Palladium-catalyzed cyclizative carbonylation of azobenzenes toward 3H-Indazol-3-ones using formic acid as CO source

A palladium-catalyzed cyclizative carbonylation of azobenzenes has been developed to access 1-acyl 2-aryl 3H-indazol-3-ones in moderate to good yields with good functional compatibility. This procedure proceeded with the sequential ortho-C–H carbonylation and cyclization, where formic acid served as the CO source. The practicability of this transformation was further increased by the employment of facilely available azobenzenes derivatives as one-handled starting materials.     

Cu(I) mediated one-pot synthesis of azobenzenes from bis-Boc aryl hydrazines and aryl halides

N,N^′-bis-Boc aryl hydrazines underwent Cu(I) catalyzed couplings with aryl halides to provide N,N^′-bis-Boc diaryl hydrazines. The resulting N,N^′-bis-Boc diaryl hydrazines are readily oxidized to the azobenzenes in the presence of Cu(I) and a base. A prolonged heating of the initial coupling reactions directly provides the corresponding azobenzenes in one pot.     

Rh(III)-catalyzed [4 + 1]-annulation of azobenzenes with α- carbonyl sulfoxonium ylides toward 3-acyl-(2H)-indazoles

A Rh(III)-catalyzed [4?+?1]-annulation of azobenzenes with α- carbonyl sulfoxonium ylides was developed to access 2H-indazoles in moderate to excellent yields with good functional group compatibilities. It proceeded with the sequential insertion of the Rh(III) carbene to the C?H bond and cyclization steps, where sulfoxonium ylides served as efficient and stable carbene precursor.     

Photochemical behavior in azobenzene having acidic groups. Preparation of magnetic photoresponsive gels

The photochemistry of three azobenzenes representing contrasting photochemical behaviors is described in the present work. Thus, Methyl Orange (MO, 4-[[(4-dimethylamino)phenyl]-azo]benzenesulfonic acid sodium salt, hereinafter (1) and 4-hydroxyazobenzene-4′-sulfonic acid (2) undergo in water fast photochemical proton shift, with decays in the microsecond timescale. In contrast to the previous cases, azobenzene-4,4′-dicarboxylic acid (3) undergoes photoisomerization in water. This photochemical behavior allows the preparation of aqueous gels with Aerosil as gelating agent (5% weight) exhibiting high cyclability and photoreversible isomerization of the trans to cis (300 nm irradiation) and cis to trans (visible white light irradiation). Gels containing azobenzene 3 as photoresponsive molecule and well-dispersed magnetite (Fe₃O₄) nanoparticles exhibit simultaneously high photoresponse and magnetic properties. Photoisomerization of compound 3 in these gels leads to small but reliable changes (25 G coercive field) in the magnetic hysteresis loop of magnetite nanoparticles. This novel concept of optical modulation of magnetism in iron oxide nanoparticles paves the way for the study of new systems, with a stronger coupling between trans–cis photoisomerization and magnetic properties.     

The kinetics of radical polymerization—XXVII. Investigation of the radical reactivity of 3,3′- and 4,4′-disubstituted azobenzenes in the polymerization of vinylacetate

A study was made on the inhibition by some 3,3′- and 4,4′-disubstituted azobenzenes of the polymerization of vinylacetate initiated by azoisobutyronitrile at 50°. The inhibitory effects of these substances can be attributed to their ability to engage in radical addition giving a less reactive hydrazyl type radical. The mechanism of the inhibition has been established by ESR and kinetic (stoichiometric) measurements. The value of k₅/k₂ (characteristic of the reactivity of an inhibitor) was determined for 9 substituents. The radical reactivity of the aromatic azo group was decreased by electron donor substituents and increased by electron acceptors. The substituent effect can be well interpreted by the Hammett equation; the value of the reaction constant was ? = +0.53.     

