29 research outputs found
Origin of the Regioselective Reduction of Chlorins
The
reduction of a free-base chlorin generally forms a bacteriochlorin
(BC), while the reduction of the corresponding metallochlorin forms
a metalloisobacteriochlorin (M-iBC). This regioselectivity has been
long known but was never fully rationalized. In the free-base case,
this regioselectivity can be explained using resonance arguments,
but the explanations for the regioselectivity in the metallochlorin
reactions requires a more sophisticated approach. A combination of
DFT-calculated average local ionization energies (ALIEs), thermodynamics
of the products, and the transition-state trajectories of reduction
reactions of <i>meso</i>-tetraaryl- and β-octaethylchlorins,
as their free bases and zinc complexes, now fully delineate the theoretical
basis of the reduction regioselectivity. The reactions are kinetically
controlled. Steric effects originating in the conformational flexibility
of the chlorin macrocycle direct the reactions toward the formation
of iBCs. Only when electronic effects are strong enough to override
the steric effects are BCs formed. Depending on the substituents present
on the chlorin, this regioselectivity may change, but ALIE calculations
provide reliable guidelines to predict this. The practical value of
this work lies in the presentation of a simple predictive method toward
synthetic tetrahydroporphyrins by reduction of chlorins
<i>meso</i>-Tetrakis(pentafluorophenyl)porphyrin-Derived Chromene-Annulated Chlorins
The synthesis of mono- and bis-chromene-annulated <i>meso</i>-(pentafluorophenyl)chlorins from <i>meso</i>-tetrakis(pentafluorophenyl)porphyrins by an OsO<sub>4</sub>-mediated
dihydroxylation reaction, followed by an intramolecular nucleophilic
aromatic substitution reaction, is described. The reaction sequence
is applicable to the free base systems as well as their Zn(II), Ni(II),
Pd(II), and Pt(II) complexes. The optical properties (UV–vis
and fluorescence spectra) of the (metallo)chlorin-like chromophores
that possess slightly red-shifted optical spectra compared to the
corresponding 2,3-dihydroxychlorins are reported. Molecular modeling
and <sup>1</sup>H–<sup>19</sup>F-HOESY NMR spectroscopy provide
indications for the conformation of the chromene-annulated chromophores.
Using <sup>1</sup>H–<sup>1</sup>H COSY and <sup>19</sup>F–<sup>19</sup>F QF-COSY NMR spectra, we interpret the <sup>1</sup>H and <sup>19</sup>F NMR spectra of the porphyrins and chlorins, thus providing
a refined reference point for the use of <sup>19</sup>F NMR spectroscopy
as a diagnostic tool in the analysis of <i>meso</i>-pentafluorophenyl-substituted
porphyrinoids
Reaching across the Divide: How Monometalation of One Binding Pocket Affects the Empty Binding Pocket in a Siamese-Twin Porphyrin Palladium Complex
Siamese-twin porphyrin
is a pyrazole-containing expanded porphyrin incorporating two porphyrin-like
binding pockets. The macrocycle, however, does not possess an aromatic
π system but rather two separated conjugation pathways that
are isolated by the pyrazole junctions. Mono- and bimetallic complexes
of the Siamese-twin porphyrin are known. This work addresses in detail
the electronic consequences that monometalation (with Pd<sup>II</sup>) has on the electronic properties of the nonmetalated binding pocket
by studying the solid-state structure, acid/base, and electrochemical
properties of the monopalladium twin-porphyrin complex. Specifically,
metalation leads to a switch of the protonation sites of the free-base
pocket. The unusual location of the protons at adjacent pyrrolic nitrogen
atoms was revealed using X-ray diffraction and 1D/2D NMR spectroscopy.
The one-electron oxidation and reduction events are both ligand-centered,
as derived by spectroelectrochemical and electron paramagnetic resonance
measurements, but are located on different halves of the molecule.
