Jan C. Namyslo

Selective Mono- to Perarylations of Tetrabromothiophene by a Cyclobutene-1,2-diylbisimidazolium Preligand

(Cyclobut-1-ene-1,2-diyl)bis(1-methylimidazolium)tetrafluoroborate is applied as preligand in palladium-catalyzed cross-coupling reactions starting from tetrabromothiophene for the synthesis of mono-, bi-, tri-, and tetraaryl-substituted thiophenes bearing up to four different aryl rings. A synthetic kit for preparations of nine different substitution patterns of arylated thiophenes is presented by application of only one single catalyst system. In agreement with DFT calculations, which predict energetically low rotational barriers in triaryl-3-bromothiophenes and tetraarylthiophenes, no NOE effects between adjacent aryl rings are detectable. The regioselectivity of their syntheses has therefore been elucidated by reduction of triaryl-3-bromothiophene to 2,3,5-triarylthiophene followed by HMBC, HSQC, and NOESY NMR measurements. Additionally, results of an X-ray single structure analysis are presented.

2-(Het)aryl-Substituted 7-Azabicyclo[2.2.1]heptane Systems

Epibatidine (1) is a recently discovered trace alkaloid found in the skin of a Latin-Amencan poisonous frog. Its remarkably high analgetic activity is accompanied by high toxicity. Therefore, in order to tune its biological activity, a convergent and efficient synthetic pathway was sought to synthesize epibatidine derivatives with different (het)aryl substituents. The hydro(het)arylation of the key intermediate 7-azabicycloheptene (10) represents such an approach. The synthesis of 10 by a Diels-Alder reaction of an N-activated pyrrole (7) with ethynyl p-tolyl sulfone (6) and subsequent steps has been optimized. The crucial last step, the reductive cleavage of the vinyl sulfone 9, has been replaced by a high-yield fluoride-induced degradation of the s-silylated sulfone 12 to give 10. A number of structurally different racemic epibatidine analogs (16b-e) can be prepared by palladium-catalyzed hydro(het)arylation of 10 with iodo(het)arenes 15b-e in good yields.

Chemistry of polyhalogenated nitrobutadienes, 8: Nitropolychlorobutadienes—Precursors for insecticidal neonicotinoids

Nitropolychlorobutadienes are valuable precursors for highly functionalized acyclic or (hetero)cyclic compounds. In this 8th part of our synthetically oriented series we focus on the application of these versatile starting materials in the synthesis of analogs of the heterocyclic insecticides imidacloprid and thiacloprid, and the acyclic counterpart clothianidin. In addition to the main synthetic part, leading to imidazolidines or oxazolidines, further promising types of compounds derived by subsequent chemical modifications, are introduced. Most of the new compounds show high insecticidal activity.

The Hydroarylation Reaction—Scope and Limitations

The synthetic potential of stereoselective, palladium-catalyzed hydro(het)arylation reactions of bi-, tri- and tetracyclic (hetero)alkenes in the presence of phospines and arsines as highly efficient ligands was studied. The mechanism of this reductive Heck reaction becomes more complex in the case of benzonorbornenes. Hydroarylation of diazabicyclo-[2.2.1]heptenes provides a stereoselective access to aryldiaminocyclopentanes. Electron-deficient arylpalladium complexes shift the reaction towards the product of a formal 1,2-hydrazidoarylation reaction of 1,3-cyclopentadiene by a stereoselective C-N cleavage. Due to steric reasons, rigid bicyclo[2.2.2]octenes react slower in hydroarylation reactions than the corresponding bicyclo[2.2.1]heptenes. The more flexible bicyclo[4.2.2]decene system already tends to undergo domino-Heck reactions, even under reductive conditions. When a tetracyclic cis-allylcyclopropane is carbopalladated in the presence of formates, the neighboring cyclopropane ring is attacked in the first reported example of a π,σ domino-Heck reaction.

Crown-ether enclosure generated by ionic liquid components—synthesis, crystal structure and Raman spectra of compounds of imidazolium based salts and 18-crown-6

Bis(1,3-dimethylimidazolium methylsulfate) [18-crown-6] (1), ([DMIm]2[CH3SO4]2[18-crown-6], bis(1-butyl-3-methylimidazolium methylsulfate) [18-crown-6] (2), ([BMIm]2[CH3SO4]2[18-crown-6]), bis(1-ethyl-3-methylimidazolium methanesulfonate) [18-crown-6] (3), ([EMIm]2 [CH3SO3]2[18-crown-6]) and bis(1-ethyl-3-methylimidazolium trifluoromethanesulfonate) [18-crown-6] (4), ([EMIm]2[CF3SO3]2[18-crown-6]) were prepared and characterized by single-crystal X-ray diffraction, NMR and Raman spectroscopy. Coulomb interactions between the ionic (liquid) components as well as hydrogen bonding are important. No significant close contacts are observed between the ionic components and the 18-crown-6 molecules. Crystal structures of all compounds consist of alternated layers of crown-ether molecules and respective ionic units compounds. Within the layers of the ionic components (compounds 1, 3 and 4), two cations are linked to each other by C–H⋯π interactions between one methyl carbon and the imidazolium ring of another cation.

