Synthesis and Steric Structure of 1, 5-Benzothiazepine-Phenyl-β-Lactams

l,5-BenzothiazepinesIpossesspotentialbiologicalactivitives.'-'Cycloadditionreactionsofthecompoundswithacidhandespermittheconstructionofnewcyclicsystemf0-lactamring.The0-lactammoietymaymodifythebiologicalactivityofi,5-benzothiazepinesormaybringneweffe...     

Ligand Steric Effects on Naphthyl-α-diimine Nickel Catalyzed α-Olefin Polymerization

Naphthyl-α-diimine nickel complexes with systematically varied ligand sterics, activated by modified methylaluminoxane(MMAO), were tested in the polymerization of higher α-olefin(1-hexene, 1-decene and 1-hexadecene) under suitable conditions. The polymerization results indicated the possibility of precise microstructure control, depending on catalyst structure, polymerization temperature, monomer concentration and types of monomers, which in turn strongly affects the resultant polymer properties. Naphthyl-α-diimine nickel complex bearing chiral bulky sec-phenethyl groups in the o-naphthyl position showed good catalytic activity, and resulted in branched polymers(42-88/1000 C) with high molecular weights(M_n:(4.3-15.2) × 10~4 g·mol~(-1)) and narrow molecular weight distribution(M_w/M_n = 1.13-1.29, RT), which suggested a living polymerization. The increasing steric hindrance of catalyst leads to enhance insertion for 2,1-insertion of α-olefin and the chain-walking reaction.     

Steric interactions between physically adsorbed polymer-coated colloidal particles: soft or hard?

The steric interaction potential between colloidal particles imparted by adsorbed polymer layers is directly related to their structure. Due to the complexity of these interfacial structures, the steric potential may behave differently at different interparticle separations. In this study, we proposed a combined model of the equivalent hard-sphere model (EHS) and the Hayter-Penfold/Yukawa model (HPY) to describe the steric potential due to adsorbed homopolymers on colloidal particles. The EHS potential describes the dense train/small-loop region and the HPY potential the more diffuse tail/long-loop region. The steric potential was extracted from the structure factors measured by small-angle neutron scattering (SANS). It was found that this combined model gave better agreement with experimental data than either of its component models alone. This study also shows that the adsorbed polymer layer in a good solvent partially collapses when the layers approach one another, which is also supported by an NMR solvent relaxation study.     

Steric hindrance versus asymmetric induction: new PN-ligands for carbon–carbon coupling reactions

Two new aminophosphine ligands 4 and 5 with increased steric interaction in proximity to the N-coordination site have been prepared. Their asymmetric induction in five model reactions was tested and compared to results obtained with the parent ligand 2. This resulted in improved enantioselectivity in only one case, from 5 to 68% e.e., while in all the other cases the more crowded ligands exhibited significantly lower reactivity accompanied by decreased enantioselectivity.     

Synthesis and Steric Structure of β-Lactam Derivatives of 2,4-Diaryl-2,3-dihydro-1,5-benzothiazepines

2,4-Diaryl-2,3-dihydro-l,5-benicthiazepines1posscsspotentbiologicalactivities1-3.CycloadditionreactionsatC=Ndoublebondmaypeaesttheconstructionofnewcyclicsystemsthatmayhaveneweffectsonstereochemistryandbiologicalactivitiesof1,5benzothiazepines.Followingourstudyonaseriesofreactionsofcompounds1,westudiedthecycloadditionreactionof1withchloroacetylchloride.Thetitlecompoundsarepresented.Especially,theirconfiguration(therelativestericpositionoftheCIandphenylgroups)andconformationwereestablishedbyX-…     

Azine Steric Hindrances Switch Halogen Bonding to N-Arylation upon Interplay with σ-Hole Donating Haloarenenitriles

