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Coordination-Mediated Optical Resolution of Carboxylic Acids with O O-Dibenzoyltartaric Acid.

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observed in 2 (Figure 2). The
Li-H(Si) distance in 3 (2.360 A)
compares well to that in 2.
The experimental and computational results clearly provide
evidence for a SiH-Li agostic
interaction. The dilithium dianion with Si-H bonds undergoes
twisting due to the SiH-Li agostic interaction caused by the polarity of the Si'+-H'- bond.[21
Experimental Section
2: Crystalline 1 (76 mg, 0.29 mmol) and
lithium metal (10 mg, 1.4 mmol) were
Figure 2. Optimized structure of 3
added to a Schlenk tube with a magnetic
at the HF/6-31+G* level as
stirrer. Degassed, dry, oxygen-free diethyl
viewed along the C-C bond.
ether ( z1 mL) was introduced by vacuum
Selected bond lengths [A] and
transfer, and the mixture stirred at room
1.581, C-Si
angles ["I: C-C
temperature to give a yellow solution of the
1.828, C-Li 2.105, 0-Li 1.933,
dianion within 1 h. The solvent was reSi-C-Si 119.3,Si-C-C 120.4,C-Li-C
moved in vacuo, and degassed pentane in44.0, Li-C-C 67.9, Li-C-Li 136. 0,
troduced by vacuum transfer. The lithium
Si-C-SiiSi-C-Si 35.
metal was removed, and the solution concentrated and cooled to afford yellow crystals of 2 in quantitative yields. 'H NMR
([D,]toluene, TMS): 6 = 0.45 (d, J = 4.0Hz, 24H, SiMe,), 1.12 (t. J =7 . 0 H z ,
12H, CH,CH,O), 3.30 (q, J = 7 . 0Hz , 8H, CH,CH,O), 4.50 (br. s, 4H, SiH);
l3C(lH} NMR ([D,]toluene, TMS): 6 = 6.3 (SiMe,), 14.6 (quint,
J(13C,6Li)= 2.3 Hz, CLi), 15.7 (CH,CH,O), 66.0 (CH,CH,O); "Si{'H} NMR
([D,]toluene, TMS): 6 = - 19.7;6L~{'H)NMR ([D,Jtoluene, LiCl in MeOH. external): 6 = 0.10 (quint, J('Li,'HSi) = 0.49 Hz).
Received: September 6, 1996
Revised version: January 7, 1997 [ZZ9533IE]
German version: Angew C k m . 1997, 109,1577- 1579
-
Keywords: agostic interactions * lithium reductions
solid-state structures
- silicon -
[I] Reviews: a) W. N. Setzer, P. von R. Schleyer, Adv. Orgunomer. Chem. 1985,
24,353; b) C. Schade, P. von R. Schleyer, ibid. 1987,27, 169; c) A. B. Sannigrahi, T. Kar, B. G. Niyogi, P. Hobza, P. von R. Schleyer, Chem. Rev. 1990, YO,
1061; d) H. Bock, K. Ruppert, C. Nather, Z. Havlas, H. F. Herrmann, C.
Arad, I. Gobel, A. John, J. Meuret, S. Nick, A. Rauschenbach, W. Seitz, T.
Vaupel, B. Solouki, Angew. Chem. 1992,104,564; Angew. Chem. Int. Ed. Engl.
1992, 31, 550; e) A.-M. Sapse, P. von R. Schleyer in Lithium Chemistry:
A Theoretical and Experimental Overview, Wiley, New York, 1995.
I21 P. von R. Schleyer, T. Clark, J. Chem. SOC.Chem. Commun. 1986, 1371.
[3] H. Pritzkow, T. Lobreyer, W. Sundermeyer, N. J. R. van Eikema Hommes,
P. von R. Schleyer, Angew. Chem. 1994,106,221; Angew. Chem. Int. Ed. Engl.
1994, 33, 216.
[41 a) A. J. KOS,E. D. Jemmis, P. von R. Schleyer, R. Gleiter, U. Fischbach, J. A.