Effect of the ortho Modification of Azobenzene on the Photoregulatory Efficiency of DNA Hybridization and the Thermal Stability of its cis Form

We synthesized various azobenzenes methylated at their ortho positions with respect to the azo bond for more effective photoregulation of DNA hybridization. Photoregulatory efficiency, evaluated from the change of T_m (ΔT_m) induced by trans–cis isomerization, was significantly improved for all ortho‐modified azobenzenes compared with non‐modified azobenzene due to the more stabilized trans form and the more destabilized cis form. Among the synthesized azobenzenes, 4‐carboxy‐2′,6′ ‐ dimethylazobenzene ( 2′,6′‐Me‐Azo ), in which two ortho positions of the distal benzene ring with respect to carboxyl group were methylated, exhibited the largest ΔT_m, whereas the newly synthesized 2,6‐Me‐Azo (4‐carboxy‐2,6‐dimethylazobenzene), which possesses two methyl groups on the two ortho positions of the other benzene ring, showed moderate improvement of ΔT_m. Both NMR spectroscopic analysis and computer modeling revealed that the two methyl groups on 2′,6′‐Me‐Azo were located near the imino protons of adjacent base pairs; these stabilized the DNA duplex by stacking interactions in the trans form and destabilized the DNA duplex by steric hindrance in the cis form. In addition, the thermal stability of cis‐ 2′,6′‐Me‐Azo was also greatly improved, but not that of cis‐ 2,6‐Me‐Azo . Solvent effects on the half‐life of the cis form demonstrated that cis‐to‐trans isomerization of all the modified azobenzenes proceeded through an inversion route. Improved thermal stability of 2′,6′‐Me‐Azo but not 2,6‐Me‐Azo in the cis form was attributed to the retardation of the inversion process due to steric hindrance between lone pair electrons of the π orbital of the nitrogen atom and the methyl group on the distal benzene ring.     

The conversion of azobenzenes to phenazines: A new molecular rearrangement

When azobenzenes arc heated with ferrous oxalate or with a fused mixture of aluminum chloride and sodium chloride the corresponding phenazines are formed in low yield. Azobenzene itself is exceptional in giving benz[c]cinnoline with AlCl₃-NaCl. Azoxybenzenes and ferrous oxalate give azobenzenes and some phenazines, while azoxybenzene itself also gives 2-hydroxyazobenzene via a Wallach-type rearrangement. The rearrangements are thought to occur at the surfaces of the reagent. No rearrangement to the phenazines occurs in the absence of the reagent.     

Reversible trans/cis photoisomerization in Langmuir-Blodgett multilayers from polyfunctional azobenzenes

Four novel, amphiphilic, N-acylaminoazobenzene derivatives containing a sulfonyl group and a β-alanine moiety were synthesized and their mono- and multilayers were prepared by the Langmuir-Blodgett (LB) technique. The opto-physical properties of the LB multilayers were investigated. Some relationship between the chemical structure of the azobenzenes and photosensitivity of their LB multilayers has been found. Reversible trans/cis photoisomerization was observed on alternate irradiation with ultraviolet and visible light of LB multilayers from azobenzenes modified by the introduction of a second aliphatic chain in the N-acylamino fragment as well as by introduction of the bulky N,N′-dicyclohexyl urea moiety.     

Photofunctional supramolecular solution systems of chiral Schiff base nickel(II), copper(II), and zinc(II) complexes and photochromic azobenzenes

Preparations, crystal structures, electronic and CD spectra are reported for new chiral Schiff base complexes, bis(N-R-1-naphthylethyl-3,5-dichlorosalicydenaminato)nickel(II), copper(II), and zinc(II). Nickel(II) and copper(II) complexes adopt a square planar trans-[MN₂O₂] coordination geometry with Δ(R,R) configuration. While zinc(II) complex adopts a compressed tetrahedral trans-[MN₂O₂] one with Δ(R,R) configuration and exhibits an emission band around 21 000 cm⁻¹ (λ_ex = 27 000 cm⁻¹). Absorption and CD spectra were recorded in N,N′-dimethylformamide, acetone, methanol, chloroform, and toluene solutions to discuss relationships between spectral shifts of d–d and π–π^∗ bands by structural changes of the complexes and physical properties of the solvents. Moreover, we have attempted to investigate conformational changes of the complexes induced by photoisomerization of azobenzene, 4-hydroxyazobenzene, or 4-aminoazobenzene, in various solutions under different conditions. Weak intermolecular interactions between complexes and azobenzenes are important for the phenomenon by conformational changes of bulky π-conjugated moieties of the ligands.     