An interplay between 4-bromo- and 4-iodo-5-nitrophthalonitriles (XNPN, X=Br or I) and any one of the azines (pyridine 1 , 4-dimethylaminopyridine 2 , isoquinoline 3 , 4-cyanopyridine 4 , 2-methylpyridine 5 , 2-aminopyridine 6 , quinoline 7 , 1-methylisoquinoline 8 , and 2,2’-bipyridine 9 ) proceeds differently depending on steric and electronic effects of the heterocycles. Sterically unhindered azines 1–3 underwent N-arylation to give the corresponding azinium salts (characterized by ¹H and ¹³C{H} NMR and high-resolution ESI-MS). In contrast, azines 4 – 9 with sterically hindered N atoms or bearing an electron-withdrawing substituent, form stable co-crystals with XNPN, where two interacting molecules are bound by halogen bonding. In all obtained co-crystals, X⋅⋅⋅N structure-directed halogen bonds were recognized and theoretically evaluated including DFT calculations (PBE0-D3/def2-TZVP level of theory), QTAIM analysis, molecular electrostatic potential surfaces, and noncovalent interaction plot index. Estimated energies of halogen bonding vary from −7.6 kcal/mol (for 6 ⋅ INPN) to −11.4 kcal/mol ( 5 ⋅ INPN).     

Kinetics of radical polymerization—XLV. Steric effects in the radical reactivity of quinones

An attempt has been made to establish the dependence on structural parameters of the reactivity of substituted 1,4-benzoquinones as inhibitors in the radical polymerization of styrene. The relative reactivity depends on the redox potential of the quinone and on the size of the ortho-substituents. The steric factor has been separated from the overall reactivity and correlated with the extent of shielding of the reaction centre in the case of substituents of spherical symmetry.     

Steric Effects in the Cl＋CHD3（v1=1） Reaction

A recent study has revealed a full 3-dimentional reactive scattering picture of the reaction CI＋CHD3（v1=1） as the C1 atoms attack CHD3 from various directions respective to the C-H stretching bond. The reported polarization-dependent differential cross sections provide the most detailed characterization of the influences of reagent alignments on reactivity. To convey the stereo-specific information more accessible to general chemists, we show here, by proper symmetry considerations, how to retrieve from the measurements the relative integral and differential cross sections of two most common collision geometries： the end-on versus side-on attacks. The results, albeit coarse-grained, provide an appealing picture that not only reinforces our intuition about chemical reactivity, but also sheds more light on the conventional （unpolarized） attributes.     

Steric effect on the formation of columnar phases in β-diketonate copper(II) complexes

A systematic study of the mesomorphic properties of three series of copper(II) complexes based on β-diketonate ligands containing branched side chains is reported. These disc-like compounds have four, six and eight flexible alkoxy side chains appended to the central core, in which two or four side chains were substituted by bulkier secondary alkoxy groups: 1-methylbutyloxy R ' = C₅(2°) or 1-methylheptyloxy R ' = C₈(2°). The mesomorphic results indicated that at least eight side chains are required to form stable columnar mesophases; other compounds with four or six side chains are not mesogenic regardless of the combination of the carbon length on the alkoxy or secondary alkoxy groups of the side chains. The compounds 3 with shorter R ' = C₅(2°) side chains were all non-mesogenic regardless of the carbon length of three alkoxy side chains (R = C₈, C₁₀, C₁₂) used. However, when the longer 1-methylheptyloxy side chain R ' = C₈(2°) was substituted, the compounds 3b-3e with various alkoxy groups (R = C₆, C₇, C₈, C₁₀, C₁₂) exhibited columnar phases. The mesophases were characterized and identified as columnar hexagonal phases (Col_h), as expected, by thermal analysis and optical polarized microscopy. The presence of the introduced secondary alkoxy groups apparently appeared to influence the formation of columnar phases. The clearing points were relatively lower than other similar copper(II) compounds not substituted by secondary alkoxy side chains.     

Steric effects in the catalytic amination of γ-, δ-, and ε-glycols

The amination of butane-1,4-diol, isomeric dipropylene glycols, and cyclohexane-1,4-diyldimethanol in the presence of nickel/copper/chromium catalysts has been studied. The effect of the initial glycol structure on the reaction selectivity has been estimated.     

Steric Considerations in Supramolecular Inclusion of Modified β-cyclodextrins with Triton X-100 and α-bromonaphthalene

Supramolecular inclusion of modified β-cyclodextrin (β-CD) with Triton X-100 (TX) and α-bromonaphthalene (BN) was studied by fluorescence and phosphorescence measurements. Major differences were observed in the magnitude of the apparent stability constants and quenching constants of the inclusion complexes. Methyl substitution on the rims of β-CD increased the binding of TX with β-CD but was unfavorable to the protection of the phenyl group of TX from fluorescence quenching and further accommodation of BN for steric considerations. According to the overall molecular size of β-CD, TX and BN, further inclusion of BN in the cavity of β-CD occupied by TX may force the flexible tert-octyl chain of TX to deform to a greater extent and close packing complexes were obtained. Phosphorescence of BN arising from intermolecular energy transfer between BN and the phenyl group of TX was observed when the phenyl group of TX was irradiated. In the case of heptakis(2,6-di-O-methyl)-β-CD, BN failed to penetrate into the cavity because of the steric hindrance of the methyl substituents at the rim of the β-CD cavity.     

