Practical method and novel mechanism for optical resolution of BINOL by molecular complexation with N-benzylcinchoninium chloride

Article abstract of DOI:10.1016/S0040-4020(00)00368-9

A highly efficient and practical resolution of racemic 1,1'-bi-2- naphthol (BINOL) has been achieved through molecular complexation with N- benzylcinchoninium chloride, which can be readily prepared in 85% yield using an improved procedure through the reaction of cinchonine with benzyl chloride in dimethylformamide (DMF). It is found that the absolute configuration of BINOL included in the molecular crystals is R, which is the same as in the case of using N-benzylcinchonidinium chloride. This result is discussed in terms of molecular recognition in the solid state. X-Ray structural analysis of molecular crystal formed between (R)-BINOL and N-benzylcinchoninium chloride indicates that the additive effect of the chiral features of the host, the interaction pattern between the functional groups of the host and guest, and the complementary molecular packing in the crystal lattice dominates the stereochemistry of the guest in the molecular crystal. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4020(00)00368-9

TETRAHEDRON
Pergamon
Tetrahedron 56 (2000) 4447±4451
Practical Method and Novel Mechanism for Optical Resolution
of BINOL by Molecular Complexation
with N-Benzylcinchoninium Chloride
Yang Wang,b Jie Suna and Kuiling Dinga,
*
aLaboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Lu,
Shanghai 200032, People's Republic of China
bDepartment of Chemistry, Zhengzhou University, Zhengzhou 450052, People's Republic of China
Received 14 March 2000; revised 13 April 2000; accepted 27 April 2000
AbstractÐA highly ef®cient and practical resolution of racemic 1,10-bi-2-naphthol (BINOL) has been achieved through molecular
complexation with N-benzylcinchoninium chloride, which can be readily prepared in 85% yield using an improved procedure through
the reaction of cinchonine with benzyl chloride in dimethylformamide (DMF). It is found that the absolute con®guration of BINOL included
in the molecular crystals is R, which is the same as in the case of using N-benzylcinchonidinium chloride. This result is discussed in terms of
molecular recognition in the solid state. X-Ray structural analysis of molecular crystal formed between (R)-BINOL and N-benzylcincho-
ninium chloride indicates that the additive effect of the chiral features of the host, the interaction pattern between the functional groups of the
host and guest, and the complementary molecular packing in the crystal lattice dominates the stereochemistry of the guest in the molecular
crystal. q 2000 Elsevier Science Ltd. All rights reserved.
Introduction
Results and Discussion
Optical resolution
Optically active 1,10-bi-2-naphthol (BINOL, 1) and its deri-
vatives have found many applications, ranging from chiral
ligands of catalysts for asymmetric reactions1 to hosts for
molecular recognition and enantiomer separation,2 to the
intermediates for the synthesis of chiral materials.3 Several
protocols have been developed for the access to its enantio-
pure form, including classical crystallization of diastereo-
isomeric derivatives,4 enantioselective formation of
inclusion crystals with chiral host molecules,5 enzymatic
hydrolysis of esters,6 and asymmetric coupling of
2-naphthol derivatives.7 So far, the enantioselective forma-
tion of inclusion crystals with chiral host molecules is one of
the most practical methods because of its very high
ef®ciency5 and the ready availability of the racemic
BINOL8 (Fig. 1). In the present work, we wish to report a
new practical method for the resolution of BINOL by mole-
cular complexation with N-benzylcinchoninium chloride (3)
and its novel molecular recognition pattern in the inclusion
crystals.
Molecular complexation has proven to be one of the most
effective methods for the resolution of chiral organic mole-
cules.9 In particular, chiral ammonium salts were reported to
be effective host molecules for the resolution of racemic
phenol derivatives by Toda's group.5a±c In the case of
BINOL, N-benzylcinchonidinium chloride (2) readily
Keywords: biaryls; molecular recognition; resolution; N-benzylcincho-
ninium chloride.
* Corresponding author. Fax: 186-21-6416-6128;
e-mail: kding@pub.sioc.ac.cn
Figure 1.
0040±4020/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(00)00368-9

