Chemistry of 1,3-diarylpropynones in superacids

In superacids with H0=-14 to -20, it has been found that 1,3-diarylpropynones ArC triple bond CCOAr' are either protonated on oxygen of carbonyl groups with the formation of stable ions ArC triple bond CC(O+H)Ar' or undergo further transformations when the highly conjugated system is electron-rich enough. In the latter case, 3-arylindenones are produced very rapidly and with high efficiency (up to 95% yield in less than 30 min). The influence of the substituents Ar, Ar' and of the reaction conditions on the behavior of 1,3-diarylpropynones and on the intramolecular cyclisation have been studied. From the collected data, a mechanism has been proposed involving vinyl cations ArC+=CHCOAr' and/or dications ArC+=CHC(O+H)Ar'.     

A new, fast and efficient synthesis of 3-aryl indenones: intramolecular cyclization of 1,3-diarylpropynones in superacids

1,3-Diarylpropynones were cleanly converted to the corresponding 3-arylindenones in various superacidic media. This new, simple, one-pot reaction proved to be efficient (yields up to 95%) and very fast (reaction time less than 30 min).     

Protonation of 1,3- and 1,4-dinitrobenzene dianions

The protonation of the electrochemically generated (0.1 M solution of tetrabutylammonium perchlorate in DMF) 1,3-dinitrobenzene (1) dianions with phenol, ethanol, tert-butanol, and 1-butyl-3-methylimidazolium cation was studied by cyclic voltammetry and chronoamperometry at the carbositall working electrode. The rate constants for the protonation (k) of the dianion of compound 1 by phenol, 1-butyl-3-methylimidazolium cation, ethanol, and tert-butanol were estimated by the comparison of the data of cyclic voltammetry and chronoamperometry with the digital simulation results, being 100, 50, 1.5, and 0.01 L mol⁻¹ s⁻¹, respectively. The similarly obtained k value for the protonation of the dianion of 1,4-dinitrobenzene (2) with phenol is 13 L mol⁻¹ s⁻¹. According to the quantum chemical calculation results, the protonation of the dianions of compounds 1 and 2 with phenol and 1-butyl-3-methylimidazolium cation can be classified as orbital-controlled reactions. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1463–1466, July, 2008.     

Synthesis of 2-sulfenylindenones by visible-light-mediated addition of sulfur-centered radicals to 1,3-diarylpropynones

A novel, visible-light-mediated method for the synthesis of 2-sulfenylindenones from easily available thiophenols (2.0 equiv.) and 1, 3-diarylpropynones (1.0 equiv.) in the presence of eosin Y (0.02 equiv.) under air atmosphere has been developed. The reaction proceeded smoothly, for a wide range of derivatives of thiophenols and 3-diarylpropynones, to give the expected products in moderate to good yields. Compared with traditional methods, this method is more convenient and avoids using stoichiometric amounts of I₂ and K₂S₂O₈.     

Protonation behaviour of 2-phenyl-1,3-diazaazulene derivatives

Three 2-phenyl-1,3-diazaazulene derivatives were synthesized and their protonation behaviours were investigated systematically via UV–vis absorption titration and ¹H NMR titration, as well as theoretical calculations. One of them exhibited a monoprotonation process while the others displayed prominent halochromic diprotonation responses. Interestingly, upon protonation of 2-phenyl-1,3-diazaazulene derivatives, the coplanarity and conjugation of the 16-π-conjugated backbones were well kept, while the electronic structures were controllably adjusted. The response mechanism of 1,3-diazaazulene derivatives towards acid is through the attachment of acid proton to the nitrogen atom in the diazaazulene ring, resulting in the change of the hybridization of protonated-N from sp² to sp³, which differed from that of the well-known azulene (analogue of 1,3-diazaazulene, protonation at carbon atom). This work would provide a new insight into the protonation research of the organic functional molecules.     

Chemistry of superacids

Superelectrophilic nitration of deactivated aromatics with NO₂Cl−3MX _n complexes in aprotic nonpolar solvents such as CH₂Cl₂ makes it possible to obtain the corresponding nitro derivatives in good to almost quantitative yields under mild conditions. For part 34, see Ref. 1. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 953–956, May, 1998.     

