Phthalocrowns: Isoindoline–Crown Ether Macrocycles

The reaction of diiminoisoindoline with amine-terminated polyethers results in the formation of phthalocrown macrocycles. For <i>n</i> = 1 (where <i>n</i> is the number of ether units), a 2 + 2 condensation takes place, but for <i>n</i> = 2 and 3, a 1 + 1 macrocycle formation occurs. The <i>n</i> = 2 phthalocrown is particularly stable due to a strong intramolecular hydrogen bond, but the <i>n</i> = 3 ring hydrolyzes to form a 3-imino-1-oxoisoindoline derivatized crown ether species. For the <i>n</i> = 1 phthalocrown, we observed dynamic behavior in the ¹H NMR spectrum, and using VTNMR were able to measure a Δ<i>G</i>^⧧ = 44.6 kJ/mol for proton exchange

Phthalocrowns: Isoindoline–Crown Ether Macrocycles

The reaction of diiminoisoindoline with amine-terminated polyethers results in the formation of phthalocrown macrocycles. For <i>n</i> = 1 (where <i>n</i> is the number of ether units), a 2 + 2 condensation takes place, but for <i>n</i> = 2 and 3, a 1 + 1 macrocycle formation occurs. The <i>n</i> = 2 phthalocrown is particularly stable due to a strong intramolecular hydrogen bond, but the <i>n</i> = 3 ring hydrolyzes to form a 3-imino-1-oxoisoindoline derivatized crown ether species. For the <i>n</i> = 1 phthalocrown, we observed dynamic behavior in the ¹H NMR spectrum, and using VTNMR were able to measure a Δ<i>G</i>^⧧ = 44.6 kJ/mol for proton exchange

Phthalocrowns: Isoindoline–Crown Ether Macrocycles

The reaction of diiminoisoindoline with amine-terminated polyethers results in the formation of phthalocrown macrocycles. For <i>n</i> = 1 (where <i>n</i> is the number of ether units), a 2 + 2 condensation takes place, but for <i>n</i> = 2 and 3, a 1 + 1 macrocycle formation occurs. The <i>n</i> = 2 phthalocrown is particularly stable due to a strong intramolecular hydrogen bond, but the <i>n</i> = 3 ring hydrolyzes to form a 3-imino-1-oxoisoindoline derivatized crown ether species. For the <i>n</i> = 1 phthalocrown, we observed dynamic behavior in the ¹H NMR spectrum, and using VTNMR were able to measure a Δ<i>G</i>^⧧ = 44.6 kJ/mol for proton exchange

Phthalocrowns: Isoindoline–Crown Ether Macrocycles

The reaction of diiminoisoindoline with amine-terminated polyethers results in the formation of phthalocrown macrocycles. For <i>n</i> = 1 (where <i>n</i> is the number of ether units), a 2 + 2 condensation takes place, but for <i>n</i> = 2 and 3, a 1 + 1 macrocycle formation occurs. The <i>n</i> = 2 phthalocrown is particularly stable due to a strong intramolecular hydrogen bond, but the <i>n</i> = 3 ring hydrolyzes to form a 3-imino-1-oxoisoindoline derivatized crown ether species. For the <i>n</i> = 1 phthalocrown, we observed dynamic behavior in the ¹H NMR spectrum, and using VTNMR were able to measure a Δ<i>G</i>^⧧ = 44.6 kJ/mol for proton exchange

Magnetic Circular Dichroism Spectroscopy of <i>N-</i>Confused Porphyrin and Its Ionized Forms

