β,β-Diborylated Subporphyrinato Boron(III) Complexes as Useful Synthetic Precursors

BIII-Subporphyrins Bearing Distorted Metal-Coordinating Straps: CuII-Assisted meso-Fabrications

BIII-subporphyrins 4, 5, and 6 possessing metal-coordinating carbaporphyrin-like pockets were synthesized by Suzuki-Miyaura coupling reactions. Compounds 4 and 5 gave PdII complexes 4-Pd and 5-Pd upon metalation with Pd(OAc)2 but did not give either their NiII or CuII complexes. Conversely, 6 was expected to induce distorted square planar coordination because of its 2,5-di(pyrid-2-yl)pyrrole strap. Indeed reaction of 6 with Cu(OAc)2 did not give its CuII complex but produced meso-alkoxy and meso-phenoxy products in the presence of alcohols and phenol, possibly via CuII-mediated C-H bond functionalization, which was further extended to meso-C-C bond-forming fabrications by using organoboronic acids. These CuII-mediated C-H bond meso-fabrications are the first example for porphyrinoid substrates.

Transition-Metal-Catalyzed Silylation and Borylation of C-H Bonds for the Synthesis and Functionalization of Complex Molecules

The functionalization of C-H bonds in organic molecules containing functional groups has been one of the holy grails of catalysis. One synthetically important approach to the diverse functionalization of C-H bonds is the catalytic silylation or borylation of C-H bonds, which enables a broad array of downstream transformations to afford diverse structures. Advances in both undirected and directed methods for the transition-metal-catalyzed silylation and borylation of C-H bonds have led to their rapid adoption in early-, mid-, and late-stage of the synthesis of complex molecules. In this Review, we review the application of the transition-metal-catalyzed silylation and borylation of C-H bonds to the synthesis of bioactive molecules, organic materials, and ligands. Overall, we aim to provide a picture of the state of art of the silylation and borylation of C-H bonds as applied to the synthesis and modification of diverse architectures that will spur further application and development of these reactions.

Borylated Subphthalocyanines: Versatile Precursors for the Preparation of Functional Bowl-Shaped Aromatics

The peripheral borylation of porphyrinoids has become a key step to prepare advanced functional materials. This study reports the synthesis, electronic properties, and reactivity of borylated subphthalocyanines. These compounds, which are prepared by Suzuki-Miyaura borylation in excellent yields, are easily purified, display a great stability, and serve as powerful starting materials for the post-functionalization of SubPcs via cross-coupling reactions. Remarkably, this novel approach is more efficient than the methodologies already described and enables the preparation of exotic systems, such as SubPc dimeric species linked by C-C bonds, which are not accessible so far and present promising properties for optoelectronic devices.

Recent advances in subphthalocyanines and related subporphyrinoids

Subporphyrinoids constitute a class of extremely versatile and attractive compounds. Herein, a comprehensive review of the most recent advances in the fundamentals and applications of these cone-shaped aromatic macrocycles is presented.

meso-Free BIII Subporphyrins with Electron-donating Groups

meso-Free B^III 5,10-bis(p-dimethylaminophenyl)subporphyrins were synthesized. They display red-shifted absorption and fluorescence spectra, bathochromic behaviors in polar solvents, a high fluorescence quantum yield (Φ_F =0.57), and a small HOMO-LUMO gap mainly due to destabilized HOMO as compared with meso-free B^III 5,10-diphenylsubporphyrin. This subporphyrin serves as a nice precursor of various meso-substituted B^III subporphyrins such as B^III meso-nitrosubporphyrin, B^III meso-aminosubporphyrin, and meso-meso' linked B^III azosubporphyrin dimer. Reactions of meso-free B^III subporphyrins with NBS or bis(2,4,6-trimethylpyridine)bromonium hexafluorophosphate gave meso-meso' linked subporphyrin dimers, often as a major product along with meso-bromosubporphyrins.

Singly and Doubly Quinoxaline-Fused BIII Subporphyrins

B-Phenyl B^III subporphyrin-α-diones prepared in a three-step reaction sequence from the parent subporphyrin were condensed with 1,2-diaminobenzenes to give the corresponding quinoxaline-fused subporphyrins in variable yields. Quinoxaline-fused B-phenyl-5,10,15-triphenyl B^III subporphyrin was transformed to the corresponding subporphyrin-α-dione in the same three-step reaction sequence, which was then condensed with 1,2-diaminobenzene to give doubly quinoxaline-fused subporphyrin. These quinoxaline-fused subporphyrins exhibit redshifted absorption and fluorescence spectra compared with the parent one. A singly quinoxaline-fused subporphyrin bearing three meso-bis(4-dimethylaminophenyl)aminophenyl substituents shows blueshifted fluorescence in less polar solvent, which has been ascribed to emission associated with charge recombination of intramolecular charge transfer (CT) state.

