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Gas-Phase Enantiodifferentiation of Chiral Molecules Chiral Recognition of 1-Phenyl-1-propanol2-Butanol Clusters by Resonance Enhanced Multiphoton Ionization Spectroscopy.

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7 . Trimer 5 (65.7 mg, 0.0181 mmol) was dissolved in benzene (30 mL), MeOH
(1.5 mL), andacetone (1.5 mL)in aglass bomb, and 10% Pd/C(66mg)added. The
bomb was subjected to three freeze-thaw cycles under vacuum, then cooled
to - 196°C and opened to H, (1 atm) for 15 min, closed, and stirred for 48 h at
25 "C. The bomb was then cooled to - 196 "C, and opened to air. The reaction
mixture was thawed, added to a pad of Celite, and rinsed with THE The solvent was
removed from the filtrate under vacuum to give an off-white residue, which was
recrystallized from CHCIJMeOH to give trimer 7 (50.1 mg, 0.0162 mmol, 90°/,).
'H NMR (CDCI,/CD,OD 2011): 6 = 2.41 (m, 24H, CH,CH,C,H,), 2.59 (m,24H,
CH,CH,C,H,),4.31 (d. J = 7 . 1 Hz, 12H, H,,),4.76(t, J =7. 8Hz , 12H,H,,,,,,,),
569 (s. 6H. H,,,,,,), 585 (d. J =7. 1 Hz, 12H, H,,,, ), 6.59 (s, 6H, para-H,,,,,
6.79 (s. 6H. para-Ha&, 7.10-7.30 (br m, CH,CH,C,H, CH,C,Hs, CHCI,);
MS:m/;= 3111 (100%j[M+Na+], 3110calcd forC,,,H,,,O,,+Na+; elemental analysis calcd for C , 9 s H , 6 8 0 3 6 ~ H ~CO75.42,
H 5.52; found: C 75.18,
H 5.66.
8.2pyr: Tetramer 6 (32.8 mg, 0.00797 mmol), K,CO, (150 mg, 1.08 mmol,
135 equiv), CH,CIBr (2.7 pL, 0.0415 mmol, 5 equiv), and 5 mol% pyrazine
(626 mg. 982 equiv) were added to N-methylpyrrolidinone (15 mL). The reaction
mixture was stirred under an N, atmosphere for 24 h at 25°C and then for 24 h at
60 'C Additional CH,CIBr (2.7 pL) was added after the first day. The solvent was
removed under vacuum. and Z M HCI (30 mL) added to the residue. The aqueous
layer was extracted with CHCI, (3 x 10 mL), and the combined CHCI, fractions
were dried over MgSO, and filtered. The solvent was removed under vacuum. The
residue was filtered through a pad of silica gel with CHCI, and recrystallized from
CHCI,/EtOAc to give bis(carcep1ex) 8.2pyr (25.4 mg, 0.00587 mmol, 74%).
'HNMR i j = 2.51 (m, 32H. CH,CH,C,H,), 2.67 (m, 32H, CH,CH,C,H,), 4.07
(d, J =7.6 Hz, 8H , H,"). 4.25 (d, J =7.4 Hz, 8H, H,,), 4.30 (5, 8H, encapsulated
pyrazine).4.77(t. J=7.6Hz,8H,H,,,,,,,),4.92(t,
(~,4H,H,,,,.,),6.03(d.J=7.6H~,16H,H,,,),6.06(d,J= 5.8Hz,4H,H,,,,,,),6.26
(d, J = 5.8 Hz. 4H, H,,,,,,), 6.51 (s, 4H, H,,,,,,), 6.87 (s, 4H,para-H,,,,,,), 6.91 (s,
8H, para-H,,,,,,). 7.02 (s, 4H. para-H,,,,,,), 7.10-7.30 (br m, CH,CH,C,H,
CH,C,H,, CHCI,), MS: m!z=4345 (100%) [M+Na+], 4348 calcd for
+ Na+; elemental analysis calcd for C264Hzz4048.(C4H,N,),: C 75.45. H 5.41, N 1.30; found: C 75.33, H 5.32, N 1.19.
