Oxidative Coupling of 1,7,8-Unsubstituted BODIPYs: Synthesis and Electrochemical and Spectroscopic Properties

We report the synthesis of BODIPYs with unsubstituted 1,7,8-positions and their dimerization by oxidative coupling with phenyliodine(III)-bis(trifluoroacetate) (PIFA). This dimerization was achieved for BODIPYs substituted in the 3,5-positions with either methyl or thienyl groups. The position and the type of the linkage in the resulting dimers depended on the nature of the substituent. The 3,5-dimethyl-BODIPY dyes were linked either via direct 1,1'-pyrrole-pyrrole coupling or via a 1,3'-methylene bridge. The 3,5-dithienyl-BODIPY dyes provided, in excellent yields, unique compounds linked exclusively via the α-thienyl positions. All dyes were unreactive in the 8-position. Electrochemical and spectroscopic measurements on the monomers and dimers provided evidence of interactions between the two halves of the dimers. Thus, oxidation and reduction potentials were split by up to 210 mV, and modest excitonic coupling and an internal charge transfer were observed in some cases.     

Synthesis, spectral, electrochemical, and anion binding properties of 3,5-bis(dipyrromethanyl) boron-dipyrromethenes

Four new boron-dipyrromethenes (BODIPYs) containing dipyrromethanyl substituents at 3,5-positions, bis(3,5-dipyrromethanyl) BODIPYs 5-8, were synthesized by treating their corresponding 3,5-diformyl BODIPYs 1-4 with excess pyrrole under mild acid catalyzed reaction conditions. The compounds 5-8 are stable and freely soluble in common organic solvents. One-dimensional, two-dimensional NMR, high resolution mass spectrometry (HRMS), absorption, fluorescence, and electrochemical techniques were used to characterize the compounds. The spectral and electrochemical studies indicated that dipyrromethanyl groups at 3,5-positions of BODIPY are less electron deficient compared to formyl groups at the same positions. The anion binding studies indicated that bis(3,5-dipyrromethanyl) BODIPY compounds containing four pyrrole NH groups showed preferential binding with F(-) ion over other anions, as confirmed by using NMR, fluorescence, and electrochemical studies.     

Pd-catalyzed reactions on pyridinium N-heteroarylaminides. Step-by-step synthesis of 3,5-unsymmetrically disubstituted 2-aminopyridines

Suzuki-Miyaura cross-coupling processes on N-pyridinium bromoazinyl aminides allow access to 3,5-disubstituted N-alkyl-2-aminopyridines. The synthetic pathway involves a regioselective bromination of pyridinium N-(pyridin-2-yl)aminide and a subsequent reaction with boronic acids to afford monosubstituted aminides in good yields. An additional bromination in the 5-position of the pyridine ring followed by a coupling reaction gives pyridinium N-(3,5-diarylpyridin-2-yl)aminides. Finally, a regioselective alkylation on the exo-nitrogen and reduction of the N-N bond yields highly substituted 2-aminopyridines.     

Synthesis of directly connected BODIPY oligomers through Suzuki-Miyaura coupling

Treatment of a meso-arylboron dipyrrin (BODIPY) with NBS provides mono- and dibrominated BODIPYs at the 2- and 6-positions in excellent yields with high regioselectivity. Brominated products can be employed as a nice building block for the synthesis of a variety of BODIPY derivatives through Suzuki-Miyaura coupling. Because of a lack of substituents at the 1,3,5,7-positions, a directly β-β-linked BODIPY dimer exhibits a completely coplanar conformation of BODIPY units, offering effective π-conjugation.     

Regioselective Aromatic Electrophilic Bromination with Dioxane Dibromide Under Solvent‐Free Conditions

Highly regioselective ring bromination of aromatic compounds has been accomplished with high yields and good purity using dioxane dibromide (DD) under solvent‐free conditions. Notable features of this methodology include operational simplicity, rapid reactions, excellent control over the degree of bromination, and tolerance of various functional groups during the reaction.     

