Synthesis and crystal structures of extremely crowded oligophenylenes as model precursors to ‘cubic graphite’

By using drastic conditions for a Diels-Alder cycloaddition reaction, it was possible to synthesize an oligophenylene with an extremely dense packing of the benzene rings. Crystallographic data could be obtained and a projection of the structure on the plane of the central phenyl ring reveals that the molecule retained its theoretical threefold symmetry with only minor deviations. Due to its dense packing of interlocked benzene rings, this oligophenylene could be furthermore used as a suitable precursor for constructing a subunit of ‘cubic graphite’.     

Imidazol(in)ium carboxylates as N-heterocyclic carbene ligand precursors for Suzuki-Miyaura reactions

Simple catalysts formed in situ from palladium acetate and a variety of imidazolium and imidazolinium carboxylates and dithiocarboxylates have been screened in the coupling of aryl halides with trans-2-phenylvinylboronic acid. Imidazol(in)ium carboxylates show an excellent activity, which compares to that displayed by the parent imidazol(in)ium chlorides, whereas imidazol(in)ium dithiocarboxylates are poorly efficient. Interestingly, the base employed exerts a profound influence on the trans/cis stereochemistry of the coupling product.     

Intra- and intermolecular trapping of cyclopentapyrazine carbenes derived from 1,2-dialkynylimidazoles

The thermolysis of 1,2-dialkynylimidazoles in benzene solution affords high yields of 7-phenyl-5H-cyclopentapyrazines, which presumably form by solvent trapping of cyclopentapyrazine carbene intermediates. In cases where dialkynylimidazole contains side chains that can participate in intramolecular carbene C-H insertion or olefin addition, these processes compete with solvent addition to afford novel tri- and tetracyclic pyrazines, which can be obtained in good yield when the thermolysis is carried out in hexafluorobenzene.     

Controlled structure and density of “living” polystyrene brushes on flat silica surfaces

Thin layers of polystyrene were grown from surface-grafted nitroxide initiators via controlled “living” free radical polymerization. The “reactive” Langmuir-Blodgett deposition method allowed an effective control of the initiator layer density leading to PS brushes with different and high grafting density and stretching. The influence of the grafting density on the layer structure was studied. Comparison with theoretical predictions for monodispersed brushes in bad solvent was discussed. The thickness was found to vary linearly with molecular weight and the density dependence was shown using wetting measurements. Special features of controlled radical nitroxide polymerization from a surface were discussed. A direct comparison of the molecular weight and polydispersity between surface and bulk polymers was made by de-grafting the brushes into a toluene/HF solution. Finally, some evidence of a “surface Fischer” effect was shown from re-initiated layers. Received 20 December 2001     

On the topological representation of catalytic cycles with nonlinear steps

Reaction route analysis is applied to visualize reaction networks in several heterogeneous catalytic reactions. Combination of the theory of complex reactions with the notion of catalytic cycles results in a topological representation of complex mechanisms with the nodes comprising all possible surface species including free sites and branches indicating interconnections between reactions.     

Synthesis and crystal structure of 1,3,5-tris [4-(phenylethynyl)phenyl]benzene

The crystal structure of 1,3,5-tris[4-(phenylethynyl)phenyl]benzene (1) has been investigated. Compound1 represents a model of the repeating unit of the most typical polyphenylene, which contains 1,3,5-trisubstituted benzene rings (chain centers) and acetylenic groups (complex-forming and cross-linking centers) in the main chain. The acetylene groups of neighboring molecules have a tendency to close mutual arrangement, which is favorable for their topochemical interaction. However, the relative conformational rigidity of molecules1 restricts not only the possibility of the optimal adjustment of the reactive sites of neighboring molecules to one another, but also hampers the close packing of molecules in the crystal, which contains channels filled by the solvent molecules (chloroform).Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 1986–1992, November, 1994.The work was carried out with the financial support of the International Science Foundation and the American Crystallographic Association.     

Influence of charge distribution on the conformation of neutral polyampholytes

Summary Two polyampholyte copolymers based on sodium-2-acrylamido-2-methylsulfonate (NaAMPS) and methacryloyloxyethyltrimethylammonium chloride (MADQUAT) have been prepared either in homogeneous solution or by an inverse microemulsion polymerization technique. The copolymer microstructure was shown to depend on the method of preparation. The microemulsion polymerization yields copolymers with a monomer sequence distribution not far from random while those obtained by polymerization in solution have a strong tendency to alternation. The aqueous-solution properties of the two samples have been investigated by viscometry. The results show that the charge distribution along the copolymer chain affects considerably its conformation, in good agreement with recent theoretical studies. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994     

19F NMR study of relative polarities and reactivities of N-H,N-Hg,and N-Au bonds in 2-(4-fluorophenyl)benzimidazole and its PhHg and Ph3PAu derivatives in intermolecular exchange

It has been shown by the¹⁹F NMR method that the relative polarities of nitrogenelement bonds in 2-(4-fluorophenyl)benzimidazole and its PhHg and PPh₃Au derivatives increase in the order N-H19F NMR. It has been found that these reactions occur by a bimolecular associative mechanism and that the N-H bond is substantially less reactive than the N-Hg and N-Au bonds, which have identical reactivities within the limits of sensitivity of the method used.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1574–1580, August, 1995.This work was carried out with financial support from the Russian Foundation for Basic Research (Project No. 93-03-5528).