1981, 103, 4996; b) S. P. So, J. Organomet. Chem.
Pople, J Am. Chem. SOC.
1989, 361,283.
[ S ] a) M. Walczak, G. D. Stucky, J Am. Chem. SOC. 1976, Y8, 5531; b) A.
Sekiguchi, T. Nakanishi, C. Kabuto, H. Sakurai, ibid. 1989, 111, 3748; c) A.
Sekiguchi, M. Ichinohe, C Kabuto, H Sakurai, Organornerallics 1995, 14,
1092; d) A. Sekiguchi, M. Ichinohe, C. Kabuto, H. Sakurai. Bull. Chem. Soc.
Jpn. 1995, 68, 2981 ; e) A. Sekiguchi, M. Ichinohe. T. Nakanishi, C. Kabuto,
H. Sakurai, ibid. 1995, 68, 3215.
[6] Ethylene dianion disodium and diceslum derivatives: a) H. Bock, K. Ruppert,
D. Fenske, Angen Chem. 1989,101,1717; Angen. Chem Int. Ed. Engl. 1989,
28, 1685; b) H. Bock, T. Hauck, C. Nather, Orgunometulh 1996, 15, 1527.
[7] H. Sakurai, K. Ebata, K. Sakamoto, Y: Nakadaira, C. Kabuto, Chem. Lett.
1988,965.
IS] A single crystal (0.25 x 0.30 x 0.40 mm) of 2 was sealed in a capillary glass tube,
and diffraction data collected at 150 K on a Rigaku Denki AFC-5R diffractometer with a rotating anode (45 kV, 200 mA) employing graphite-monochromatized Mo,,radiation ( j . = 0.71069 A). A total of 3247 reflections
(3<20<5S') werecollected. Crystal data for C,,H,,Li,O,Si, ( M ,= 422 80):
u =18.551(7), b = 9.890(8), c =17.183(6)&
=113.61(2), V = 2889(3) A',
monoclinic, space group C2/c, Z = 4, pCslid
= 0.972 gem-,. The positions of
the hydrogen atoms connected to silicon atoms were determined by a difference
Fourier calculation, and those of the other hydrogen atoms calculated. When
the hydrogen atoms were included in the refinement, the final R factor con-
1534
8 VCH Verlagsgesellschaft mbH, D-69451 Weinheim, 1997
verged at 0.0698 (R,= 0.0717) for 2114 reflections with F,>3o(F,). Further
details of the crystal-structure investigation may be obtained from the Fachinformationszentrum Karlsruhe, D-76344 Eggenstem-Leopoldshafen (Germany), on quoting the depository number CSD-59427.
1974, 96,6048.
[9] R. Zerger, W Rhine, G . Stucky, J. Am. Chem. SOC.
[lo] TheGIAOcalculationsof3attheHF/6-31 + G*level suggestthat theSiH-Li
interaction causes the 29SiNMR signal to shift downfield by A6 = 5.62, and
the 6Li NMR signal to shift upfield by A6 = 0.46 with respect to the corresponding signals for the planar structure without this interaction. The calculated Si-H frequency for the twisted structure of 3 is 1925cm-' (DFT/631 + G*). These predictions are fully consistent with the experimental
observations.
[I 11 E. A. Williams, J. D. Cargioli in Annual Reporls on NMR Spectroscopy, Vol. 9
(Ed.: G. A. Webb), Academic Press, New York, 1979, p. 221.
[12] The twisted structure A with the SiH-Li interaction is calculated to be more
stable than planar B by 3.93 kcalmol-' (A - 1405.3569491a.u., B
- 1405.3506778a.u.),
3.51 kcalmoi-'
(A
- 1405.346044a.u., B
- 1405.3404548 a u.), and 6.06 kcalmol-'
(A -1406.3602892 a.u., B
- 1406.350628 a.u) at the HF/6-31 +G*, HF/6-31G*, and MP2/6-31G*
levels, respectively.