Received: January 21, 1997 [Z10018IE]
German version: Angew. Chem. 1997, 109.1828- 1830
Keywords: carceplexes - cavitands host -guest chemistry selfassembly - supramolecular chemistry
[I] M. Goodman. Y. Feng,G. Melacini, J. P. Taulane,J Am. Chem. Sue. 1996,118,
51 56
[2] G. R. Desiraju, Angeu. Chem. 1995, 107, 2541 ; Angew. Chem. In/. Ed. Engl.
1995, 34, 231 1
[3] G. M. Whitesides. E. E. Simanek, J. P. Mathias, C. T. Seto, D. N. Chin, M.
Mammen, D. M. Gordon, Acc. Chem. Res. 1995, 28, 37.
[4] a) D. J. Cram, Nature 1992,356, 29; b) J. C. Sherman, Tetrahedron 1995,51,
3395; c) A. Miiller, E. Diemann, E. Krickemeyer, S. Che, Naturwissenschafen '
1993, 80, 77; d) B:H. Huisman, D. M. Rudkevich, F. C. J. M. van Veggel,
D. N. Reinhoudt. J. Am. Chem. Soc. 1996, 118, 3523.
[ S ] R. G. Chapman, N. Chopra, E. D. Cochien, J. C. Sherman, J Am. Chem. SUC.
1994. 116. 369; template effect of hemicarceplex formation: N. Chopra, J. C.
Sherman, Suprumul Chem. 1995, 5 , 31.
[6] a) Cyclic array of two cavitands and two calixarenes: P. Timmerman, W.
Verboom, F. C. J. M.van Veggel. W. P. van Hoorn, D. N. Reinhoudt, Angew.
Chem. 1994,106,1313;Angew. Chem. Int. Ed Engl. 1994,33,1292;cycIicarray
of 3-8 calixarenes: b) P. D. Beer, A. D. Keefe, A.M. Z. Slawin, D. J.
Williams. J. Chrni. Soc. Dufton Trans. 1990,3675; c) D. Kraft, J.-D. van Loon,
M. Owens, W. Verboom, W. Vogt, M. A. McKervey, V. Bohmer, D. N. Reinhoudt, Tetrahedron Lerr. 1990, 31, 4941; d) P. Lhotak, M. Kawdguchi, A.
Ikeda, S. Shinkai. Tetrahedron 1996, 52, 12399.
[7] J C. Sherman, C. B. Knobler, D. J. Cram, J. Am. Chem. SOC.1991, 113,2194.
[8] For other hosts that contain a large cavity, see ref. [6] and a) J. H. Small, D. J.
McCord, J. Greaves, K. J. Shea, J Am. Chem. SUC.1995, 117, 11588; b) M.
Fujita. D. Oguro. M. Miyazawa, H. Oka, K. Yamaguchi, K. Ogura, Nature
1995,378,469; c j J. W. Steed, C. P. Johnson, C. L. Barnes, R. K. Juneja, J. L.
Atwood, S. Reilly. R. L. Hollis, P. H. Smith, D. L. Clark, J. Am. Chem. SUC.
1995, 117, 11426; d) S. Watanabe, K. Goto, T. Kawashima, R. Okazaki,
Tetrahedron Lett. 1995, 36, 7677; e) K. Araki, K. Akao, A. Ikeda, T. Suzuki,
S. Shinkai, h i d . 1996,37,73; f) A. Arduini. A. Pochini, A. Secchi, R. Ungaro,
J Chem. Soc. C h m Commun. 1995. 879: g) P. A. Brady, R. P. Bonar-Law,
S. J. Rowan. C. J. Suckling, J. K. M. Sunders, Chem. Cummun. 1996, 319.
[9] We recently reported a reversible complex between two molecules of tetrol 1 in
which the bowls are linked by four charged hydrogen bonds (ArO- . . . HOAr):
R. G. Chapman, J. C. Sherman, J. Am. Chem. SUC.1995,117,1081. Tetramer
6 can potentially oligomerize into channels over such linkages; investigations
are currently underway.