Conformationally Restricted Aza‐Dipyrromethene Boron Difluorides (Aza‐BODIPYs) with High Fluorescent Quantum Yields

A simple approach to the highly fluorescent near‐infrared aza‐BODIPY dyes with higher fluorescence quantum yields (up to 0.81 in toluene) in comparison with their known analogues is presented. Our approach is based on the restricted rotations of the 1,7‐phenyl groups to the mean plane of the aza‐BODIPYs, which is achieved through the installation of bulky substituents on the 1,7‐phenyl groups of aza‐BODIPYs and results in a reduced nonradiative relaxation process in solution. The large torsion angles between the 1,7‐phenyl groups and the aza‐BODIPY core (?₁ and ?₂ in these novel conformationally restricted aza‐BODIPYs) were confirmed by X‐ray diffraction studies.     

Preparation and chromatographic evaluation of β-cyclodextrin derivative CSPs bearing substituted phenylcarbamate groups for HPLC

Five β-cyclodextrin (β-CD) derivatives bearing substituted phenylcarbamate/3-(triethoxysilyl)propylcarbamate groups at the 2-, 3-, and 6-positions of glucose unit and another five derivatives containing benzoate at the 2-position and substituted phenylcarbamate/3-(triethoxysilyl)propylcarbamate groups at the 3- and 6-positions were synthesized using the regioselective esterification method. The obtained β-CD derivatives were efficiently immobilized onto the silica gel through the intermolecular polycondensation of a small amount of the triethoxysilyl groups, which were used as the chiral stationary phases (CSPs) for high-performance liquid chromatography (HPLC). The chiral separation properties of these CSPs were evaluated under the normal-phase HPLC. The effects of solvent polarity and the side chain structures of β-CD derivatives on the chiral recognition ability of the immobilized CSPs were investigated. Among these β-CD derivative CSPs, 2,3,6-tris(3,5-dichlorophenylcarbamate)-β-CD CSP showed a relatively high chiral recognition ability for the studied racemates. The regioselective esterification at the 2-position of glucose unit in the β-CD decreased the chiral recognition ability at the same conditions. For some racemates, the β-CD derivative CSPs showed chiral recognition abilities comparable or better to some chemical bonded β-CD derivative CSPs and 3,5-dichloro- and 3,5-dimethylphenylcarbamates of cellulose and amylose CSPs.     

3,5-Diformylboron dipyrromethenes as fluorescent pH sensors

A series of boron dipyrromethene (BODIPY) dyes containing two aldehyde functional groups at the 3 and 5 positions have been synthesized in low-to-decent yields in two steps. In the first step, the meso-aryl dipyrromethanes were treated with POCl(3) in N,N-dimethylformamide to afford 1,9-diformylated dipyrromethanes. In the second step, the diformylated dipyrromethanes were first in situ oxidized with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone and then reacted with BF(3)·OEt(2) to afford 3,5-diformylboron dipyrromethenes. The X-ray structural analysis indicated that the aldehyde groups are involved in intramolecular hydrogen bonding with fluoride atoms, which may be responsible for the stability of the diformylated BODIPY compounds. The presence of two formyl groups significantly alters the electronic properties, which is clearly evident in downfield shifts in the (1)H and (19)F NMR spectra, bathochromic shifts in the absorption and fluorescence spectra, better quantum yields, and increased lifetimes compared to 3,5-unsubstituted BODIPYs. Furthermore, 3,5-diformylboron dipyrromethenes are highly electron-deficient and undergo facile reductions compared to unsubstituted BODIPYs. These compounds exhibit pH-dependent on/off fluorescence and thus act as fluorescent pH sensors.     

An effective and regioselective bromination of 1,4,5,8-naphthalenetetracarboxylic dianhydride using tribromoisocyanuric acid

A highly efficient and cost-effective reagent for the bromination of 1,4,5,8-naphthalenetetracarboxylic dianhydride under mild reaction conditions is reported. Bromination of 1,4,5,8-naphthalenetetracarboxylic dianhydride using tribromoisocyanuric acid (TBCA) in concentrated H₂SO₄ is very effective and regioselective. 1,4,5,8-Naphthalenetetracarboxylic dianhydride was brominated smoothly under optimized reaction conditions to give mono-, di- and tetra-brominated products in good to excellent yields using TBCA. As a proof of principle, the potential of this bromination methodology is demonstrated by converting brominated naphthalenetetracarboxylic dianhydrides into N-imide and core functionalized 1,4,5,8-napthalenetetracarboxylic diimides by treating with n-butylamine to yield corresponding mono-, di- and tetra-(n-butylamino)-naphthalene diimides in good yields in one-step reactions.     