Coordination-Mediated Optical Resolution of
Carboxylic Acids with
0,O'-Dibenzoyltartaric Acid**
AndrLs Mravik,' Zsolt Bocskei, Zoltan Katona,
Imre Markovits, and Elemer Fogassy
Racemic carboxylic acids are generally resolved by optically
active bases from natural or synthetic sources. Some of the
naturally occurring amines (brucine, strychnine) are extremely
toxic and available only in one enantiomeric form.
In contrast, tartaric acid and its derivatives are relatively
cheap, nontoxic resolving agents that are available in both enantiomeric forms, even on a large scale. Although derivatives of
unnatural tartaric acid are more expensive than the enantiomers, the stereoisomers of 0,O'-dibenzoyltartaric acid can be
produced by preferential crystallization at the same cost on a
fully synthetic basis.['] We report here new applications of 0,O'dibenzoyltartaric acid, which is used for the resolution of bases,
as a resolving agent for carboxylic acids. Conversion of 0,Odibenzoyltartaric acid into its neutral calcium salt (2) results in
a compound of basic character that can form salts with acids. In
fact, 2 readily reacts with enantiomers of suitable acids 1 to form
crystallizing, mixed calcium salts 3; this provides excellent resolution of the acids (Scheme 1).
A coordinating group attached to the chiral center has an
important role in crystallization and, furthermore, in the discrimination of the enantiomers. The resolving agent 2 itself
forms a bulky crystalline mass upon separation that cannot be
removed by filtration. However, it readily dissolves in hot
ethanol or methanol, in which it can be prepared in situ by
heating dibenzoyltartaric acid and calcium oxide.
[*I
[**I
A. Mravik, 2.Katona, I. Markovits, Prof. Dr. E. Fogassy
Department of Organic Chemical Technology
Technical University of Budapest
P. 0 Box 91. H-1521 Budapest (Hungary)
Fax: Int. code +(1)257-7128
e-mail: mravik(a,oct.bme.hu
Dr. Z. Bocskei
CHINOIN Pharmaceuticals, Budapest
The work at the Technical University of Budapest was supported by the Hungarian Research Fund (OTKA T 014887) and the Varga Jozsef Foundation.
We thank Prof. Gy Pokol, Budapest. for the thermoanalytical determinations.
0570-0833/97/3613-1534$17.50+ S O / O
AngeW. Chem. Int. Ed. Engl. 1997,36, No. 13/14
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0
H'
0
'H
RLCK-COOH
I
OR2
1
2
Scheme 1. Formation of a mixed calcium salt upon resolution of carboxylic acids
with dihenzoyltartaric acid.
Racemic acids 1 a or 1 band then water were added to the hot
solution of 2 in the suitable alcohol. On cooling (or seeding)
salts 3 a and 3 b began to sepaCI+-C,KOOH
n
rate. Thus, l a was resolved in
O
I Cb
'o"c0otl
aqueous ethanol, and pure
(R)-(
+)-1 a-2 ((R)-3a) was obla
Ib
tained in 51 % yield after three
recrystallizations (ethanol/water and methanol/water). The acid (R)-(
+)-1 a liberated from
= + 74.5 (c = 1 in
this salt showed a specific rotation of [cc]i3
methanol), [cc]i3
= + 71.5 (neat). (Traditional resolution of l a
with (S)-(-)-I -phenylethylamine required five recrystallizations from benzene and gave a salt (63 YO)from which (S)-1 a
was obtained with [cc]i3
= -70.5 (neat).r21)Accordingly, ( S ) (-)-1 b-2 ((S)-3b) was obtained (methanol/water) in 89% yield
after one recrystallization, and the acid (S)-(-)-lb had
[a]i3= - 34.1 (c = 1 in chloroform). Resolution of l b with
brucine afforded the brucine salt (crystallization from ethyl acetate followed by two recrystallizations from acetonitrile) in
51 % yield, and the isolated (R)-(+)-lb had [4i3= + 30.4
(c = 1.01 in chloroform) .[31 Yields were calculated based on half
of the starting racemic acids.
