Kinetic resolution of racemic secondary alcohols mediated by N-Methylimidazole in the presence of optically active acyl chlorides

Article abstract of DOI:10.1002/ejoc.200901075

N-Methylimidazole was used to promote the acylation of secondary racemic alcohols and to carry out their kinetic resolution through intermediate chiral acyl imidazolium chlorides. The kinetic resolution could be turned into a catalytic process in the presence of a catalytic amount of N-methylimidazole.

Full text of DOI:10.1002/ejoc.200901075

FULL PAPER
DOI: 10.1002/ejoc.200901075
Kinetic Resolution of Racemic Secondary Alcohols Mediated by
N-Methylimidazole in the Presence of Optically Active Acyl Chlorides
Loïc Leclercq,[a,b] Isabelle Suisse,[a,c,d] and Francine Agbossou-Niedercorn*[a,c,d]
Keywords: Acylation / Nitrogen heterocycles / Alcohols / Carboxylic acids / Kinetic resolution
N-Methylimidazole was used to promote the acylation of sec-
ondary racemic alcohols and to carry out their kinetic resolu-
tion through intermediate chiral acyl imidazolium chlorides.
The kinetic resolution could be turned into a catalytic process
in the presence of a catalytic amount of N-methylimidazole.
Introduction
Results and Discussion
The reaction of N-methylimidazole with optically pure
acyl chloride (S)-1a produced quantitatively the stable (S)-
N-acyl N-methylimidazolium chloride (S)-2a (Ͼ99% iso-
lated yield; Scheme 1). Compound (S)-2a is reactive
towards water[30] and alcohols. For example, the reaction of
(S)-2a with one equivalent of racemic secondary alcohol
(Ϯ)-3a, in THF, produces chiral ester 4a,a (20 °C, 2 h, 20%
conversion determined by 1H NMR spectroscopy). Ester
4a,a is composed of two diastereomers (S,S/S,R) in a 98:2
ratio (96%de; Table 1, Entry 1; Scheme 1). The optical pu-
rity of recovered alcohol 3a was 26 %ee (R). As no special
care had been taken to avoid trace amounts of water for the
initial experiments and because of the sensitivity of the acyl
imidazolium species towards water,[30] we carried out the
same reaction under rigorously anhydrous conditions. The
Achiral acyl transfer reactions have been promoted by a
number of highly active nucleophiles such as 4-(dimeth-
ylamino)pyridine (DMAP),[1,2] N-methylimidazole[3] and
phosphanes.[4] Afterwards, successful chiral acylations[5]
have been reported in the presence of stoichiometric
amounts of chiral acylating agents such as DMAP[6] and
imides.[7] Then, catalysed enantioselective acylations, ac-
companied by kinetic resolution of secondary alcohols,
have been performed in the presence of chiral catalysts.[8,9]
Finally, chiral nucleophiles[8] such as DMAP,[10–20] pyr-
idines,[21] amidines,[22] phosphanes,[23] N-heterocyclic carb-
enes,[24] tetramizole derivatives[25] and alcohol deriva-
tives,[26] as well as small peptides,[27] have also been applied
in such reactions.
Prior studies from this laboratory were devoted to the conversion reached 47% and the selectivity increased as
synthesis and use of imidazolium derivatives as ionic liquid well (4a,a: S,S/S,R 99:1; recovered 3a: 91%ee; Table 1, En-
media for organometallic catalysis.[28] As a result of obser- try 2).
vations made during the synthesis of new chiral imid-
azolium salts, we observed a peculiar behaviour of N-
methylimidazole. Thus, we wondered if the latter could be
able to promote kinetic resolution of secondary aryl
alcohols in the presence of chiral acylating agents.[29] Here,
we report on our investigations of the potential of N-
methylimidazole to mediate the transfer of chiral acyl resi-
dues with concomitant resolution of the reacting racemic
secondary alcohols.
Scheme 1.
[a] Université Lille Nord de France,
59000 Lille, France
[b] USTL, LCOM,
A control experiment carried out with optically pure acyl
59655 Villeneuve d’Ascq, France
[c] ENSCL, SOMC,
chloride (S)-1a and racemic (Ϯ)-3a in THF provided ester
4a,a as a mixture of diastereomers (S,S/S,R, 1:1) with 9%
conversion within 2 h at 20 °C (Table 1, Entry 3). A reac-
tion was carried out also in the presence of pyridine
(1 equiv., THF, 0 °C, 2 h). Ester 4a,a was obtained quanti-
59652, Villeneuve d’Ascq, France
[d] CNRS, UMR 8181,
59655 Villeneuve d’Ascq, France
Fax: +33-3-20-43-65-85
E-mail: francine.agbossou@ensc-lille.fr
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Kinetic Resolution of Racemic Secondary Alcohols by N-Methylimidazole
Table 1. Selectivities in the kinetic resolution of racemic secondary alcohols.