Angew Chem Inr Ed Etig( 1997, 36, No 16
a) A. Harada, J. Li, M. Kamachi, Nature 1994, 370, 126; b) C. Wu, T. Bein,
R. Granja, L. K. Buehler, Nature
1994, 369, 301; d) J.-A. Pirez-Adelmar, H. Abraham, C. Sinchez, K. Rissanen, P. Prados, J. de Mendoza, Angew. Chem. 1996,108,1088;Angew Chem.
In/. Ed. Engl. 1996, 35, 1009.
The "gating" mechanism for egress of guests from carceplexes and hemicarceplexes: a) K. N. Houk, K. Nakamura, C. Sheu, A. E. Keating, Science 19%.
273,627; b) C. Sheu, K. N. Houk, J. Am. Chem. Soc. 1996. 118.8056; c) K.
Nakamura, K. N. Houk, ibid. 1995, 117, 1853.
J. R. Fraser, B. Borecka, J. Trotter, J. C. Sherman, J Org. Chem. 1995,60,1207
a) CS Chem 3D, Cambridgesoft Corp , Cambridge, MA, USA, 1995; b) U.
Burkert, N. L. Allinger, Molecular Mechanics. American Chemical Society,
Washington, DC, USA, 1982.
Science 1994,266,1013; c) M. R. Ghadiri, J.
Gas-Phase Enantiodifferentiation of
Chiral Molecules: Chiral Recognition
of l-Phenyl-l-propanol/2-ButanolClusters by
Resonance Enhanced Multiphoton Ionization
Susanna Piccirillo, Cesare Bosman, Daniela Toja,
Anna Giardini-Guidoni, Marco Pierini, Anna Troiani,
and Maurizio Speranza*
Enantioselective complexation is a basic tool for molecular
recognition. In the condensed phase chiral molecules (M) may
establish short-range electrostatic, orbital, and steric interactions with suitable chiral partners (C) to form diastereomeric
molecular complexes (MC), which are discriminable by virtue of
their different physical and chemical properties. Determination
of melting point and solubility, vapor-pressure measurements,
as well as X-ray analysis"] are commonly used to discriminate
diastereomeric MC crystals. In solution calorimetric, colorimetric,C2] spectroscopic, and transport-rate measurements can be
used, although the presence of the solvent may sometimes hinder their differentiation.". 3J
A way to eliminate this undesired interference and to evaluate
the intrinsic interactions in diastereomeric MC is to extend their
study to the gas phase; this, however, is by no means an easy
task. Mass spectrometry has been occasionally employed to
enantiodifferentiate chiral ions by complexation with suitable
chiral hosts.14' The inherent limitation of mass spetrometry to
discriminate between diastereomeric or enantiomeric species demands the frequent use of isotopic markers, which may somewhat alter the relative stability of diastereomeric ion-neutral
[*] Prof. M. Speranza, Dr. M. Pierini, Dr. A. Troiani
Facolta di Farmacia
Dipartimento di Studi di Chimica e Tecnologia delle
Sostanze Biologicamente Attive
Universita di Roma "La Sapienza"
P.le A. Moro 5 , 1-00185 Rome (Italy)
Fax: Int. code +(6)4991-3602
e-mail: speranza(
Dr. S. Piccirillo, Dr. C. Bosman
Istituto Materiali Speciali, CNR, Tito Scalo (Italy)
Dr. D. Toja, Prof. A. Giardini-Guidoni
Dipartimento di Chimica, Universita di Roma "La Sapienza"
I**] This work was supported by the Minister0 della Universita e della Ricerca
Scientifica e Tecnologica (MURST) and the Consiglio Nazionale delle
Ricerche (CNR). Help and advice from Professor F. Cacace. Professor F.
Gasparnni, and Dr. M. Aschi are gratefully acknowledged.
