Preparation of enantiomerically pure C76 with a general electrochemical method for the removal of Di(alkoxycarbonyl)methano bridges from methanofullerenes: The retro-bingel reaction

Article abstract of DOI:10.1002/(SICI)1521-3773(19980803)37:13/14<1919::AID-ANIE1919>3.0.CO;2-X

Serendipitously discovered during electrochemical investigations on the stability of anion 1, a preparatively useful electrochemical procedure, the retro-Bingel reaction, has been identified as a method for removing di(alkoxycarbonyl)methano bridges from methanofullerenes. This procedure was applied to prepare the first samples of enantiomerically pure D₂-C₇₆ with unambiguous optical purity.

Full text of DOI:10.1002/(SICI)1521-3773(19980803)37:13/14<1919::AID-ANIE1919>3.0.CO;2-X

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partially resolved EtOAc molecules (C120.75
Å
H
64
N
2
� 3
O
20, M
r
ˆ 1862.74):
Bingel protocol, occurs by treating the carbon allotrope with
Triclinic space group P1, Z ˆ 4, 1calcd ˆ 1.368 gcm , a ˆ 14.218(7), b ˆ
2
-bromomalonate esters in the presence of base. Modified
2
5.562(13), c ˆ 25.916(13) Š, a ˆ 84.18(3), b ˆ 74.84(3), g ˆ 89.81(4)8,
procedures allowing in situ formation of the 2-bromo- or 2-
3
Vˆ 9042(8) Š . The structure was solved starting with a single
fullerene oriented by PATTSEE orientation search and subsequent
tangent expansion in P1. Refinement by full-matrix least-squares
analysis (SHELXTL PLUS; heavy atoms anisotropic, H atoms fixed,
whereby H positions are based on stereochemical considerations) to
iodomalonates have also been reported.[
4, 5]
Cyclopropane
rings fused to fullerenes are highly stable and cannot be
removed thermally or by wet-chemical methods, although
[6]
mass spectrometric fragmentation occurs rather readily.
give R(1) ˆ 0.0964 and R (2) ˆ 0.2564 for 2554 variables, 756 re-
w
The majority of methanofullerenes are also known to be
very stable under the conditions of cyclic (CV) and steady-
state voltammetry (SSV).[ A notable exception was reported
straints, and 18545 observed reflections with I > 2s(I) and 1.74 ꢀ q ꢀ
16c]
5
0
0.01. b) X-ray crystal data for 1:[ Yellow rhomboid crystals (0.1 Â
7]
.1 Â 0.015 mm) were obtained by slow diffusion of hexane into a
solution of 1 in EtOAc/PhH and contained a large amount of
[8]
by Haddon, Wudl, and co-workers for the monoanion of 1,2-
unresolved solvent molecules (C200.5
H
5
N
2
O
20
P
2
PtS,
M
r
ˆ 3050.16):
methano[60]fullerene-61,61-dinitrile, which was unstable un-
der CV conditions and, based upon voltammetric observa-
� 3
Monoclinic space group P2(1)/c, Z ˆ 4,
1
calcd ˆ 1.580 gcm
,
a ˆ
2
9.17(2), b ˆ 17.986(9), c ˆ 27.37(2) Š, a ˆ 90, b ˆ 116.75(5), g ˆ 908,
3
[9]
Vˆ 12821(13) Š . The structure was solved by Patterson methods and
refined by full-matrix least-squares analysis (SHELXTL PLUS; heavy
atoms isotropic, bond distances and angles of the partially disordered
diethyl malonate groups constrained by SHELX-DFIX-instructions)
tions, was reported to revert back to C . Also, the electro-
60
chemistry group of M. Gross at Strasbourg had noted a
decomposition of dianions of specific bis(methano)fullerenes
with a cis-2 addition pattern during CV and SSV, and this
to give R ˆ 0.109 and R
w
ˆ 0.092 (based on F) for 756 variables and
5
549 observed reflections with I > 2 s(I) and 1.70 ꢀ q ꢀ 35.10. c) Data
observation provided the impetus for the work reported
here.[
7c, 10]
In this paper, we describe the retro-Bingel reaction
collection at Tˆ 293 K: Picker± Stoe diffractometer, CuKa radiation
using w scans, l ˆ 1.54178 Š. Crystallographic data (excluding struc-
ture factors) for the structures reported in this paper have been
deposited with the Cambridge Crystallographic Data Centre as
supplementary publication no. CCDC-101112. Copies of the data
can be obtained free of charge on application to CCDC, 12 Union
Road, Cambridge CB21EZ, UK (fax: (‡44)1223-336-033; e-mail:
deposit@ccdc.cam.ac.uk).