Entry
Acyl donor
Alcohol (Ϯ)-3
4
Conversion
[%][a]
% de of 4[a]
(config.)
% ee of recovered 3
S[c]
(config.)[b]
1
2
2a
2a
3a
3a
3a
3a
3a
3b
3b
3a
3a
3c
3d
3a
3b
3a
4a,a
4a,a
4a,a
4a,a
4a,a
4a,b
4b,b
4c,a
4d,a
4a,c
4a,d
4a,a
4a,b
4c,a
20
47
9
96 (S,S)
98 (S,S)
–
26 (R)
91 (R)
–
–
–
90 (R)
5 (S)
94 (R)
2 (R)
11 (R)
0
92 (R)
91 (R)
95 (R)
63
316
–
–
–
151
1
98
1
1.3
–
327
124
120
3[d]
4[e]
5[f]
6
1a
1a
100
100
48
48
47
48
33
15
48
48
47
–
–
1a
2a
96 (S,S)
4 (R,R)
93 (S,S)
2 (S,S)
21 (S,S)
0
98 (S,S)
95 (S,S)
94 (S,S)
7
8
9
10
11
12
13
14
2b
2c
2d
2a
2a
1a[g]
1a[g]
1c[g]
1
[a] Determined by H NMR spectroscopy. [b] Determined by chiral HPLC by employing a Chiralcel OJ® column. [c] Values were calcu-
lated according to the method of Kagan. See ref.[31] [d] Compounds 1a and 3w were reacted in THF. [e] Prior to the addition of racemic
alcohol, 1a was treated with a stoichiometric amount of pyridine. [f] Prior to the addition of the racemic alcohol, 1a was treated with a
stoichiometric amount of DMAP. [g] Reactions performed in the presence of 2 mol-% of N-methylimidazole.
tatively as a 1:1 mixture of diastereomers (Table 1, Entry 4).
An identical result was obtained in the presence of DMAP
(Table 1, Entry 5). Thus, as expected, the three nucleophilic
nitrogen aromatic species promote the acylation of alcohol
3a compared to the control experiment (Table 1, Entry 3).
Nonetheless, a discriminating ability between the enantio-
mers of alcohol (Ϯ)-3a was observed exclusively in the pres-
ence of N-methylimidazole.
Next, three other optically pure acyl chlorides were pre-
pared, namely, (S)-1b, (S)-1c and (S)-1d, and they were con-
verted similarly into the corresponding acyl imidazolium
Figure 1. Acyl transfer reaction: substrates and products.
chlorides 2b–d (97 – Ͼ99% isolated yields; Figure 1). Acyl
derivatives 2a–d were treated with a series of racemic sec-
ondary alcohols (Ϯ)-3a–e (Figure 1) to afford esters 4 and incoming alcohol.[32,33] Cation–π interactions cannot be
alcohols 3 were recovered in an optically enriched form totally ruled out in pyridine-assisted acylation.[34] However,
(Table 1, Entries 6–11). Very high selectivities were reached no selectivity was observed. The specific conformation of
only for three combinations of acyl imidazolium chlorides an intermediate cation–π complex (parallel stacking) en-
and alcohols, namely, (S)-2a/(Ϯ)-3a (Table 1, Entry 2), (S)- ables the selective “intramolecular” attack of one alcohol
2a/(Ϯ)-3b (Table 1, Entry 6) and (S)-2c/(Ϯ)-3a (Table 1, En- enantiomer onto the acyl imidazolium chloride, as is sug-
try 8). The selectivities thus largely depend upon the sub- gested in complex 5 (Scheme 2). More selective acylations
stituents on the alcohol but also on those on the chiral acyl are found with intermediates exhibiting the highest nonco-
imidazolium precursor. Acyl imidazolium chlorides bearing valent binding strengths. Such noncovalent interactions can
an aromatic residue α to the C=O group combined with account for the classification of the alcohol given above
secondary alcohols bearing a naphthyl residue induce the with respect to the selectivity of their resolution. Conse-
highest selectivities during acyl transfer. From the alcohol quently, alcohol 3d is acylated with no selectivity (Table 1,
standpoint, the following order of selectivity has been Entry 11). The selectivity achieved with alcohol (Ϯ)-3c
found: 3a (2-naphthyl) ≈ 3b (1-naphthyl) Ͼ 3c (phenyl) Ͼ bearing a phenyl group is significantly lower than that ob-
3d (ethyl). For the latter, no selectivity was observed at all tained with alcohols (Ϯ)-3a and (Ϯ)-3b bearing a naphthyl
(Table 1, Entry 11).