Formation of polyfluorinated azobenzenes and azophenols in the reactions of chloropentafluoro-cyclohexa-2,5- and -2,4-dienones with phenyl- and pentafluorophenylhydrazines

4-Chloropentafluorocyclohexa-2,5-dienone reacted with phenyl- and pentafluorophenylhydrazines in the presence of AlCl₃ via addition to the carbonyl group with formation of the corresponding azobenzenes. The reaction of 6-chloropentafluorocyclohexa-2,4-dienone with phenyl- and pentafluorophenylhydrazines both in the presence and in the absence of AlCl₃ afforded mainly 3-arylazotetrafluorophenols as products of nucleophilic replacement of fluorine atom at the double C=C bond.     

Cluster chemistry: L. Trapping of arylnitrenes formed by cleavage of azoarenes in reactions with M3(CO)12 (M = Fe or Ru): X-ray structure of Ru3(μ3-NPh)2(μ-dppm)(CO)7

Trinuclear products obtained from reactions between M₃(CO)₁₂ (M = Fe or Ru) and azobenzenes are shown to have the structure M₃(μ₃-NAr)₂(CO)₉, rather than the o-semidine formulation proposed earlier. ETC CO-substitution reactions are similar to those of Ru₃(CO)₁₂, with isocyanides occupying axial sites and tertiary phosphines and phosphites occupying equatorial sites on the Ru₃(μ₃-NPh)₂(μ-dppm)(CO)₇, in which the dppm ligand spans the non-bonded Ru … Ru vector.     

Rh(III)-catalyzed annulation of azobenzenes and α-Cl ketones toward 3-acyl-2H-indazoles

Rhodium(III)-catalyzed [4 + 1] cyclization of azobenzenes with α-Cl ketones has been developed. 3-Acyl-2H-indazoles could be easily afforded in up to 97% yields for more than 30 examples. The obtained products are potentially valuable in organic synthesis and drug discovery. This protocol featured with high efficiency, extensive functional group tolerance and mild reaction conditions. The one-step efficient construction of an anti-inflammatory agent confirms the practicability of this procedure.     

Reduction of nitro- and nitroso-benzenes to azobenzenes promoted by tris(acetonitrile)tricarbonylchromium

Treatment of various nitrobenzenes and nitrosobenzene with (MeCN) ₃Cr(CO)₃ in refluxing cyclohexane affords low yields of the corresponding symmetrical azobenzenes.     

Dual-mode optical switching property of copolymers containing pendant nitro and cyano substituted azobenzenes and fulgimide units

Homo/co-poly(decyloxymethacrylate)s containing thermally reversible nitro and cyano substituted azobenzenes and thermally irreversible fulgimide units in the pendant respectively were synthesized by free radical addition polymerization method and investigated their photochromic property. The dual-mode optical switching property of copolymers F-co-N and F-co-C was investigated and revealed C-form of fulgimide in F-co-N altered the electron withdrawing nature of nitro group in the terminal azobenzene. The UV exposed films of F-co-N and F-co-C were annealed around their T_g and found that thermally reversible cis-form of azobenzene isomerized to trans-form and thermally irreversible C-form of fulgimide unaltered. Both photochromic units in the resultant film were converted into planar configurations with good fluorescence property.     

Direct ortho-mercuration reactions of azobenzene and ortho-substituted azobenzenes

The mercuration of azobenzene occurred exclusively in the ortho-position to yield 2-chloromercuriazobenzene and a mixture which on iodination gave 2,2'and 2,6-diiodoazobenzene. Ten other ortho-substituted azobenzenes were mercurated and mercuration was found to occur predominately in an ortho-position. The regiospecificity of these reactions suggests that the mercury is directed into an ortho-position by coordination of the mercury to an azo nitrogen and then subsequent electrophilic substitution.     

High‐Yield Lithiation of Azobenzenes by Tin–Lithium Exchange

The lithiation of halogenated azobenzenes by halogen–lithium exchange commonly leads to substantial degradation of the azo group to give hydrazine derivatives besides the desired aryl lithium species. Yields of quenching reactions with electrophiles are therefore low. This work shows that a transmetalation reaction of easily accessible stannylated azobenzenes with methyllithium leads to a near‐quantitative lithiation of azobenzenes in para, meta, and ortho positions. To investigate the scope of the reaction, various lithiated azobenzenes were quenched with a variety of electrophiles. Furthermore, mechanistic ¹¹⁹Sn NMR spectroscopic studies on the formation of lithiated azobenzenes are presented. A tin ate complex of the azobenzene was detected and characterized at low temperature.     

