Search results for "Effects"

13C-NMR Based Evaluation of the Electronic and Steric Interactions in Aromatic Amines

2005

Abstract: Chemical shifts of the para carbon atoms, δ(13C-4), in a series of aromatic amines were used to calculate the Ãp, ÃR and ÃOR substituent constants for different amino groups. 1-Pyrrolidino, N,N-di-n-butylamino and N,N-diethylamino groups were found to be the most strong electron-donors. ortho-Substitution decreases the donor properties of the amino group. The amino groups in 2,6-di-i-propylaniline and N,N-2,6-tetramethylaniline have very weak electron-donor properties. The nitrogen atom in benzoquinuclidine and N,N-dimethyl-2,6-di-i-propylaniline have an electron-acceptor character. The calculated substituent constants of the amino groups studied are consistent with the s…

Exploration of the Solid-State Sorption Properties of Shape-Persistent Macrocyclic Nanocarbons as Bulk Materials and Small Aggregates.

2020

Porous molecular materials combine benefits such as convenient processability and the possibility for atom-precise structural fine-tuning which makes them remarkable candidates for specialty applications in the areas of gas separation, catalysis, and sensing. In order to realize the full potential of these materials and guide future molecular design, knowledge of the transition from molecular properties into materials behavior is essential. In this work, the class of compounds termed cycloparaphenylenes (CPPs)-shape-persistent macrocycles with built-in cavities and radially oriented π-systems-was selected as a conceptually simple class of intrinsically porous nanocarbons to serve as a platf…

Mbandakamine-Type Naphthylisoquinoline Dimers and Related Alkaloids from the Central African Liana Ancistrocladus ealaensis with Antiparasitic and An…

2018

Four new dimeric naphthylisoquinoline alkaloids, michellamine A5 (2) and mbandakamines C–E (4–6), were isolated from the Congolese plant Ancistrocladus ealaensis, along with the known dimer mbandakamine A (3). They represent constitutionally unsymmetric dimers, each consisting of two 5,8′-coupled naphthylisoquinoline monomers. While the molecular halves of michellamine A5 (2) are linked via C-6′ of both of the naphthalene moieties, i.e., via the least-hindered positions, so that the central biaryl axis is configurationally unstable and not an additional element of chirality, the mbandakamines 3–6 possess three consecutive stereogenic axes. Their monomeric units are linked through an unprece…

Copper(I) Halides (X = Br, I) Coordinated to Bis(arylthio)methane Ligands: Aryl Substitution and Halide Effects on the Dimensionality, Cluster Size a…

2014

Bis(phenylthio)methane (L1) reacts with CuI to yield the 1D-coordination polymer [{Cu4(μ3-I)4}(μ-L1)2]n (1) bearing cubane Cu4I4 clusters as connecting nodes. The crystal structures at 115, 155, 195, and 235 K provided evidence for a phase transition changing from the monoclinic space group C2/c to P21/c. The self-assembly process of CuI with bis(p-tolylthio)methane (L2), bis(4-methoxyphenylthio)methane (L3), and bis(4-bromo-phenylthio)methane (L4) affords the 1D-coordination polymers [{Cu4(μ3-I)4}(μ-Lx)2]n (x = 2, 3, or 4). Compounds 2 and 4 are isostructural with C2/c low temperature polymorph of 1, whereas the inversion centers and 2-fold axes are lost in 3 (space group Cc). The use of b…

Overcoming Steric Hindrance in Aryl‐Aryl Homocoupling via On‐Surface Copolymerization

2019

On-surface synthesis is a unique tool for growing low-dimensional carbon nanomaterials with precise structural control down to the atomic level. This novel approach relies on carefully designed precursor molecules, which are deposited on suitable substrates and activated to ultimately form the desired nanostructures. One of the most applied reactions to covalently interlink molecular precursors is dehalogenative aryl-aryl coupling. Despite the versatility of this approach, many unsuccessful attempts are also known, most of them associated to the poor capability of the activated precursors to couple to each other. Such failure is often related to the steric hindrance between reactants, which…