residue (11%ee vs. 90–91%ee; Table 1, Entry 10 vs. En-
Conformational features play a key role in the kinetic tries 2 and 6). On the other side, even in the presence of
resolution of secondary alcohols during acylation. The favourable alcohol (Ϯ)-3a bearing a naphthyl residue, the
alcohols can approach the acyl imidazolium intermediate small steric differentiation exhibited by acyl intermediate
along different orientations. In order to observe significant (S)-2d (Me and OMe substituents on the stereogenic centre)
selectivity, one orientation has to be productive within the does not lead to any selectivity during acyl transfer
transition state. A reasonable rationale for the selectivity (Table 1, Entry 9).
achieved with chiral acyl imidazolium salts would be the
Next, to obtain more insight into the structure of the
participation of strong noncovalent cation–π interactions acyl imidazolium chloride/alcohol [2a/(Ϯ)-3w] intermediate
between the imidazolium cation and an aryl group of the that is expected to form during acylation, we carried out a
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L. Leclercq, I. Suisse, F. Agbossou-Niedercorn
FULL PAPER
shielding of one face of the acyl pyridinium cation. This
allows the diastereoselective attack of a nucleophile on the
opposite, less-hindered side as is schematised for
6
(Scheme 3). In this study, we have a quite different situa-
tion, as π–π stacking interactions orient the approach of
the “incoming” nucleophile, as in 5. In the latter, a combi-
nation of steric and electronic factors thus dictates the con-
formation of the stable and active chiral acyl imidazolium
intermediate. In the case of the Miller short peptides[27a]
bearing an imidazole residue, the formation of intramolecu-
lar π–π stacking within the folded peptide is rather detri-
mental to the selectivity of the acyl transfer process. In the
case of Miller, other interactions such as hydrogen bonding
through an amide group contribute to the selectivity of the
reaction, as can be seen for 7 (Scheme 3). Thus, trans-2-
(N-acetylamino)-cyclohexan-1-ol was found to be the best
alcohol substrate for attaining high resolution. It is also
noteworthy that no selectivity was achieved with alkyl aryl
carbinols.
Scheme 2.
2D NMR spectroscopy experiment and acquired T-ROESY
data. The existence of intense cross-peaks, particularly be-
tween the protons of the methyl residue on the imidazolium
cation (H-10) and protons of the methyl group of the
alcohol (H-1Ј) proves the close proximity between the acyl-
ating agent and the nucleophile as shown for compound 5
(Scheme 2 and Figure 2). Thus, the stability of that interme-
diate within the NMR timescale has to be quite noteworthy.
Another intense cross-peak is attributed to the interaction
between the methyl residue (H-10) of the imidazolium cat-
ion and the methyl group H-d of ester 4a,a (Scheme 1),
which results from the occurrence of the acylation process
during NMR spectroscopic data acquisition. Cation–π in-
teractions between the imidazolium cation and the naphthyl
residue of the alcohol as well as those between the imid-
azolium cation and the naphthyl group of the ester product
account thus for the observed 2D NMR cross-peaks.
Scheme 3.
Next, we attempted to run a catalytic experiment while
reacting acyl chloride (S)-1a with racemic alcohol (Ϯ)-3a in
the presence of a catalytic amount of N-methylimidazole
(2 mol-%; Table 1, Entry 12) and DBU as the auxiliary
base. The latter was introduced in small amounts to neu-
tralise the HCl produced. The progress of the reaction was
1
followed by H NMR spectroscopy. Relative to the noncat-
alysed background reaction of 1a with racemic (Ϯ)-3a, an
initial rate enhancement by a factor of 2350 was obtained in
the presence of the catalytic amount of N-methylimidazole.
After 2 h, the conversion reached 48%, and 4a,a was ob-
tained with a selectivity of 98%de. The optical purity of
recovered alcohol 3a was 92% (Table 1, Entry 12), which
is thus close to the results obtained with preformed acyl
imidazolium chloride (Table 1, Entry 2). This suggests that,
in the catalysed reaction, the mechanism is also going quite
fast through the acyl imidazolium intermediate rather than
through an uncatalysed process, which is a slow process
anyhow (see above). The same trend was obtained for the
reaction of other key acyl chlorides with racemic alcohols
under the catalytic conditions (Table 1, Entries 13 and 14).
