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The porphyrin handbook - Kadish K.M.

Kadish K.M. The porphyrin handbook - Academic press, 2000. - 368 p.
Download (direct link): kadishsmishgulilard2000.djvu
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NH groups of the bulky calixpyrrole 6.
Furusho and Aida have recently shown that crystals of wc'.vi9-
octaethylcalix[4]pyrroIe can undergo reversible exchange of solvent.'7
Crystals of 5-MeOH, 5-2,
264
Sessler and Gale
Figure 11. Crystal packing diagrams of 5 without guest (a), 5-MeOH (b)
and 5-2'PrOH (c). (Reprinted with permission from Chem. Commun. 1997,
2205. Copyright 1997 The Royal Society of Chemistry.)
5-HOCH2OH, 5-HOCH2CH2OH, 5-jPrOH and 5 without solvent were prepared and
it was found that when solvent-free crystals of 5 were powdered and
exposed to an alcoholic vapor, the vapor would include in the crystal (as
judged from powder X-ray diffraction data). Additionally, Furusho and
Aida found that substrate exchange would take place when crystals
containing an initial guest alcohol were powdered and exposed to the
vapor of a different alcohol (Figure 11). Conversion between 5, 5-MeOH,
5-2 and
5-2'PrOH was found to occur reversibly without crystal decomposition.
When solvent-free crystals of 5 were exposed to a mixed methanol/ethanol
vapor, inclusion of methanol was found to occur selectively unless the
mole fraction of methanol fell below 30%. Under these latter conditions
the selectivity was found to be reversed, with ethanol being exclusively
included into the calix[4]pyrrole
crystals. Thus a type of allosteric effect, presumably due to packing
effects in the crystalline phase, was observed that appears to have a
profound effect on the kind of solvent included in the crystal.
The complexes of calixpyrroles with neutral substrates characterized
so far have the neutral guest molecule bound in an exo-fashion, that is,
outside the aromatic cavity of the macrocycle. This mode of binding, seen
also with anionic guests, contrasts with the many neutral complexes of
calixarenes, a class of host that generally binds neutral guests within
the bucket-shaped cavity of the receptor.61 The propensity for
calixarenes to form endo-complexes with neutral species may be attributed
to solvatophobic effects providing a driving force for inclusion and the
inherent stability of the cone conformation of the macrocycle (due either
to a hydrogen-bonding array, in the case of the parent macrocycles, or to
steric effects in lower-rim-substituted derivatives) "preorganizing" the
aromatic cavity. Such a stabilization is not present in the case of the
calixpyrroles, a conclusion that is perhaps best underscored by the fact
that exo-complexes have been observed exclusively so far.
Synthesis of Functionalized Calixpyrroles
10
A. ^-SUBSTITUTED SYSTEMS
Several calix[4]pyrrole molecules containing functional groups appended
to the carbon- or C-rim of the calix[4]-pyrrole have recently been
synthesized. These, were prepared using one of two synthetic strategies.
The first involves a direct condensation approach and was used to
synthesize //-octamethoxy-me.vo-tetraspirocyclohexylca-lix[4]pyrrole 7
from 3,4-dimethoxypyrrole and cyclo-hexanone.62 The resulting electron-
rich calixpyrrole was then purified by column chromatography and isolated
in 8% yield.
The second strategy involves modifying the C-rim of a presynthesized
calix[4]pyrrole. Specifically in this case, meso-
octamethylcalix[4]pyrrole 1 was dissolved in dry THF and cooled to - 78
C before being treated with four equivalents of n-butyllithium and four
equivalents of ethyl
45 / Calixpyrroles: Novel Anion and Neutral Substrate Receptors
265
bromoacetate. Purification by column chromatography afforded two
products, a monoester 8 ("/i-hook") in 26% yield and a diester 9 in 3%
yield.
In chemistry somewhat related to the above, /i-octa-bromo-w<'.vo-
octamethylcalix[4]pyrrole 10 was synthesized in 90% yield by reacting
wc'.v<?-octaniethylcalix[4|pyrroIe with iV-bromosuccinimide in hot, dry
THF. In this instance, an X-ray structure (Figure 12) revealed the
calixpyrrole product (10) exists in a chair-like flattened 1.2-alternate
conformation in the solid state (that is, the dihedral angles between
pyrrole rings and plane through the calixpyrrole
Figure 12. X-ray crystal structure of /(-oclabromo-meso-octamethyl-
calix[4]pyrrole compound 10. The molecule adopts a chair-like flattened
1,2-alternate conformation in the solid state. Crystal structure
originally published in Gale, P. A.; Sessler, |. L.; Allen, W. E.;
Tvermoes, N. A.; Lynch, V. Chem. Commun. 1997, 665. Diagram produced
using data from the Cambridge Cryslallographic Database.
me.w-carbon atoms are 66.8 . 5.8 , - 66.8 and -5.8 , respectively).62
B. meso-SUBSTITUTED SYSTEMS
Functional groups may also be appended to the meso-positions of
caIix[4]pyrroIes by co-condensing a keto-ester with pyrrole and acetone.
For example, a "we.w-hook" eaIix[4Jpyrrole monoester 11 was synthesized
by co-condensing methyI-4-acetyIbutyrate, cyclohexanone and pyrrole.62'
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