Microwave-Assisted Synthesis and Spectral Properties of Pyrrolidine-Fused Chlorin Derivatives

In this work we pursued research involving the microwave-assisted N-alkylation of a NH pyrrolidine-fused chlorin with methyl 4-(bromomethyl) benzoate and subsequent ester hydrolysis as a straightforward strategy to obtain carboxylic acid functionality in the pyrrolidine-fused chlorin, as a single reaction product. We studied the reaction's scope by extending the N-alkylation of the free-base chlorin and its corresponding Zn(II) complex to other alkyl halides, including 1,4-diiodobutane, N-(2-bromoethyl)phthalimide, and 2-bromoethanaminium bromide. In addition, two new chlorin-dansyl dyads were synthesized by reacting dansyl chloride with the 2-aminoethyl pyrrolidine-fused chlorin (dyad 6) and NH pyrrolidine-fused chlorin (dyad 7). According to spectral studies, the linker length between the two fluorophores influences the response of the dyads to the solvent polarity. Because of the simplicity of these approaches, we believe it will enable access to a vast library of custom-tailored N-functionalized chlorins while preserving their important absorption and emission spectra as photosensitizers in photodynamic therapy (PDT) of cancer and photodynamic inactivation (PDI) of microorganisms.

[3 + 2]-Cycloadditions with Porphyrin β,β'-Bonds: Theoretical Basis of the Counterintuitive meso-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.

Ohmic heating-assisted synthesis and characterization of Zn(II), Cu(II) and Pd(II) complexes of heterocyclic-fused chlorins

Chlorins are highly interesting compounds due to their spectroscopic properties in both UV-Vis and NIR regions. Upon coordination to a metal ion, the corresponding metallochlorins exhibit more valuable physicochemical properties that enable a broader range of applications, such as in photodynamic therapy (PDT), water splitting catalysis, optical sensor devices and dye-sensitized solar cells. Synthetic chemistry has been in a continuous quest to fulfil most green chemistry requirements through the development of efficient reactions. Being a heating process that does not depend on heat transfer to the reaction medium, ohmic heating accomplishes the mentioned requirements and allows a fast and uniform heating regime thanks to the ionic conductivity of the reaction medium. Herein, we report the metallation of pyrrolidine- and isoxazolidine-fused chlorins with Zn(II), Cu(II) and Pd(II) salts by ohmic heating, using non-toxic aqueous solutions, and their corresponding physico-chemical characterization. All pyrrolidine-fused chlorins showed higher yields, when compared with isoxazolidine ones. From the thermogravimetric analysis performed it is possible to infer that the metal enhances the steadiness of the macrocycle, making it easier to cause the thermal decomposition of the pyrrolidine- and isoxazolidine-fused chlorins. The Zn(II) complexes showed high absorption in the NIR spectral region, a low fluorescence quantum yield and a short excited singlet state, which indicate the high efficiency of intersystem crossing to the triplet state, making them very promising candidates as photosensitizers for PDT.

Pyrrole-Modified Porphyrins Containing Eight-Membered Heterocycles Using a Reversal of the "Breaking and Mending" Strategy

The conversion of meso-aryl-porphyrins/chlorins to porphyrinoids containing nonpyrrolic heterocycles (so-called pyrrole-modified porphyrins, PMPs) along an approach we dubbed "the breaking and mending of porphyrins" is well known. However, examples are limited to the synthesis of PMPs containing up to six-membered heterocycles; the syntheses of larger rings failed. We report here hitherto unavailable eight-membered chlorin-type PMPs using an inverted "mending and breaking" approach. All examples are based on the addition of N,N'-dimethylurea derivatives to a meso-phenyl-β,β'-dioxoporphyrin, followed by oxidative cleavage of the intermediate diol adduct. We correlate the extremely nonplanar solid-state structures of three crystallographically characterized PMPs containing an eight-membered ring with their solution-state optical properties. The first examples of bis-modified, bacteriochlorin-type PMPs containing either two eight-membered rings or an eight-membered ring and an imidazolone ring are also detailed. Using other N,N'-nucleophiles failed to either generate chlorins containing a β,β'-dihydroxypyrroline, a prerequisite for the "breaking step," or the cleavage of those substrates that did generate a diol underwent subsequent reactions that thwarted the generation of the desired PMPs. This contribution adds novel PMPs containing eight-membered rings, highlights the effects these derivatizations have on the macrocycle conformation, and how that affects their optical properties.

