Effect Of Conformational Rigidity Research Articles

Intramolecular N-H⋅⋅⋅F Hydrogen Bonding Interaction in a Series of 4-Anilino-5-Fluoroquinazolines: Experimental and Theoretical Characterization of Electronic and Conformational Effects.

The 4-anilino-6,7-ethylenedioxy-5-fluoroquinazoline scaffold is presented as a novel model system for the characterization of the weak NH⋅⋅⋅F hydrogen bonding (HB) interaction. In this scaffold, the aniline NH proton is forced into close proximity with the nearby fluorine (dH,F ∼2.0 Å, ∠∼138°), and a through-space interaction is observed by NMR spectroscopy with couplings (1h JNH,F ) of 19±1 Hz. A combination of experimental (NMR spectroscopy and X-ray crystallography) and theoretical methods (DFT calculations) were used for the characterization of this weak interaction. In particular, the effects of conformational rigidity and steric compression on coupling were investigated. This scaffold was used for the direct comparison of fluoride with methoxy as HB acceptors, and the susceptibility of the NH⋅⋅⋅F interaction to changes in electron distribution and resonance was probed by preparing a series of molecules with different electron-donating or -withdrawing groups in the positions para to the NH and F. The results support the idea that fluorine can act as a weak HB acceptor, and the HB strength can be modulated through additive and linear electronic substituent effects.

ChemInform Abstract: Effect of Conformational Rigidity on the Stereoselectivity of Nucleophilic Additions to Five‐Membered Ring Bicyclic Oxocarbenium Ion Intermediates.

AbstractOn the basis of four substrates, the influence of structural perturbations of a fused seven‐membered ring on the selectivity of nucleophilic substitution of bicyclic acetals is studied.

Effect of conformational rigidity on the stereoselectivity of nucleophilic additions to five-membered ring bicyclic oxocarbenium ion intermediates.

Nucleophilic substitution reactions of five-membered ring acetals bearing fused rings reveal that subtle changes in the structure of the fused ring can exert dramatic influences on selectivity. If the fused ring did not constrain the five-membered ring undergoing substitution, selectivity was comparable to what was observed for an unconstrained system (≥92% diastereoselectivity, favoring the product of inside attack on the oxocarbenium ion). If the ring were more constrained by including at least one oxygen atom in the ring, selectivity dropped considerably (to 60% diastereoselectivity in one case). Transition states of the nucleophilic addition of allyltrimethylsilane to selected oxocarbenium ions were calculated using DFT methods. These computational models reproduced the correlation between additional conformational rigidity and selectivity.

Effect of Conformational Rigidity on Physical Properties of Some Poly(Imide–Amide)s Containing Dimethylsilane Units

A series of poly(imide–amide)s containing dimethylsilane units has been synthesized by low temperature solution polycondensation of various aromatic diamines that have ether bridges between phenylene rings with a diacid chloride incorporating a dimethylsilane group and two imide rings, namely, bis[N‐(4‐chlorocarbonylphenyl)phthal‐imidyl]dimethylsilane. The polymers were easily soluble in polar amidic solvents and show high thermal stability with initial decomposition temperature being above 410°C and glass transition temperature in the range of 251–281°C. The polymer solution can be cast into flexible films with the dielectric constant in the range of 3.66–4.05. Conformational rigidity parameters of these polymers have been calculated by the Monte Carlo method with allowance for hindered rotation. Several physical properties such as solubility, glass transition temperature, initial decomposition temperature, and dielectric constant are discussed in relation with the structure of the polymers and rigidity of their chains.

Effect of conformational rigidity on the glass transition and initial decomposition temperatures of polyimides

The dependence of the glass transition and initial decomposition temperatures of polyperyleneimide and polynaphthylimide on the conformational rigidity was studied by the Monte Carlo, Kuhn segment, and quantum chemical AM1 methods. The corresponding linear plots can be used for estimation of the glass transition and initial decomposition temperatures when experimental determination is difficult.

The Effect of Conformational Rigidity on Several Physical Properties of Polymers

The phenomenological dependences of several physical properties such as the softening temperature, friction coefficient, elasticity modulus, fire-resistance parameters and quantum luminescence yield on the conformational rigidity of a polymeric chain are revealed for polyheteroarylenes. The contribution of the conformational rigidity of a polymer chain to the physical properties of this class of polymers is shown to be significant. It should be taken into account in estimation of the above-mentioned properties together with the contribution of intermolecular interactions.

Hydrogen Bonding and π-Stacking Interactions as Organizing Elements in Lariat Ethers Containing Nucleotide Bases

Two new bibracchial lariat ethers have been prepared as flexible model systems for nucleotide H-bonding and π-stacking interactions. In both compounds, nucleotide bases terminate side arms attached at the nitrogen atoms of a diaza-18-crown-6 macroring. Each side arm is terminated by one or more nucleotide bases. The unsymmetrical structure is ade-CH2CH2 CH2CH2CH2-thy-CH2CH2-ade (A−O−T−A). The symmetrical compound is ade-CH2CH2-thy-CH2CH2CH2 CH2CH2CH2-thy-CH2CH2-ade (A−T−O−T−A). NMR studies show that the thy−ade side arm organizes by π-stacking in both A−O−T−A and A−T−O−T−A. Both compounds exhibit hydrogen bonding between the stacked base pair and the opposite adenine. A−O−T−A exhibits a stronger interaction resulting from the enforced interplay of noncovalent forces, as evidenced by one- and two-dimensional NMR experiments. This compound also presented the opportunity to detect and examine single hydrogen-bonded interactions between two adenine bases. The effects of conformational rigidity, side...

Asymmetric cyclopalladation of 1′-dimethylamino-[3] ferrocenophane: central and planar chirality and the effect of conformational rigidity

1′-Dimethylamino[3]ferrocenophane undergoes asymmetric cyclopalladation in the presence of the sodium salt of N-acetyl- D- or - L-leucine to give an optically active organopalladium derivative in nearly quantitative yield. Due to the restriction of the conformational mobility of the bridged molecule, the S-configuration of the chiral centre induces preferentially S p planar chirality. An enantiomeric excess of 84.0% was achieved when 0.5 equivalent of Na 2PdCl 4 was used with the simultaneous kinetic resolution of amine, affording nearly 50% of enantiomeric purity.