Synthesis and stereochemical properties of "extended" biphenols bridged by ortho-, meta-, and para-phenylene spacers

2009

A series of isomeric biphenols based on para- (1), meta- (2), and ortho- (3) terphenyl backbones was synthesized. Suzuki cross-coupling methodology was employed for the construction of the terphenyl backbone of their methyl-protected precursors (respectively 8, 13, and 16). Using K2CO3 as the base, the best reaction conditions involved DMF at 100 °C as solvent. Anhydrous conditions greatly improved the yields of the sterically crowded systems (particularly 16). ortho-Terphenyls 3 and 16 exist as mixtures of syn/anti atropisomers in solution. Compound 16 crystallizes in the anti-in form. DFT calculations at the B3LYP/6-311+G(d,p) level indicate that for both compounds the anti-in form is mor…

Self-Sorting Effects in the Self-Assembly of Metallosupramolecular Rhombi from Chiral BINOL-Derived Bis(pyridine) Ligands

2013

Four BINOL-based bis(4-pyridyl) ligands were synthesised in enantiopure and racemic form. These ligands form metallosupramolecular [(dppp)2M2L2] rhombi with cis-protected [(dppp)Pd]2+ and [(dppp)Pt]2+ ions. In principle, racemic ligands can self-assemble into three stereoisomeric rhombi. The degree of self-sorting in the self-assembly process crucially depends on the substitution pattern and the resulting bend angle of the V-shaped ligands as well as the degree of steric crowding within the assembly when racemic ligands are used. Thus, these processes either lead to homochiral assemblies in a narcissistic self-recognition manner, to heterochiral assemblies in a social self-discriminating ma…

2015

Propargyl groups are attractive functional groups for labeling purposes, as they allow CuAAC-mediated bioconjugation. Their size minimally exceeds that of a methyl group, the latter being frequent in natural nucleotide modifications. To understand under which circumstances propargyl-containing oligodeoxynucleotides preserve base pairing, we focused on the exocyclic amine of cytidine. Residues attached to the exocyclic N4 may orient away from or toward the Watson-Crick face, ensuing dramatic alteration of base pairing properties. ROESY-NMR experiments suggest a uniform orientation toward the Watson-Crick face of N(4)-propargyl residues in derivatives of both deoxycytidine and 5-methyl-deoxyc…

Gas chromatography of homologous esters

1984

Abstract The gas chromatographic retention behaviour of the propanoyl and monochloropropanoyl esters of saturated C3C5 branched-chain and unsaturated C3C6 alcohols on SE-30 and OV-351 capillary columns at several temperatures is reported. Retention index increments at 80 and 120°C show the effect of branching in the alkyl chain and also the effect of unsaturation and of its position in the chain with chlorination at the two possible positions in the acyl chain. The various incremental effects are discussed and compared with the behaviour of the corresponding n-alkyl esters.

Reactivity of dialkoxydibutylstannanes toward carbon dioxide: A DFT study of electronic and steric effects

2012

Abstract DFT calculations were performed for the reaction of CO 2 with the monomeric species, R′ 2 Sn(OR) 2 , (R = R′ = CH 3 ; R = CH 3 , CH 2 CH 3 , CH(CH 3 ) 2 , R′ =  n -Bu) for assessing the role of electronic and steric effects in the kinetics and thermodynamics of CO 2 insertion into Sn–OR bonds. The reaction pathways are exothermic and involve the successive insertion into the two Sn–OR bonds. The driving force for insertion is ascribed to a charge-transfer between the HOMO of the complexes, mainly localized on the oxygen atom of the alkoxy ligands, and the LUMO of CO 2 . Interestingly enough, the energy barrier of the second insertion is much lower by around 27 kJ mol −1 , and quite…