5,6-Dihydropyrrolo[2,1-b]isoquinolines as scaffolds for synthesis of lamellarin analogues

Efficient modular synthetic routes to open chain marine alkaloids such as lamellarins have been developed. 5,6-Dihydropyrrolo[2,1-b]isoquinoline scaffolds were prepared, and protocols enabling regioselective bromination followed by Suzuki cross-coupling were established for the introduction of aryl groups onto the 2- and 3-positions.     

Cycloaddition of germylenes to 3,5-di-t-butyl orthoquinone

The germylenes X₂Ge, RGeX and R₂Ge (X = halogen, OR; R = alkyl or aryl) react at room temperature with 3,5-di-t-butyl orthoquinone by regioselective cycloaddition. The corresponding substituted 2-germa-1,3-dioxolans are formed similarly in good yields, but their stabilities depend on the substituents on the metal. Some of them have also been synthesized by nucleophilic substitution from the corresponding chlorogermanes and 3,5-di-t-butyl catechol. 2-Halo(6,8-di-t-butyl)-4,5-benzo-2-germa-1,3-dioxolans undergo redistribution reactions, while the corresponding dialkyl or diaryl derivatives are very stable.     

One-pot selective syntheses of 5-azaindoles through zirconocene-mediated multicomponent reactions with three different nitrile components and one alkyne component

5-Azaindoles either with three different substituents at their 2-, 4-, and 6-positions or with two identical substituents at their 2- and 6-positions and a different one at the 4-position, were obtained in good to excellent isolated yields by a zirconocene-mediated multicomponent process. Each reaction involved four organic partners, comprising a Si-tethered diyne, one tBuCN component, and two (either different or identical) nitriles. All these four components were combined through the action of a Cp(2)Zr(II) species into a three-ring fused Zr/Si-containing organometallic complex in a perfectly chemo- and regioselective manner. This multicomponent reaction process consisted of three reaction steps, all of which were made clear through the isolation and characterization of their corresponding organometallic intermediates: the zirconacyclopropene-azasilacyclopentadienes 2, the allenyl-aza-zirconacycles 3, and the three-ring fused complexes 6. X-ray single-crystal structural analyses of two three-ring fused Zr/Si-containing intermediates and two 5-azaindoles unambiguously showed the positions of the different substituents and the regioselectivity. Iminopyrrole derivatives could be also highly selectively prepared from a Si-tethered diyne and two different nitriles.     

On the bromination of 1,7- and 1,8-Naphthyridine in nitrobenzene

By application of the Kress procedure for bromination it has been found that from the hydro-bromide of 1,7-naphthyridine with 1.1 equivalents of bromine in nitrobenzene a mixture of 3-and 5-bromo- and 3,5-dibromo-1,7-naphthyridine is obtained in reasonable yield. With an excess of bromine 3,5-dibromo-1,7-naphthyridine is nearly exclusively formed. Similar brominations of the hydrobromide of 1,8-naphthyridine with 1.1 equivalents of bromine gave 3-bromo- and 3,6-dibromo-1,8-naphthyridine. By using an excess of bromine a high-yield conversion into 3,6-dibromo-1,8-naphthyridine is observed. Bromination of the hydrochloride salt of 1,7- and 1,8-naphthyridine affords the same bromo derivatives.     

Palladium bis(2,2,6,6-tetramethyl-3,5-heptanedionate): an efficient catalyst for regioselective C-2 arylation of heterocycles

Palladium bis(2,2,6,6-tetramethyl-3,5-heptanedionate), a structurally well defined O-containing transition metal complex is reported as an efficient catalyst for regioselective direct C-2 arylation of heterocycles with aryl halides. The present protocol is applicable to a wide variety of heterocycles providing good to excellent yields of products.     