X-ray crystal structure analyses were carried out on (S)-3bC4]
and (R)-3a to investigate the interactions controlling the optical
resolution (in particular the role of coordination around the
calcium atom). The molecular structure of (S)-3b is shown in
Figure 1. Tetrahydrofuroic acid (1 b) has (S)-(-) configuration.
m
Figure I . Stereoview of the crystal structure of (S)-3b.
A stereoview of the packing in (S)-3 b is given in Figure 2. The
crystal structure of (S)-3b (Figure 1) is made up of alternating
hydrophobic layers, composed of the phenyl moieties of the
benzoyl groups, and hydrophilic layers, composed of the Ca2
ions and their coordinating ligands. The hydrophylic layers can
be further divided into alternating stripes of hydrogen bonds
and Ca2+-coordination polyhedrons (Figure 2). The bond
lengths and angles demonstrate that the coordination geometry
at Ca2' is distorted trigonal bipyramidal ( C a 2 + - 0 2.272.50 A). The tetrahydrofurane carboxylate (THFC) ion is
+
A n g m Chrm Int. Ed. Engl. 1997, 36. N o . 13/14
Figure 2. Stereoview of the packing of (S)-3b.
a bis(bidentate) ligand : Its carboxylate group is a bidentate
Ca2+-binding unit ( 0 2 . . ' 0 3 nonbonding distance 2.18 A),
and one of the carboxylate oxygen atoms (02) and the ring ether
oxygen atom (01; 0 1 . . ' 0 2 nonbonding distance 2.66 A) form
another bidentate moiety. The equatorial positions of the trigonal bipyramid are occupied by the two bidentate ligands of the
THFC moiety as well as the oxygen atom 0 1 2 from water. One
of the two oxygen atoms of the carboxylate groups (02) simultaneously coordinates to two neighboring Ca2 ions to form an
endless Ca-0-Ca-0 chain through the crystal lattice. The axial
positions of the bipyramid are occupied by the carbonyl oxygen
atoms of one of the two benzoyl groups (01 1) and of the deprotonated carboxylate group (04). Since the ligands at the axial
positions are less bulky they are substantially closer to the Ca2+
ions than the equatorial ligands. The hydrogen-bonding system
of the crystal also shows interesting features. The tartaric acid
moieties are arranged in a head-to-tail fashion by hydrogen
bonds between deprotonated and protonated carboxylate
groups. Two stripes of Ca2 bipyramids are interconnected by
several hydrogen bonds mediated by water molecules.
Due to the crowded structure of these mixed salts, the scope
of the method is limited to relatively small racemic acids. The
alkyl ether functionality seems to be essential. cc-Chloro acids do
not form mixed salts, whereas cc-hydroxy acids, due to the low
solubility of their calcium complexes, separated as their simple
calcium salts under the above conditions.
Because of these limitations we directed our interest to the
resolution of simple derivatives of carboxylic acids (for example
esters). The preparative-scale separation of an ester from the
resolving acid can be easily accomplished, whereas problems
might be encountered in certain cases upon separation of a free
acid.
Hydroxy acids represent an important class of chiral compounds. We developed a simple, generally applicable method for
resolving hydroxy acid esters (5, see Table 1 for examples).
As the esters lack an acid functionality, saIt formation should
be entirely replaced by coordination bonds. The acidic calcium
salt of 0,O'-dibenzoyltartaric acid Ca(HL,), (4) readily coordinates hydroxy acid esters to form crystalline complex
This phesaltsc5]of the general formula 6 (see Table
+
+
nomenon serves as a basis for the resolution of hydroxy esters,
since selection between the enantiomers of 5 occurs during crystallization. In the formula for 6 L stands for 5, and A, and A,
stand for auxiliary ligands (A, is generally a simple ester such as
ethyl or propyl acetate, and A, water). The complex salts
formed can be divided into three major types. Type I: n, = 2; no
auxiliary ligand is cocomplexed, but water may be present.
