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Steric Demands (steric + demand)
Selected AbstractsNovel Synthetic Strategy towards the Efficient Synthesis of Substituted Bis(pyrazolyl)(2-pyridyl)methane Ligands,EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 21 2010Alexander Hoffmann Abstract A general one-pot synthesis of new substituted heteroscorpionate ligands is presented. These mixed-functionality ligands were obtained in a catalyzed Peterson rearrangement starting from the substituted pyrazole, thionyl chloride, and an aldehyde. Thus, the synthesis of polyfunctional tridentate ligands is enabled, and they contain, besides the two pyrazole groups, other functionalities relevant for coordination chemistry. Additionally, the steric hindrance is easily defined in the ligands by the substitution of the pyrazole rings. By combination of the versatility in donor function and steric demand, a systematic tuning of the properties of the bis(pyrazolyl)methane ligands is possible. The synthesis and full characterization of 11 bis(pyrazolyl)methane ligands are reported. Two of these were structurally characterized as well. [source] Reactions of Di(tert -butyl)diazomethane with Acceptor-Substituted Ethylenes,HELVETICA CHIMICA ACTA, Issue 5 2007Rolf Huisgen Abstract Di(tert- butyl)diazomethane (4) is a nucleophilic 1,3-dipole with strong steric hindrance at one terminus. In its reaction with 2,3-bis(trifluoromethyl)fumaronitrile ((E)- BTE), a highly electrophilic tetra-acceptor-substituted ethene, an imino-substituted cyclopentene 9 is formed as a 1,:,2 product. The open-chain zwitterion 10, assumed as intermediate, adds the second molecule of (E)- BTE. The 19F- and 13C-NMR spectra allow the structural assignment of two diastereoisomers, 9A and 9B. The zwitterion 10 can also be intercepted by dimethyl 2,3-dicyanofumarate (11) and furnishes diastereoisomeric cyclopentenes 12A and 12B; an X-ray-analysis of 12B confirms the ,mixed' 1,:,1,:,1 product. Competing is an (E)- BTE -catalyzed decomposition of 4 to give 2,3,4,4-tetramethylpent-1-ene (7)+N2; the reaction of (E)- BTE with a trace of water appears to be responsible for the chain initiation. The H2SO4 -catalyzed decomposition of diazoalkane 4, indeed, produced the alkene 7 in high yield. The attack on the hindered diazoalkane 4 by 11 is slower than that by (E)- BTE; the zwitterionic intermediate 21 undergoes cyclization and furnishes the tetrasubstituted furan 22. In fumaronitrile, electrophilicity and steric demand are diminished, and a 1,3-cycloaddition produces the 4,5-dihydro-1H -pyrazole derivative 25. The reaction of 4 with dimethyl acetylenedicarboxylate leads to pyrazole 29+isobutene. [source] Blue Emission of a Soluble Poly(p -phenylene) with a Cross-Conjugated Bisimidazole-Based ChromophoreMACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 1 2007Frank Dierschke Abstract A novel poly(p -phenylene) (6) has been synthesized by a cruciform combination of a polyphenylene backbone with 2,2,-(p -phenylene)-bis(4,5-diphenylimidazole) (5) as an additional, orthogonal chromophore. Polymer 6 showed in solution and in the solid state, a blue-green emission, which is obviously arising from the second bisimidazole-based chromophore. UV-Vis spectroscopy, and cyclic voltammetry revealed that the optical and electronic properties of 6 were fully determined by the incorporated 2,2,-(1,4-phenylene)bisimidazole structure. The bisimidazole unit led to high solubility and, despite the steric demand of the substituents, at the same time to the blue-green emission of the poly(p -phenylene) (6). In addition, the oxidation of 6 with potassium ferricyanide yielded a low bandgap polymer with a quinoid-type structure and a bandgap of 1.6 eV. [source] 9,10-Diarylanthracenes as Molecular Switches: Syntheses, Properties, Isomerisations and Their Reactions with Singlet OxygenCHEMISTRY - A EUROPEAN JOURNAL, Issue 36 2008Daniel Zehm Abstract A series of 9,10-diarylanthracenes with various substituents at the ortho positions have been synthesised by palladium-catalysed cross-coupling reactions. Such compounds exhibit interesting physical properties and can be applied as molecular switches. Despite the high steric demand of the substituents, products were formed in moderate-to-good yields. In some cases, microwave conditions further improved yields. Bis-coupling afforded two isomers (syn and anti) that do not interconvert at room temperature. These products were easily separated and their relative stereochemistries were unequivocally assigned by NMR spectroscopy and X-ray analysis. The syn and anti isomers exhibit different physical properties (e.g., melting points and solubilities) and interconversion by rotation around the aryl,aryl axis commences at <100,°C for fluoro-substituted diarylanthracenes and at >300,°C for alkyl- or alkoxy-substituted