(
od
Scheme 1) as a general, preparative electrochemical meth-
[11, 12]
for the removal of di(alkoxycarbonyl)methano
bridges from methanofullerenes and illustrate its power with
[13, 14]
the preparation of the pure enantiomers of C .
76
[
17] F. Cardullo, P. Seiler, L. Isaacs, J.-F. Nierengarten, R. F. Haldimann, F.
Diederich, T. Mordasini-Denti, W. Thiel, C. Boudon, J.-P. Gissel-
brecht, M. Gross, Helv. Chim. Acta 1997, 80, 343 ± 371.
Preparation of Enantiomerically Pure C with
7
6
Scheme 1. The Bingel and retro-Bingel reactions.
a General Electrochemical Method for the
Removal of Di(alkoxycarbonyl)methano
Bridges from Methanofullerenes:
The Retro-Bingel Reaction**
Electrochemical investigations on diethyl 1,2-methano[60]-
fullerene-61,61-dicarboxylate (1) and similar C60 monoad-
ducts have shown that the first two reduction steps are
electrochemically and chemically reversible on the voltam-
Roland Kessinger, Jeanne Crassous, Andreas
Herrmann, Markus Rüttimann, Luis Echegoyen,* and
François Diederich*.
�
1 [7b, 8]
metric time scale at a scan rate of 100 mVs .
Our CV
data are in agreement with these earlier findings (Figure 1).
Chemical irreversibility of the second reduction is only
evident at very slow scan rates. We reduced 1 electrochemi-
Since its discovery in 1993, the Bingel reaction[1] has been
widely used as a mild, versatile method to introduce one or
cally in CH
Cl containing 0.1m Bu NPF at 293 K at
2
2
4
6
[
2]
[3]
more methano bridges into C60 and the higher fullerenes.
Cyclopropanation of a fullerene, according to the original
controlled potential, initially to its monoanionic state (at
[15]
� 1.2 V versus a Ag wire pseudo reference electrode). The
charge transferred corresponded exactly to one electron per
molecule, and the resulting anion was very stable under these
conditions, with no apparent decomposition after 30 min. The
formation of the dianion (at � 1.55 V versus Ag) by controlled
potential electrolysis (CPE, Figure 1), on the other hand, was
surprising, because the charge transferred corresponded to
four electrons per molecule and not just one. Clear changes in
the CV indicated that some chemical reaction had taken
place. After 30 min of electrolysis, reoxidation of the solution
at 0 V (versus Ag), followed by column chromatography
[
*] Prof. Dr. L. Echegoyen
Department of Chemistry, University of Miami
Coral Gables, FL 33124, USA
Fax: (‡1)305-284-4571
E-mail: lechegoyen@umiami.ir.miami.edu
Prof. Dr. F. Diederich, Dr. J. Crassous, Dr. A. Herrmann,
Dipl.-Chem. R. Kessinger, Dipl.-Chem. M. Rüttimann
Laboratorium für Organische Chemie, ETH-Zentrum
Universitätstrasse 16, CH-8092 Zürich (Switzerland)
Fax: (‡41)1-632-1109
E-mail: diederich@org.chem.ethz.ch
(
SiO , toluene) yielded pure C in over 80% yield (Table 1).
2
6
0
[
**] This work was supported by the Swiss National Science Foundation,
the US National Science Foundation (CHE-9313018), an NIH Fogarty
Fellowship (1 F06 TW-02231 ± 01 to L.E.), and Hoechst AG.
It must be stressed that the two first reduction steps are
perfectly reversible on the CV time scale at a scan rate of
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We monitored the retro-Bingel reaction of 4 by CV. The
first reduction step is fully reversible (Figure 2a), whereas the
second reduction process in this case is electrochemically
irreversible (Figure 2b), and, not surprisingly, bulk electrolysis
Figure 1. CV of 1 in CH
2
Cl
2 4 6
containing 0.1m Bu NPF , with a scan rate of
�
1
100 mVs . The working electrode was a glassy carbon mini-electrode. The
dotted line corresponds to a scan with a switching potential of � 1 V versus
the Ag wire pseudo reference electrode, and the solid line to one with a
switching potential of � 1.4 V. Shown are the potentials applied in the
controlled potential coulometry.