We discovered thus a peculiar behaviour of imidazole in
Figure 2. Partial contour plot of the T-ROESY spectrum of a mix-
ture containing 2a and (Ϯ)-3w 20% (m/m) diluted in CDCl3,
300 K. Mixing time = 300 ms.
Such interactions have been pursued in an “induced-fit” assisting the acylation of secondary alcohols. The ability of
acylation mechanism involving pyridinium acylating high medium organisation of imidazolium salts, which is a
agents.[11–13,33] Upon acylation, a conformational change in key property of imidazolium-based ionic liquids, can ac-
the pyridinium residue produces a rigidified structure and count for the results obtained in this study.
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Eur. J. Org. Chem. 2010, 2696–2700

Kinetic Resolution of Racemic Secondary Alcohols by N-Methylimidazole
(CH2), 42.3 (N-CH3), 122.2 and 126.2 (Cimid), 127.4 (Cpara), 127.7
(Cmeta), 128.7 (Cortho), 142.4 (NCN), 146.9 (Carom-CH), 163.8
(C=O) ppm. C14H17ClN2O (264.75): calcd. C 63.52, H 6.47, N
Conclusions
In summary, we have shown, for the first time, that N-
methylimidazole can mediate the kinetic resolution of race- 10.58; found C 63.12, H 6.27, N 10.70.
mic secondary alcohols during transfer of a chiral acyl moi-
(S)-3-(2-Methoxy-2-phenylacetyl)-1-methyl-1H-imidazol-3-ium Chlo-
ety via intermediate chiral acyl imidazolium chlorides. Such
a property is not exhibited at all by pyridine- and DMAP-
mediated acylations. The process, which is quite selective
for some acyl chloride/secondary alcohol combinations and
which has been turned into an organocatalytic process, is
expected to be applicable to a variety of aryl acyl chlorides
in combination with a diversity of naphthyl-based second-
ary alcohols. Research is under way to design a catalytic
system able to perform dynamic kinetic resolution while
combining an imidazole-based catalytic acyl transfer system
with an alcohol racemisation promoter.
ride [(S)-2c]: Prepared by a method similar to that outlined for (S)-
1
2a. Yield: Ͼ99%. H NMR (300 MHz, CDCl3): δ = 3.68 (s, 3 H,
OCH3), 3.87 (s, 3 H, N-CH3), 5.23 (s, 1 H, CH), 7.53 (m, 5 H,
Harom), 7.97 (s, 1 H, Himid) 8.24 (s, 1 H, Himid), 9.67 (s, 1 H, NCHN)
ppm. 13C NMR (75 MHz, CDCl3): δ = 35.4 (N-CH3), 54.9 (CH3O),
89.5 (CH-O), 121.2 and 125.3 (Cimid), 128.6 (Cmeta), 129.5 and
130.3 (Cortho), 139.1 (Carom-CH), 140.6 (NCN), 164.7 (C=O) ppm.
C13H15ClN2O2 (266.73): calcd. C 58.54, H 5.67, N 10.50; found C
58.17, H 5.55, N 10.67.
(S)-3-(2-Methoxypropanyl)-1-methyl-1H-imidazol-3-ium Chloride
[(S)-2d]: Prepared by a method similar to that outlined for (S)-2a.
Yield: Ͼ99%. 1H NMR (300 MHz, CDCl3): δ = 1.53 (d, 3J =
7.3 Hz, 3 H, CH3), 3.56 (s, 3 H, N-CH3), 3.90 (s, 3 H, OCH3), 4.02
(m, 1 H, CH), 7.96 (s, 1 H, Himid), 8.02 (s, 1 H, Himid), 9.61 (s, 1
H, NCHN) ppm. 13C NMR (75 MHz, CDCl3): δ = 5.4 (CH3-CH),
35.4 (N-CH3), 55.5 (CH3O), 85.3 (CH-O), 120.5 and 130.3 (Cimid),
140.1 (NCN), 164.3 (C=O) ppm. C8H13ClN2O2 (204.66): calcd. C
46.95, H 6.40, N 13.69; found C 46.67, H 6.23, N 13.42.