8 WILEY-VCH Verlag GmbH, D-69451 Weinhem, 1997
0570-0833/97/3616-1729$17 50+ 50/0
Enantiodifferentiation of neutral chiral molecules in the gas
phase can be obtained by high-resolution molecular spectroscopy of their MC complexes with chiral partners. So far
there has only been one study on the laser induced fluorescence
(LIF) excitation spectra of a single jet-cooled van der Waals
diastereomeric MC pair.r5]Here we illustrate the first application of resonant multiphoton ionization spectroscopy
(REMPI),r61combined with time-of-flight (TOF) mass spectrometry,['] to discriminate supersonically expanded
van der Waals diastereomeric MC complexes, and to study their
fundamental properties at the microscopic level. The complexes
were obtained by combining a chiral chromophore, in this case
+ )-I-phenyl-I-propanol (P,), with the enantiomers of 2butanol (B, or Bs). Their MC complexes (P,B, or P,BJ are
expected to display different spectral signatures with respect to
the electronic band origin of each pair, their appearance potential, and their vibronic bands, in particular those due to intermolecular vibrations of PRwith the partner (B, or Bs).[*]
The PRBRand P,Bs clusters were discriminated by one-color
resonance enhanced multiphoton ionization (IcREMPI) experiments, in which the complex is excited to a discrete S, state by
absorption of one photon of frequency v , , and then to the
continuum by absorption of several other photons with the
same frequency v , . The IcREMPI excitation spectra are taken
by recording the TOF mass spectra of PRBR(or P,B,) as a function of v, . The appearance potential of pure PRis measured by
two-color resonance enhanced two photon ionization (2cR2PI)
experiments, in which the molecule is excited to a discrete S,
state by one photon of a fixed frequency v l , and then ionized
by a second photon of variable frequency vz. A value of
9.01 0.01 eV is obtained for pure P,, which compares well with
the ionization potential of 8.9 eV for benzyl alcohol at room
The IcREMPI excitation spectrum of pure PRis characterized
by the electronic band origin of the S,+So transition of the
molecule placed at 37 577 cm-' and followed by two intense
peaks that are shifted by 41 and 47 cm-'. Other strong vibronic
transitions are observed at 530, 575, 581, 629, 772, and
976cm-' from the origin. The low-frequency modes are assigned to torsional motion of the ring substituents. The other
intense features are at frequencies typical for motion of the
aromatic ring and ethyl or OH groups, and were not completely
identified." O1
Figure 1 reports the IcREMPI excitation spectra of the
diasteromeric pairs PRBRand P,Bs around their electronic band
origin. Both clusters display a complex vibronic spectrum due to
numerous van der Waals vibrations, which may be coupled with
low-frequency modes of the partners.'". 12]The spectra are red
shifted relative to the S, +So origin of the isolated PRmolecule,
indicating an enhancement of binding energy for the clusters in
the S, state relative to the SOstate. The shifts of the most intense
bands, assigned to the electronic band origin of the corresponding clusters, differ by 13 cm-' as a result of a P,B,-band shift
of -79 cm-' and of a P,B,-band shift of -92 cm-'. This reflects an S, +So energy gap for P,Bs that is smaller than that for
The IcREMPI excitation spectrum of the mixture with
= 1, obtained by clustering PRwith 2-butanol racemate, essentially corresponds to the superimposed spectra of the
separate PRBRand P,Bs clusters. However, when equilibration
between PRBRand P,Bs is favored by increasing the stagnation
pressure from 2 to 5 x 10' Pa, the relative intensity of the PRBR
bands increases, and that of the P,Bs bands decreases by about
7 %.[I3] These findings corroborate molecular mechanics calcul a t i o n ~ [ ' that
~ ] predict that the ground-state homochiral PRBR
0 WILEY-VCH Verlag GmbH, D-69451 Weinhelm, 1997
Av / Cm-'
Figure 1. IcREMPI excitation spectra of diasteromeric PRBR(a) and PRBs(b) clusters measured at m/z = 210 and a total stagnation pressure of 2 x lo5 Pa. The origin
of the frequency scale is relative to the electronic band origin of the S I +So transition
of PRat 37577 cm-'. I = intensity (arbitrary units).
adduct is more stable than the heterochiral P,Bs adduct. Therefore, the decrease in the S,+So energy gap in going from PRBR
to P,Bs points to an excited state in which the stability difference
of the clusters tends to be cancelled out or even reversed relative
to the ground state.
