Investigation of electrochemically induced Michael addition reactions. Oxidation of some dihydroxybenzene derivatives in the presence of azide ion

In aqueous solution containing azide ion as a nucleophile, electrochemical oxidation of hydroquinone and some dihydroxybenzoic acids have been studied using cyclic voltammetry and controlled-potential coulometry. The voltammetric data show that electrochemically generated para and ortho-benzoquinones participate in Michael addition reactions with azide ions to form the corresponding diazido or diaminobenzoquinones. In this work, we have proposed various mechanisms for the electrode process and we report an efficient and one-pot method for the synthesis of 2,5-diazido-1,4-benzoquinone, 2,5-diamino-1,4-benzoquinone, 4,5-diamino-1,2-benzoquinone, and 2,3-diamino-5,6-dioxocyclohexa-1,3-dienecarboxylic acid based on the Michael reaction of electrochemically generated ortho and para-benzoquinones with azide ion in an undivided cell using an environmentally friendly reagent-less method in ambient conditions. An estimation of the observed homogeneous rate constant (k_obs) of the reaction of electrochemically generated para-benzoquinone with azide ion by the digital simulation method is also presented.     

Fries rearrangement of dibenzofuran-2-yl ethanoate under photochemical and Lewis-acid-catalysed conditions

The Fries rearrangement of dibenzofuran-2-yl ethanoate as a route to o-hydroxyacetyldibenzofurans has been investigated, both under thermal Lewis-acid catalysed and non-catalysed photochemical conditions. The reactions were examined theoretically at semi-empirical (PM3 and ZINDO/S) and density functional theory (DFT) levels. The correct selection of reaction conditions provides viable preparative routes to ortho-acylated hydroxydibenzofurans.     

Recent development of ortho-C–H functionalization of aryl sulfoxides through [3,3] sigmatropic rearrangement

Sulfoxide-promoted ortho-C–H functionalization reactions of aromatic compounds have emerged as powerful tools for organic synthesis. Among them, [3,3] sigmatropic rearrangement induced by interrupted Pummerer reaction or electrophilic activation of sulfoxides has proved to be a fruitful strategy due to its versatility and high reaction efficiency. This digest paper mainly focuses on recent progress on C–H functionalization reactions of aryl sulfoxides via [3,3] sigmatropic rearrangement of sulfonium-tethered intermediates.     

Alkylation of olefins with “alkylpalladium acetates” lacking β hydrogen substituents. Observation of an intramolecular shift of an acetatopalladium group in neophylpalladium acetate

Methyl-, benzyl-, neopentyl- and (2-methyl-2-phenylpropyl)palladium acetates, prepared in situ by exchange reactions of the corresponding mercurials with palladium acetate, alkylate monosubstituted ethylene derivatives in fair to good yields. The “(2-methyl-2-phenylpropyl)palladium acetate” apparently underwent an unusual rearrangement during reaction with methyl acrylate. The palladium acetate group was partially transferred from the side-chain to the ortho position of the aromatic ring and produced methyl o-tert-butylcinnamate in 49% yield. The “normal product”, methyl 5-methyl-5-phenyl-2-hexenoate was also obtained, in 16% yield. A similar rearrangement occurred in the reaction with styrene.     

One-step novel synthesis of methylene bisphenols and methylene bisnaphthols using Lewis acid mediated rearrangement

Mono- and di-substituted isomeric methylene bisphenols and methylene bisnaphthols have been synthesized by rearrangement of the corresponding O-methoxyacetyl derivatives of phenols and naphthols, respectively, in presence of aluminium chloride under dry conditions. The chemistry observed is different from the usual Fries rearrangement reaction and involves an intermolecular rearrangement. The reactions reported here also reflect the influence of substituents present in the substrate as is supported by the substitution of the bridging methylene at a position meta to the phenolic hydroxyl in some of the minor products formed along-side the majorly formed ortho substituted products.     