2 2 4 6
Figure 2. CV data for 4 in CH Cl containing 0.1m Bu NPF , with a glassy
2 2
Table 1. Results of the retro-Bingel reaction by CPE in CH Cl .
�
1
carbon working electrode and a scan rate of 100 mVs . a) Initial scan
showing the reversibility of the first reduction process. b) Scan with a
switching potential of � 2 V (versus Ag), clearly showing the irreversibility
of subsequent reductions, even the second one. c) Voltammogram recorded
after electrolyzing the solution with a net charge corresponding to a four-
electron transfer per molecule. The scan was started at the rest potential,
around � 1.25 V (versus Ag). d) Same as in (c), but after waiting
approximately 30 min. Notice the almost exclusive observation of the first
Starting material
Product
Yield[%]
1
2
3
4
5
6
C
C
C
C
C
60
60
60
70
76
82
75
75
70
5[
9
a]
[a]
ent-C76
three reduction waves of pure C70
.
[
a] Not optimized.
at the potential corresponding to the irreversible second
reduction process (� 1.3 V versus Ag) led to a net charge
transfer of four electrons per molecule, transforming 4 into
C . This conversion occurs stepwise since new waves between
�
1
00 mVs 1 (Figure 1). It was thus surprising to see that
controlled potential bulk electrolysis at the second reduction
potential led to complete chemical irreversibility yielding
predominantly C . The mechanism of this reaction, which we
call the retro-Bingel reaction, is not yet fully understood but is
under investigation in our laboratory.
70
60
0
and � 0.5 V (versus Ag) started appearing during electrol-
ysis (Figure 2c), providing evidence for the formation of
intermediates. These intermediates eventually disappeared
when the conversion into pure C70 was complete (Figure 2d).
These intermediate species are under current investigation as
possible clues to the mechanism of the retro-Bingel reaction.
Hawkins and Meyer had performed the kinetic optical
resolution on a small scale of the inherently chiral fullerene
D -C by employing the asymmetric Sharpless osmylation
The generality of the retro-Bingel reaction was proven by
CPE (under the conditions described above) of the bis-Bingel
[
2] [16]
[7c]
adducts
2
and 3 , which were converted cleanly and in
over 75% yield into C (Table 1). As in the case of 1, CPE was
6
0
conducted at the second reduction potential, but the number
of electrons transferred per molecule was approximately six
instead of four. The reaction is not limited to C60 derivatives
and was shown to work also for the C70 monoadduct 4,[
which reacted cleanly to give C70 in 70% yield.
2
76
[14a]
reaction.
A comparison of the circular dichroism (CD)
17a]
spectra reported by Hawkins and Meyer for the C7 enantio-
6
[14a]
mers
to those of a variety of optically active, covalent
derivatives of C76 prepared by us[ had revealed a large,
unexpected difference in the magnitude of the Cotton effects.
Whereas the covalent C76 derivatives displayed bands whose
18]
�
1
� 1
De values can be up to 250 M cm , the enantiomers of the
pure fullerene displayed bands with De-values up to only
�
1
� 1 [14a, 19]
3
2 M cm .
In order to reinvestigate the chiroptical
properties of C , we applied the retro-Bingel reaction to the
76
preparation of the enantiomerically pure higher fullerene
Scheme 2). When the optically active, diastereoisomerically
(
pure C monoadducts 5 and 6, with enantiomeric carbon
76
cores,[ were submitted to CPE and subsequent purification
18]
by HPLC,[ the two enantiomers, C76 and (ent-C ) were
20]
76
1
920
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Figure 3. CD Spectra of C76 and ent-C76 in toluene (see Table 2).
introduction of the cyclopropane ring as a new protecting
group in fullerene chemistry, which should be selectively
removable in the presence of many other addends. With the
Scheme 2. Retro-Bingel reaction of the diastereoisomerically pure, opti-
cally active C76 monoadducts 5 and 6. The location of the malonate addend
is not exactly known although tentative constitutional assignments have
been made. The CD spectra, however, provide unambiguous evidence that
the two compounds are associated pairs of diastereoisomers.[
capability of selectively reversing both fullerene cyclopropa-
18, 21]
[7c, 23]
nation and, by an indirect method,
the Diels ± Alder
addition, our tether-directed remote functionalization,[
which introduces both cyclopropane and cyclohexene rings,
should now lead to an even greater diversity of hitherto
unknown multiple addition patterns on the fullerene surface
that are not available by other methods.
2c]
obtained in low yields (about 5 ± 10%) which have not yet
been optimized because of the limited availability of the
starting materials.
