Supramolecular ferroelectric liquid crystalline complexes have been obtained from 4-alkoxybenzoic acids and optically active truns-4-substituted-4-stilbazolesC. hiral smectic C phases are induced by the formation of supramolecular mesogenic structure through the selective intermolecular hydrogen bond between the achiral benzoic acids and the chiral nonmesogenic stilbazoles.
Aliphatic small saturated carbocycles and azacycles are increasingly used as bioisosteres and structural cores in medicinally active compounds due to the beneficial pharmacological and physicochemical properties they can impart. Therefore, a need exists to modify these motifs and to install groups that enable their incorporation into organic structures; these goals can be accomplished by introducing functional groups at the position of the C-H bonds on the rings. However, functionalization of secondary C-H bonds in strained rings, such as cyclopropanes and cyclobutanes, confronts several challenges, including the greater strength of these bonds than those in unstrained rings. Although catalytic, undirected borylation has been reported to functionalize the C-H bonds of selected strained rings, the examples of such reactions in earlier studies are limited in scope, principally involving rings with a small number and size of substituents. We report the borylation of fused, spirocyclic, and polysubstituted cyclopropanes, cyclobutanes, azetidines, and β-lactams with high diastereoselectivity with the most recent generation of catalysts for the borylation of alkyl C-H bonds. Important for the formation of more complex structures, reactions occur, unlike reactions of larger rings, with steric hindrance on adjacent carbon atoms. The stoichiometry of the diboron reagent, the <i>s</i>-character of the C-H bond, and the disposition of substituents influence reactivity and product distribution in ways charted by our results. With these advances, the borylation of C-H bonds becomes a viable approach for the modification and incorporation of these ring systems into pharmaceutically active structures.
Copolymers containing Ru(bpy)3 and coumarin-2 chromophores prepared using both grafting and copolymerization approaches exhibit Förster-type energy-transfer efficiencies ranging from 70% to above 98%.
Abstract Selective recognition between a benzoic acid derivative and nonmesogenic 4,4′-bibyridine through intermolecular hydrogen bonds results in a novel molecular structure with liquid crystalline properties, in which 4,4′-bipyridine functions as a core unit.
We introduce here a novel approach to highly EUV transparent, carbon dense polymers for application as photoresist materials. The backbone of the prototype polymer consists of bicyclic hydrocarbons spiro-fused to cyclohexane moieties decorated with pendant t-butyl esters. This high polymer is formed through the free radical cyclopolymerization of functionalized norbornane derivatives. Imaging experiments conducted at 193 nm demonstrate features below 0.15 microns.
Several monolithic chiral stationary phases for reversed-phase electrochromatography have been prepared within the confines of untreated fused silica capillaries by the direct copolymerization of the chiral monomer 2-hydroxyethyl methacrylate (N-L-valine-3,5- dimethylanilide) carbamate with ethylene dimethacrylate, 2-acrylamido-2-methyl-1-propanesulfonic acid and butyl or glycidyl methacrylate in the presence of a porogenic solvent. The hydrophilicity of the stationary phase, which may be enhanced further by the hydrolysis of the epoxide functionalities of the glycidyl methacrylate moieties within the monolith, was found to have a pronounced effect on the enantioseparation. Using the most hydrophilic monolithic capillary column and optimized elution conditions, a separation of N-(3,5-dinitrobenzoyl)leucine diallylamide enantiomers with an efficiency of 61000 plates m–1 and a resolution of 2.0 was achieved.
Supramolecular liquid-crystalline networks have been prepared by self-assembly of multifunctional H-bond donor and acceptor molecules through the formation of intermolecular hydrogen bonds. Two tricarboxylic acids, 1,3,5-tris(2-(2-(4-carboxyphenoxy)ethoxy)ethoxy)benzene (1) and 3,4-bis(2-(2-(4-carboxyphenoxy)ethoxy)ethoxy)benzoic acid (2), have been synthesized for the use as trifunctional H-bond donors. These trifunctional H-bond donors have been complexed with bifunctional H-bond acceptors, such as 4,4'-bipyridine (3), 1,2-bis(4-pyridyl)ethane (4), trans-1,2-bis(4-pyridyl)ethylene (5), and bis(2-(2-(4-(2-(4-pyridyl)ethenyl)phenoxy)ethoxy)ethyl) ether (6), maintaining a 1:1 donor/acceptor group stoichiometry. All individual components are nonmesogenic. Self-assembly of these multifunctional compounds results in the formation of liquid-crystalline network structures. For example, the H-bonded complex 1/5 shows a smectic A phase from 176 to 156 °C, while 2/5 exhibits a nematic phase between 200 and 87 °C on cooling. This behavior is attributed to the dynamics of the hydrogen bonds. These results suggest that the trifunctional H-bond donors adopt linear conformations that induce calamitic mesomorphic behavior with bifunctional H-bond acceptors.
ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTHeterocyclic polymers as catalysts in organic synthesis - effect of macromolecular design and microenvironment on the catalytic activity of polymer-supported (dialkylamino)pyridine catalystsAndre Deratani, Graham D. Darling, Daniel Horak, and Jean M. J. FrechetCite this: Macromolecules 1987, 20, 4, 767–772Publication Date (Print):April 1, 1987Publication History Published online1 May 2002Published inissue 1 April 1987https://doi.org/10.1021/ma00170a011RIGHTS & PERMISSIONSArticle Views290Altmetric-Citations43LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (819 KB) Get e-Alerts Get e-Alerts
Read moreThe highly compact and globular shape, as well as the uniform size and plurifunctionality of dendrimers make them ideal molecular building blocks for a wide range of interfacial materials involving self-assembled monolayers, Langmuir films, multilayers, and other surface-confined assemblies. Moreover, the study of the behavior of dendrimers at surfaces and interfaces provides unique insight into their chemical and physical properties. Dendritic macromolecules play an increasingly important role in the materials and surface sciences, where applications utilizing polymer thin films can benefit from their distinctive chemical and physical properties. Recent investigations have highlighted the use of dendrimers as functional surfaces and as interfacial materials for applications in membranes, adhesion, microelectronics or in chemical and biological sensing.
Read moreThe photolysis of 9-hydroxy-10-methyl-9-phenyl-9,10-dihydroacridine 1 or its 9-methoxy derivative 2 occurs with heterolytic cleavage of a C–O bond and formation of the desired strong base together with a cation that stabilizes itself through aromatization.
Read moreAbstract Hydrogen-bonded ferroelectric liquid-crystalline complexes have been obtained by self-assembly of S-(+)-4-(2-methylbutoxy)benzoic acid and trans-4-alkoxy-4'-stilbazoles. Chiral smectic C phases are induced by the formation of the hydrogen bonds between the chiral nonmesogenic benzoic acid and the achiral stilbazoles. The chiral smectic C phase of the complex has been characterized by X-ray measurement. The hydrogen-bonded complex shows stable ferroelectric behavior in electric fields.
Read moreThe synthesis of new poly(siloxysilanes) by polyhydrosilation of AB2, AB4, and AB6 monomers using a solid-supported catalyst is described. Analysis of the polymers by gel permeation chromatography shows weight-average molecular weight values near 5−10 000 amu with polydispersities of ∼2. Addition of fresh monomer to the polymerized material only leads to a slight increase in molecular weight. In all cases, polymerization is accompanied by significant intramolecular cyclization side reaction. These results can be explained by considering factors such as the unequal reactivity of the different oligomeric species and their tendency to form cyclic species, but no clear evidence for a growth-limiting effect arising from steric crowding could be seen. End capping of the terminal silicon hydride groups by hydrosilation with a variety of reagents has been demonstrated, affording a diversity of functionalized poly(siloxysilane) materials.
Read moreThe polycondensation of bis-allylic or bis-benzylic diols with dibromomethane under phase transfer catalysis may be used to generate some polyformal polymers and copolymers which depolymerize readily when heated or when subjected to acidolysis. The polymers are best prepared by condensation of diols such as 1,4-dihydroxy-2-cyclohexene or 1,4-dihydroxy-1,2,3,4-tetrahydronaphthalene and CH2Br2 under phase transfer conditions. The polymers which are obtained by this process generally have fairly broad polydispersities and are stable to temperatures higher than 150°C. Under acid catalysis, they decompose below 80°C to afford mainly the aromatic elimination product (benzene or naphthalene), and formaldehyde hydrate. As the polymers absorb very weakly even in the deep-UV (Emax < 300), imaging is possible through formulations incorporating both the polyformals and a small amount of a triarylsulfonium salt or similar photoacid generator. The polyformals can be used as self-developing and chemically amplified imaging systems which operate in the deep-UV and possess good sensitivities due to the catalytic nature of their photoinitiated thermal decomposition.
