- N,N-Dialkylhydrazones as Versatile Umpolung Reagents in Enantioselective Anion-Binding Catalysis
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An enantioselective anion-binding organocatalytic approach with versatile N,N-dialkylhydrazones (DAHs) as polarity-reversed (umpolung) nucleophiles is presented. For the application of this concept, a highly ordered hydrogen-bond (HB) network between a carefully selected CF3-substituted triazole-based multidentate HB-donor catalyst, the ionic substrate and the hydrazone in a supramolecular chiral ion-pair complex was envisioned. The formation of such a network was further supported by both experimental and computational studies, which showed the crucial role of the anion as a template unit. The asymmetric Reissert-type reaction of quinolines as a model test reaction chemoselectively delivered highly enantiomerically enriched hydrazones (up 95:5 e.r.) that could be further derivatized to value-added compounds with up to three stereocenters.
- Gómez-Martínez, Melania,del Carmen Pérez-Aguilar, María,Piekarski, Dariusz G.,Daniliuc, Constantin G.,García Manche?o, Olga
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supporting information
p. 5102 - 5107
(2021/01/21)
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- Synthesis, in-situ membrane preparation, and good gas permselectivity of insoluble poly(substituted acetylene)s loosely cross-linked with short and soft siloxane and silanol linkages
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Insoluble but membrane-formable poly(substituted acetylene)s loosely cross-linked by short and soft Si–O–Si and SiOH---(OH)Si linkages have been synthesized from the corresponding monomers having one or two SiOH groups. These monomers have been polymerized in homogeneous systems to produce homogeneous polymer solutions in toluene. The resulting solutions were cast on polytetrafuluoroethylene sheets followed by evaporation of the solvent to give tough and flexible membranes which were completely insoluble. The membranes showed good oxygen and carbon oxide permselectivity and their plots in PO2 vs PO2/PN2 graph exceeded the 1991 Robeson's upper bound and very close to the 2008 Robeson's upper bound. The excellent selectivity are caused by enhancement of diffusion selectivity by the soft network structure which can create small pores (i.e., ultramicro pores) based on the short linkages. No drop in permeability may be due to the soft Si–O–Si and SiOH---(OH)Si linkages. In addition no change in PO2 and PO2/PN2 by aging was observed.
- Wang, Jianjun,Aoki, Toshiki,Kaneko, Takashi,Teraguchi, Masahiro
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- Hydrogen-bond-assisted helical folding of propeller-shaped molecules: Effects of extended π-conjugation on chiral selection, conformational stability, and exciton coupling
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Cooperative interaction between multiple chiral centers dictates the absolute handedness of structural folding. We have designed and prepared a series of chiral C3-symmetric tris(N-salicylidenamine) derivatives that adopt three-blade propeller-like conformations. Synthetic access to an expanded family of such constructs was aided by enzymatic resolution and C-C cross-coupling reactions of aryl-substituted chiral propargylic alcohol derivatives. These key structural components were integrated into molecular propellers of predetermined helical screw sense. Through comparative studies on a homologous set of molecules, we found that installation of phenylene-ethynylene-derived π-conjugation profoundly affected the stabilities of the helically folded structures, as evidenced by UV/Vis and circular dichroism (CD) studies. Increasing the number of hydrogen bonds through additional substitution also enhanced the populations of the folded conformations in solution. In addition to introducing steric bias to control structural folding, linearly π-conjugated groups function as spatially well-defined chromophores that give rise to characteristic exciton-coupled circular dichroism. Absolute configurations of chiral centers could thus be further confirmed by comparing the torsional relationships between pairs of chromophores on adjacent subunits, which are fully consistent with the computationally predicted structural models. Twist to fold: Chiral alcohol groups effectively direct the absolute helicities of three-bladed propeller-shaped C3-symmetric molecules through tight O-H...O-H contacts. The stabilities of the conformations of these systems depend upon the numbers of hydrogen bonds and the steric bulk of the π-conjugated substituents on thestereogenic centers. Copyright
- Opsitnick, Elizabeth A.,Jiang, Xuan,Hollenbeck, Andrew N.,Lee, Dongwhan
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scheme or table
p. 708 - 720
(2012/03/22)
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- An effective, orthogonal deprotection strategy for differentially functionalized, linear and Y-shaped oligo phenylene ethynylenes
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Several methodologies for the selective deprotection acetylenes have been reported previously. However, as is shown here, they are often not reliable or convenient. Here, an approach is reported that is efficient and general. Use of this approach to synth
- Chandra, Kusum L.,Zhang, Sheng,Gorman, Christopher B.
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p. 7120 - 7132
(2008/02/07)
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- Synthesis of nanostructures based on 1,4- and 1,3,5-phenylethynyl units with π-extended conjugation. Carbon networks dendrimer base units
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A convenient and efficient synthesis of 3,5-di(silylethynyl)phenylacetylene and p-[3,5-di(silylethynyl)phenylethynyl]phenylacetylene has been carried out. These compounds serve to prepare nanometer-sized conjugated 1,4- and 1,3,5-phenylethynyl oligomers,
- Rodríguez, J. Gonzalo,Esquivias
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p. 4831 - 4834
(2007/10/03)
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- Synthesis of nanostructures based on 1,4- and 1,3,5-ethynylphenyl subunits with π-extended conjugation. Carbon dendron units
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Nanometer-sized conjugated 1,4- and 1,3,5-ethynylphenyl oligomers were synthesized starting from 3,5-di(trimethylsilylethynyl)phenylacetylene and p-[3,5-di(trimethylsilylethynyl)-1-ethynylphenyl]-phenyl acetylene by cross-coupling reaction with a convenient haloaryl derivative, catalyzed by palladium(II)/copper(I), in excellent yield. The terminal acetylenes were efficiently prepared by a specific protection-deprotection methodology. All ethynylphenyl homologues obtained show fluorescence emission, with the bathochromic shift of approximately 20 nm by each ethynylphenyl unit increasing the conjugate chain. Parallel conjugated ethynylphenyl chains were prepared through the insertion of a 1,5-naphthalene subunit, and the compounds exhibit fluorescence radiation emission.
- Gonzalo Rodriguez,Esquivias, Jorge,Lafuente, Antonio,Diaz, Cristina
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p. 8120 - 8128
(2007/10/03)
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