- Enhanced Basicity of the 2-Position of 1,3-Dialkoxybenzenes in S1: Acid Catalyzed Photochemical Proton/Deuteron Exchange
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The photochemistry of several 1,3-dialkoxy-substituted benzenes 1-8 has been studied in aqueous H2SO4 (D2SO4) solution.In contrast to ground state behaviour, all of these compounds (with the exception of 7) are photoprotonated efficiently (via S1) and almost exclusively at the 2-position (and to a much lesser extent at the 5-position) to give dialkoxy-substituted cyclohexadienyl cation intermediates (2,6-dialkoxybenzenonium ions), resulting in proton/deuteron exchange of the 2-position of the benzene ring for all systems except 4 (and 7).In the ground state, protonation takes place predominantly at the 4-position for 1-4 and at the 6-position for 5 and 6.In the case of 4, photoprotonation at the 2-position is implicated by observation of fluorescence quenching by acid but deuterium exchange at this position is not possible.Photoprotonation (quenching) rate constants (kH) were calculated by Stern-Volmer analysis of steady state fluorescence quenching by proton and are in the range 0.9-3E9 M-1 s-1.Quantum yields for deuterium incorporation in 4:1 aqueous D2SO4-CH3CN are reported for several substrates at selected acidities.The sigmoid dependence of plots of fluorescence quenching vs acidity is used to give estimated pKBH(1+)* of ca. 0.1-0.5 for these substrates, indicating that they are much more basic in S1 than in S0.
- Mosi, Renee,Zhang, Guangzhong,Wan, Peter
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- Self-assembly and self-organization of self-recognizing cyclophanes
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An analysis is presented of the different contributions that give rise to the packing observed in the crystal structures of a wide range of bipyridinium-based molecular assemblies and supramolecular arrays. It is demonstrated how the various interactions - electrostatic, van der Waals, and π-π interactions - that contribute to the solid-state arrangement of these molecules and supermolecules can be utilized in order to design a series of tetracationic cyclophanes that can potentially self-organize in a highly ordered way in the solid state by virtue of the fact that they contain π-electron donors as well as π-electron acceptors. The syntheses of these cyclophanes is outlined and the tunability of the self-assembly methodology in their construction is demonstrated. One of these tetracationic cyclophanes - comprising π-electron-rich hydroquinone rings and π-electron-deficient bipyridinium units - has been shown to pack as highly ordered two-dimensional, mosaic-like sheets in the solid state. Its dicationic precursor also forms extended π-π-stacked layers in the solid state. An analogous cyclophane - containing two π-electron-rich resorcinol rings in place of the two hydroquinone rings - forms, in the solid state, one-dimensional arrays wherein the component resorcinol rings interact through then: parallel π-π stacking. It has also been established that the first of the aforementioned tetracationic cyclophanes forms a 1:1 adduct with ferrocene in both the solution and solid states. X-ray crystallography, performed on the 1:1 adduct, reveals that not only is the ferrocene molecule complexed in a π-π slacking sense within the tetracationic cyclophane, but the 1:1 adduct also packs in a manner that is remarkably similar to the supramolecular organization of the free cyclophane in the crystalline state.
- Ashton, Peter R.,Chemin, Alexandre,Claessens, Christian G.,Menzer, Stephan,Stoddart, J. Fraser,White, Andrew J. P.,Williams, David J.
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- A new class of flavonoids bearing macrocyclic polyethers by stereoselective photochemical cycloaddition reaction
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A new class of flavonoids bearing cyclic polyethers involving a phenyl ring was conveniently provided by the intramolecular photochemical dimerization of 2-chromonecarboxylic esters. Irradiation of 2-chromonecarboxylate with a polyether tethered at both ends promoted intramolecular [2 + 2] cyclobutane formation leading to 14- to 27-membered cyclic polyethers. The efficiency depended on the substituted position of the phenyl ring, with ortho- and meta-substituted derivatives giving cycloadducts in good chemical yields and quantum efficiencies, whereas the para-derivatives were inert toward photolysis. X-ray crystallographic analysis revealed that the stereochemistry of the macrocyclic cycloadducts exhibited C2-symmetry.
