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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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interaction modes. Interesting results on the strength and features of
the 1CD derived from complex formation of achiral multifunctional
porphyrins and chiral substrates have been accumulated. In the case of
amino-acid esters, the strength and feature of the LCD depend upon the
number of interactions, the amino-acid side chain, and the binding
constant (Figure 5). However, multichiral compounds such as saccharides
show a complicated spectral pattern.22 More elaborate theoretical
treatments of 1CD are required to clarify the relationship of the 1CD
spectrum and interaction between the large chromo-phore and a chiral
compound.
Solubility of porphyrins and metalloporphyrins in organic solvents or
water is one of the crucial problems in molecular spectroscopic
investigations. Such solubility of porphyrin hosts in organic solvents
can be controlled by auxiliary modification of wtwo-aryl groups. Highly
polar porphyrins prepared from condensation of pyrrole and an
arylaklehyde having hydroxy or carboxylate groups are usually insoluble
in organic solvents. Alkylation of meso-aryl groups improves markedly the
solubility of the resulting porphyrin in organic solvents, as exemplified
in 8.2i~2:i Auxiliary modification enables one to solubilize polar
porphyrins and their corresponding metalloporphyr-
CD active CD inactive
Figure 5. Recognition of amino-acid esters by (a) bifunctional and (b)
monofunctional porphyrin receptors. Only ditopic complexes, which fix the
guest orientation rigidly, showed induced CD in the Soret band.
ins, even in hexane, by spectroscopic techniques that monitor chemical
changes due to molecular recognition. Atropisomerization in a nonpolar
solvent has been noted for two particular porphyrins, w<?.sr>tetrakis-(2-
hydroxy-4-non-y!phenyl)porphyrin 8 and |_//j<?.w-tetrakis(2-carboxyl-4-
non-ylphenyl)porphyrinato]zinc 9. Addition of tetramethoxy-benzoquinone
to a solution of 8 causes dramatic changes in the population of isomers
from the initial statistical ratio of isomers. Conversion into the
preferred a,a,a.a-atropisomer proceeds via formation of a stable host-
guest complex with a face-to-face orientation via multihydrogen bonding.
Thermal atropisomerization of 9 in toluene gives rise to dimeric self-
assembly of the <x,a.".a-isomer of 9 which leads to formation of an
inclusion complex with pyrazine derivatives.26
It should be noted that condensation of pyrrole and chiral cyclopropyl
derivatives affords the .//,,/ isomer of meso-tetrakis-(m(p)-
nitrophenylchiro)porphyrin 10 as the sole product. Coordination of 3,5-
lutidine to the zinc complex of 10 causes complete conversion from the
,/?,,/^ isomer into the ",,, isomer due to specific interaction of
the amine and peripheral groups. X-ray studies indicate a domed structure
for the zinc porphyrin core for the pyridine adduct.27
Shinkai and coworkers reported porphyrins bearing boronic acid, which
bind sugars via covalent bonds. M-2S Although these systems do not
strictly involve molecular recognition, it is demonstrated that they are
good sensory systems for sugars in water.
On the other hand, water-soluble porphyrins have usually been
obtainable by introduction of polar groups such as amino, carboxylate and
sulfate groups to the terminal positions of the meso and /?-pyrrolic
substituents. Aggregation phenomena should be taken into account in host
and guest chemistry in aqueous solution.
Divergent functional groups are readily introduced at the meso and/or
/^-positions by: (1) condensation of [i-substituted pyrroleaklehydes; (2)
condensation of pyrrole with aldehydes having terminal functional groups;
and (3) condensation of pyrrole with p- or m-substituted benzalde-hydes.
Utilization of divergent terminal functional groups
10
284
Ogoshi et al.
enables one to synthesize water-soluble porphyrins and dendrimer
porphyrins.
Using these synthetic strategies, a variety of monofunctional
porphyrins,14-24-32 difunctional porphyrins,33-34 trifunctional
porphyrins,35-36 and tetrafunctional porphyrins37 34 have been reported.
D. STRAPPED PORPHYRINS
A variety of strapped porphyrins have been prepared as biomimetic models
of heme proteins and light-harvesting systems. Most functional groups in
the bridged strap were aimed at fixing the axial ligand to
metalloporphyrin and at stabilizing dioxygen complexes of
metalloporphyrins. In earlier work, porphyrins strapped with
nonfunctional polymethylene chains were prepared to control the axial
ligation of amines to central metal ions by steric hindrance between the
bridged linkage and ligand.40 Single and double-strapped porphyrins with
functional and nonfunctional groups have been synthesized either by
bridging across /i-pyrrolic positions or via functional groups present at
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