4448
Y. Wang et al. / Tetrahedron 56 (2000) 4447±4451
So far, a new, ef®cient and practical resolution of BINOL
has been successfully achieved. It is interesting to note that
the absolute con®guration of BINOL included in the
molecular crystals is (R) in the present work as described
above. However, in Toda's procedure5a,b and our previous
optical resolution of 2-amino-20-hydroxy-1,10-binaphthyl
(NOBIN) with N-benzylcinchonidinium chloride (2),5f the
absolute con®guration of the molecules included in the
molecular crystals were also found to be (R)-(1)-1. This
unique result must be caused by the speci®c molecular
recognition pattern between host and guest molecules in
the lattice of the molecular crystals, which prompted us to
elucidate the structure of molecular crystal 5 by single
crystal X-ray crystallography.
Scheme 1. The preparation of 3.
forms inclusion crystals with its R-enantiomer, which pre-
cipitate from the solution and the (S)-enantiomer remains in
the mother liquid. With the improvement of this procedure
by Pu and Cai, both the (R) and (S) enantiomers can be
obtained ef®ciently in .99% ee.5d,e However, the commer-
cially available N-benzylcinchonidinium chloride (2) is
expensive and its preparation seems dif®cult because of
long reaction time and poor reproducibility of the pro-
cedure.10 Therefore, we investigated the use of N-benzyl-
cinchoninium chloride (3), an epimer of 2, as the chiral host
for the resolution of BINOL. One reason is that 3 can be
readily prepared in 85% yield via an improved procedure
through the reaction of cinchonine (4) with benzyl chloride
in dimethylformamide (DMF) at 808C for 3 h as shown in
Scheme 1. It was found that racemic BINOL can also be
effectively resolved with N-benzylcinchoninium chloride
through molecular complexation.
Structural analysis
The single crystals of molecular complex 5 were obtained
by slow evaporation from acetonitrile±methanol mixed
solvent. The intermolecular organization and association
in the molecular complex are shown in Fig. 2. The structural
parameters exhibited for the host and guest molecules are in
good agreement with standard values.11
As shown in Fig. 2, the structure of the molecular complex
network between (1)-1 and 3 can be described as in®nite
chains of interlinked species that are aligned in an alter-
nating manner through hydrogen bonds. The hydrogen
bonding pattern includes two OH groups of neighboring
BINOLs as the proton donors and the chloride anion,
which lies between BINOLs, as the proton acceptor (at
p
As shown in Scheme 2, re¯uxing acetonitrile solution
containing 1 equiv. of (^)-BINOL (1) and 0.6 equiv. of
N-benzylcinchoninium chloride (3) resulted in the forma-
tion of a white solid which was collected by ®ltration after
cooling and washed thoroughly with acetonitrile to remove
the opposite enantiomer. The white solid was characterized
by elemental analysis to be 1:1 molar ratio of molecular
crystal (5) between BINOL and 3. The ®ltrate was concen-
trated to dryness, redissolved in ethyl acetate, and washed
with dilute HCl and brine. The organic layer was dried over
anhydrous MgSO4, ®ltered, and concentrated until solvent
free. Further recrystallization from benzene gave enantio-
merically pure (S)-(2)-BINOL (S-1) in 76% yields based on
one enantiomer (.99% ee). The molecular crystals could be
decomposed with dilute HCl and extracted with ethyl
acetate. Following the same procedure for the workup of
the (S)-1 as mentioned above, the enantiomerically pure
(R)-(1)-1 was obtained in 80% yield based on one enantio-
mer with .99% ee. The aqueous phase was neutralized with
NaHCO3 and concentrated to give N-benzylcinchoninium
chloride in .90 % recovery.
Ê
Ê
Cl´´´O2 and Cl´´´O3 of 3.017(6) A and 3.024(6) A, respec-
tively). Simultaneously, one of the hydrogen-bonded OH
groups of BINOL to chloride anion provides the hydrogen
acceptor to associate with the proton of OH group at cincho-
Ê
ninium moiety (at O1´´´O2 of 2.899(8) A). However, the
H-bond between cinchoninium OH and halide anion
disclosed in Toda's report5b and our previous work5f is not
found in the present molecular complex because of the
Ê
distant separation of Cl and O1 atoms (3.979(6) A). There-
fore, it is both the electrostatic and H-bonding interactions
that predominate in the formation of the bridging link
between host±guest pairs. The dihedral angle of the
binaphthyl unit is 97.88 and its absolute con®guration is
found to be R unambiguously through relating with the
absolute con®guration of cinchoninium moiety. The
C±H´´´p interactions are found to be additional inter-
molecular non-bonding forces for the packing of host±guest
pairs in the crystal lattice. The shortest relevant nonbonding
Ê
H´´´C distances are within the range of 2.69±3.00 A. Based
on the facts mentioned above, we may conclude that the
chirality of host molecule is not the only factor which deter-
mines the stereochemistry of molecular recognition between
the interacting partners. A combination of the chiral features
of the host, the interaction pattern between the functional
groups of the host and guest, and complementary molecular
packing in the crystal lattice controls the stereochemistry of
the guest molecule in the crystal lattice.
Conclusion
A new practical and ef®cient method for the resolution of
Scheme 2. The resolution of rac-BINOL by molecular complexation.

