506
J. Am. Chem. Soc. 1997, 119, 506-512
Conformational Analysis of Some
trans-4,5-Diaryl-1,3-dioxolanes by CD Spectroscopy and
Induction of Cholesteric Mesophases in Nematic Solvents: A
Correlation between Twisting Power and Structure of the
Dopant
,†
,‡
‡
‡
Carlo Rosini,* Gian Piero Spada,* Gloria Proni, Stefano Masiero, and
Simone Scamuzzi§
Contribution from the UniVersit a` della Basilicata, Dipartimento di Chimica, Via N. Sauro 85,
8
5100 Potenza, Italy, UniVersit a` di Bologna, Dipartimento di Chimica Organica “A. Mangini”,
Via S. Donato 15, 40127 Bologna, Italy, and UniVersit a` di Pisa, CNR-CSMSOA, Dipartimento di
Chimica e Chimica Industriale, Via Risorgimento 35, 56126 Pisa, Italy
X
ReceiVed July 16, 1996
Abstract: A series of trans-4,5-diaryl-1,3-dioxolanes (compounds 1-10) has been prepared from the corresponding
1
,2-diols obtained by asymmetric syn-dihydroxylation of (E)-olefins, in the presence of derivatives of quinidine.
Compounds 1-10, when dissolved in MBBA or E7, induce left-handed cholesterics, showing twisting powers which,
in absolute value, are strongly dependent on the structure of the solute and on the nature of the nematic mesophase.
By means of molecular mechanics calculations and the analysis of CD spectra of 1-10, it is possible to determine
the most stable conformation of these derivatives which is characterized by a negative twist of the aryl moiety.
With this information a reliable cholesteric induction model has been developed.
Introduction
of molecular shape anisotropy of the dopant opens new
perspectives for configurational assignments by the liquid
crystals technique. We decided to undertake a more detailed
analysis of the cholesteric induction ability of these dioxolanes
by measuring the twisting powers of compounds 1-13 in two
different nematic solvents and investigating in detail the
induction mechanism for two main reasons: first of all, only a
knowledge of the correct induction mechanism can allow a
reliable correlation between the helicity of the cholesteric and
the absolute configuration of the dopant; second, this knowledge
could help in the design of some 4,5-diaryl-1,3-dioxolane
dopants to induce chiral mesophases with interesting techno-
1
Modern techniques of configurational assignment (analysis
of circular dichroism (CD) spectra, of cholesteric mesophases
induced in nematic liquid crystals, and of the elution order upon
chiral stationary phases (CSP) for HPLC) have been recently
2
employed to characterize several 1,2-diarylethane-1,2-diols after
their transformation into the corresponding 4,5-diaryl-1,3-
dioxolanes, i.e., derivatives that are rigid and nonassociated and
whose molecular conformation is more reliably determinable
than that of the starting compounds. Both the analysis of the
CD spectra and of the induced cholesteric mesophases have been
2
previously carried out at a qualitative level; in particular, the
7
logical applications. This paper describes also a quantitative
cholesteric mesophases have been characterized only by rec-
ognizing the handedness (P or M) of the cholesteric helix. A
better understanding of the ability of a chiral dopant to twist
analysis of the CD spectra of such derivatives which, together
with some molecular mechanics calculations, provide a detailed
description of the molecular conformation of the chiral solute,
a fundamental piece of information required to formulate a
reliable cholesteric induction mechanism.
3
the nematic phase is given by its twisting power â ) 1/(pcr),
where p is the cholesteric pitch, c the molar fraction of solute,
and r its enantiomeric excess. The sign of â is taken as positive
for right-handed (P) cholesteric phases and negative for left-
handed (M) ones. The twisting power (and its sign) is sensitive
to the molecular shape, and it has been extensively applied to
stereochemical studies.4 Furthermore, the recent presentation
Results and Discussion
Synthesis. Compounds 1-3 and 5-10 have been prepared
,5
2
and characterized as previously described by asymmetric syn-
6
by Nordio and co-workers of a theoretical model for calculation
of the magnitude and the handedness of helical pitch in terms
dihydroxylation of some (E) olefins in the presence of quinidine
derivatives, providing the corresponding dextrorotatory 1,2-diols.
By treatment with 2,2-dimethoxypropane/TsOH they have been
†
Universit a` della Basilicata.
Universit a` di Bologna.
Universit a` di Pisa.
Abstract published in AdVance ACS Abstracts, December 15, 1996.
‡
§
(5) Gottarelli, G.; Spada, G. P.; Bartsch, R.; Solladi e´ , G.; Zimmermann,
R. J. Org. Chem. 1986, 51, 589. Gottarelli, G.; Spada, G. P.; Seno, K.;
Hagishita, S.; Kuriyama, K. Bull. Chem. Soc. Jpn. 1986, 59, 1607. Arnone,
C.; Gottarelli, G.; Spada, G. P.; Spinelli, D.; Exner, O. J. Mol. Struct. 1986,
147, 307. Naciri, J.; Spada, G. P.; Gottarelli, G.; Weiss, R. G. J. Am. Chem.
Soc. 1987, 109, 4352. Suchod, B.; Renault, A.; Lajzerowicz, J.; Spada, G.
P. J. Chem. Soc., Perkin Trans. 2 1992, 1839. Rosini, C.; Franzini, L.;
Salvadori, P.; Spada, G. P. J. Org. Chem. 1992, 57, 6820. Rosati, I.; Rosini,
C.; Spada, G. P. Chirality 1995, 7, 353. Gottarelli, G.; Proni, G.; Spada, G.
P.; Fabbri, D.; Gladiali, S.; Rosini, C. J. Org. Chem. 1996, 61, 2013.
(6) Ferrarini, A.; Moro, G. J.; Nordio P. L. Mol. Phys. 1996, 87, 485.
(7) Jang, K.; Lemieux, R. P. Mol. Cryst. Liq. Cryst. 1995, 260, 247.
X
(1) Eliel, E. L.; Wilen, S. Stereochemistry of Organic Compounds; John
Wiley & Sons: New York, 1993.
2) Rosini, C.; Scamuzzi, S.; Pisani Focati, M.; Salvadori, P. J. Org.
Chem. 1995, 60, 8289.
3) Gottarelli, G.; Spada, G. P.; Solladi e´ , G. NouV. J. Chim. 1986, 10,
91. Gottarelli, G.; Spada, G. P. Mol. Cryst. Liq. Cryst. 1985, 123, 377.
Solladi e´ , G.; Zimmermann, R. G. Angew. Chem., Int. Ed. Engl. 1984, 23,
48.
4) Gottarelli, G.; Hibert, M.; Samor `ı , B.; Solladi e´ , G.; Spada, G. P.;
Zimmermann, R. J. Am. Chem. Soc. 1983, 105, 7318.
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