The CD spectra of C76 and ent-C76 displayed the expected
mirror image shapes (Figure 3, Table 2) with band positions in
full agreement with those reported by Hawkins and Meyer.[
In contrast, however, the Cotton effects in our spectra are
almost a full magnitude larger than those previously reported.
19]
Experimental Section
�
1
� 1
The De values can be as large as 210 M cm and are
therefore much more in agreement with the values measured
for optically pure C76 derivatives such as 5 and 6.[ Two
possible reasons may account for the noticeably low values
reported by Hawkins and Meyer: Either the enantiomeric
purity of their samples was not very high or their concen-
Fullerene monoadduct 1 (11 mg, 12.5 mmol) and supporting electrolyte
[
12b]
Bu
4
NPF
6
(0.8 g) were added into a home-built electrochemical cell,
and
21]
� 6
the cell was degassed and pumped to 10 Torr before the dried CH Cl2
2
(17 mL), which had also been degassed and pumped to the same pressure,
was vapor-transferred directly into the cell. CPE at 293 K on a Pt mesh
2
working electrode (100mesh, 6.5 cm ) at � 1.55 V versus a Ag wire pseudo
reference electrode, separated from the solution by a Vycor tip, was
performed for 30 min after which the solution was reoxidized at 0 V versus
Ag for the same length of time. The electrolyte was removed by passing the
[
22]
trations were not accurately determined.
We now intend to apply the retro-Bingel reaction also to the
formation of other enantiomerically pure higher fullerenes
such as D -C or D -C , starting from diastereoisomerically
mixture through a short column of SiO
pure C60 (7.4 mg, 82%).
2
, and elution with toluene yielded
3
78
2
84
pure malonate adducts. Furthermore, the sequence of the
Bingel followed by the retro-Bingel reaction also permits the
Received: February 18, 1998 [Z11492IE]
German version: Angew. Chem. 1998, 110, 2022 ± 2025
�
6
Table 2. Circular dichroism spectra of C76 (c ˆ 3.4  10 m in toluene) and
Keywords: chiral resolution ´ cyclic voltammetry ´ electro-
chemistry ´ fullerenes ´ retro-Bingel reaction
�
6
ent-C76 (c ˆ 8.0  10 m in toluene) in the range 290 ± 800 nm. Both
compounds were prepared from the retro-Bingel reaction.
C
76
ent-C76
l[nm]
De[M 1 cm
�
� 1
]
l[nm]
De[M cm ]
� 1
� 1
[
1] C. Bingel, Chem. Ber. 1993, 126, 1957 ± 1959.
[2] a) A. Hirsch, I. Lamparth, H. R. Karfunkel, Angew. Chem. 1994, 106,
53 ± 455; Angew. Chem. Int. Ed. Engl. 1994, 33, 437 ± 438; b) I.
[
a]
[a]
315
331
352
391
406
463
519
541
578
629
694
� 73
316
331
351
391
407
462
520
549
578
629
700
‡ 80
4
‡ 53.4
� 18.8
� 52.3
‡ 23.7
Lamparth, G. Schick, A. Hirsch, Liebigs Ann.Chem./Recueil 1997,
253 ± 258; c) F. Diederich, C. Thilgen Science 1996, 271, 317 ± 323; L.
Isaacs, F. Diederich, R. F. Haldimann, Helv. Chim. Acta 1997, 2, 317 ±
[
a]
[a]
‡ 186
� 186
‡ 210.4
‡ 74.1
� 215.5
� 66.0
3
42.
[3] C. Thilgen, A. Herrmann, F. Diederich, Angew. Chem. 1997, 109,
362 ± 2374; Angew. Chem. Int. Ed .Engl. 1997, 36, 2268 ± 2280.
[a]
[a]
� 32
‡ 45
2
� 55.8
‡ 69.5
[
4] a) C. Bingel, presentation at the conference New Perspectives in
Fullerene Chemistry and Physics, Rome, 1994; b) J.-F. Nierengarten, V.
Gramlich, F. Cardullo, F. Diederich, Angew. Chem. 1996, 108, 2242 ±
� 106.3
‡ 117.0
[
a]
[a]
� 6
‡ 6
‡ 9.7
� 9.7
2
244; Angew. Chem. Int. Ed. Engl. 1996, 35, 2101 ± 2103.
[
a] Shoulders; De values have greater uncertainties.
[5] X. Champs, A. Hirsch, J. Chem. Soc. Perkin Trans. 1 1997, 1595 ± 1596.