Diversity-oriented synthesis of 1,2,3,5-tetrahydrobenzo[e][1,2,4]oxadiazepines and 2,3-dihydro-1H-benzo[e][1,2,4]triazepines by base-induced [4 + 3] annulation reactions

The base-induced formal [4 + 3] annulation reactions of N-(ortho-chloromethyl)aryl amides with nitrones or hydrazonoyl chlorides have been reported. When nitrones are used as the 1,3-dipole, the corresponding reaction afforded a series of 1,2,3,5-tetrahydrobenzo[e][1,2,4]oxadiazepine derivatives. Highly functionalized 2,3-dihydro-1H-benzo[e][1,2,4]triazepine derivatives were also synthesized via an unprecedented tandem aza-Michael addition/rearrangement aromatization between N-(ortho-chloromethyl)aryl amides and hydrazonoyl chlorides.     

New route for the synthesis of benzimidazoles by a one-pot multistep process with mono and bifunctional solid catalysts

One-pot multistep reactions involving a new environmentally friendly catalytic procedure have been developed for the synthesis of benzimidazoles. Benzimidazole derivatives with biological and pharmaceutical interest have been prepared by a one-pot four step process with a solid catalyst containing basic and oxidation sites. The four steps refer to: (a) oxidation of the alcohol; (b) cyclocondensation of the aldehyde formed with ortho-phenylenediamines, (c) oxidation of the carbon-nitrogen bond, (d) N-alkylation reaction. The process is illustrated by the synthesis of 1,2-disubstituted benzimidazole derivative with antiviral activity.     

Rh(I)-catalyzed solvent-free ortho-alkylation of aromatic imines under microwave irradiation

The synthesis of ortho-alkylated ketones through a chelation-assisted Rh (I) catalyzed ortho-alkylation reaction of aromatic imines under microwave activated solvent-free conditions in monomode reactors was performed. These conditions have been also applied to hydroacylation and ortho-alkylation reactions with aldimines.     

Diphosphonioiminobornane diperchlorate: Electrosynthesis, crystal structure, and hydrolysis

On of the products of electrochemical phosphorylation of camphene was 2-(C-methyl-C-trialkylphosphonio) methyleneimino-10-trialkylphosphoniomethyl)bornane diperhlorates. A mechanism of their formation was suggested consisting in the rearrangement of the intermediate camphenylphosphonium dication followed by selective addition of acetonitrile and a second trialkylphosphine molecules. The diperhlorate hydrolysis was found to lead to the synthesis of 2-acetamido-10-trialkylphosphoniobornane perchlorates. The 2-(C-methyl-C-tripropylphosphonio)methyleneimino-10-tripropylphosphoniomethyl)bornane and 2-acetamidoyl-10-tripropylphosphoniobornane structures were established by the X-ray diffraction study.     

Electrochemical characterization of small organic hole-transport molecules based on the triphenylamine unit

A series of substituted triphenylamine-containing organic compounds are synthesized and their hole-transport properties are examined by electrochemical and spectroelectrochemical methods. Several substituted tirphenylamines exhibited irreversible electron-transfer reactions both in the oxidative and reductive scan. On the other hand, the cyclic voltammograms of the p-phenylenediamine series are well defined. N,N^′-bis(4-nitrophenyl)-N,N^′-diphenyl-1,4-phenylenediamine (NPD) exhibited two reversible oxidation redox couples at +1.00 and +1.28 V vs. Ag/AgCl in dichloromethane solution. There is one reversible reduction redox couple at −1.12 V and one irreversible wave with E_p,c at −1.87 V. Cyano-substituted p-phenylenediamine (CPD) exhibited similar oxidation redox couples. Amino-substituted p-phenylenediamine (APD) is easier to oxidize than NPD and CPD. APD exhibits two reversible oxidation redox couples at +0.40 and +0.70 V and two extra irreversible oxidation waves at +1.26 and +1.52 V. Optically transparent thin-layer electrode (OTTLE) coupled with UV/Vis/NIR spectroscopy was used to examine the oxidation products of the above reactions. The electrogenerated cation and dication of the substituted p-phenylenediamine are very stable in the spectroelectrochemical studies. Oxidation of the compound APD exhibited a distinguished absorption pattern, which is different from those of compound NPD and compound CPD.     

