A simple, straightforward [2+1] condensation of 5,6-diaryldipyrroethene dicarbinols with pyrrole under mild acid-catalyzed conditions resulted in the formation of highly desirable aromatic β-free meso-tetraaryl [14]triphyrins(2.1.1) in 15-18% yields. The triphyrins(2.1.1) are very novel monoanionic tridentate ligands that form metal complexes readily as demonstrated here by preparing Re(I) complexes.
Functionalized meso-tetraaryl triphyrins(2.1.1) containing two meso-iodophenyl groups or a bromo group at the β-pyrrole carbon were synthesized over a sequence of steps.
Three
novel doubly fused unsymmetrical calixdicarbahexaphyrins
were synthesized by mild acid-catalyzed (4+2) condensation of dicarbatetrapyrrane
with dipyrroethene diol followed by oxidation. The condensation formed
doubly fused calixdicarbahexaphyrins instead of π-conjugated
dicarbahexaphyrins, due to the unusual fusion of the pyrrole N with
the α-carbon of the adjacent pyrrole ring to form a tripentacyclic
ring and one usual fusion of the pyrrole N with the adjacent phenylene
C to form a fused moiety containing two pentacycles and one hexacycle
ring. Both fusions occurred on one side of the macrocycle, making
the macrocycles unsymmetric. The crystal structure obtained for one
of the macrocycles exhibited a saddle-shaped structure with two benzene
rings and four pyrrole rings connected via two ethylene and four methene meso-carbon atoms. The crystal structure also revealed unusual
fusions in the macrocyclic framework and the presence of one sp3 carbon that disrupts the π-electron delocalization. 1H, 1H–1H COSY, NOESY, 13C, and HMBC NMR techniques were used to characterize the macrocycles.
The absorption spectra of the macrocycles showed one intense sharp
band at ∼485 nm along with a shoulder in the lower-energy region,
suggesting its non-aromatic nature. Electrochemical studies indicated
their electron rich nature, and DFT/TD-DFT studies corroborated the
experimental observations.
Various
core-modified tellurophene-containing pentaphyrin(2.1.1.1.1)s
were synthesized via (3 + 2) condensation of 16-telluratripyrrane
with different heterodiols under mild acid catalyzed conditions in
5–12% yields. The formation of pentaphyrin (2.1.1.1.1) with
a N2O2Te core was not successful due to its
inherent instability. The new pentaphyrins were characterized and
studied by HR-MS, 1D and 2D NMR, X-ray crystallography for one of
the pentaphyrins, absorption and DFT/TD-DFT techniques. The NMR studies
indicated their nonaromatic nature. The X-ray structure obtained for
pentaphyrin(2.1.1.1.1) with N4Te core revealed that the
macrocycle exists in a highly distorted nonplanar structure. The DFT
studies showed that the macrocycles are nonaromatic and exists in
highly distorted nonplanar geometry. Furthermore, as the core heterocyclic
groups at ethene moiety were changed from pyrrole to furan to thiophene
to benzene, the macrocycles tended toward more planar structures.
The absorption spectra of pentaphyrins showed one strong sharp band
in the region of 450–540 nm along with a broad band in the
region of 700–800 nm. The pentaphyrin(2.1.1.1.1) with N4Te
core upon protonation showed distinct color change in solution and
large bathochromic shifts in absorption bands with an absorption in
the NIR region.
A series of hetero analogues of pentaphyrins(2.1.1.1.1) such as oxapentaphyrins(2.1.1.1.1) and thiapentaphyrins(2.1.1.1.1) were synthesized by 3 + 2 condensation of dipyrroethenedicarbinol with 16-oxatripyrrane/16-thiatripyrrane under mild acid-catalyzed reaction conditions. The stable macrocycles are freely soluble in organic solvents, and their identities were confirmed by a corresponding molecular-ion peak in highresolution mass spectrometry, 1D and 2D NMR spectroscopy, and X-ray structure obtained for one of the oxapentaphyrin(2.1.1.1.1) macrocycles. The crystal structure and NMR studies indicated that the heterocyclic ring, such as furan in oxapentaphyrins(2.1.1.1.1) and thiophene in thiapentaphyrins(2.1.1.1.1), was inverted. In absorption spectra, the macrocycles showed one sharp band at ∼516 nm and one broad band at ∼744 nm. The spectral and X-ray studies supported the nonaromatic nature of these macrocycles. This is in contrast to the recently reported aza analogue of pentaphyrins(2.1.1.1.1), which showed antiaromatic behavior. Upon protonation, the core-modified pentaphyrins(2.1.1.1.1) macrocycles exhibited bathochromically shifted absorption bands with a distinct change in the color of the solution. The 1 H NMR, nucleus-independent chemical shift, and anisotropy-induced current density studies indicated the presence of Mobius aromaticity in the protonated macrocycles. The core-modified pentaphyrins(2.1.1.1.1) can act as good coordinating ligands, as shown here by synthesizing a bis(difluoroborane) complex of one of the thiapentaphyrins(2.1.1.1.1).
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