Y. Wang et al. / Tetrahedron 56 (2000) 4447±4451
4449
Figure 2. Perspective view of molecular complex 5.
racemic 1,10-bi-2-naphthol (1) has been developed through
molecular complexation with the readily available chiral
ammonium salt, N-benzylcinchoninium chloride, which
provides a more convenient access to the enantiopure
BINOL. The novel stereochemistry in the molecular recog-
nition between host and guest molecules is revealed by
X-ray diffraction analysis of the molecular crystals 5.
Improved procedure for the preparation of
N-benzylcinchoninium chloride
Cinchonine (4) (11.76 g, 40 mmol) was added to the
solution of benzyl chloride (7.62 g, 60 mmol) in dimethyl-
formamide (DMF) (80 mL). The mixture was heated at
808C and stirred for 3 h. The white needle crystals were
collected by ®ltration after cooling, and washed with
acetone (2£10 mL) to give N-benzylcinchoninium chloride
(3) (14.24 g) in 85% yield. Mp 2568C (dec.) (lit.10b Mp
2488C). [a]2D7ˆ1165.9 (cˆ0.50, H2O) (lit.10b [a]2D2ˆ
1164.8 (cˆ0.716, H2O)).
Experimental
General
Resolution of racemic 1,10-bi-2-naphthol (1) using
N-benzylcinchoninium chloride (3)
Racemic 1,10-bi-2-naphthol was prepared by coupling of
2-naphthol using FeCl3´6H2O as the oxidant in aqueous
suspension followed by recrystallization from toluene.8c
Cinchonine was purchased from Fluka and used directly.
1H NMR and 13C NMR spectra were taken on Bruker
AM300 and Bruker DRX400 spectrometers at 258C, respec-
N-benzylcinchoninium chloride (3) (20.21 g, 48 mmol) was
added to the solution of racemic 1,10-bi-2-naphthol (1)
(22.88 g, 80 mmol) in acetonitrile (300 mL). The mixture
was heated to re¯ux for 4 h and then allowed to cool to room
temperature. The resulting white solids were collected by
®ltration and washed with acetonitrile (3£20 mL). The
solids were characterized to be two-component molecular
crystals of (R)-(1)-1 and 3 in a 1:1 molar ratio. Mp 2488C
(dec.). Anal. Calcd for C46H43ClN2O3: C, 78.11; H, 6.13, N,
3.96. Found: C, 78.26; H, 6.06, N, 4.02. This is also
con®rmed by X-ray diffraction analysis. The enriched
(S)-(2)-1 was left in the mother liquor.
1
tively. Chemical shifts of H NMR are expressed in ppm
with tetramethylsilane as an internal standard (dˆ0) in
CDCl3 and chemical shifts of 13C NMR in ppm with residual
signal of CDCl3 as an internal standard (dˆ77). Coupling
constants are reported in hertz (Hz). Elemental analysis was
performed on a Foss±Heraeus Vario EL instrument. Melt-
ing points are uncorrected. Enantiomeric excesses were
calculated from [a]D values measured on a PE 341 auto-
matic polarimeter. They were also con®rmed by HPLC
analysis on a Chiralcel AS column (4.6 mm£250 mm) at
room temperature with hexane-iso-propanol (3:1) as eluent.
A suspension of molecular crystals in acidic water (200 mL
of 1 N HCl and 800 mL of H2O) and ethyl acetate (300 mL)

4450
Y. Wang et al. / Tetrahedron 56 (2000) 4447±4451
was stirred for 30 min until the white solids disappear. The
organic layer was separated, washed with brine, and then
dried over MgSO4. After removal of the solvent, the residue
obtained from organic phase was recrystallized from
benzene to afford (R)-(1)-1 (9.15 g) in 80% yield based
on one enantiomer with .99% ee (Chiralcel AS,
0.8 mL min21, S: 12.43 min; R: 16.06 min.) as colorless
prisms. Mp 208±2108C (lit.5d 208±2108C). [a]D27ˆ132.1
(cˆ1.0, THF) (lit.5d [a]2D1ˆ134.3 (cˆ1.0, THF)); 1H
NMR (300 MHz, CDCl3): d 5.05 (s, 2H), 7.15 (d, Jˆ
8.11 Hz, 2H), 7.29±7.41 (m, 6H), 7.88 (d, Jˆ8.40 Hz,
2H), 7.96 (d, Jˆ8.92 Hz, 2H). 13C NMR (100.61 MHz,
CDCl3): d 110.9, 117.8, 124.1, 124.3, 128.5, 129.5, 131.5,
133.5, 152.8. The mother liquor was concentrated to
dryness, then redissolved in ethyl acetate (300 mL), and
washed with HCl (100 mL, 1 N) and brine (50 mL). The
organic layer was dried over MgSO4. Following the same
procedure for the recrystallization of (R)-(1)-1, (S)-(2)-1
(8.92 g) was obtained in 78% yield based on the other
Crystal data for 5 (C46H43ClN2O3): 2umaxˆ51.08; formula
weight 707.31, tetragonal, space group P43212(#96), aˆ
Ê
Ê
Ê 3
12.997(3) A, cˆ43.199(6) A, Vˆ7297(3) A , Zˆ8, Dcacl
ˆ
1.288 g cm23, m(Mo-Ka)ˆ1.50 cm21, F(000)ˆ 2992.00,
index ranges 0#h#15, 0#k#11, 0#l#48.
Acknowledgements
We thank the National Natural Science Foundation of China,
Chinese Academy of Sciences, Laboratory of Organo-
metallic Chemistry, and Shanghai Institute of Organic
Chemistry for ®nancial support of this work.
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