Intensely Fluorescent Azobenzenes: Synthesis,Crystal Structures,Effects of Substituents,and Application to Fluorescent Vital Stain

2‐[Bis(pentafluorophenyl)boryl]azobenzenes bearing hydrogen, methoxy, dimethylamino, trifluoromethyl, fluoro, n‐butyl, and tert‐butyldimethylsiloxy groups at the 4′‐position or methoxy and bromo groups at the 4‐position have been synthesized. The 4‐bromo group of the 2‐boryl‐4‐bromoazobenzene derivative was converted to phenyl and diphenylamino groups by palladium‐catalyzed reactions. The absorption and fluorescence properties have been investigated using UV/Vis and fluorescence spectroscopy. The 2‐borylazobenzenes emitted an intense green, yellow, and orange fluorescence, in marked contrast to the usual azobenzene fluorescence. The 4′‐siloxy derivative showed the highest fluorescence quantum yield (0.90) among those reported for azobenzenes to date. The correlation between the substituent and the fluorescence properties was elucidated by studying the effect of the substituent on the relaxation process and from DFT and TD‐DFT calculations. An electron‐donating group at the 4′‐position was found to be important for an intense emission. Application of fluorescent azobenzenes as a fluorescent vital stain for the visualization of living tissues was also investigated by microinjection into Xenopus embryos, suggesting these compounds are nontoxic towards embryos.     

Synthesis of Redshifted Azobenzene Photoswitches by Late‐Stage Functionalization

Azobenzenes are versatile photoswitches that can be cycled between their trans‐ and cis‐configuration with light. The wavelengths required for this isomerization are substantially shifted from the UV to the visible range through tetra‐ortho‐chlorination. These halogenated azobenzenes display unique photoswitching characteristics, but their syntheses remain limited and inefficient. A new general method for the synthesis of tetra‐ortho‐chloro azobenzenes has been developed, which relies on direct palladium(II)‐catalyzed C?H activation of pre‐existing standard azobenzenes. This late‐stage functionalization has a broad substrate scope and can be used to create a variety of useful building blocks for the construction of more elaborate redshifted photopharmaceuticals. This method is used to prepare red‐ AzCA‐4 , a photoswitchable vanilloid that enables optical control of the cation channel TRPV1 with visible light.     

Azopolyesters with Intrinsic Crystallinity and Photoswitchable Reversible Solid-to-Liquid Transitions

Herein, we introduce a variety of azopolyesters (azobenzene-based polyesters) with remarkable intrinsic crystallinity and photoinduced reversible solid-to-liquid transition abilities from copolymerization of azobenzene-based epoxides with cyclic anhydrides. The length of the soft alkyl side-chain inlaid with azobenzenes and stereoregularity of main-chain of azopolymers have tremendous effects on crystallization properties of the resulting polyesters with melting temperature (T_m) in the range of 51–251 °C. Moreover, some of azopolyesters possess excellently photoinduced reversible solid-to-liquid transition performance thanks to trans-cis photoisomerization of azobenzenes. Trans-azopolyesters are yellow solids with T_ms or glass transition temperatures (T_gs) above room temperature, whereas cis-polymers are red liquids with T_gs below −20 °C. These azopolyesters could be applied as novel light-switchable adhesives for quartz/quartz, wood/wood and quartz/wood adhesion, with the strength in the range of 0.73–0.89 MPa for trans-polymers. Conversely, the adhesion strength of liquefied cis-azopolyesters generated from the irradiation of trans-polymers by UV light was about 0.1 MPa, which shows light enable to control the adhesion process with high spatiotemporal resolution.     

Palladium-catalyzed direct ortho CX bond construction via CH activation of azobenzenes: Synthesis of (E)-1,2-b(2,6-dibromo(chloro)-phenyl)diazene

A palladium-catalyzed direct CX bond formation of azobenzenes with 1,3-dibromo-5,5-dimethylhydantoin or trichloro isocyanuric acid via CH activation has been developed. The reaction could proceed smoothly under an air atmosphere at 110 °C in the presence of Pd(Ph₃P)₄ in DCE and 13 examples of products were obtained.