Single-electron oxidation (−0.32 V vs Fc/Fc<sup>+</sup>) generated
an organic radical centered on the metal-coordinating side of the
ligand, while single-electron reduction (−1.59 V vs Fc/Fc<sup>+</sup>) led to the formation of an organic radical on the free-base
side of the macrocycle. Density functional theory calculations corroborated
the redox chemistry observed. The possibility of selectively preparing
the monometallic complexes carrying two distinct redox sitesa
metal-containing oxidation site and a metal-free reduction sitefurther
expands the potential of Siamese-twin porphyrins to serve as an adjustable
platform for multielectron redox processes in chemical catalysis or
molecular electronics applications
Mono- and Bisquinoline-Annulated Porphyrins from Porphyrin β,β′‑Dione Oximes
An
acid-induced reaction of <i>meso</i>-tetraphenyl-2-hydroxyimino-3-oxoporphyrin
leads, with concomitant loss of water, to a formal electrophilic aromatic
substitution of the ortho-position of the phenyl group adjacent to
the oxime, forming a quinoline moiety. Owing in part to the presence
of a π-extended chromophore, the resulting <i>meso</i>-triphenylmonoquinoline-annulated porphyrin (λ<sub>max</sub> = 750 nm) possesses a much altered optical spectrum from that of
the starting oxime (λ<sub>max</sub> = 667 nm). An oxidative
DDQ-induced ring-closure process is also possible, generating the
corresponding <i>meso</i>-triphenylmonoquinoline-annulated
porphyrin quinoline <i>N</i>-oxide, possessing a slightly
shifted and sharpened UV–vis spectrum (λ<sub>max</sub> = 737 nm). The connectivity of the chromophores was conclusively
shown by NMR spectroscopy. Both ketone functionalities in <i>meso</i>-tetraphenyl-2,3-dioxoporphyrin can be converted, via
the oxime and using the acid- or oxidant-induced reaction pathways,
either in one step or in a stepwise fashion, to bisquinoline-annulated
porphyrin (λ<sub>max</sub> = 775 nm) and its <i>N</i>-oxide (λ<sub>max</sub> = 779 nm), respectively. This process
is complementary to a previously established pathway toward bisquinoline-annulated
porphyrins. Their zinc(II), nickel(II), and palladium(II) complexes
are also described. Several examples of the quinoline-annulated porphyrins
were crystallographically characterized, proving their connectivity
and showing their conformations that are extremely distorted from
planarity. The work presents a full account on the synthesis, structure,
and spectroscopic properties of these classes of NIR-absorbing dyes
Fusion and Desulfurization Reactions of Thiomorpholinochlorins
An unusually nonplanar, ruffled structure
that had been suspected
for the previously reported [2,3-bismethylenethiomorpholinochlorinato]nickel(II)
complex was confirmed by determination of its crystal structure. Treatment
of this thiomorpholinochlorin with acid converts the exocyclic double
bonds to direct links to the <i>ortho</i>-positions of both
adjacent <i>meso</i>-phenyl groups. The crystal structure
of this product indicated that the introduction of these linkages
did not change the overall conformation of the macrocycle. The reactivity
of the bis-linked thiomorpholine moiety with respect to Raney-nickel-induced
(hydro)desulfurization reactions was probed, forming a bis-phenyl-linked
2,3-dimethylchlorin, also characterized by X-ray diffraction, and
a bis-indene-annulated porphyrin. We also report on the synthesis
of the oxygen analogue to the bis-linked thiomorpholine by reaction
of a secochlorin bisketone nickel complex with Woollins’ reagent.
We thus introduce novel methodologies toward the synthesis of porphyrinoids
carrying β-to-<i>ortho</i>-phenyl fusions and expand
on the scope and limits of the chemistry and interconversion of pyrrole-modified
porphyrins
Reaching across the Divide: How Monometalation of One Binding Pocket Affects the Empty Binding Pocket in a Siamese-Twin Porphyrin Palladium Complex
Siamese-twin porphyrin
is a pyrazole-containing expanded porphyrin incorporating two porphyrin-like
binding pockets. The macrocycle, however, does not possess an aromatic
π system but rather two separated conjugation pathways that
are isolated by the pyrazole junctions. Mono- and bimetallic complexes
of the Siamese-twin porphyrin are known. This work addresses in detail
the electronic consequences that monometalation (with Pd<sup>II</sup>) has on the electronic properties of the nonmetalated binding pocket
by studying the solid-state structure, acid/base, and electrochemical
properties of the monopalladium twin-porphyrin complex. Specifically,
metalation leads to a switch of the protonation sites of the free-base
pocket. The unusual location of the protons at adjacent pyrrolic nitrogen
atoms was revealed using X-ray diffraction and 1D/2D NMR spectroscopy.
The one-electron oxidation and reduction events are both ligand-centered,
as derived by spectroelectrochemical and electron paramagnetic resonance
measurements, but are located on different halves of the molecule.