1,7-Palladium migration via C-H activation, followed by intramolecular amination: regioselective synthesis of benzotriazoles

A novel 1,7-palladium migration-cyclization-dealkylation sequence for the regioselective synthesis of benzotriazoles has been developed. These reactions proceed in excellent yields with high regioselectivities. The mechanism of the reaction has also been investigated.     

Synthesis and reactivity of dimethoxy-functionalised Tröger’s base analogues

Tröger’s base analogues were prepared bearing methoxy groups in the 1,7-, 2,8-, 3,9- or 4,10-positions. These compounds were converted to their dihydroxy analogues in excellent yields upon treatment with boron tribromide and the 4,10-dihydroxy analogue could be prepared by directly from 4-hydroxyaniline. The synthetic utility of the dihydroxy-functionalised compounds as building blocks was demonstrated by the synthesis of a dialkoxy and a diester Tröger’s base analogue.     

Regioselective one-pot bromination of aromatic amines

[reaction: see text] Treatment of aniline with n-butyllithium and then trimethyltin chloride gave the tin amide (PhNH-SnMe(3)) in situ. Without isolation of the tin amide, reaction with bromine and workup with aqueous fluoride ion gave p-bromoaniline in 76% yield, with no dibromoaniline or o-bromoaniline. Application of this sequence to 11 different aromatic amines gave selective bromination in 36-91% yields, without formation of dibromides. This constitutes a good general method for the regioselective bromination of aromatic amines.     

Singlet oxygen generation ability of squarylium cyanine dyes

The quantum yields for singlet oxygen generation of several squarylium cyanine dyes derived from benzothiazole, benzoselenazole and quinoline, displaying absorption within the so-called “phototherapeutic window” (600–1000 nm), were determined, envisioning their potential usefulness for photodynamic therapy (PDT). The determination was performed by a direct method measuring the luminescence decay of the dyes in the near infrared. Considering the absorption and the quantum yields displayed by some of the dyes, these seemed to be potential candidates as sensitizers for PDT.     

Diiodinated Mono- and Dipyridylvinyl BODIPY Dyes: Photophysicochemical Properties,in vitro Antibacterial Studies,Molecular Docking and Theoretical Calculations

In this study, novel mono- and dipyridylvinyl boron dipyrromethene dyes are prepared to compare their photodynamic antimicrobial chemotherapy (PACT) activities against Staphylococcus aureus to the corresponding core dyes. Pyridylvinyl substitution at the 3- or 3,5-positions of a meso-4-bromophenylBODIPY core dye via a Knoevenagel reaction with an aromatic 2-bromopyridinecarboxaldehyde shifts the major BODIPY spectral band to longer wavelength. The extended π-conjugation red shifts the main spectral band into the 602–618 nm region in CHCl₃, THF, ethanol and DMSO after monopyridylvinyl substitution and to 685–704 nm after dipyridylvinyl substitution. An enhancement of the population of the T₁ state through the incorporation of iodine atoms at the 2,6-positions results in moderately high singlet oxygen quantum yields in DMSO. The π-extended dyes were found to have significantly lower PACT activities than the diiodinated core dye.     

A new, efficient glycosylation method for oligosaccharide synthesis under neutral conditions: preparation and use of new DISAL donors

Efficient, stereoselective glycosylation methods are required for the synthesis of complex oligosaccharides as tools in glycobiology. All glycosylation methods, which have found wide acceptance, rely on Lewis acid activation of glycosyl donors prior to glycosylation. Here, we present a new and efficient method for glycosylation under neutral or mildly basic conditions. Glycosides of methyl 2-hydroxy-3,5-dinitrobenzoate (DISAL) and its para regioisomer, methyl 4-hydroxy-3,5-dinitrobenzoate, were prepared by nucleophilic aromatic substitution. In a first demonstration of their potential as glycosyl donors, stereospecific glycosylation of methanol was achieved. In the glycosylation of more hindered alcohols, the beta-donor proved more reactive, and alpha-glucosides were predominantly formed. Glycosylation of protected monosaccharides, with free 6-OH or 3-OH, proceeded smoothly in 1-methyl-2-pyrrolidinone (NMP) at 40-60 degrees C in the absence of Lewis acids and bases in good to excellent yields. Glycosylation of 3-OH gave the alpha-linked disaccharide only.