Multi-channel transformations of 1,3-diarylpropynones under superelectrophilic activation conditions: concurrence of intra- and intermolecular reactions. Experimental and theoretical study

1,3-Diarypropynones undergo concurrent multi-channel transformations in Bronsted (FSO₃H, TfOH) and Bronsted–Lewis (HF–SbF₅, TfOH-SbF₅) superacids, leading intramolecularly to 3-arylindenones, or intermolecularly to 1,3,3-triarylpropenones, and dimeric structures. The outcome depends on the electronic donor-acceptor properties of the substituents in the aromatic rings of the 1,3-diarylpropynones, and on the added aromatic external π-nucleophiles. The orbital energies, atom charges, orbital contributions, and global electrophilicity indices of the cationic intermediates have been calculated by DFT methods to explain these transformations.     

Nitro and nitroso transformations in superacids

Novel transformations involving the nitro and nitroso groups in superacid media are discussed and summarized. NO₂-diprotonation is a key reaction for nitro PAHs, forming N,N-dihydroxyiminium–arenium dications whereas nitrosoarenes are N,O-diprotonated to form hydroxyiminium–arenium dications. In nitropyrenes a facile cyclization involving the nitro group leads to five- and/or six-membered ring heterocyclic cations. Nitro cyclization also occurs in nitroalkylbenzenes but at higher temperatures. Multinuclear NMR data for the resulting persistent NO₂-diprotonated dications and their cyclized analogs are gathered and the structural requirements to bring about nitro cyclization are assessed. Conjugated nitroalkenes such as nitrostyrene and nitroethene are NO₂-diprotonated to form hydroxyiminium–carbenium dications. β-Heteroatom-substituted nitroethylenes are C,O-diprotonated and subsequently form hydroxynitrilium ions. Nitronate salts and unsubstituted nitroalkanes are activated in superacids via their protonated nitronic acid. β-ethoxycarbonyl-substituted nitroalkanes are activated via the O,O-diprotonated aci-nitro species with further O-protonation to give dioxonium–carbenium trications or via hydroxynitrilium ions. The NMR characteristics for the dihydroxyiminium–carbenium dications and hydroxynitrilium ions from various 2-nitroalkenes and their nucleophile quenching-derived cations are also gathered. The mechanistic aspects emphasizing dicationic (and tricationic) intermediates, their interplay and the potential synthetic benefits of these transformations which greatly extend the chemistry of the nitro group are highlighted.     

Oxidative cyclization of some 1,3-dioximes with lead tetraacetate

Oxidation of 1,3-dioximes 1 with lead tetraacetate gives di-N-oxides of pyrazolines 4 or pyrazoles 5 or N-oxides of oxadiazines 7 as the main reaction products. The reaction mechanism is discussed.     

Equilibrium acidities of superacids

In this paper, we report the most comprehensive equilibrium superacidity scale that is available to date. Contrary to most of the past works, this scale is set up in a medium of constant composition and the obtained acidity values characterize the acidities of molecules rather than acidities of media. The current scale is thus complementary to the well-known H(0) scale in the information that it provides. The solvent used is 1,2-dichloroethane (DCE). DCE has very weak basic properties (but sufficiently high polarity) and is an appropriate solvent for measuring acidities of very strong acids of diverse chemical nature. DCE acidities of well-known superacids (CF(3)SO(2)OH, (CF(3)SO(2))(2)NH, cyanocarbon acids, etc.) as well as common mineral acids (H(2)SO(4), HI, HBr, etc.) are reported. Acidities of altogether 62 acids have been determined from 176 interlinked relative acidity measurements. The scale spans 15 orders of magnitude (from picric acid to 1,1,2,3,3-pentacyanopropene) and is expected to be a useful tool in design, use, and further acidity measurements of superacidic molecules.     

Reactions of acetylene ketones in superacids

Vinyl type cations ArC⁺=CHCOR generated from acetylene ketones ArC≡CCOR in superacids HSO₃F and CF₃SO₃H react with diverse benzene derivatives to form alkenylation products, E-/Z-isomers of diarylpropenone structures Ar(Ar’)C=CHCOR. The alkenylation of aromatic compounds with acetylene ketones in superacids occurs with the primary syn-addition of a hydrogen and an aryl residue to the acetylene bond followed by transformation of the product into anti-isomer under the conditions of the reaction.     