Steric Exclusion Liquid Chromatography Studies in Urea on the Denaturation of the Bovine Eye Lens Protein α-Crystallin

Abstract

Steric exclusion liquid chromatography in the presence of intermediate urea concentrations with lowangle laser light scattering detection was used to investigate the stepwise dissociation of the multimeric bovine eye lens protein α-crystallin. The change in the quaternary structure of α-crystallin as a function of increasing urea concentration clearly resembled dissociation by increasing alkaline pH, urea or guanidine-HC1 concentrations when studied by sedimentation velocity analysis. Next to native and native-like threelayer aggregates (M_r 6.5 ? 7.5 × 10⁵), the first dissociation products (two-layer molecules M_r 4 ? 5.5 × 10⁵), the second dissociation products (core molecules M_r 2.5 ? 3 × 10⁵), and monomeric subunits (M_r 20 000) could be characterized. In the range from 2.6 to 4.4 M urea, we found a gradual decrease in the proportion of the remaining three-layer aggregates and an increase in that of monomeric subunits. The fluorescence emission maxima showed increasing solvent exposure of the tryptophan residues going from three-layer aggregates to monomeric subunits. The subunit compositions for most dissosiation products did not significantly differ from that of native α-crystallin. The interpretation of earlier results on Sephacryl-S200 steric exclusion chromatography in 3.8 M urea appeared to be an oversimplification.     

The Influence of Various Cationic Group on Polynorbornene Based Anion Exchange Membranes with Hydrophobic Large Steric Hindrance Arylene Substituent

A series of brominated polynorbornene derivatives,including bulky steric hydrophobic groups and highly physical and chemical stable backbones,were synthesized via ring-opening metathesis polymerization and post-functionalized with trimethylammonium(QA),Nmethylimidazole(Im),N-methylpyrrolidinium(Pyr) or N-methylpiperidinium(Pip) to construct the entire anion exchange membranes(AEMs).Benefited from prominent phase-separated morphology,PBO-x%-y(x=66,68,70;y=QA,Im,Pyr,Pip) AEMs with ion exchange cap...     

Four- and Five-Coordinate Oxovanadium(V) Alkoxides: Do Steric Effects or Electronic Properties Dictate the Geometry?

Sterically hindered ligands generally form metal complexes with lower coordination numbers than less hindered ligands. In contrast to dogma, the solid state vanadium(V) complexes with ethylene glycol and pinacol contain four- and five-coordinate vanadium atoms, respectively. Ab initio and electrostatic potential distribution calculations were conducted on both experimental and optimized geometries of the four- and five-coordinate oxovanadium chloroalkoxides. Ab initio energy calculations favor the five-coordinate species for all ligands at all levels of theory examined. No significant differences in the electrostatic properties of the vanadium atoms in the two types of molecules were observed by using electrostatic potential distribution analysis. Methyl group substitutions on the ligand did not change the electronics of the vanadium atoms sufficiently to be a factor. Thus, we conclude that neither electronics nor sterics at the metal center explains the experimental geometries. In the absence of a significant observable electronic effect at the vanadium, we note that the experimental observations can be attributed to ligand geometric effects such as the Thorpe-Ingold and/or gem-dialkyl effects.     

