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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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various noncharged substrates was investigated.35 Polyfluorobiphenyls are
compounds that serve as models for the analogous, albeit much more toxic,
polychlorobiphenyls (PCBs). As illustrated in Figure 22, mixtures of
these fluorinated species are readily separated on column with the more
highly substituted substrate, perfluorobiphenyl, being eluted last
(similar results were obtained with column M). Such findings are
consistent with those from a mechanism wherein separation is achieved, at
least in part, on the basis of hydrogen-bonding interactions. In
particular, the increased polarization arising from the presence of
electronegative fluorine substituents allows for increased calixpyrrole-
to-substrate interactions analogous to those seen in the case of
polyanionic oligonucleotide species.
Table 7. Stability Constants (M ') for Compounds 16 and 17 with Anionic
Substrates in Dichloromethane-d2 at 298K
Anion0 K"(M ')
Compound 16 Compound 17
Chloride 415 (45) 405 (10)
Dihydrogenphosphate 62 (6) 80 ( 15)''
Hydrogensulfate < 10 < 10
Source: Data from Sessler, J. L.; Gale. P. A.; Genge, J. W. Chem. Ear. J.
1998. 4.
um.
"Anions were added as 0.1 M dichloromethanc-Ji solutions of their tetra-
butylammonium salts to 10 mM solutions of the receptor in
dichloromcthane-r/i with concentration changes being accounted for by
EQNMR. In determining the stability constants, the possible effects of
ion pairing (if any) were ignored. hEstimated value. The NH-proton
resonance broadened considerably during the titration, forcing the
frequency of the resonance to be noted manually. This value should,
therefore, be treated with caution.
15.5 18.7 35.0 47.2
Elution Time (min.)
Figure 22. Separation of polyfluorobiphenyls on a calixpyrrole-modified
silica gel column, B, Flow rate 0.2ml/min., mobile phase 76/24 H20/ CH3CN
(v/v), column temperature 25 C, UV detection at 254 nm. (Reprinted with
permission from Chem. Eur.}. 199B, 4,
1095. Copyright (c) 1998 Wiley-VCH.)
VIII. Expanded Calixpyrroles
Today it is possible to synthesize calix[n]arenes with 4 <n< 11.71-74
However, this is not the case for calixpyrroles. Indeed, before recent
work in this area by Sessler et al, calix [] pyrroles with n>4 were
(apparently) unknown. It was quickly found that such species were often
byproducts of condensations leading to the generation of
calix[4]pyrroles, and that they were difficult to isolate cleanly.75
Therefore a template-type strategy was explored that might allow the
synthesis and isolation of a higher-order calixpyrrole. To test this
strategy, /?-fc/Y-butylcalix[4]arene tetramethyl ketone76 was condensed
with pyrrole in the presence of methanesulfonic acid. This afforded the
cylindrical calix[4]arene-calix[4]pyiTole pseudo dimer 18 in 32% yield.77
The X-ray structure of 18 revealed, as expected, that the calixarene
adopts a cone conformation in the solid state. It also served to show
that two of the pyrrole-NH groups form hydrogen bonds to the phenolic
oxygen atoms at the lower rim of the calixarene (Figure 23). These
noncovalent interactions, in turn, act to induce a level of structural
rigidity that reinforces that which was established by the covalent
skeleton.
The hydrogen bonding between the pyrrole-NH groups and phenolic oxygen
atoms is maintained in solution as
16
17
272
Sessler and Gale
18 19
Figure 23. X-ray crystal structure of the calix[4]arene-calix[4]pyrrole
pseudo-dimer 18. Two pyrrole-NH groups form hydrogen bonds to the
phenolic oxygen atoms at the lower rim of the calixarene. Crystal
structure originally published in Gale, P. A.; Sessler, J. L.; Lynch, V.;
Sansom, P. I. Tetrahedron Lett. 1996, 37, 7881. Diagram produced using
data from the Cambridge Crystallographic Database.
evidenced by the 'H NMR spectrum (Figure 24). The pyrrole-NH protons
resonate at 11.22 ppm, a deshielded value that is consistent with their
being involved in the proposed hydrogen bonds. Addition of polar solvents
such as methanol-c/., or anions such as fluoride does not serve to
disrupt the hydrogen-bonding array (as judged from 'H NMR studies). This
leads to the inference that molecule
18 retains a cylindrical conformation in solution under a wide range of
conditions.
The next step in testing this strategy was to use a larger
calix[n]arene polyketone (n > 4) as the template. While odd-numbered
calix["]arenes are more challenging to synthesize than calixarenes
containing even numbers of phenolic residues, recent advances have
allowed various calix[5]ar-enes, including /?-ferf-butylcalix[5]arene
pentamethyl ketone, to be synthesized in gram quantities.72-78 Using this
latter substance as the starting material (along with pyrrole) it was
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