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Steric Properties (steric + property)
Selected AbstractsIntroduction of Aromatic and Heteroaromatic Groups in the 2- and 8-Positions of the Tröger's Base Core by Suzuki, Stille and Negishi Cross-Coupling Reactions , A Comparative StudyEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 16 2005Carlos Solano Abstract A comparative study on the bis(functionalization) of the Tröger's base core by aromatic and heteroaromatic groups using palladium-catalyzed cross-coupling reactions is presented. Three different reactions, the Suzuki, Stille, and Negishi couplings, were investigated using Tröger's base analogues equally substituted in the 2,8-positions with (HO)2B, Bu3Sn and ZnCl groups, respectively, as the metallated component. Six aryl halides with different electronic and steric properties were employed as coupling partners. The presence of the bulky and electron-rich phosphane P(tBu)3 as co-catalyst was found to play an important role. In addition, the palladium source, [Pd(PPh3)4] or [Pd2(dba)3], was also found to be an important factor for the yield of the reactions. The Suzuki coupling was found to be the best method in general, giving excellent yields for most aryl halides, whereas the Stille and Negishi couplings gave moderate to good yields. Finally, the crystal structures of the 4-nitrophenyl- and the 2-pyridyl-appended analogues of Tröger's base, 7d and 7f, are presented in the Supporting Information. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] New Carbohydrate-Based Phosphite-Oxazoline Ligands as Highly Versatile Ligands for Palladium-Catalyzed Allylic Substitution ReactionsADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 15 2005Yvette Mata Abstract We have designed and synthesized a new family of readily available phosphite-oxazoline ligands for Pd-catalyzed asymmetric allylic substitution reactions. These ligands can be tuned in two regions to explore their effect on catalytic performance. By carefully selecting the ligand components, we obtained high enantioselectivities in the Pd-catalyzed allylic substitution in substrates with different steric properties [source] New energy terms for reduced protein models implemented in an off-lattice force fieldJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 12 2001Tommi Hassinen Abstract Parameterization and test calculations of a reduced protein model with new energy terms are presented. The new energy terms retain the steric properties and the most significant degrees of freedom of protein side chains in an efficient way using only one to three virtual atoms per amino acid residue. The energy terms are implemented in a force field containing predefined secondary structure elements as constraints, electrostatic interaction terms, and a solvent-accessible surface area term to include the effect of solvation. In the force field the main-chain peptide units are modeled as electric dipoles, which have constant directions in ,-helices and ,-sheets and variable conformation-dependent directions in loops. Protein secondary structures can be readily modeled using these dipole terms. Parameters of the force field were derived using a large set of experimental protein structures and refined by minimizing RMS errors between the experimental structures and structures generated using molecular dynamics simulations. The final average RMS error was 3.7 Å for the main-chain virtual atoms (C, atoms) and 4.2 Å for all virtual atoms for a test set of 10 proteins with 58,294 amino acid residues. The force field was further tested with a substantially larger test set of 608 proteins yielding somewhat lower accuracy. The fold recognition capabilities of the force field were also evaluated using a set of 27,814 misfolded decoy structures. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1229,1242, 2001 [source] Substituent effects on conformational preference in , -substituted , -fluorophenylacetic acid methyl ester model systems for chiral derivatizing agentsJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 10 2009Riadh Sahnoun Abstract In connection with study of chiral derivatizing agents (CDAs) for NMR determination of absolute configuration of organic compounds, factors controlling the conformational preference between syn - and anti -forms in , -substituted , -fluorophenylacetic acid methyl ester (FC(X)(Ph)COOMe) model systems were theoretically investigated. Substituents X at the stereogenic carbon atom were X,=,H, CCH and CH3, the electronic and steric properties of which were significantly different from each other. The model system with X,=,CCH and that with X,=,CH3 were found to be possible candidates for fluorine-containing CDAs. The syn conformation is stable compared with the anti one by 0.7,kcal,mol,1 for the ester with X,=,CCH. On the other hand, the anti conformation is stable compared with the syn one by 0.5,kcal,mol,1 for the ester with X,=,CH3. Both natural