J. Am. Chem. Soc. 1998, 120, 11627-11632
11627
Induced Circular Dichroism and UV-Vis Absorption Spectroscopy of
Cyclodextrin Inclusion Complexes: Structural Elucidation of
Supramolecular Azi-adamantane (Spiro[adamantane-2,3′-diazirine])
Daniel Krois and Udo H. Brinker*
Contribution from the Institut f u¨ r Organische Chemie, UniVersit a¨ t Wien, W a¨ hringer Strasse 38,
A-1090 Wien, Austria
ReceiVed July 1, 1998
Abstract: The first induced circular dichroism (ICD) analyses of diazirine@cyclodextrin inclusion complexes
are reported. The stoichiometries and association constants of the guest@host complexes with R-, â-, and
γ-cyclodextrin were determined. In addition, with the R-cyclodextrin complex, UV-vis spectroscopy of water-
ethanol solutions showed remarkable fine structure, probably indicating that the diazirine experiences a nonpolar
microenvironment. These analytical methods provide details about the architecture and nature of these
supramolecular carbene precursors.
In recent years, the chemistry of reactive species entrapped
within host molecules has become the object of much research
activity.1 One strategy makes use of the unique ability of the
cyclodextrins to modify the physical and chemical properties
of guest molecules by molecular encapsulation. We have
succeeded in preparing molecular noncovalent host-guest
motion-restrictive, three-dimensional cyclodextrin inclusion
complexes. This may give insight into how cyclodextrins
modify the reactivities of carbenes generated in the supramo-
lecular state. The exact stoichiometry and association constants
for the formation of the inclusion complexes is of rudimentary
concern. A number of spectroscopic methods have been applied
2
inclusion complexes of various diazirines with cyclodextrins
for the measurement of association constants of complexes with
2
b,3
4
6,7
and zeolites.
Diazirines are increasingly popular precursors
cyclodextrin as host molecules.
The chromophore of an
for the generation of carbenes.5 The cylodextrin hosts have been
used to modify intramolecular reactions of the entrapped
carbenes in order to manipulate product formation. The
achiral guest may exhibit an induced circular dichroism (ICD)
in the chiral environment of a cyclodextrin. The ICD has been
2
8,9
used to determine association constants in general and for
10
reaction products obtained after photolysis of these supramo-
lecular complexes in the solid state differed considerably, in
some cases, from the results obtained when only physical
cyclodextrins in particular, even though the effects in the latter
1
0,11
case are often very weak.
Until now, to our knowledge, an
ICD of diazirines has never been observed, or used for the
determination of association constants.
2
a,b,d
mixtures of diazirines and host molecules were used.
It is of particular interest to obtain more detailed information
about the microenvironment of the diazirine within different
Because the solubility of the guest and also that of the
complexes in water is generally poor, a solvent system with
better solubility for all components had to be used in order to
collect data for a wider range of different concentrations of the
components. This is necessary for a reliable determination of
association constants. A mixture of water - ethanol (30% v/v)
was chosen as the system most suited for the formation of the
complexes between azi-adamantane (spiro[adamantane-2,3′-
(
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1
009. (d) Smith, D. R. Chem. Ind. 1994, 14. (e) Dowden, J.; Kilburn, J.
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(
2a,c,12
Rosenberg, M.; Poliks, M. D.; Orlando, M.; Gross, M. L. Angew. Chem.,
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diazirine]) (1)
and R-, â-, and γ-cyclodextrin (6-, 7-, 8-Cy),
respectively. For each of the three complexes, an ICD spectrum
with a distinct fine structure could be observed in the absorption
region of the diazirine chromophore (300-400 nm) which
increased in intensity with increasing concentration of the
components. While the sign of the ICD Cotton effect for 1@-
1
995, 1721. (c) Rosenberg, M. G.; Kam, S. M.; Brinker, U. H. Tetrahedron
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2
1
1
9.
(
16, 7393.
(
3) Kupfer, R.; Poliks, M. D.; Brinker, U. H. J. Am. Chem. Soc. 1994,
4) (a) Chemistry of Diazirines; Liu, M. T. H., Ed.; CRC: Boca Raton,
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(9) For a detailed investigation concerning the applicability of the ICD
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Chem. Soc., Perkin Trans. 2 1995, 489. Krois, D.; Lehner, H. Monatsh.
Chem. 1995, 126, 349.
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(
d) Carbenes; Jones, M., Jr.; Moss, R. A., Eds.; Wiley: New York, 1973,
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0.1021/ja982299f CCC: $15.00 © 1998 American Chemical Society
Published on Web 10/29/1998