Single-electron oxidation (−0.32 V vs Fc/Fc<sup>+</sup>) generated
an organic radical centered on the metal-coordinating side of the
ligand, while single-electron reduction (−1.59 V vs Fc/Fc<sup>+</sup>) led to the formation of an organic radical on the free-base
side of the macrocycle. Density functional theory calculations corroborated
the redox chemistry observed. The possibility of selectively preparing
the monometallic complexes carrying two distinct redox sitesa
metal-containing oxidation site and a metal-free reduction sitefurther
expands the potential of Siamese-twin porphyrins to serve as an adjustable
platform for multielectron redox processes in chemical catalysis or
molecular electronics applications
Octaethyl-1,3-oxazinochlorin: A β‑Octaethylchlorin Analogue Made by Pyrrole Expansion
Treatment
of the oxime of octaethyloxochlorin <b>4</b>, available
from octaethylporphyrin <b>3</b>, under Beckmann conditions
provided not the expected lactam, but octaethyl-1,3-oxazinochlorin <b>8</b>, in which a pyrrole moiety of the parent oxochlorin was
expanded by an oxygen atom to an 1,3-oxazinone moiety. Its mechanism
of formation was demonstrated to occur along an “abnormal Beckmann”
pathway, followed by intramolecular ring closure and hydrolysis. The
work expands the methodologies known to convert octaethylporphyrin
to pyrrole-modified porphyrin analogues
Fusion and Desulfurization Reactions of Thiomorpholinochlorins
An unusually nonplanar, ruffled structure
that had been suspected
for the previously reported [2,3-bismethylenethiomorpholinochlorinato]nickel(II)
complex was confirmed by determination of its crystal structure. Treatment
of this thiomorpholinochlorin with acid converts the exocyclic double
bonds to direct links to the <i>ortho</i>-positions of both
adjacent <i>meso</i>-phenyl groups. The crystal structure
of this product indicated that the introduction of these linkages
did not change the overall conformation of the macrocycle. The reactivity
of the bis-linked thiomorpholine moiety with respect to Raney-nickel-induced
(hydro)desulfurization reactions was probed, forming a bis-phenyl-linked
2,3-dimethylchlorin, also characterized by X-ray diffraction, and
a bis-indene-annulated porphyrin. We also report on the synthesis
of the oxygen analogue to the bis-linked thiomorpholine by reaction
of a secochlorin bisketone nickel complex with Woollins’ reagent.
We thus introduce novel methodologies toward the synthesis of porphyrinoids
carrying β-to-<i>ortho</i>-phenyl fusions and expand
on the scope and limits of the chemistry and interconversion of pyrrole-modified
porphyrins
[3 + 2]-Cycloadditions with Porphyrin β,β′-Bonds: Theoretical Basis of the Counterintuitive <i>meso</i>-Aryl Group Influence on the Rates of Reaction
Removal of a β,β′-bond from meso-tetraarylporphyrin using [3 + 2]-cycloadditions generates meso-tetraarylhydroporphyrins. Literature evidence indicates
that meso-tetraphenylporphyrins react more sluggishly
with 1,3-dipoles such as ylides and OsO4 (in the presence
of pyridine) than meso-tetrakis(pentafluorophenyl)porphyrin.
The trend is counterintuitive for the reaction with OsO4, as this formal oxidation reaction is expected to proceed more readily
with more electron-rich substrates. This work presents a density functional
theory–based computational study of the frontier molecular
orbital (FMO) interactions and reaction profile thermodynamics involved
in the reaction of archetypical cycloaddition reactions (a simple
ylide, OsO4, OsO4·py, OsO4·(py)2, and ozone) with the β,β′-double bonds
of variously fluorinated meso-arylporphyrins. The
trend observed for the Type I cycloaddition of an ylide is straightforward,
as lowering the LUMO of the porphyrin with increasing meso-phenyl-fluorination also lowers the reaction barrier. The corresponding
simple FMO analyses of Type III cycloadditions do not correctly model
the reaction energetics. This is because increasing fluorination leads
to lowering of the porphyrin HOMO–2, thus increasing the reaction
barrier. However, coordination of pyridine to OsO4 preorganizes
the transition state complex; lowering of the energy barrier by the
preorganization exceeds the increase in repulsive orbital interactions,
overall accelerating the cycloaddition and rationalizing the counterintuitive
experimental findings
Oxazolochlorins. 9. <i>meso</i>-Tetraphenyl-2-oxabacteriochlorins and <i>meso</i>-Tetraphenyl-2,12/13-dioxabacteriochlorins
The
formal replacement of one or two pyrrole groups in <i>meso</i>-tetraphenylporphyrin by oxazole moieties is described,
generating inter alia the bacteriochlorin-type chromophores oxazolobacteriochlorins
(oxabacteriochlorins) and bisoxazolobacteriochlorins (dioxabacteriochlorins).
The key step is the conversion of a β,β′-dihydroxy-functionalized
pyrroline group into an oxazolone or (substituted) oxazole. Depending
on the substitution pattern on the oxazole or oxazoline moieties,
mono- and dioxabacteriochlorins may have chlorin- or bacteriochlorin-like
spectra. The optical properties (as measured by UV–vis and
fluorescence spectroscopies) of the novel oxa- and dioxabacteriochlorins
are described and contrasted against benchmark chlorins and bacteriochlorins.
The conformations of a representative number of mono- and dioxabacteriochlorins,
as their free bases or Zn<sup>II</sup> complexes, were determined
by single-crystal X-ray diffractometry. They proved to be essentially
planar, showing that the modulation of their optical properties is
primarily due to their intrinsic electronic structures and electronic
substituent effects and are not largely affected by conformational
effects. The mono- and bisoxazolobacteriochlorins are a novel class
of readily prepared and oxidatively stable chlorin and bacteriochlorin
analogues with tunable optical spectra that, in part, reach into the
NIR