bond orbital (NBO) analysis and deletion of selected orbitals based on the donor,acceptor NBO scheme were adopted for semi-quantitative estimation of factors responsible for the conformational preference as well as a qualitative inspection of occupied canonical molecular orbitals (MOs). It was shown that [,,(,*,+,,*)(CO)] and [,,,*(Ph) and ,(Ph),,*] hyperconjugations are the main factors controlling the conformational preferences between the syn and anti conformations. Other types of effects such as electrostatic effects were also investigated. The role of the fluorine atom was also clarified. Copyright © 2009 John Wiley & Sons, Ltd. [source] Living polymerization of substituted acetylenesJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 23 2005Martin G. Mayershofer Abstract For many years, considerable research efforts have been dedicated to ,-conjugated polymers because of their extraordinary electronic, optical, and structural properties. The employed transition-metal-based initiating systems comprise not only simple transition-metal salts but also rather sophisticated mixtures of two, three, or four compounds and even highly defined single-component systems such as transition-metal alkylidene complexes. Extensive fine-tuning of the electronic and steric properties of initiator,monomer systems eventually allowed the tailor-made synthesis of conjugated materials via living polymerization techniques. This article focuses on recent developments in the field of the living polymerization of substituted acetylene derivatives. Ill-defined group 5 and 6 transition metal halide-based initiators, well-defined transition-metal alkylidene complexes, and rhodium(I)-based systems that induce the living polymerization of numerous substituted acetylenes are reviewed. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5723,5747, 2005 [source] Two novel silver(I) coordination polymers: poly[(,2 -2-aminopyrimidine-,2N1:N3)bis(,3 -thiocyanato-,3S:S:S)disilver(I)] and poly[(2-amino-4,6-dimethylpyrimidine-,N)(,3 -thiocyanato-,3N:S:S)silver(I)]ACTA CRYSTALLOGRAPHICA SECTION C, Issue 10 2009Geng-Geng Luo 2-Aminopyrimidine (L1) and 2-amino-4,6-dimethylpyrimidine (L2) have been used to create the two novel title complexes, [Ag2(NCS)2(C4H5N3)]n, (I), and [Ag(NCS)(C6H9N3)]n, (II). The structures of complexes (I) and (II) are mainly directed by the steric properties of the ligands. In (I), the L1 ligand is bisected by a twofold rotation axis running through the amine N atom and opposite C atoms of the pyrimidine ring. The thiocyanate anion adopts the rare ,3 -,3S coordination mode to link three tetrahedrally coordinated AgI ions into a two-dimensional honeycomb-like 63 net. The L1 ligands further extend the two-dimensional sheet to form a three-dimensional framework by bridging AgI ions in adjacent layers. In (II), with three formula units in the asymmetric unit, the L2 ligand bonds to a single AgI ion in a monodentate fashion, while the thiocyanate anions adopt a ,3 -,1N,,2S coordination mode to link the AgL2 subunits to form two-dimensional sheets. These layers are linked by N,H...N hydrogen bonds between the noncoordinated amino H atoms and both thiocyanate and pyrimidine N atoms. [source] Insights into CO/Styrene Copolymerization by Using PdII Catalysts Containing Modular Pyridine,Imidazoline LigandsCHEMISTRY - A EUROPEAN JOURNAL, Issue 15 2004Amaia Bastero Dr. Abstract Continuing our studies into the effect that NN, ligands have on CO/styrene copolymerization, we prepared new C1 -symmetrical pyridine,imidazoline ligands with 4,,5,- cis stereochemistry in the imidazoline ring (5) and 4,,5,- trans stereochemistry (6,10) and compared them with our previously reported ligands (1,4). Their coordination to neutral methylpalladium(II) (5,a,10,a) and cationic complexes (5,b,10,b), investigated in solution by NMR spectroscopy, indicates that both the electronic and steric properties of the imidazolines determine the stereochemistry of the palladium complexes. The crystal structures of two neutral palladium precursors [Pd(Me)2,nCln(NN,)] (n=1 for 8,a; n=2 for 9,a,) show that the PdN coordination distances and the geometrical distortions in the imidazoline ring depend on the electronic nature of the substituents in the imidazoline fragment. Density functional calculations performed on selected neutral and cationic palladium complexes compare well with NMR and X-ray data. The calculations also account for the formation of only one or two stereoisomers of the cationic complexes. The performance of the cationic complexes as catalyst precursors in CO/4- tert -butylstyrene copolymerization under mild pressures and temperatures was analyzed in terms of the productivity and degree of stereoregularity of the polyketones obtained. Insertion of CO into the PdMe bond, which was monitored by multinuclear NMR spectroscopy, shows that the N ligand influences the stereochemistry of the acyl species formed. [source] | |||||||||||||||||||