A novel and efficient synthesis of DOPA and DOPA peptides by oxidation of tyrosine residues with IBX

An efficient route to 3,4-dihydroxylphenylalanine (DOPA) and DOPA peptides was described by oxidation of l-tyrosine and l-tyrosine derivatives with 2-iodoxybenzoic acid (IBX). DOPA was obtained after an situ reduction of the corresponding ortho-quinone with sodium dithionite. Oxidation reactions proceeded in good yields and high chemo- and regio-selectivity. The chirality of the DOPA residue was retained under the reaction conditions. The efficiency and the selectivity of the reaction were successfully tested using recyclable polymer-supported IBX.     

Reactivity of the radical cation of 4-(N,N-dialkylamino)biphenyl derivatives as probed by resonance Raman spectroscopy

The cw resonance Raman spectra of the products of electrochemical oxidation of 4-(N,N-dialkylamino)biphenyl compounds, with alkyl = methyl, methyl-d₃ or ethyl, are analyzed and compared with the time-resolved resonance Raman spectra of the photogenerated radical cation of these amines. The anodic oxidation is shown to lead to the formation of the respective N,N,N′,N′-tetraalkylbenzidine radical cation and dication, and a mechanism scheme for the oxidation is proposed.     

1,3-Dipolar cycloaddition of azomethine imines to ethynyl hetarenes: A synthetic route to 2,3-dihydropyrazolo[1,2-a]pyrazol-1(5H)-one based heterobiaryls

π-Deficient ethynyl hetarenes were used as dipolarophiles in a 1,3-dipolar cycloaddition reaction with azomethine imines (2-arylidene-5-oxopyrazolidin-2-ium-1-ides). Both CuI-catalyzed and catalyst-free thermally induced reactions proceeded with high regioselectivity providing 6-hetaryl-5-aryl-2,3-dihydropyrazolo[1,2-a]pyrazol-1(5H)-ones in moderate to excellent yields. The ethynyl hetarenes (pyridines, pyrazines, quinoxalines, pteridines and pyrimido[4,5-c]pyridazines) with ortho-methyl, ortho-cyano and ortho-alkynyl substituents were applicable to this reaction. 1,3-Dipolar cycloaddition reactions of alkynyl hetarenes with azomethine imines or other 1,3-dipole reagents can be considered as an alternative synthetic approach to heterobiaryls.     

Substituent position effect on the optoelectronic properties of photochromic diarylethenes

Photochromic diarylethenes bearing fluorine atoms at the ortho-, meta-, or para-position of both terminal phenyl rings have been synthesized. Effect of the substituent positions on their optoelectronic properties, including photochromism in solution, fluorescence, and electrochemical properties were investigated for the first time. The results indicated that the position of the fluorine substituent had remarkable impacts on molar absorption coefficient of the closed-ring forms of diarylethenes, quantum yields of the cyclization and cycloreversion reactions, fluorescence intensity, as well as the oxidation potential.     

Preparation and Structures of Dication Salts of Phenyl-Substituted TTF Vinylogues

Some TTF vinylogues 1 containing phenyl groups at the vinyl positions were synthesized. The redox properties were investigated by cyclic voltammograms, showing that they are strong electron donors and the Coulomb repulsion is decreased in the dication states. Some of the donors afforded their dication salts as single crystals by electrochemical oxidation or the reaction with CuCl₂. X-ray structure analysis has revealed that the dication molecules have structures with planar TTF vinylogue units and twisted phenyl groups. The crystal structures are unusual to avoid the steric interactions of the phenyl groups.     

AuCl-catalyzed reaction of ortho-alkynyl(oxo)benzene with benzenediazonium 2-carboxylate as a synthetic method towards anthracene, triptycene, and phthalazine derivatives

The AuCl-catalyzed benzannulation of ortho-alkynylphenyl ketones with benzenediazonium 2-carboxylate proceeded efficiently at 40 °C in (CH₂Cl)₂ and a variety of anthracene derivatives, having a ketone group at 9-position, were produced in good to high yields. On the other hand, the reaction of ortho-alkynylbenzaldehydes with benzenediazonium 2-carboxylate afforded triptycyl ketones. The reactions most probably proceed through the formation of a zwitterionic intermediate by the gold-induced electrophilic cyclization of ortho-alkynyl(oxo)benzenes, followed by the cycloaddition of benzyne. In contrast, when the above reaction was carried out at rt in 1,4-dioxane, phthalazine derivative was produced without the generation of benzyne.     

