typical Procedure for the asymmetric addition of Et2Zn to
benzaldehyde with Ti(O-iPr)4
To a solution of ligand CL-1 (24.5 mg, 0.1 mmol) dissolved in
toluene (1.5 mL), Ti(O-iPr)4 (1 ml, 1 mmol) was added dropwise
and was stirred for 30 min. Then Et2Zn (2 ml, 1 M solution in hex-
ane) was added dropwise at 0 °C. After the mixture was stirred for
1 h, benzaldehyde (0.1 ml, 1 mmol) was added, and the reaction
mixture was again stirred for 24 h at 0 °C. Then the reaction mix-
ture was quenched with 2 M HCl and extracted with chloroform.
The optically active 1-phenyl propan-1-ol was obtained in 90%
yield after purification by column chromatography with silicagel
as an adsorbent using 98:2 hexane/ethyl acetate as an eluent.
4.2. SynthesisofN-((1S,2R)-1-hydroxy-1-phenylpropan-2-yl)furan-
2-carboxamide CL-1
Compound CL-1 was prepared according to the following proce-
dure. 2-Furoic acid (0.561 g, 5 mmol) taken in a round-bottomed
flask flushed with nitrogen gas was dissolved in dry THF. To this,
triethylamine (0.1 ml, 5 mmol) was added dropwise. Ethylchloro-
formate (0.7 ml, 5 mmol) in dry THF was slowly added into the
flask at 0° C and stirred for 30 min. After the formation of a white
precipitate, that is, the highly reactive anhydride, norephedrine
(0.936 g, 5 mmol) dissolved in dry THF cooled to 0 °C was added
dropwise into the reaction mixture and the stirring was continued
for 24 h. The reaction mixture was then filtered. The solvent was
removed by rotary evaporator, and the residue was dissolved in
ethylacetate. The organic layer was washed with water, followed
by saturated sodium bicarbonate and finally with brine and dried
over anhydrous sodium sulfate. The crude product was purified
by column chromatography with silica gel as the adsorbent and
90:10 hexane/ethylacetate as the eluent. The amide was obtained
Acknowledgments
The authors thank Dr. M. Chidambaram, Director, NITT for con-
stant encouragement, support, and research grant and DST for the
financial support in the form of a fast track sponsored project (SR/
FTP/CS-142-2006). Authors N.A. and U.B. thank Mr. P. Ilayaraja
from Madras University, Chennai for giving the chiral column
and also Mr. P. Senthil Kumaran and Mr. S. Sivaguru for giving
the starting materials and for their help in characterization of the
sample. They also thank Dr. R. Karvembu, NITT for helping in HPLC
analysis. This work was partially supported by the Ministry of Edu-
cation, Culture, Sports, Science, and Technology (MEXT) under the
Strategic Program for Building an Asian Science and Technology
Community Scheme and World Premier International Research
as a colorless solid in 80% yield. Melting point: 129–130 °C.
589
30
½
aꢁ
¼ þ70:4 (c 0.125, CHCl3); FTIR (cmꢀ1): 3289 (–OH), 3042
(–NH), 2872 (–CH), 1679 (C@O); 1H NMR (400 MHz, CDCl3,
25 °C), d (ppm): 1.1 (d, 3J = 6.8 Hz, 3H, CH3), 3.33 (br s, 1H, OH),
4.5 (sep, 3J = 7.9 Hz, 1H, CHNH), 4.96 (d, 3J = 2.0, 1H, CHOH), 6.51
(t, 3J = 1.6, 1H, CH), 6.56 (d, 3J = 8.8 Hz, 1H, CH), 7.13 (d,

N. Ananthi et al. / Tetrahedron: Asymmetry 20 (2009) 1731–1735
1735
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