- An unusual conformation of α-haloamides due to cooperative binding with zincated porphyrins
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CD and NMR spectroscopic evidence of cooperative binding between an α-halogen atom and a carboxamide group with a zinc porphyrin leads to an unprecedented conformation for the determination of the absolute stereochemistry of α-haloamides (α-halocarboxylic acids derivatized with 1,4-phenylenediamine) through the use of exciton-coupled circular dichroism (ECCD). With the use of chiral lactams, whose rotomeric contributions are minimized, both ECCD and NMR spectroscopy demonstrate that the porphyrin favors binding to the side of the sterically more demanding halogen atom as compared to the smaller hydrogen atom. In all, the data is strongly suggestive of an unusual conformation not observed before for α-chiral amides. A mnemonic for determining the absolute stereochemistry of α-halogenated carboxylic acids is provided.
- Tanasova, Marina,Yang, Qifei,Olmsted, Courtney C.,Vasileiou, Chrysoula,Li, Xiaoyong,Anyika, Mercy,Borhan, Babak
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supporting information; experimental part
p. 4242 - 4253
(2011/02/25)
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- Synthesis and study of new α-haloacid ferroelectric liquid crystal derivatives. MM2 approach to the molecular structure-ferroelectric activity relationship
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In order to understand the structural factors that influence ferroelectric properties, three new series (F, Cl, and Br) of chiral naphthalene-ring derived compounds were synthesized, and their ferroelectric properties [spontaneous polarization (Ps) and response time (τ)] were evaluated in the pure compound. The chiral tails are α-halo acids derived from L-α-amino acids: L-α-alanine (1), L-leucine (2), L-isoleucine (3), and L-valine (4), with a fluorine, chlorine, or bromine atom in the chiral center. The highest Ps values were obtained for compounds containing a fluorine or chlorine atom in their asymmetric center and with chiral tail derived from L-isoleucine (3) (F-3, 102 nC/cm2; Cl-3, 100 nC/cm2). The steric requirements of the halogen atom and the bulky alkyl group in the asymmetric center determine the most stable conformations of these chiral tails, which have been studied by molecular mechanic empirical calculations, MM2. MM2 calculations prove to be a successful tool for understanding how the structure of the lateral chiral tail affects molecular arrangement and, as a consequence, the ferroelectric properties of the materials.
- Sierra,Serrano,Ros,Ezcurra,Zubía
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p. 7645 - 7651
(2007/10/02)
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- Synthesis and Ferroelectric Properties of 2,5-Diphenylpyrimidines with Different α-Fluorocarboxylic Acids
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Chiral esters of 2--5-pyrimidine and 2--5-pyrimidine with four different α-fluorocarboxylic acids have been synthesized.The spontaneous polarization and the tilt angles in the smectic C* phase were measured as a function of temperature.All compounds exhibit wide range smectic C* phases, the highest values of the spontaneous polarization being about 400 nC/cm2.Additionally the spontaneous polarization in a higher ordered smectic phase was determined, the values ranging up to about 600 nC/cm2.The spontaneous polarization/tilt angle ratio strongly increases with decreasing temperature. - Keywords: Liquid Crystals, Smectic Liquid Crystals, Ferroelectric Properties, Chiral α-Fluorocarboxylic Acid Esters, 2,5-Diphenylpyrimidines
- Boemelburg, Jan,Heppke, Gerd,Ranft, Andreas
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p. 1127 - 1131
(2007/10/02)
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