Read moreAbstract A novel separation medium based on uniform size beads of 4-hydroxystyrene-divinylbenzene can be used for both the size exclusion chromatography of small molecules and their separation by reversed-phase chromatography. The new 5 μm material is prepared by the controlled swelling of monodispersed polystyrene particles, that serve both as porogen and shape template, with a polymerization mixture consisting of the monomers and dibutylphthalate, followed by a suspension polymerization. Removal of the acetoxy groups by hydrolysis with aqueous base leads to the final poly(4-vinylphenol-co-divinylbenzene) monodispersed beads. Polymerization conditions that favor the formation of very small pores were developed to optimize the beads for the SEC separation of small molecules. The SEC calibration curve confirms that the optimized beads contain a large volume of pores suitable for the separation of solutes with a molecular weight of up to about 1000. The ability to separate alkylbenzenes according to their hydrodynamic sizes has been demonstrated using a column packed with this material. In addition, the phenol chemistry used in combination with a hydrophobic crosslinking monomer provides this separation medium with an unusual versatility that allows both reversed phase and normal phase chromatography to be run in the same column after a simple change of the mobile phase.
Read moreA variety of strong bases and hydrocarbon-soluble superbases (SB) have been used for the selective metalation of poly(isobutylene-co-p-methylstyrene) at the benzylic p-methyl position. The reaction was monitored by quenching the metalated polymer with chlorotrimethylsilane and measuring the resulting functionalization by 1H NMR spectrometry. A study of different SB's prepared from butyllithium and a heavier alkali metal alkoxide showed the importance of the alkali metal used in the alkoxide. Both the degree of metalation and the selectivity increase with the size of the metal cation (Li+ < Na+ < K+ < Cs+). Best results for the quantitative metalation of all of the benzylic p-methyl groups of the copolymer were obtained using a 2-fold molar excess of the SB derived from s-BuLi and cesium l-(−)-menthoxide in the ratio of 1:3. Ring metalation is estimated to less than 2%, and the polymer backbone is unreactive. Time-dependent metalation reactions carried out between −78 and +65 °C reveal that, regardless of the reaction temperature, maximum metalation takes place in less than 2 min. Other superbases such as [(trimethylsilyl)methyl]potassium or alkali metal amides exhibit negligible reactivities under the reaction conditions used.
Read moreAbstract : A novel approach to thermoresponsive polymeric membranes based on molded monolithic sheets of macroporous polymers has been demonstrated. The macroporous sheet prepared from glycidyl methacrylate ethylene dimethacrylate is functionalized either by free-radical grafting of N-isopropylacrylamide, or, better, by copolymerization of N-isopropylacrylamide onto the vinylated surface of the pores of the polymer sheet. The macroporous polymers modified with poly(N-isopropylacrylamide) were shown to be thermoresponsive. Changes in the temperature of a flow system incorporating the modified macroporous material were accompanied by significant changes in the backpressure required to effect flow. This study confirms the feasibility of thermoresponsive membranes for water purification.
Read moreWe investigate the design of new photoprecursors of organic bases and the steric and electronic factors that control their photocleavage to give free amines. The design strategy involves the protection of an amine with novel [(benzoinyl)oxy]carbonyl groups and substituted analogs. The resulting masked amines owe their photosensitivity to the rich photochemistry of the benzoinyl chromophore. The photochemistry of this chromophore allows for the clean photogeneration of free amine upon irradiation with UV light below 400 nm in both the solid state and in solution. The structure of the benzoinyl chromophore was varied to determine the optimal chromophore design. By varying the chromophore design, the influence of various steric and electronic effects on the photoliberation of free amines from α-keto carbamates could be gauged. Structural modification of the aryl rings was intended to probe the electronic factors of the photocyclization. Substitution at the 2 position was varied to investigate the steric factors involved in photocyclization. The practical potential of these photoactive carbamates as organic sources of photogenerated base was demonstrated spectroscopically (UV, IR, and NMR). GC-MS product studies also proved diagnostic in identifying photogenerated base. The thermal properties of this class of base photogenerators were also determined. α-Keto carbamates derived from 3',5'-dimethoxybenzoin and its substituted analogs appear particularly attractive. These carbamates undergo near quantitative photocleavage to give free amine along with the corresponding substituted benzo[b]furan photocyclization product. Preliminary evaluation of the solid state quantum yield for cyclohexylamine photogeneration from 3,3',5,5'-tetramethoxybenzoin cyclohexyl carbamate ranged from 0.03 to 0.08 depending on the exposure wavelength. The variation in photoefficiency correlates with the UV absorbance of the keto chromophore indicating direct excitation of the carbonyl group is the preferred pathway for photocleavage.
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