<i>N-</i>Confused porphyrin (NCP) and its externally methylated variant (MeNCP) were investigated using UV–vis and magnetic circular dichrosim (MCD) spectroscopies. In addition to evaluating the spectroscopy of the neutral compounds, the acid/base chemistry of these macrocycles was examined by the same methods. NCP exhibits two tautomeric states depending on the polarity of the solvent, and their protonation/deprotonation chemistries also differ depending on solvent polarity. DFT and TDDFT calculations were employed to evaluate the observed spectroscopic changes. Using both experimental and calculated results, we were able to determine the sites of protonation/deprotonation for both tautomeric forms of NCP. Inspection of the MCD Faraday B terms for all of the macrocycles presented in this report showed that the ΔHOMO > ΔLUMO condition is maintained in all cases, and these observations were in good agreement with the DFT calculations

Enhanced Helical Folding of <i>ortho</i>-Phenylenes through the Control of Aromatic Stacking Interactions

The <i>ortho</i>-phenylenes are a simple class of foldamers, with the formation of helices driven by offset aromatic stacking interactions parallel to the helical axis. For the majority of reported <i>o</i>-phenylene oligomers, the perfectly folded conformer comprises perhaps 50–75% of the total population. Given the hundreds or thousands of possible conformers for even short oligomers, this distribution represents a substantial bias toward the folded state. However, “next-generation” <i>o</i>-phenylenes with better folding properties are needed if these structures are to be exploited as functional units within more complex architectures. Here, we report several new series of <i>o</i>-phenylene oligomers, varying both the nature and orientation of the substituents on every repeat unit. The conformational behavior was probed using a combination of NMR spectroscopy, DFT calculations, and X-ray crystallography. We find that increasing the electron-withdrawing character of the substituents gives oligomers with substantially improved folding properties. With moderately electron-withdrawing groups (acetoxy), we observe >90% of the perfectly folded conformer, and stronger electron withdrawing groups (triflate, cyano) give oligomers for which misfolded states are undetectable by NMR. The folding of these oligomers is only weakly solvent-dependent. General guidelines for the assessment of <i>o</i>-phenylene folding by NMR and UV–vis spectroscopy are also discussed

Re(CO)₃‑Templated Synthesis of α‑Amidinoazadi(benzopyrro)methenes

α-Amidinoazadi­(benzopyrro)­methenes were synthesized using the Re­(CO)₃ unit as a templating agent. The products of these template reactions are six-coordinate rhenium complexes, with a facial arrangement of carbonyls, a noncoordinating anion, and a tridentate α-amidinoazadi­(benzopyrro)­methene ligand. The tridentate ligand shows the conversion of one diiminoisoindoline sp² carbon to a sp³ carbon, which has been seen in the “helmet” and bicyclic phthalocyanines. The bidentate diiminoisoindoline fragment tilts out of the plane of coordination. Five examples of α-amidinoazadi­(benzopyrro)­methenes produced from these reactions using different nitrile solvents, including the nitrile activation of acetonitrile, propionitrile, butyronitrile, cyclohexanecarbonitrile, and benzonitrile

The Role of Arene–Arene Interactions in the Folding of <i>ortho</i>-Phenylenes

The <i>ortho</i>-phenylenes are a simple class of helical oligomers and representative of the broader class of sterically congested polyphenylenes. Recent work has shown that <i>o</i>-phenylenes fold into well-defined helical conformations (in solution and, typically, in the solid state); however, the specific causes of this folding behavior have not been determined. Here, we report the effect of substituents on the conformational distributions of a series of <i>o</i>-phenylene hexamers. These experiments are complemented by dispersion-corrected DFT calculations on model oligomers (B97-D/TZV­(2d,2p)). The results are consistent with a deterministic role for offset arene–arene stacking interactions on the folding behavior. On the basis of the experimental and computational results, we propose a model for <i>o</i>-phenylene folding with two simple rules. (1) Conformers are forbidden if they include a particular sequence of biaryl torsional states that causes excessive steric strain. These “ABA” states correspond to consecutive dihedral angles of −55°/+130°/–55° (or +55°/–130°/+55). (2) The stability of the remaining conformers is determined by offset arene–arene stacking interactions that are easily estimated as an additive function of the number of well-folded torsional states (±55°) along the backbone. For the parent, unsubstituted poly­(<i>o</i>-phenylene), each interaction contributes roughly 0.5 kcal/mol to the helix stability (in chloroform), although their strength is sensitive to substituent effects. The behavior of the <i>o</i>-phenylenes as a class is discussed in the context of this model. They are analogous to α-helices, with axial aromatic stacking interactions in place of hydrogen bonding. The model predicts that the overall folding propensity should be quite sensitive to relatively small changes in the strength of the arene–arene stacking. In a broader sense, these results demonstrate that polyphenylenes may exhibit folding behavior that is amenable to simple models, and validate the use of diffusion-corrected DFT methods in predicting their three-dimensional structures