- Ishikawa, Hiroki,Uemura, Naohiro,Taira, Ryo,Sano, Kento,Yoshida, Yasushi,Mino, Takashi,Kasashima, Yoshio,Sakamoto, Masami
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p. 3911 - 3916
(2019/06/18)
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- One-pot alkoxylation of phenols with urea and 1,2-glycols
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A one-pot epoxide-free alkoxylation process has been developed for phenolic compounds. The process involves heating phenols and urea in 1,2-glycols at 170-190 °C using Na2CO3/ZnO as co-catalysts under atmospheric conditions. During the course of this new alkoxylation reaction, a five-membered ring cyclic carbonate intermediate, ethylene carbonate (EC) or propylene carbonate (PPC), was produced in-transit as the key intermediate and was subsequently consumed by phenols to form alkoxylated ether alcohols as final products in excellent yields. For instance, phenol, bisphenol A (BPA), hydroquinone and resorcinol were converted into their respective mono-alkoxylated ether alcohols on each of their phenolic groups in 80-95% isolated yields. In propoxylation of phenols, this approach shows great product selectivity favoring production of high secondary alcohols over primary alcohols in isomeric ratios of nearing 95/5. Since ammonia (NH3) and carbon dioxide (CO2) evolving from the reaction can be re-combined in theory into urea for re-use, the overall net-alkoxylation by this approach can be regarded as a simple condensation reaction of phenols with 1,2-glycols giving off water as its by-product. This one-pot process is simple, safe and environmentally friendlier than the conventional alkoxylated processes based on ethylene oxide (EO) or propylene oxide (PO). Moreover, this process is particularly well-suited for making short chain-length alkoxyether alcohols of phenols.
- Lin, Hsing-Yo,Dai, Shenghong A.
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experimental part
p. 167 - 173
(2011/04/19)
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- Path selection for conformational interconversions in [2]catenanes
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The conformational interconversions of several [2]catenanes containing a dibenzo-34-crown-10 ether (BPP34C10) interlocked with rings containing two 4,4′-dipyridyls tethered by different aryl spacers have been studied. Blocking groups on the tethers enabled the two pathways for circumrotation of the BPP34C10 to be open or blocked. The activation barrier for migration along the open tethers varied from 11 to 13 kcal/mol. This study demonstrates an ability to select the pathway for conformational interconversions in [2]catenanes.
- Halterman, Ronald L.,Martyn, David E.,Pan, Xingang,Ha, Diana B.,Frow, Michael,Haessig, Kathryn
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p. 2119 - 2121
(2007/10/03)
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- New Efficient Procedure for the Use of Diethoxyphosphoryl as a Protecting Group in the Synthesis of Polyazamacrocycles. Preparation of Polyazacyclophanes Derived from Resorcinol
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The synthesis of polyazamacrocycles containing an electron-rich aromatic subunit derived from resorcinol is described. The reported synthetic procedure is based on the use of diethoxyphosphoryl (Dep) as an amine protecting group. The new conditions employed for the cyclization reaction allow for a generalized use of Dep in the synthesis of polyazamacrocycles.
- Burguete, M. Isabel,Lopez-Diago, Laura,Garcia-Espana, Enrique,Galindo, Francisco,Luis, Santiago V.,Miravet, Juan F.,Sroczynski, Dariusz
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p. 10169 - 10171
(2007/10/03)
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- Enhanced basicity of 1,3-dialkoxy-substituted benzenes: cyclophane derivatives
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The photoprotonation of four dialkoxy-substituted benzenes in their excited singlet states has been studied.The parent systems 4 and 5 are regioselectively photoprotonated at the 2-position, with significant quantum efficiencies for deuterium incorporation at acidities greater than pH 2.The structurally related cyclophane derivatives 6 anf 7 did not show any deuterium exchange over the same acidity range but fluorescence quenching by proton (in aqueous solution) and laser flash photolysis structural constraints imposed by the most stable conformation adopted by these m olecules (6 and 7), the attacking proton (deuteron) is also the proton (deuteron) that is deprotonated from the cyclohexadienyl cation (2,6-dialkoxybenzenonium ion) intermediate, resulting in the absence of incorporation of deuterium.
- Zhang, Guangzhong,Shi, Yijian,Mosi, Renee,Ho, Thao,Wan, Peter
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p. 2388 - 2395
(2007/10/02)
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- Hydroxyalkylation of phenols or thiophenols with cyclic organic carbonates using triorganophosphine catalysts
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This invention relates to the use of triorganophosphine compounds as catalyst for the reaction of phenols or thiophenols with cyclic organic carbonates.
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