Migration of trimethylsilyl group in the reaction of sodium bis(trimethylsilyl)amide with bromobenzene

The reaction of sodium bis(trimethylsilyl)amide with bromobenzene gave a mixture of N,N-bis-(trimethylsilyl)aniline and N,2-bis(trimethylsilyl)aniline, the latter being a rearrangement product formed via 1,3-migration of trimethylsilyl group from the nitrogen atom to the ortho-carbon atom in the benzene ring.     

Directed ortho-metalation versus reductive amination in the preparation of polytopic,highly substituted,and sterically congested amine-S-arylthiocarbamates as thiophenol precursors

The synthesis of polydentate amine-S-arythiocarbamates was tested by directed ortho-lithiation of 2,4-disubstituted thiophenols, or the corresponding O- and S-arylthiocarbamates by deprotonation or lithium–bromine exchange, followed by addition of the electrophiles N(CH₂CH₂X)₃ (X=Br, I). In the case of the thiophenol, deprotonation resulted in a trithioether-amine upon addition of the electrophile. With the O- and S-thiocarbamates, the reactions resulted in the migration of the thiocarbamoyl group to the ortho-position (Fries rearrangement), or nucleophilic attack of the carbonyl group by butyllithium, respectively. An alternative route employed 2,4-disubstituted phenols to obtain the corresponding salicylaldehydes, and subsequently the O-arylthiocarbamates for Newman–Kwart thermal rearrangement (NKR). Finally, the formyl group on the S-arylthiocarbamates allowed reductive amination to assemble polytopic compounds with amine and S-thiocarbamate groups.     

Tandem molecular rearrangement in the alkylation of phenol with camphene

The alkylation of phenol with camphene in the presence of boron trifluoride in glacial acetic acid was accompanied by tandem molecular rearrangement with formation of a mixture of ortho- and para-substituted phenols having 1,7,7-trimethylbicyclo[2.2.1]hept-exo-2-yl and 5,5,6-trimethylbicyclo[2.2.1]hept-exo-2-yl substituents. The same products were obtained by rearrangement of 1,7,7-trimethylbicyclo[2.2.1]hept-exo-2-yloxybenzene under analogous conditions. Similar reactions performed in the presence of aluminum phenoxide as catalyst resulted in predominant formation of the corresponding ortho-substituted phenols.     

Synthesis and two-electron redox behavior of diazuleno[2,1-a:1,2-c]naphthalenes

The Diels-Alder reaction of di-2-azulenylacetylene with tetraphenylcyclopentadienone afforded 7,8,9,10-tetraphenyldiazuleno[2,1-a:1,2-c]naphthalene in one pot via autoxidation of the presumed 1,2-di-2-azulenylbenzene derivative. In contrast, a similar reaction of bis(1-methoxycarbonyl-2-azulenyl)acetylene with tetraphenylcyclopentadienone gave the 1,2-di-2-azulenylbenzene derivative. The following cyclodehydrogenation reaction of the benzene derivative with iron(III) chloride afforded diazuleno[2,1-a:1,2-c]naphthalene 6,11-bismethoxycarbonyl derivative. The redox behavior of these novel diazuleno[2,1-a:1,2-c]naphthalenes was examined by cyclic voltammetry (CV). These compounds exhibited two-step oxidation waves at +0.22 to +0.71 V upon CV, which revealed the formation of a radical cation and dication stabilized by the fused two azulene rings under the electrochemical oxidation conditions. Since the 1,2-di-2-azulenylbenzene derivative was oxidized at higher oxidation potentials (+0.83 and +1.86 V), the fusion of the two azulene rings to naphthalene increased electron-donating properties because of the formation of a closed-shell dicationic structure. Formation of the radical cation was characterized by UV-vis spectroscopy under the electrochemical oxidation conditions, although no evidence was obtained for the presumed dication under the conditions of the UV-vis spectroscopy measurement.