Figure 2 displays the highly reproducible IcREMPI/TOF
mass spectra of diastereomeric PRBR(Figure 2a) and P,Bs (Figure 2b) complexes taken at a fixed frequency, namely at the
electronic origin frequency of the corresponding cluster (Figure 1). In addition to the parent ion [PRBR]+(or [P,B,]+) at
m/z = 210 the spectra show the formation of fragments [P,]'
(m/z = 136), [PRBR- C,H,]+ (or [PRB,- C,HSIC;m/z =181),
and [P, - C,H,]+ (m/z = 107). That all these species and
[PRBR]'(or [P,B,]+) actually have the same origin is testified by
their identical IcREMPI excitation patterns.
The distinct difference between the IcREMPI/TOF mass
spectra of the diastereomers PRBRand P,Bs (Figure 2a, b)
provides a further tool for their discrimination. In particular
the spectrum of the homochiral cluster PRBR (Figure 2a)
shows a distribution of [PR]+:[PRBR
- C,H,]':[P,
- C2Hs]+:
[PRBR]+= 0.6:2.5:0.7: 1.0, which clearly diverges from that
of the heterochiral cluster P,Bs ([PR]+:[PRB,- C2H,]+ :
[PR- C,H,]+:[P,B,]+ =1.4:9.7:3.2:1.0; Figure 2b). The more
extensive fragmentation observed in the spectrum of P,Bs (93 YO
(P,B,) versus 79% (P,B,)) is responsible for the comparatively
low intensity of its IcREMPI excitation spectrum (see Figure la, b), and suggests that, despite the slightly higher energy
absorbed by PRBR,[I higher-energy dissociative states are more
populated in heterochiral P,Bs than in homochiral PRBRfor the
same 1cREMPI process.
The REMPIiTOF technique has been applied for the first
time to enantiodifferentiate neutral chiral molecules in the gas
phase. Chiral recognition is based on highly reproducible differ-
0570-0833/97/3616-1730$17 50+ 50/0
Angeu Chem Int Ed Engl 1997,36,No 16
the TOF source were mass discriminated and detected by a channeltron after a
SO-cm flight path.
Received: December 18, 1996
Revised version: March 27, 1997 [Z9911 IE]
German version: Anfen.. Chem. 1997. 109. 1816-1818
Keywords: chrality
enantiomeric resolution * laser spectroscopy mass spectrometry - molecular recognition
[l] A. Findlay, A. N. Campbell, The Phase Rule and Its Applications. Dover. New
York, 1945, pp. 136,256.
[2] Y Kubo, S. Maeda, S. Tokita, M. Kubo, Nature 1996, 382, 522.
[3] See for instance E. M. Arnett, S. P. Zingg. J Am Chem. Soc. 1981, fO3,1221.
[4] M Sawada, Y. Takai, H. Yamada, S. Hirayama, T. Kaneda. T.Tanaka, K.
Kamada, T. Mizooku, S. Takeuchi, K. Ueno, K. Hirose, Y Tobe. K. Naemura,
J Am. Chem. Soc. 1995, 117, 7726, and references therein.
[ 5 ] A. R. Al-Rabaa, E. Breheret, F. Lahmani, A. Zehnacker, Chem Phys. Lett.
1995, 237, 480.
[6J B. Brutschy, Chem. Rev. 1992, 92, 1567.
[7] S . Piccirillo, M. Coreno. A. Giardini-Guidoni, G. Pizzella. M. Snels, R. Teghil,
J Mol. Srruct. 1993, 293, 197.
[Sl S. Piccirillo, D. Consalvo, M. Coreno, A. Giardini-Guidoni. S Duin, P
Parneix, P Brechignac, Chem. Phys. 1994, 187, 97
[9] R. E. Ballard, J. Jones, E. Sutherland, D. Read, A. Inchley, Chem. Phys. Lett.
1986. 126, 311.
[lo] S. Piccirillo, A. Giardini-Guidoni, unpublished results.
[ l l ] D. Consalvo, A. van der Avoird, S. Piccirillo, M. Coreno, A Giardini-Guidoni, A. Mele, M. Snels, J Chem. P h w 1993, 99, 8398.