Angew. Chem. Int. Ed. 1998, 37, No. 13/14
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1433-7851/98/3713-1921 $ 17.50+.50/0
1921

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[
6] For a hint that the Bingel-addend may be removed at very high
temperatures, see A. Hirsch, I. Lamparth, T. Grösser, H. R. Karfun-
kel, J. Am. Chem. Soc. 1994, 116, 9385 ± 9386.
Enantiomeric Enrichment of Stereolabile
Chiral Spiro Compounds by Dynamic HPLC on
Chiral Stationary Phases**
[
7] a) D. M. Guldi, H. Hungerbühler, K.-D. Asmus, J. Phys. Chem. 1995,
9
9, 9380 ± 9385; b) C. Boudon, J.-P. Gisselbrecht, M. Gross, L. Isaacs,
H. L. Anderson, R. Faust, F. Diederich, Helv. Chem. Acta 1995, 78,
334 ± 1344; c) F. Cardullo, P. Seiler, L. Isaacs, J.-F. Nierengarten, R. F.
Klaus Lorenz, Eiji Yashima, and Yoshio Okamoto*
1
Haldimann, F. Diederich, T. Mordasini-Denti, W. Thiel, C. Boudon, J.-
P. Giesselbrecht, M. Gross, Helv. Chim. Acta 1997, 80, 343 ± 371.
8] M. Keshavarz-K., B. Knight, R. C. Haddon, F. Wudl, Tetrahedron
Chromatographic enantioseparation on chiral stationary
[1±5]
phases (CSPs)
has become a very useful method for the
[
[
analysis of chiral compounds and the preparative separation
of enantiomers because of recent progress in the development
of CSPs with high chiral recognition ability. In addition, the
newly developed dynamic high-performance liquid chroma-
tography (DHPLC) has received considerable attention as a
1996, 52, 5149 ± 5159.
9] For the ring opening of cyclopropanes by electrochemical reduction,
see J. Y. Becker in The Chemistry of the Cyclopropyl Group, Part 2
(ed.: Z. Rappoport), Wiley, Chichester, 1987, pp. 915 ± 958.
[
10] R. Kessinger, M. G o mez-L o pez, C. Boudin, J.-P. Gisselbrecht, M.
Gross, L. Echegoyen, F. Diederich, J. Am. Chem. Soc., submitted.
11] For the loss of fullerene addends (H, Cl, Pt, or Pd) by electrochemical
reduction during CV, see a) P. Boulas, F. D'Souza, C. C. Henderson,
P. A. Cahill, M. Thomas Jones, K. M. Kadish, J. Phys. Chem. 1993, 97,
powerful tool for investigating dynamic processes of inter-
[
converting enantiomers;[
6±11]
kinetic data and enantiomeriza-
tion barriers for stereolabile compounds can be obtained from
a series of temperature-dependent plateaus and peak shapes
by chiral DHPLC. Here, we focus on the influence of CSPs on
the equilibrium of interconverting enantiomers which will
result in nonracemic mixtures. In the case of interconverting
enantiomers, the use of CSPs opens the possibility for
combining separation techniques and equilibrium shift in
one step. By employing quite simple new chromatographic
procedures we may, in principle, obtain one enantiomerically
pure enantiomer in 100% yield from a racemate with
standard HPLC equipment.
13435 ± 13437; b) T. F. Guarr, M. S. Meier, V. K. Vance, M. Clayton, J.
Am. Chem. Soc. 1993, 115, 9862 ± 9863; c) F. N. Tebbe, J. Y. Becker,
D. B. Chase, L. E. Firment, E. R. Holler, B. S. Malone, P. J. Krusic, E.
Wasserman, J. Am. Chem. Soc. 1991, 113, 9900 ± 9901; d) S. A. Lerke,
B. A. Parkinson, D. H. Evans, P. J. Lerke, D. H. Evans, P. J. Fagan, J.
Electroan. Chem. 1995, 383, 127 ± 132; e) S. A. Lerke, B. A. Parkinson,
D. H. Evans, P. J. Fagan, J. Am. Chem. Soc. 1992, 114, 7807 ± 7813. f) H.
Nagashima, M. Nakazawa, T. Furukawa, K. Itoh, Chem. Lett. 1996,
405 ± 406.
[
12] For electrochemical synthesis of fullerene adducts, see a) C. Caron, R.
Subramanian, F. D'Souza, J. Kim, W. Kutner, M. T. Jones, K. M.
Kadish, J. Am. Chem. Soc. 1993, 115, 8505 ± 8506; b) P. L. Boulas, Y.
