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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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calix[4]pyrrole configurational isomer. Additionally, these workers co-
condensed methyl ethyl ketone and acetone with pyrrole to produce a mixed
we^o-hexamethyl-bisethylcalix[4]pyrrole of unknown structure.48 Later
Rothemund and Gage used methanesulfonic acid as the acid catalyst and
obtained improved yields,44 and in 1971 Brown and coworkers reported a
refined procedure that permitted them to obtain
tetraspirocyclohexylcalix[4]pyr-role 2,50 a compound that had previously
been reported by Chelintzev, Tronov and Karmanov in 1916,51 in good
yield. Calix[4]pyrroles with functionalizable groups in the meso
positions have been reported by Lehn and coworkers.52 54 A me,w-
octachloroalkylcalix[4Jpyrrole 3 was synthesized and was subsequently
converted into a we.vo-octacyanoalkyl derivative 4.
III. Structural Studies of Free Ligands
The X-ray crystal structure of 1 was elucidated by Sessler and coworkers
in 19953x and revealed that the macrocycle adopts a 1,3-alternate
conformation in the solid state wherein adjacent rings are oriented in
opposite directions (Figure la). This structure is conformationally
similar to the acetone solvate of[4]pyrrole 555 and
that of the dichloromethane solvate of meso-tetraspirocy-
clohexylcalix[4]pyrrole 238 (Figure lb). In contrast, the structure of p-
tert-butylcalix[4]arene adopts a cone conformation, wherein all the
phenol rings are oriented in the same direction. This conformation is
stabilized by an array of hydrogen bonds between the phenol OH groups.
Clearly no such internal hydrogen bonding array is possible in the case
of calix[4]pyrrole; presumably the 1,3-alternate conformation is adopted
in the solid state due to packing forces in the crystal and as a result
of a need to minimize steric interactions within the macrocycle.

In the solid state, the nitrogen-nitrogen cross-ring distances for
compound 1 are N1-N3 4.824(4) and
45 / Calixpyrroles: Novel Anion and Neutral Substrate Receptors
Figure 1. X-ray crystal structures of (a) meso-octamethylcalix[4]pyrrole
1 and (b) meso-tetraspirocyclohex-ylcalix[4]pyrrole 2-CH2CI2. Crystal
structures originally published in Gale, P. A.; Sessler, J. L.; Krai, V.;
Lynch, /. Am. Chem. Soc. 1996, 7 18, 5140. Diagram produced using data
from the Cambridge Crystallographic Database.
N2-N4 4.838(4) A. The dihedral angles between the root mean square
(r.m.s.) plane delined by the mew-like carbon atoms and the pyrrole rings
are 72.2(2) ; 70.8(2); 70.4(2) ; and 72.6(2) lor pyrrole rings 1 to 4,
respectively. The mew-like carbon atoms are in fact not eoplanar. Rather,
(hey deviate from the r.m.s. plane by 0.203(3) A on average. The C-C-
angles at the mew-like carbons are 109.5 (8) (i.e. the ring system
appears to be strain free). The box formed by the mew-like carbons is
5.028(2) A long and 7.088(3) A diagonally across.
In 2 2 the nitrogen-nilrogen crass-ring distances are NI-N3
4.812(6) and N2-N4 5.177(5) A. In this structure, as in the structure of
compound 1, the mew-like carbon atoms are also not eoplanar (the average
deviation from 1 he r.m.s. plane is 0.130(6) A). The dihedral angles
between the r.m.s. plane through the mew-like carbon atoms and the
pyrrole rings are 58.6(3). 84.1(3), 56.6(3) and 81.2(3) with the
alternation in dihedral angle being due, presumably, to the influence of
the 2 solvent. The average C-C-C angle al the mew-like carbon atoms
is 109.5(11) . The hydrogen atoms of ihe dichloromethane solvent are
2.49(3) A from the centroid of pyrrole rings 1 and 3.
IV. Coordination Chemistry of Simple Systems
The transilion-metal coordinalion chemistry of a number of deprotonated
mew-octaalkyIporphyrinogens, particularly mew-oclaethylcalix[4[pyrrole 5
has been studied in detail by Floriani and coworkers. 2-211 The results
of their extensive work are detailed elsewhere in this book.,ft
Early work by Sessler and coworkers had demonstrated that sapphyrin, a
pentapyrrolic macrocycle lirst synthesized by Woodward and coworkers,-''7
is capable of coordinating to anions when protonaied via a combination of
hydrogen bonding and electrostatic interactions.24 Once Ihe generality of
this result was confirmed by the study of other expanded porphyrins,^
attention turned to the calix[4]pyrroles. mew-Octamethylcalix[4]pyrrole 1
contains four NH groups and is neutral, confomiationally flexible, and
nonaromalic (and therefore quite different in nature to sapphyrin). Thus
it was speculated that it and other calix[4]pyrroles could serve as
anion-binding agents by virtue of possessing hydrogen-bonding-donor
functionality alone.
In order to test (his supposition, the solution-binding properties of
ealix[4]pyrroles I and 2 were studied using 'H NMR titration
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