Secondary Interactions or Ligand Scrambling? Subtle Steric Effects Govern the Iridium(I) Coordination Chemistry of Phosphoramidite Ligands

The like and unlike isomers of phosphoramidite (P*) ligands are found to react differently with iridium(I), which is a key to explaining the apparently inconsistent results obtained by us and other research groups in a variety of catalytic reactions. Thus, the unlike diastereoisomer (aR,S,S)‐[IrCl(cod)( 1 a )] ( 2 a ; cod=1,5‐cyclooctadiene, 1 a =(aR,S,S)‐(1,1′‐binaphthalene)‐2,2′‐diyl bis(1‐phenylethyl)phosphoramidite) forms, upon chloride abstraction, the monosubstituted complex (aR,S,S)‐[Ir(cod)(1,2‐η‐ 1 a ,κP)]⁺ ( 3 a ), which contains a chelating P* ligand that features an η² interaction between a dangling phenyl group and iridium. Under analogous conditions, the like analogue (aR,R,R)‐ 1 a′ gives the disubstituted species (aR,R,R)‐[Ir(cod)( 1 a′ ,κP)₂]⁺ ( 4 a′ ) with monodentate P* ligands. The structure of 3 a was assessed by a combination of X‐ray and NMR spectroscopic studies, which indicate that it is the configuration of the binaphthol moiety (and not that of the dangling benzyl N groups) that determines the configuration of the complex. The effect of the relative configuration of the P* ligand on its iridium(I) coordination chemistry is discussed in the context of our preliminary catalytic results and of apparently random results obtained by other groups in the iridium(I)‐catalyzed asymmetric allylic alkylation of allylic acetates and in rhodium(I)‐catalyzed asymmetric cycloaddition reactions. Further studies with the unlike ligand (aS,R,R)‐(1,1′‐binaphthalene)‐2,2′‐diyl bis{[1‐(1‐naphthalene‐1‐yl)ethyl]phosphoramidite} ( 1 b ) showed a yet different coordination mode, that is, the η⁴‐arene–metal interaction in (aS,R,R)‐[Ir(cod)(1,2,3,4‐η‐ 1 b ,κP)]⁺ ( 3 b ).     

Steric constraints against [3,3]-sigmatropic rearrangement of allylic azides. A convenient approach to β-l-4-aminopent-2-enoglyceropyranosides

Starting from alkyl α-d-4-O-methanesulphonylpent-2-enoglyceropyranosides 13a–c, nucleophilic substitution carried out with polymer-supported azide ion led to regioisomeric mixtures of the azides 14a–c and 15a–c. An analogous result, due to a [3,3]-sigmatropic rearrangement, was observed starting from methyl α-d-hex-2-enoerythropyranoside derivatives 6a and 6b. On the contrary, starting from alkyl β-d-4-O-methanesulphonylpent-2-enoglyceropyranosides 21a–c, azides 22a–c were exclusively obtained, and subsequently converted into the corresponding amino derivatives 23a–c. The behaviour of β-anomers 21a–c was ascribed to steric interactions in the cyclic transition state, as supported by ab initio calculations.     

Steric,Electronic and Conformational Synergistic Effects in the Gold(I)-catalyzed α-C−H Bond Functionalization of Tertiary Amines**

Direct C−H bond functionalization is a useful strategy for the straightforward formation of C−C and C−Heteroatom bonds. In the present work, a unique approach for the challenging electrophilic Au-catalyzed α-C−H bond functionalization of tertiary amines is presented. Electronic, steric and conformational synergistic effects exerted by the use of a malonate unit in the substrate were key to the success of this transformation. This new reactivity was applied to the synthesis of tetrahydro-γ-carboline products which, under oxidative conditions, could be converted into valuable structural motifs found in bioactive alkaloid natural products.     

Flexibility versus Rigidity: How Conformational Constraint,Steric Hindrance and Solvent Impact Binding between π-Electron Poor Tweezer-Type Hosts and π-Electron Rich Guests

Tweezer-type receptors that form π−π stacked supramolecular complexes are important components in functional polymeric materials and molecular machines. Herein, we study how varying specific structural components of tweezer-receptors impacts their binding. A library of tweezer receptors, each containing two π-electron poor receptor residues and differing by the nature of the linking unit which was either a flexible 2,2′-(ethylenedioxy)bis(ethylamine) residue or a rigid 3,3’’-diamino-m-terphenyl diamine structure, were synthesised. Each tweezer formed 1 : 1 supramolecular complexes with π-electron rich residues (1,5-dihydroxynapthalene and pyrene) as confirmed by UV/Vis and ¹H NMR spectroscopic studies. Binding constants were determined to be between 2.3×10⁻⁵ and 71 M⁻¹ in organic solvents and were one magnitude greater in aqueous solvents for water soluble systems. The nature of the linker had variable effects on the binding constants, showing the design of tweezer type supramolecular receptors with targeted K_a values is non-trivial and requires structural optimisation supported by binding constant determination studies.