Zuo, L. Echegoyen, Chem. Commun. 1996, 1547 ± 1548.
We investigated the racemic spiro compounds 1 and 2 as
model compounds. Both enantiomerize thermally and photo-
[
13] R. Ettl, I. Chao, F. Diederich, R. L. Whetten, Nature 1991, 353, 149 ±
153.
[
14] a) J. M. Hawkins, A. Meyer, Science 1993, 260, 1918 ± 1920; b) J. M.
Hawkins, M. Nambu, A. Meyer, J. Am. Chem. Soc. 1994, 116, 7642 ±
7645.
[
15] All controlled potential electrolyses were executed in a home-built
1
2b
electrochemical cell,
and all electrochemical experiments were
performed with an EG&G Princeton Applied Research model 263 A
potentiostat/galvanostat.
[
[
16] J.-F. Nierengarten, T. Habicher, R. Kessinger, F. Cardullo, F. Die-
derich, V. Gramlich, J.-P. Gisselbrecht, C. Boudon, M. Gross, Helv.
Chim. Acta 1997, 80, 2238 ± 2276.
17] a) A. Herrmann, M. Rüttimann, C. Thilgen, F. Diederich, Helv. Chim.
Acta 1995, 78, 1673 ± 1704; b) C. Bingel, H. Schiffer, Liebigs. Ann.
chemically through an electrocyclic [1,6]-ring opening of the
C ± O bond next to the spiro center and consecutive ring
closure.[
12]
1995, 1551 ± 1553.
Figure 1a shows a standard HPLC chromatogram for the
[
[
18] A. Herrmann, F. Diederich, Helv. Chim. Acta 1996, 79, 1741 ± 1756.
19] CD spectrum of enantiomerically pure C76 (c ˆ 3.8  10 M) in
�
5
[13]
resolution of 1 on Chiralcel OD.
Compound 1 can be
toluene:[
(
(
14a]
l[nm] (De [M cm ]: 315 (� 11.7), 330 (‡8.9), 354
� 4.2), 394 (‡29.4), 405 (‡31.6), 460 (‡12.6), 541 (� 11.1), 573
� 18.2), 639 (‡2.5).
� 1
� 1
completely resolved at 208C without any enantiomerization.
However, at higher temperatures (ca. 508C), the enantiome-
rization processes described in Scheme 1 (DHPLC on
[
20] Preparative HPLC on a Regis Buckyclutcher I Trident-Tri-DNP
CSPs)[
14,15]
lead to plateaulike elution profiles (Figure 1b).
[16]
(
4
10 mm) 500 mm  21.1 mm column with an n-hexane/toluene (60/
�
1
0) mobile phase at 8 mLmin
.
In the heterogeneous system (CSP/mobile phase) the adsor-
bed enantiomer showing a longer retention time (B) should
be enriched, whereas the eluent may contain a racemic
mixture under equilibrium conditions.
[
21] The chiroptical properties of both functionalized inherently chiral
fullerene derivatives and derivatives with an inherently chiral addition
pattern are largely determined by the chirality of the carbon sphere
and not by the chirality of the addends.[
17a, 18]
[
22] We determined the concentrations of the enantiomeric solutions
based on their known UV/Vis absorptions (see reference [13] and A.
Herrmann, doctoral dissertation, ETH Zürich, 1997).
According to the kinetic differential equations describing
the enantiomerization processes during DHPLC, only race-
[
23] a) Y.-Z. An, G. A. Ellis, A. L. Viado, Y. Rubin, J. Org. Chem. 1995, 60,
[
*] Prof. Y. Okamoto, Dr. K. Lorenz, Dr. E. Yashima
Department of Applied Chemistry, Graduate School of Engineering
Nagoya University
6353 ± 6361; b) F. Cardullo, L. Isaacs, F. Diederich, J.-P. Gisselbrecht,
C. Boudon, M. Gross, Chem. Commun. 1996, 797 ± 799.
Furo-cho, Chikusa-ku, Nagoya 464 ± 8603 (Japan)
Fax: ( ‡ 81)52-789-3188
E-mail: okamoto@apchem.nagoya-u.ac.jp
[
**] This work was partially supported by the Japan Society for the
Promotion of Science (JSPS). K.L. thanks the European Union and
the JSPS for a joint post-doctorate fellowship.
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ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1998
1433-7851/98/3713-1922 $ 17.50+.50/0
Angew. Chem. Int. Ed. 1998, 37, No. 13/14