1121 Careful adjustment of the expansion parameters allowed us to maximize the
concentration of two-body PRBRand P& clusters relative to that of larger
1131 M. D. Brookes, D. J. Hughes, B J. Howard, J Chem. P b p . 1996, 104, 5391.
[14] Molecular mechanics calculations were performed by using "automatic and
systematic quasi-flexible docking processing" (S Alcaro. F Gasparrini, 0.
Incani, M. Pierini, C. Villani, J. Compul. Chem., submitted) and applying
MMX force field, as implemented in the PC-model software (PC Model Molecular Modeling Software, Serena Software, P. 0. Box 3076, Bloomington,
IN 17102-3076 (USA)).
[I51 The energy absorbed by P,B, exceeds that ahsorbed by PRBs by nl3cm-',
where n is the number of absorbed photons, and 13 cm-' the red shift between
the two clusters.
[16] A. Giardini-Guidoni. E. Borsella, R. Fantoni, Europhys. N r a s 1985, 16, 2.
[17] M. Coreno, S . Piccirillo, A. Giardini-Guidoni, A. Mele. A Palleschi, P
Brechignac, P. Parneix, Chem. Phys. Lett. 1995, 236, 580.
m/zFigure 2. IcREMPIlTOF spectra of diasteromeric PRBRat i.= 266.68 nm (a) and
PRBsclusters at i.= 266.77 nm (b) at a total stagnation pressure of 2 x lo5 Pa. The
structures of the ground state P,B, and PRBs clusters, calculated with modified
molecular mechanics (MMX), are given in (a) and (b), respectively.
ences in the photoionization fragmentation pattern of the
diastereomeric clusters obtained from complexation of the chiral molecule with suitable chiral partners, and on the different
bathochromic shifts of their electronic band origin relative to
that of the pure chromophore. The latter indicates that attractive interactions within the clusters are more pronounced in the
excited than in the ground state. Under consideration of the
order of stability in the ground state, PRB,>PRB,, a greater
stabilization of the PRBscluster in the S, state relative to that of
PRBRtends to cancel the stability order in the excited state.
Differences in the absorption frequencies of gaseous jet-cooled
species were recently used for isotope-enrichment procedures.r'61This study demonstrates that the same approach can
be applied to jet-cooled diastereomeric clusters to enantiomerically enrich a racemate.
Multipolymer-Supported Substrate and
Ligand Approach to the Sharpless Asymmetric
Dihydrox ylation**
Hyunsoo Han and Kim D. Janda*
Since the initial solid-phase synthesis of oligopeptides by
Merrifield"] the use of polymers as supports, reagents or even
catalysts for various reactions has increased.['] Recently a high
level of activity has been devoted to this field due to the application of combinatorial chemistry to drug
a resurgence of interest in polymers for organic synthesis has
been seen, little effort has been paid to defining the structural
Experimental Section
The experimental setup for the generatJon OF van der Waals clusters and their
REMPIjTOF analysis was previously described [7,11,17]. Diastereomeric pairs
PnBRand PRBswere obtained by in a supersonic beam by adiabatic expansion of a
carrier gas (Ar) seeded with the corresponding alcohols (Aldrich) through a pulsed
400-pm nozzle kept at 85 "C. The molecular beam was allowed to pass through a
1-mm skimmer into a second chamber equipped with a TOF spectrometer. The laser
= 532 nm), which
system consisted of a frequency-doubled Nd:YAG laser (i
pumps two dye lasers. The dye frequencies were doubled and, when necessary,
mixed with residual 1064-nm radiation. The ions formed by REMPI ionization in
Angew Chem inr Ed Engl 1997,36,No. 16
[*I Prof. K. D Janda, Dr. H. Han
The Scripps Research Institute
Department of Chemistry and
The Skaggs Institute for Chemical Biology
10550 N. Torrey Pines Road, La Jolla, CA 92037 (USA)
Fax: Int. code +(619)784-2595
This research was supported by the R. W Johnson Pharmaceutlcal Institute
and The Skaggs Institute for Chemical Biology.
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butanol, molecules, phenyl, recognition, gas, phase, chiral, spectroscopy, clusters, propanol, enhance, resonance, ionization, enantiodifferentiation, multiphoton
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