Experimental Section
General Experimental Procedures: All reactions were performed un-
der anhydrous conditions and under an inert atmosphere of nitro-
gen. Reagents were used as obtained from commercial sources
without further purification. THF and diethyl ether were distilled
prior to use from sodium/benzophenone ketyl under an atmosphere
of nitrogen. Toluene was distilled from Na/Hg. Dichloromethane
was distilled from calcium hydride and stored under an atmosphere
of nitrogen. 1H and 13C NMR spectra were recorded in CDCl3 and
chemical shifts are expressed in ppm relative to internal Me4Si (δ
=0.00 ppm) and were recorded with a 300 MHz spectrometer. Chi-
ral separations were performed by HPLC with a Chiralcel OJ®
(Daicel) column. The esters prepared are known compounds.
General Procedure for the Reaction of Acyl Imidazolium Chlorides
with Alcohols: The imidazolium chloride salt (1.7 mmol) was dis-
solved in THF (5 mL) and treated with the selected racemic alcohol
(1.7 mmol) at room temperature under vigorous stirring. The acid
produced was neutralised by the addition of small amounts of
DBU. Aliquots are taken from the reaction mixture and filtered
through a small pad of 4-polyvinylpyridine. After evaporation of
the solvent under reduced pressure and drying under vacuum, the
1
crude mixture was analysed by H NMR spectroscopy and chiral
(S)-1-Methyl-3-(2-phenylpropanoyl)-1H-imidazol-3-ium
Chloride
HPLC to determine the conversion and the selectivity.
[(S)-2a]:[35] A round-bottomed flask equipped with a condenser and
a dropping funnel was charged with (S)-2-phenyl propionic acid
(1a; 250 mg, 1.7 mmol) and toluene (25 mL) and cooled to 0 °C.
Then, thionyl chloride (2.5 mmol) was added dropwise. After ad-
dition, the reaction mixture was heated at reflux for 2 h. After cool-
ing, the solvent was evaporated in vacuo. The crude residue was
dissolved in THF (10 mL), and the solution was cooled to 0 °C. N-
Methylimidazole (1.8 mmol) was added dropwise under vigorous
stirring. A white precipitate formed in the reaction medium. At
the end of the addition, the medium was stirred for 2 h at room
temperature. Acyl imidazolium chloride (S)-2a was isolated
through filtration. The solid was washed with anhydrous diethyl
ether and dried under vacuum (white solid, 425 mg, Ͼ99%). 1H
NMR (300 MHz, CDCl3): δ = 9.47 (s, 1 H, NCHN), 7.31–7.16 (m,
7 H, Harom and Himid), 3.99 (s, 1.5 H, N-CH3), 3.97 (s, 1.5 H, N-
CH3), 3.60 (d, J = 7 Hz, 0.5 H, CH), 1.36 (d, J = 7 Hz, 1.5 H,
CH3), 1.34 (d, J = 7 Hz, 1.5 H, CH3) ppm. 13C NMR (75 MHz,
CDCl3): δ = 170.1 (C=O), 142.4 (NCN), 138.1 (Carom-CH), 129.6
(Cortho), 127.7 (Cmeta), 127.6 (Cpara), 122.3 and 119.7 (Cimid), 46.3
(CH-CH3), 36.2 (N-CH3), 17.7 (CH3-CH) ppm. C13H15ClN2O
(250.73): calcd. C 62.28, H 6.03, N 11.73; found C 61.96, H 5.91,
N 11.32.
General Procedure for the Catalysed Acyl Transfer Reaction in the
Presence of N-Methylimidazole: The acyl chloride (1.7 mmol), the
racemic alcohol (1.7 mmol) and N-methylimidazole (2 mol-%) were
combined in THF (5 mL). The reaction mixture was stirred vigor-
ously at room temperature for 2 h. The progress of the reaction was
followed as described above for the stoichiometric reaction.
Acknowledgments
We thank the Centre National de la Recherche Scientifique
(CNRS) and the Ministère de l’Enseignement Supérieur et de la
Recherche (MESR) for financial support (grant to L.L.). We are
most grateful to Dr Frédéric Hapiot for T-ROESY experiments.
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[(S)-2b]: Prepared by a method similar to that outlined for (S)-2a.
Yield: Ͼ99%. 1H NMR (300 MHz, CDCl3): δ = 1.46 (d, J =
7.1 Hz, 3 H, CH3), 3.21 (m, 1 H, CH), 3.46 (m, 1 H, CHH), 3.53
(m, 1 H, CHH) 3.86 (s, 3 H, N-CH3), 7.27 (m, 5 H, Harom), 8.04
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Eur. J. Org. Chem. 2010, 2696–2700
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Received: September 22, 2009
Published Online: March 24, 2010
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