Acetylene and trans-Ethylene Bridged BIII -Subporphyrin Dimers

Acetylene and trans-ethylene bridged B^III -subporphyrin dimers were synthesized by cross-coupling reactions of meso-bromo B^III subporphyrin. These dimers display perturbed and red-shifted absorption spectra reaching around 750 nm and fluorescence reaching at around 850 nm with high quantum yields of 0.39 and 0.47, respectively. DFT calculations have revealed that the HOMOs and the LUMOs of both dimers are spread over the two subporphyrin units as an indication of effective conjugation between the two subporphyrin units. The large Stokes shifts and characteristic pico-second time-resolved transient absorption spectra indicated that the S₁ -states of the dimers relax with structural changes, which are larger for the trans-ethylene bridged dimer.

Synthesis and characterization of π-extended "earring" subporphyrins

A π-extended "earring" subporphyrin 3 was synthesized from β,β'-diiodosubporphyrin and diboryltripyrrane via a Suzuki-Miyaura coupling and following oxidation. Its Pd complex 3Pd was also synthesized and both of the compounds were fully characterized by 1H NMR, MS and X-ray single crystal diffraction. The 1H NMR spectra and single crystal structures revealed that aromatic ring current did not extend to the "ear" in both of the two compounds. Their UV-vis/NIR spectra were recorded and the absorption of both compounds is extended to the NIR region and that the absorption of 3Pd is further red-shifted and more intense.

Diarylamine-Fused Subporphyrins: Proof of Twisted Intramolecular Charge Transfer (TICT) Mechanism

Diarylamine-fused subporphyrins 1 a and 1 b were synthesized from β,β-diiodo-meso-chloro subporphyrin 2 through a one-pot procedure involving nucleophilic aromatic substitution and S_RN 1-type intramolecular fusion reaction as the first example of meso-nitrogen-embedded subporphyrin. While non-fused counterparts 3 b and 4 b display effective fluorescence quenching due to twisted intramolecular charge transfer (TICT) in CH₂ Cl₂ at room temperature, 1 b emits fluorescence with Φ_F =0.18 under the same conditions, because conformational twisting is not allowed due to the fused structure. In addition, the oxidative titration with tris(4-bromophenyl)ammoniumyl hexachloroantimonate gave cation radical 7 for 1 a and cation radical and dication for 1 b. Actually, cation radical 7 was isolated through separation over silica gel column but was found to slowly decompose in concentrated solutions.

Recent Advances in Subporphyrins and Triphyrin Analogues: Contracted Porphyrins Comprising Three Pyrrole Rings

Subporphyrinato boron (subporphyrin) was elusive until the syntheses of tribenzosubporphine in 2006 and meso-aryl-substituted subporphyrin in 2007. These novel contracted analogues possess a 14π-electron conjugated system embedded in a bowl-shaped structure. They exhibit absorption and fluorescence in the UV/vis region and nonlinear optical properties due to their octupolar structures. The unique coordination geometry around the central boron atom in the structure of subporphyrin enabled investigation of rare boron species, such as borenium cations, boron hydrides, and boron peroxides. Along with the burgeoning development of the chemistry of subporphyrins, analogous triphyrin systems have also emerged. Their rich coordination chemistry as a result of their free-base structures, which are different from the boron-coordinating structure of subporphyrins, has been intensively investigated. On the basis of the unique structures and reactivities of subporphyrins and their related triphyrin analogues, supramolecular architectures and covalently linked multicomponent systems have also been actively pursued. This Review provides an overview of the development of subporphyrin and triphyrin chemistry in the past decade and future prospects in this field, which may inspire molecular design toward applications based on their unique properties.

meso-Nitro- and meso-Aminosubporphyrinatoboron(III)s and meso-to-meso Azosubporphyrinatoboron(III)s

meso-Nitrosubporphyrinatoboron(III) was synthesized by nitration of meso-free subporphyrin with AgNO₂ /I₂ . The subsequent reduction with a combination of NaBH₄ and Pd/C gave meso-aminosubporphyrinatoboron(III). meso-Nitro- and meso-amino-groups significantly influenced the electronic properties of subporphyrin, which has been confirmed by NMR and UV/Vis spectra, electrochemical analysis, and DFT calculations. Oxidation of meso-aminosubporphyrinatoboron(III)s with PbO₂ cleanly gave meso-to-meso azosubporphyrinatoboron(III)s that exhibited almost coplanar conformations and large electronic interaction through the azo-bridge.

Synthesis and Functionalization of Porphyrins through Organometallic Methodologies

This review focuses on the postfunctionalization of porphyrins and related compounds through catalytic and stoichiometric organometallic methodologies. The employment of organometallic reactions has become common in porphyrin synthesis. Palladium-catalyzed cross-coupling reactions are now standard techniques for constructing carbon-carbon bonds in porphyrin synthesis. In addition, iridium- or palladium-catalyzed direct C-H functionalization of porphyrins is emerging as an efficient way to install various substituents onto porphyrins. Furthermore, the copper-mediated Huisgen cycloaddition reaction has become a frequent strategy to incorporate porphyrin units into functional molecules. The use of these organometallic techniques, along with the traditional porphyrin synthesis, now allows chemists to construct a wide range of highly elaborated and complex porphyrin architectures.