Electrophilic intermediates and their reactions in superacids

Acid-catalyzed reactions have played a major role in hydrocarbon chemistry involving electron-deficient intermediates such as carbocations, carbodications, onium ions, etc. The pioneering discovery of the use of superacids by George A. Olah, in the early 1960s, to characterize such intermediates under so-called long-lived stable ion conditions led to the understanding of their structures and reactivity patterns much more clearly. Continuing studies in this area in the past 30 years have resulted in a paradigm shift in comprehending the stability and reactivity of electrophilic intermediates in superacid media.     

Copper-catalyzed intramolecular cyclization to N-substituted 1,3-dihydrobenzimidazol-2-ones

An efficient and convenient method was developed for preparing N-substituted 1,3-dihydrobenzimidazol-2-ones from N'-substituted N-(2-halophenyl)ureas via a CuI/DBU-catalyzed cyclization in DMSO under microwave heating. High yields were obtained and a variety of functional groups were tolerated under these conditions, including N'-aryl, alkyl, heterocyclic, various N-(substituted 2-halophenyl) and N-(2-iodopyridyl)ureas.     

Mass spectrometric intramolecular cyclization reactions of some 2-N-phenyliminoperhydro-1,3-oxazines

Mass spectrometric fragmentation of four monocyclic and eight condensed skeleton 2-N-phenyliminoper-hydro-1,3-oxazine derivatives have been characterized by metastable ion analysis, collision induced dissociation measurements, exact mass measurements and ion structural studies. Extensive rearrangement reactions, best characterized in terms of intramolecular cyclizations, took place. Namely, when R = H the N-cyclization was almost exclusive whereas in cases where R > H steric reasons rendered the O-cyclization more feasible. Part of the fragmentation of the R = H compounds seemed to occur via the amino form. On the other hand the methane chemical ionization spectra of cis- and trans-anellated 5,6-trimethylene-2-N-phenyliminoperhydro-1,3-oxazines proved that at least some imino structure was present in the gas phase.     

Ruthenium-catalyzed diyne hydrative cyclization: synthesis of substituted 1,3-diene synthons

[reaction: see text]. A novel and versatile strategy for the synthesis of highly functionalized substituted 3-sulfolenes based on [CpRu(CH3CN)3]PF6-catalyzed hydrative cyclization has been developed. A marked ketone directing effect in ruthenium-catalyzed cyclization was observed for the first time. This provides complementary chemoselectivity for the synthesis of 3-sulfolenes and other cyclic enones. The utility of this method has been demonstrated by SO2 extrusion of 3-sulfolenes to afford 1,3-dienes and the subsequent inter- and intramolecular Diels-Alder reaction.     

Acid catalyzed intramolecular 2-aza-1,3-diene Diels-Alder cyclization processes

Preparation and acid-catalyzed Diels-Alder cyclizations of a 1-alkenyl-2-aza- 1,3-diene have been explored.     

New reactions of cyclization and recyclization of derivatives of 1,3-dimethyl-5,6-diaminouracil

The reaction of 1,3-dimethyl-5-6-diaminouracils with chalcones produced 7,9-dimethyl-2,4-diaryl-1-,7,9-triazaspiro[4,5] decene-1-6,8,10-triols. The transformation of these spiro-compounds to pyrimido[4,5-d]oxazoles was investigated.Khar'kov State University, Khar'kov 31007. Institute of Single Crystals, Academy of Sciences of Ukraine, Khar'kov 310141. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 972-983, July, 1996. Original article submitted February 20, 1996.     

Acid-mediated four-component tandem cyclization: Access to multifused 1,3-benzoxazine frameworks

An acid-mediated multicomponent reaction has been developed for the direct synthesis of multifused 1,3-benzoxazine derivatives from simple and readily available arylglyoxal monohydrates and 2-aminobenzyl alcohols under mild conditions. This novel protocol is proposed to proceed through intramolecular poly-heterocyclizations, thus leading to the formation of three new rings and six new chemical bonds, including four CN and two CO bonds.