diarylanthracenes. The reactions with singlet oxygen were studied separately and revealed different reactivities and reaction pathways. The yields and reactivities depend on the size and electronic nature of the substituents. The anti isomers form the same 9,10-endoperoxides as the syn species, occasionally accompanied by unexpected 1,4-endoperoxides as byproducts. Thermolysis of the endoperoxides exclusively yielded the syn isomers. The interesting rotation around the aryl,aryl axis allows the application of 9,10-diarylanthracenes as molecular switches, which are triggered by light and air under mild conditions. Finally, the oxygenation and thermolysis sequence provides a simple, synthetic access to a single stereoisomer (syn) from an unselective coupling step. [source] Direct Assignment of Enantiofacial Discrimination on Single Heterocyclic Substrates by Self-induced CDCHEMISTRY - A EUROPEAN JOURNAL, Issue 6 2005Carsten Siering Abstract The first direct assignment of highly dynamic enantiofacial discrimination acting on a single heterocyclic substrate has been achieved by a combination of experimental and theoretical CD spectroscopy. The interaction of chirally modified hosts based on triphenylene ketals with appropriate prochiral guests can lead to the preferential formation of one diastereomeric host,guest complex. This reversible stereoselective binding transmits the chiral information from remote chiral groups in the host to the strongly absorbing triphenylene chromophore, which gives rise to self-induced CD. This effect was exploited for the determination of the enantiofacial recognition in various host,guest systems. Inversion of the steric demand either of the chiral substituents at the host or of the prochiral guest leads to almost complete inversion of the resulting CD spectra. For the assignment of the absolute stereochemistry of the complexes, a combined molecular dynamics/quantum-chemical approach was successfully employed. Despite the size and the highly dynamic character of the supramolecular systems, fundamental properties of the systems and details of the spectra were simulated accurately, providing access to fast and reliable assignment of the enantiofacial preference. The results are highly consistent with available X-ray data. [source] 1,8-Bis(dialkylamino)-4,5-dinitronaphthalenes and 4,5-Bis(dimethylamino)naphthalene-1,8-dicarbaldehyde as "Push,Pull" Proton Sponges: When and Why Formyl Groups Become Stronger ,-Electron Acceptors than Nitro GroupsEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 8 2009Valery A. Ozeryanskii Abstract Single-crystal X-ray studies of four representatives of "push,pull" proton sponges, namely 1,8-bis(dimethylamino)-, 1,8-bis(diethylamino)-, 1,8-bis(dipropylamino)-4,5-dinitronaphthalenes and 4,5-bis(dimethylamino)naphthalene-1,8-dicarbaldehyde have been performed at low and ambient temperatures. The most interesting and unexpected result is that the formyl groups in the peri -dialdehyde display stronger ,-acceptor effects than the nitro groups. This phenomenon is ascribed to smaller steric demands of the CHO groups, their lower electrostatic repulsion, and specific packing forces. The naphthalene cores of all but one of the molecules are markedly twisted (21,26°) while that of the diethylamino derivative is not (<5°), providing different and somewhat unpredictable ways of resonance stabilization and steric relaxation. The through-conjugation in the above compounds is also discussed for gas and solution phases on the basis of theoretical calculations, UV/Vis and 1H NMR spectra.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] Alternating Copolymerization of Cyclohexene Oxide and CO2 Catalyzed by Zinc Complexes with New 3-Amino-2-cyanoimidoacrylate LigandsADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 10 2005Mario Kröger Abstract New 3-amino-2-cyanoimidoacrylate ligands with varying steric demands have been synthesized. Zinc acetate complexes of these ligands catalyze the copolymerisation of CO2 and cyclohexene oxide, showing high activities (TOF up to over 200,h,1). [source] Regio- and stereoselective synthesis of isoxazolidine derivatives by asymmetric 1,3-dipolar cycloaddition reaction of chiral nitrones with 1-propene-1,3-sultoneJOURNAL OF HETEROCYCLIC CHEMISTRY, Issue 4 2008Hong-Kui Zhang Asymmetric 1,3-dipolar cycloadditions of chiral nitrones to 1-propene-1,3-sultone (1) were investigated. Chiral nitrones 6a-e reacted with sultone 1 in toluene at 90 °C for 24-36 h to give the corresponding isoxazolidines in moderate yields with high regioselectivities and stereoselectivities. The diastereoselectivity of this reaction varied with the choice of dipolarophile and the steric demands of nitrones. When sultone 1 was allowed to react chiral nitrone 6e, a much better diastereoselectivity of up to 5.1:1 was