0 VCH Verlugsgesell.whufimbH, 0.69451
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0570-0833/97i3613-1535$17.50+ S O Y I
1535
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Table 1. Complex salts 6 obtained upon resolution of 5 a , b with (2R,3R)-0,0'dibenzoyltartaric acid.
r
Q
c m
QJ&OR
Sa
5a
5a
5a
5a
5a
5a
5a
5a
5a
5a
5a
5b
5b
C,H,OOCCH,
CH 3
Sb
C A
C,H,
C,H,CH,
CH,
CH,
C,H,
CH,
CH,
C,H,
CH,
C,H,
CH,
C,H,
I
I
I
I
I1
I1
EA
EP
1
1
1
1
0.5
0.5
1
1
2
2
1
1
2
2
1
1
111
I1
I1
I1
I1
PA
EA
EA
PA
98
90
99
99
84111
93
90
92
93
86
85
68
63
[a] EA: ethyl acetate. EP: ethyl propionate, PA: propyl acetate. [b] The number of
cocomplexed auxiliary esters. [c] The number of water molecules in the complex.
[d] Yield of the complex salt based on half of the starting racemic 5.
[el Enantiomeric excess and configuration (in parentheses) of 5 obtained from the
complex were determined by comparing the specific rotation value and the sign of
rotation to those of the pure enantiomer. [fJ Yield of the salt is based on the amount
of the (S)-enantiomer present in the starting mixture. [g] Starting from a mixture
with 76%(S) ee.
Type 11: n , = 1; auxiliary ligands (generally both A , and A,
when A, is a simple ester) are cocomplexed. Cocomplexation
can be advantageous considering the remarkable influence of A,
on the enantiomeric composition of L.['] Type I1 is characteristic for medium-sized hydroxy esters. Type 111: n , = 1; only water molecules are cocomplexed. This classification is valid for
air-dried salts.
These methods present new aspects of optical resolution.
Normally, the resolving agent should be selected to achieve
better resolution. Using this second method, however, the efficiency of the resolution and, in certain cases, the configuration
of the enantiomer obtained can be controlled by varying the
ester function and/or the applied auxiliary ligands.
Received: April 3, 1996
Revised version: January 2, 1997 [Z8993IE]
German version: Angew. Chem. 1991. 109, 1529-1531
Keywords: carboxylic acids . chiral recognition
resolution
- enantiomeric
[I] A Mravik, E. Fogassy, Z. Lepp, Hung. Pat. AppI. PO0185 1996; A. Mravik,
2. Lepp, E. Fogassy, Tetrahedron: Asymmetry 1996, 7, 2387-2390.
121 N. K. Kochetkov. A. M. Likhosherstov, V. N. Kulakov. Tetrahedron 1969, 25,
2313 - 2323.
[3] P. C. Belanger, H. W. R. Williams, Can. J. Chem. 1983,61, 1383-1386.
[4] Single crystals of (S)-3b were grown by slow concentration of a solution in
methanoI/water. A stereoview of (S)-3b is shown in Figure 1 . M , = 548.50,
orthorhombic, space group P2,2,2,, a = 11.074(10), h = 27.149(3), c = 8.220
( 5 ) A, V = 2472(3) A3, Z = 4, pcalcd
= 1.474 gcm-', p = 2.806 mm-l. Data
were collected on a colorless crystal (0.70 x 0.30 x 0.25 mm) at room temperature with a Rigaku AFC6S diffrdctometer; 4 2 0 scans with a w scan rate of
8'min-' and a scan width of(0.89+0.30 tan0)'with graphite-monochromated
Cu,, radiation. Lattice parameters were determined by least squares using 25
reflections with 46.2 < 2 6 < 55.0". Three standards monitored every 150 reflections indicated no significant decay. A full set of Friedel mates was collected for
a total of 10292 reflections (Om,, =75.77'; 1 3 t h t -1, 3 4 t k t - 34,
1 0 2 1 2 -10) of which 5109 were unique with R,,,,, = 0.082 (without Friedel
mates). The structure was solved with the TEXSAN package (Molecular Structure Co., TEXSAN. Single Crystal Structure Analysis Package, The Woodlands,
TX 77381, USA, 1992) and refined with SHELXL-93 (G. M. Sheldrick,
1536
0 VCH Verlug.~gesell.schaftmbH, 0-69451 Weinheim. 1997
SHELXL-93, Program for the Refinement of Crystal Structures, University of
Gottingen, 1993) against F 2 with all unique reflections (2788 had intensities
higher than 2u(1)). All non-hydrogen atoms were refined anisotropically. Most
hydrogen atoms were generated based on geometrical evidence with X-H bond
lengths dependant on the chemical nature of X. Some hydrogen-atom positions
(for example those belonging to the two water molecules) were taken from
difference-Fourier calculations and reinforced by forming hydrogen bonds with
proper geometry. R1 = 0.0602, wR2 = 0.1369, G O F =1.095 for those reflections with 1>2u(f). The Flack parameter 1; = 0.01(2) confirms the absolute
configuration (H. D Flack, Acta Cryslallugr. Sect. A 1983,39,876-881). Crystallographic data (excluding structure factors) for the structure reported in this
paper have been deposited with the Cambridge Crystallographic Data Centre as
supplementary publication no. CCDC-100266. Copies of the data can be
obtained free of charge on application to The Director. CCDC, 12 Union
Road, Cambridge CBZIEZ, UK (fax: int. code +(1223)336-033; e-mail:
deposit(u chemcrys.cam.ac.uk) .