Site-Selective Modification of a Porpholactone-Selective Synthesis of 12,13- and 17,18-Dihydroporpholactones

The reaction of meso-tetrakis(pentafluorophenyl)porpholactone with azomethine ylides and nitrones affords pyrrolidine-fused and isoxazolidine-fused dihydroporpholactones that display, respectively, isobacteriochlorin- and chlorin-type UV–Vis spectra. These reactions are site-selective, yielding, respectively, 17,18- or 12,13-dihydroporpholactones. The crystal and molecular features of pyrrolidine-fused and isoxazolidine-fused dihydroporpholactones were unveiled from single-crystal X-ray diffraction studies.

Introduction of an isoxazoline unit to the β-position of porphyrin via regioselective 1,3-dipolar cycloaddition reaction

Isoxazoline-linked porphyrins have been synthesized by a regioselective 1,3-dipolar cycloaddition reaction between vinylporphyrin 2 and nitrile oxides. The steric interaction directed the reaction trajectory, in which only the product with a link between the 5-position of the isoxazoline and the β-position of porphyrin was observed. The isoxazoline-porphyrins 3a,b have been characterized by absorption, emission, 1H NMR and mass spectra. Later, the crystal structure of 3a was obtained and confirmed the basic features of the NMR-derived structure. Furthermore, a pair of enantiomers of 3a presented in the crystal, which formed a dimeric complex through intermolecular coordination between the Zn2+ center and the carbonyl group of the second molecule.

Nitrobenzene method: A keystone in meso-substituted halogenated porphyrin synthesis and applications

This review article briefly describes the available synthetic approaches for meso-arylporphyrins giving particular emphasis for one-pot nitrobenzene and nitrobenzene/NaY methods regarding the synthesis of meso-halogenated arylporphyrins. The review also describes the relevant applications of these halogenated porphyrins and their metalloporphyrin counterparts, prepared via nitrobenzene method, as photosensitizers for therapy (PDT and PDI), diagnostic (molecular contrast agents) and also for catalytic oxidation and CO2 cycloaddition reactions to epoxides.

Bacterio- and Isobacteriodilactones by Stepwise or Direct Oxidations of meso-Tetrakis(pentafluorophenyl)porphyrin

Porpholactones are porphyrinoids in which one or more β,β'-bonds of the parent chromophore were replaced by lactone moieties. Accessible to varying degrees by direct and nonselective oxidations of porphyrins, the rational syntheses of all five dilactone isomers along stepwise, controlled, and high-yielding routes via porphyrin → tetrahydroxyisobacteriochlorin metal complexes → isobacteriochlorindilactone metal complexes or porphyrin → tetrahydroxybacteriochlorin → bacteriochlorindilactone (and related) pathways, respectively, are described. A major benefit of these complementary routes over established methods is the simplicity of the isolation of the dilactones because of the reduced number of side products formed. In an alternative approach we report the direct and selective conversion of free base meso-tetrakis(pentafluorophenyl)porphyrin to all isomers of free base isobacteriodilactones using the oxidant cetyltrimethylN⁺MnO₄^-. The solid-state structures of some of the isomers and their precursors are reported, providing data on the conformational modulation induced by the derivatizations. We also rationalize computationally their differing thermodynamic stability and electronic properties. In making new efficient routes toward these dilactone isomers available, we enable the further study of this diverse class of porphyrinoids.

Structure and conformation of photosynthetic pigments and related compounds. 15. Conformational analysis of chlorophyll derivatives – implications for hydroporphyrinsin vivo

Controlling the function of chlorophylls depends in part on their 3D conformation. The NSD program presents a powerful tool to identify the distortion modes in phytochlorins.

Synthesis and coordination studies of 5-(4′-carboxyphenyl)-10,15,20-tris(pentafluorophenyl)porphyrin and its pyrrolidine-fused chlorin derivative

An efficient strategy was developed to obtain carboxyphenyl porphyrin, chlorins and metal complexes, with potential applications in photonics and biology.