Search results for "steric"

(E)-2-{[1-(3,11-Dimethyl-4-methylene-10-oxo-1-phenyl-4,5,10,11-tetrahydro-1H-benzo[b]pyrazolo[3,4-f][1,5]diazocin-5-yl)ethylidene]amino}-N-methyl-N-(…

2013

The central eight-membered ring of the title compound, C40H36N8O2, deviates from the ideal boat conformation because the bond between the exo-ethylene group and the adjacent N atom is twisted by 60.0 (4)° due to steric hindrance. Its adjacent benzene and pyrazole rings are oriented almost perpendicular to each other, making a dihedral angle of 85.8 (3)°. In the crystal, the molecules are linked by C(ar)—H...O hydrogen bonds, generating a three-dimensional network.

Untersuchungen zur Konformerenvielfalt von mono- und 2,5-dialkoxysubstituierten Poly(1,4-phenylenethenylen)en

1994

Investigations on the Conformational Variety of Mono- and 2,5-Dialkoxy Substituted Poly(1,4-phenyleneethenylene)s The alkoxy substituted poly(1,4-phenyleneethenylene)s 1 and 2 can principally exist in an enormous variety of conformers (table 1). However, force field calculations (MMX) reveal a far-reaching restriction to a relatively small number of non planar conformers with minimized steric energy. The decisive criterion is due to the interaction between the oxygen atoms and the olefinic hydrogens. An increasing steric hindrance should lead to higher twist angles θ in the main chain and hence to a shorter effective conjugation length. Such an effect could play a minor role between the uns…

Two strained hexahelicenophanes

2000

The crystal structures of the [6]­helicenes 4,13-(1,10-deca­methyl­ene­dioxy)­hexahelicene, C36H34O2, (I), and 4,13-(1,8-octa­methyl­ene­dioxy)­hexahelicene, C34H30O2, (II), show strong steric interactions between the terminal benzene rings and the poly­methyl­ene­dioxy chains. The shortest ring A and F distances amount to 2.941 (3) and 2.902 (3) A, respectively. The increased steric energy of the ground state is responsible for a significantly lower racemization barrier of (I) and (II) in comparison to the unsubstituted [6]­helicene.

1976

tert-Butoxy radicals were generated by decomposing di-tert-butyl hyponitrite. The relative reactivities of substrates were measured from the ratio of tert-butanol and acetone formed. Polystyrene was found to be less reactive than model compounds. Activation parameters for the hydrogen abstraction reaction for cumene, 3-phenylpentane, and 2,4-diphenylpentane are reported and show small differences for the enthalpy of activation; large negative values for the entropy of activation demonstrate the importance of steric hindrance in the hydrogen abstracting step. 2,3-Diphenylbutane as model compound for head to head links in polystyrene exhibits low reactivity, whereas 2,5-diphenylhexane, a mode…

Unveiling the Ionic Diels-Alder Reactions within the Molecular Electron Density Theory

2021

The ionic Diels–Alder (I-DA) reactions of a series of six iminium cations with cyclopentadiene have been studied within the Molecular Electron Density Theory (MEDT). The superelectrophilic character of iminium cations, ω &gt

Ligand Effects on the Chemoselectivity of Ortho-Metalated Rhodium(II) Catalyzed α-Diazo Ketone Transformations

1997

Rh2(OOCR)2(PC)2 complexes (PC = orthometalated phosphines, OOCR = carboxylates) with very polarizable ligands, such as aromatic rings directly joined to the rhodium atoms, control chemoselectivity in competitive metal carbene transformations of α-diazo ketones. These catalysts have a mixed set of ligands that allows choosing among a big selection of ligands to gradually affect the electronic and steric properties of the catalyst. Their selectivity depends on the electrophilicity of the ligands and the polarizability of the metalated aromatic rings. Thus, Rh2(OOCR)2(PC)2 compounds [PC = (C6H4)P(CH3)(C6H5), (p-CH3C6H3)P(p-CH3C6H4)2, (C6H4)P(C6H5)2; R = C3F7 or CF3] exhibit an exceptional sele…

Uncommon coordination behaviour of P(S) and P(Se) units when bonded to carboranyl clusters: experimental and computational studies on the oxidation o…

2010

Oxidation of closo-carboranyl diphosphines 1,2-(PR(2))(2)-1,2-closo-C(2)B(10)H(10) (R=Ph, iPr) and closo-carboranyl monophosphines 1-PR(2)-2-R'-1,2-closo-C(2)B(10)H(10) (R=Ph, iPr, Cy; R'=Me, Ph) with hydrogen peroxide, sulfur and elemental black selenium evidences the unique capacity of the closo-carborane cluster to produce uncommon or unprecedented P/P(E) (E=S, Se) and P=O/P=S chelating ligands. When H(2)O(2) reacts with 1,2-(PR(2))(2)-1,2-closo-C(2)B(10)H(10) (R=Ph, iPr), they are oxidized to 1,2-(OPR(2))(2)-1,2-closo-C(2)B(10)H(10) (R=Ph, iPr). However, when S and Se are used, different reactivity is found for 1,2-(PPh(2))(2)-1,2-closo-C(2)B(10)H(10) and 1,2-(PiPr(2))(2)-1,2-closo-C(2)…

Remarkable Steric Effects and Influence of Monodentate Axial Ligands L on the Spin-Crossover Properties of trans-[FeII(N4 ligand)L] Complexes

2007

Iron(II) complexes obtained from tetradentate, rigid, linear N4 ligands have been investigated to appraise the influence of steric effects and the impact of trans-coordinated anions on the spin-transition behavior. As expected, the well-designed ligands embrace the metal center, resulting in octahedral iron(II) complexes where the basal plane is fully occupied by the pyridine/pyrazole N4 ligand, while anions or solvent molecules are exclusively axially coordinated. Precursor complexes, namely, [Fe(bpzbpy)(MeOH)2](BF4)2 (where bpzbpy symbolizes the ligand 6,6'-bis(N-pyrazolylmethyl)-2,2'-bipyridine) and [Fe(mbpzbpy)(MeOH)2](BF4)2 (where mbpzbpy symbolizes the ligand 6,6'-bis(3,5-dimethyl-N-p…

Activation of the Cyano Group at Imidazole via Copper Stimulated Alcoholysis

2019

Reactions of 4,5-dicyano-1-methylimidazole with CuX2 (X = Cl, Br) in alcohol solvents (ethanol and methanol) resulted in the formation of Cu(II) carboximidate complexes [CuCl2(5- cyano-4-C(OEt)N-1-methylimidazole)(EtOH)] (1), [Cu2(&micro

Direct High-Performance Liquid Chromatographic Separation of Peptide Enantiomers:  Study on Chiral Recognition by Systematic Evaluation of the Influe…

2002

All-R/all-S enantiomers of oligoalanines (Ala(n), n = 1-10) with N-terminal protection group have been separated by HPLC on chiral stationary phases based on various cinchona alkaloid selectors. Structure-enantioselectivity relationships derived by extensive selector structure optimization provided insights into binding mechanisms and chiral recognition. Their interpretation was supported by X-ray crystal structures of amino acid and dipeptide, respectively, in complex with chiral selector. Optimized selectors have bulky elements representing steric barriers and deep binding pockets that afforded very high enantioselectivities; e.g., for the all-R and all-S enantiomers of N-(3,5-dinitrobenz…