The Role of Arene–Arene Interactions in the Folding of <i>ortho</i>-Phenylenes

The <i>ortho</i>-phenylenes are a simple class of helical oligomers and representative of the broader class of sterically congested polyphenylenes. Recent work has shown that <i>o</i>-phenylenes fold into well-defined helical conformations (in solution and, typically, in the solid state); however, the specific causes of this folding behavior have not been determined. Here, we report the effect of substituents on the conformational distributions of a series of <i>o</i>-phenylene hexamers. These experiments are complemented by dispersion-corrected DFT calculations on model oligomers (B97-D/TZV­(2d,2p)). The results are consistent with a deterministic role for offset arene–arene stacking interactions on the folding behavior. On the basis of the experimental and computational results, we propose a model for <i>o</i>-phenylene folding with two simple rules. (1) Conformers are forbidden if they include a particular sequence of biaryl torsional states that causes excessive steric strain. These “ABA” states correspond to consecutive dihedral angles of −55°/+130°/–55° (or +55°/–130°/+55). (2) The stability of the remaining conformers is determined by offset arene–arene stacking interactions that are easily estimated as an additive function of the number of well-folded torsional states (±55°) along the backbone. For the parent, unsubstituted poly­(<i>o</i>-phenylene), each interaction contributes roughly 0.5 kcal/mol to the helix stability (in chloroform), although their strength is sensitive to substituent effects. The behavior of the <i>o</i>-phenylenes as a class is discussed in the context of this model. They are analogous to α-helices, with axial aromatic stacking interactions in place of hydrogen bonding. The model predicts that the overall folding propensity should be quite sensitive to relatively small changes in the strength of the arene–arene stacking. In a broader sense, these results demonstrate that polyphenylenes may exhibit folding behavior that is amenable to simple models, and validate the use of diffusion-corrected DFT methods in predicting their three-dimensional structures

<i>meso</i>-Aryl-3-alkyl-2-oxachlorins

The formal replacement of a pyrrole moiety of <i>meso</i>-tetraarylporphyrin <b>1</b> by an oxazole moiety is described. The key step is the conversion of porpholactones <b>4</b> (prepared by a known two-step oxidation procedure from <b>1</b>) by addition of alkyl Grignard reagent to form <i>meso</i>-tetraaryl-3-alkyl-2-oxachlorins <b>9</b> (alkyloxazolochlorins; alkyl = Me, Et, <i>i</i>Pr). Hemiacetal <b>9</b> can be converted to an acetal, reduced to an ether, or converted to bis-alkyloxazolochlorins <b>11</b>. The optical properties (UV–visible and fluorescence spectroscopy) are described. The chlorin-like optical properties of the alkyloxazolochlorins are compared to regular chlorins, such as 2,3-dihydroxychlorins and nonalkylated oxazolochlorins made by reduction from porpholactone <b>4</b>. The conformations of the mono- and bis-alkylated 2-oxachlorins, as determined by single crystal X-ray diffractometry, are essentially planar, thus proving that their optical properties are largely due to their intrinsic electronic properties and not affected by conformational effects. The mono- and bis-3-alkyl-2-oxachlorins are a class of readily prepared and oxidatively stable chlorins