observed. [source] (Acetato-,O)[tris(3,5-dimethylpyrazol-1-yl-,N2)hydroborato]zinc(II)ACTA CRYSTALLOGRAPHICA SECTION C, Issue 8 2009Azizolla Beheshti The title complex, [Zn(C15H22BN6)(C2H3O2)] or (TpMe,Me)Zn(OAc), contains a tripodal tris(pyrazolyl)hydroborate ligand, a monodentate acetate ligand and a ZnII centre in a distorted tetrahedral coordination environment capped on one triangular face by a secondary Zn...O interaction with the second O atom of the acetate ligand. The four-coordination of ZnII and the essentially monodentate character of the acetate ligand are due to the high steric demands of the ligand set, which prevent chelate formation and five-coordination and lead to relatively long Zn,O and Zn,N bonds compared with related complexes of ZnII and other metals. [source] Efficient Intramolecular Hydroalkoxylation of Unactivated Alkenols Mediated by Recyclable Lanthanide Triflate Ionic Liquids: Scope and MechanismCHEMISTRY - A EUROPEAN JOURNAL, Issue 11 2010Alma Dzudza Dr. Abstract Lanthanide triflate complexes of the type [Ln(OTf)3] (Ln=La, Sm, Nd, Yb, Lu) serve as effective, recyclable catalysts for the rapid intramolecular hydroalkoxylation (HO)/cyclization of primary/secondary and aliphatic/aromatic hydroxyalkenes in imidazolium-based room-temperature ionic liquids (RTILs) to yield the corresponding furan, pyran, spirobicyclic furan, spirobicyclic furan/pyran, benzofuran, and isochroman derivatives. Products are straightforwardly isolated from the catalytic solution, conversions exhibit Markovnikov regioselectivity, and turnover frequencies are as high as 47,h,1 at 120,°C. The ring-size rate dependence of the primary alkenol cyclizations is 5>6, consistent with a sterically controlled transition state. The hydroalkoxylation/cyclization rates of terminal alkenols are slightly more rapid than those of internal alkenols, which suggests modest steric demands in the cyclic transition state. Cyclization rates of aryl-functionalized hydroxyalkenes are more rapid than those of the linear alkenols, whereas five- and five/six-membered spirobicyclic skeletons are also regioselectively closed. In cyclization of primary, sterically encumbered alkenols, turnover-frequency dependence on metal-ionic radius decreases by approximately 80-fold on going from La3+ (1.160,Å) to Lu3+ (0.977,Å), presumably reflecting steric impediments along the reaction coordinate. The overall rate law for alkenol hydroalkoxylation/cyclization is v,k[catalyst]1[alkenol]1. An observed ROH/ROD kinetic isotope effect of 2.48 (9) is suggestive of a catalytic pathway that involves kinetically significant intramolecular proton transfer. The present activation parameters,enthalpy (,H,)=18.2 (9),kcal,mol,1, entropy (,S,)=,17.0 (1.4),eu, and energy (Ea)=18.2 (8),kcal,mol,1,suggest a highly organized transition state. Proton scavenging and coordinative probing results suggest that the lanthanide triflates are not simply precursors of free triflic acid. Based on the kinetic and mechanistic evidence, the proposed catalytic pathway invokes hydroxyl and olefin activation by the electron-deficient Ln3+ center, and intramolecular H+ transfer, followed by alkoxide nucleophilic attack with ring closure. [source] Preferred Functionalization of Metallic and Small-Diameter Single-Walled Carbon Nanotubes by Nucleophilic Addition of Organolithium and -Magnesium Compounds Followed by ReoxidationCHEMISTRY - A EUROPEAN JOURNAL, Issue 5 2008David Wunderlich Dipl.-Chem Abstract Covalent sidewall addition to single-walled nanotubes (SWNTs) of a series of organolithium and organomagnesium compounds (nBuLi, tBuLi, EtLi, nHexLi, nBuMgCl, tBuMgCl) followed by reoxidation is reported. The functionalized Rn -SWNTs were characterized by Raman and NIR emission spectroscopy. The reaction of SWNTs with organolithium and magnesium compounds exhibits pronounced selectivity: in general, metallic tubes are more reactive than semiconducting ones. The reactivity of SWNTs toward the addition of organometallic compounds is inversely proportional to the diameter of the tubes. This was determined simultaneously and independently for both metallic and semiconducting SWNTs. The reactivity also depends on the steric demands of the addend. Binding of the bulky t- butyl addend is less favorable than addition of primary alkyl groups. Significantly, although tBuLi is less reactive than, for example, nBuLi, it is less selective toward the preferred reaction with metallic tubes. This unexpected behavior is explained by fast electron transfer to the metallic SWNTs having low-lying electronic states close to the Fermi level, a competitive initial process. The NIR emission of weakly functionalized semiconducting SWNTs, also reported for the first time, implies interesting applications of functionalized tubes as novel fluorescent reporter molecules. [source] | ||||||||||||||||||||||