[5] The previously prepared acidic calcium salt or dibenzoyltartaric acid and calcium oxide were dissolved in hot ethanol, and the racemic ester was added. In
some cases crystallization can be carried out from this solution, but more frequently from a solvent mixture made by adding further solvents (such as acetone, toluene, or ethyl acetate). However, most of the complexes were separated
from an ester-type solvent after the ethanol was removed by evaporation
[6] Characterized by thermoanalytical methods (TGA, EGD), elemental analysis,
and ICP-AES. The structure of the complex formed with two molecules of
methyl mandelate was determined by X-ray crystallography (A. Mravik. 2.
Bocskei, Z. Katona, I Markovits, G. Pokol, D. K. Menyhard, E. Fogassy,
Chem. Commun. 1996,1983- 1984.) In some cases partial decomposition occurs
at slightly elevated temperatures or even at room temperature. Measurements
were performed on air-dried samples, which may contain 0.5-2% of volatile
impurities. Furthermore, the relatively high molecular weight may cause uncertainty in the results.
[7] The resolution of ethyl mandelate in the presence of various simple esters of
C,-C, alcohols afforded complex salts in which the ee values of the ethyl
mandelate were in the range of 31 -78%.
A Hydrogen-Bonding Receptor That Binds Urea
with High Affinity**
Thomas W. Bell* and Zheng Hou
Urea is an important target for molecular recognition studies,
both from fundamental and practical points of view. It is a small
molecule having hydrogen-bonding sites of well-defined geometry. The urea moiety is present in numerous biologically relevant
compounds, including barbiturates, biotin, citrulline, cytosine,
thymine, uracil, and uric acid. As an end product of nitrogen
metabolism, urea is found in the human blood stream in concentrations between 2 and 8mM and is excreted in urine. The blood
urea nitrogen (BUN) level is a primary indicator of kidney function"] and compounds having high affinity for urea are potentially useful in clinical chemistry. We have synthesized an artificial receptor that fully satisfies the hydrogen-bonding capacity
of urea. The resulting complex is stable even in pure DMSO,
from which it can be isolated by crystallization.
The novel receptor 1 nearly encircles urea from the "bottom,"
that is from the side opposite the carbonyl group, as shown by
the schematic structure I in Figure 1 a. Like previously reported
[*I Prof. T. W.
Z. Hou
Department of Chemistry
State University of New York
Stony Brook, NY 11794-3400 (USA)
[ + ] Current address: Department of Chemistry, MS216
University of Nevada, Reno, NV 89557-0020 (USA)
Fax: Int. code +(702) 784-6804
e-mail: twb(a unr.edu
[**I This work was partially funded by the National Institutes of Health (GM
32937)
OS70-0833/97/3613-1536$ 17.50+ .SO/()
Angew. Chem. Int. Ed. Engl. 1997, 36, Nu. 13/14
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