Dimerisation, rhodium complex formation and rearrangements of N-heterocyclic carbenes of indazoles.

Summary Deprotonation of indazolium salts at low temperatures gives N-heterocyclic carbenes of indazoles (indazol-3-ylidenes) which can be trapped as rhodium complexes (X-ray analysis). In the absence of Rh, the indazol-3-ylidenes spontaneously dimerize under ring cleavage of one of the N,N-bonds and ring closure to an indazole–indole spiro compound which possesses an exocyclic imine group. The E/Z isomers of the imines can be separated by column chromatography when methanol is used as eluent. We present results of a single crystal X-ray analysis of one of the E-isomers, which equilibrate in solution as well as in the solid state. Heating of the indazole–indole spiro compounds results in the formation of quinazolines by a ring-cleavage/ring-closure sequence (X-ray analysis). Results of DFT calculations are presented.

Chemical improvement of surfaces: Part 1: Novel functional modification of wood with covalently bound organoboron compounds

Abstract A novel type of chemical modification of wood was developed and applied to functionalise wood permanently with organoboron compounds. The covalent attachment of metalloid substituents via a benzotriazolyl-activated benzoic acid to wood hydroxy groups was in focus. Thus, standard modification procedures in wood chemistry – such as the acetylation – are improved by avoiding either the loss of half the reagent in the case of acid anhydrides or the release of corrosive hydrochloric acid in the case of carboxylic acid chlorides. The introduced boron moiety could be a contribution to solve a long-lasting problem in the field of wood protection: chemical fixation of an organoboron compound by means of a well-defined covalent bond. Accordingly, the hitherto insufficient long-term availability of conventionally spread boron compounds as wood protecting agents caused by leaching can now be avoided. The investigation was also extended to arylsilyl compounds, as a second type of organometal substances, which potentially allow for subsequent chemical modifications by ipso-substitution. The presented wood modification reactions yielded weight percent gain values from 14% to 31% on beech wood, spruce wood, and pine sapwood meal (approximately 500 μm in diameter) or pine sapwood veneer chips. The modified meals and chips were analysed by IR spectroscopy.

Chemistry of polyhalogenated nitrobutadienes, 10: Synthesis of highly functionalized heterocycles with a rigid 6-amino-3-azabicyclo[3.1.0]hexane moiety

Summary The nitropolychlorobutadienes 3, 4 are valuable building blocks for various amination and successive heterocyclization products. Nucleophilic substitution reactions of the partially protected, bioactive amines 1, 2 with either vinyl, imidoyl or carbonyl chlorides result in the formation of the enamines 11, 12, 13, 16, 25, the amidine 6, and the amides 20, 21, respectively. In the following, cyclization to the highly functionalized pyrazoles 27, 28, pyrimidine 26 and pyridopyrimidine 24 succeeded. Deprotection of 21, 12 and 28 proved to be only partially feasible.

Chemical improvement of surfaces. Part 2: Permanent hydrophobization of wood by covalently bonded fluoroorganyl substituents

Abstract A recently developed esterification method in our laboratory was applied to permanent hydrophobization of wood surfaces. Specifically, the covalent attachment of fluoroorganyl substituents to wood hydroxy groups via benzotriazolyl-activated, substituted benzoic acids was in focus. Weight percent gain values from 10% to 28% were obtained on Scots pine (Pinus sylvestris) sapwood veneer chips. It proved feasible to lower the temperature for wood modification from 150°C to 120°C, or even to 70°C so that thermal decomposition of wood during modification can be neglected. The modified chips were analyzed by attenuated total reflection – infrared and X-ray photoelectron spectroscopy. Covalent attachment of the fluorobenzoic acids led to a very high, permanent hydrophobicity of the wood surface, documented by outstanding contact angles of up to 143°.

Chemistry of polyhalogenated nitrobutadienes, 14: Efficient synthesis of functionalized (Z)-2-allylidenethiazolidin-4-ones

Summary The reaction of mercaptoacetic acid esters with pentachloro-2-nitro-1,3-butadiene (1) provides an appropriate precursor for the synthesis of special thiazolidin-4-ones. Applying different anilines as the second constituent for the requisite cyclization step, a series of (Z)-2-allylidenethiazolidin-4-ones was obtained in yields up to 81%. Some subsequent reactions have been examined too, such as the formation of perfunctionalized 1H-pyrazoles upon treatment with hydrazine. Thiazolidinones are as well known for their physiological activities as for their application in optoelectronics.