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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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found that the expanded calix[5]pyrrole-calix[5]arene pseudo dimer 19 was
formed in 10% yield as the result of a
-,-/ ......... ..... .. ..........-r-
12.0 11.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0
ppm
Figure 24. 'h NMR spectrum of compound 18 recorded in dichloromethane-d2.
The pyrrole-NH protons are deshielded and resonate at 11.22 ppm, a
finding that is consistent with the formation of hydrogen bonds.
(Reprinted from Tetrahedron Letters, Gale, P. A.; Sessler, J. L.; Lynch,
V.; Sansom, P. I., "Synthesis of a New Cylindrical Calix[4]arene-
Calix[4]pyrrole Pseudo Dimer/' 1996, 37, 7881-7884. Copyright (c) 1996,
with permission from Elsevier Science.)
BF3-OEt2 catalyzed condensation. Compound 19 constitutes the first
example of a higher- order calixpyrrole to be fully characterized.79
The 'H NMR spectrum of 19 in dichloromethane-^ is very similar to that
of the calix[4]arene-calix[4]pyrrole pseudo-dimer; however the NH protons
now resonate at 9.88 ppm, a finding that indicates possibly weakened
internal hydrogen-bonding interactions. Addition of metha-nol-d4 (50%) to
the NMR solution causes the NH protons of
19 to shift downfield to 10.46 ppm and is consistent with this solvent
serving to interfere with the internal hydrogen-bonding array in 19.
Similarly, addition of five equivalents of tetrabutylammonium chloride to
19 causes a 0.28 ppm shift in the NH-proton resonance, a finding that
supports the conclusion that the lower (oxygen) rim hydrogen-bonding
array in compound 19 is considerably weaker than that in compound 18.
IX. From Calixpyrroles to Calixpyridines
A very versatile and easy entry into the chemistry of calixpyridines,
recently reported,80 is predicated on the use of calixpyrroles as
starting materials. This route, illustrated in Scheme 2, involves
reaction of a cal ix [4 J pyrrole with dichlorocarbene and provides a
ready access into the previously unknown calix[3]pyridino[l]pyrrole and
calix-[4]pyridine families. It also allows for the facile synthesis of
calix[ 1 ]pyridino[3]pyrroles and calix[2Jpyridino[2]pyr-roles, species
that Floriani and coworkers have also produced from calix[4]pyrroles
using alternative organo-metallic-based procedures.80
It was found that when me so - octamet hy leal ix [ 41 py rrol e
(1) was reacted with 15 molar equivalents of sodium trichloroacetate (the
dichlorocarbene source), a 2.4:1 mixture of the mono- and dipyridine
macrocycles (21 and 22) was produced.81 When the same reaction conditions
were employed using 1,2-dimethoxyethane as the solvent, a mixture of di-
(22a or 22b), tri- (23) and tetrapyridine (20) species was obtained in a
1 : I : 1 ratio. Improved yields of
i
1
45 / Calixpyrroles: Novel Anion and Neutral Substrate Receptors
273
Scheme 2. Synthesis of chloro-derivatives of calixpyridinopyrroles 21,
22, and 23 and calix[4]pyridine 20 from meso-octamethylcalix[4]pyrrole
using a carbene insertion strategy. The chlorine atoms may be attached to
each pyridine ring either in position a or b.
:CCI2
-HCI
or
the latter products could be obtained by adding separate batches of the
dichlorocarbenc precursor. In fact, using this stepwise-addition
approach, the reaction process could be made to favor, as desired, the
formation of either 22 and 23, or just 20.
The X-ray crystal structure of 22a (Figure 25) revealed that there are
two crystallographically distinct molecules in the unit cell. Both adopt
the cone conformation in the solid state (the dihedral angles between
pyrrole and pyridine rings and plane through the calixpyridinopyrrole
mew-carbon atoms are 20.1 (2)p>n, 66.9(2)pvridmo, 23.2(2)pyir and 67.1(2)
pyndinc for one molecule and 21.8(1 )P>IT, 69.4 (2)pv,,d"K-, 17.7 (3)p>".
and 67.9(2) p>nJm, for the second). The pyrrolic-NH groups are oriented
towards the pyridine nitrogen atoms such that potential NH - N hydrogen
bonds may be formed. While not yel investigated in detail, these putative
hydrogen bonds could be serving to influence the conformational
properties of the macrocycle.
The X-ray crystal structure of 23 has also been elucidated (Figures
26a and b).M As in the case of 22. there are two molecules of 23 per
asymmetric unit. However unlike 22, where the two molecules were found to
be conformationally equivalent, the two molecules of 23 assume strikingly
different conformations. In the first of these disparate molecules, the
molecular conformation is similar to that found in compound 20 (see
below) in that alternate rings are either parallel or nearly
perpendicular (compare Figure 26a). The dihedral angles between the rings
and the plane through the bridging mew-like carbon atoms are 81.5(2),
74.9(2), 85.1(2) and 7.3(2)' for rings I to 4, respectively. In this
first molecule, the pyrrole-NH group is not hydrogen bonded to any of the
pyridine nitrogen atoms. The other molecule (Figure 26b) in the
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