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In this article, we report the computational examination of the impact of hydrogen bonding (HB) on the stereoselectivity of a series of Diels–Alder (DA) reactions. Four different types of diene/dienophile couples were studied including (a) cyclopenta-2,4-dien-1-ol and heteroatom-substituted cyclopentenes, (b) substituted cyclopentadienes and N-protonated 2,5-dihydro-1H-pyrrole, (c) furan and N-protonated 5-azabicyclo[2.1.1]hex-2-ene, and (d) N-protonated cyclopenta-2,4-dien-1-amine and α,β-unsaturated carbonyl compounds. These systems were designed such that the HB can only exist in the exo reaction pathway. The optimized structures of the transition states (TSs) and products, along with the Gibbs free energies of activation and reaction were calculated in the gas phase and dichloromethane by B3LYP/6-31G++(d,p) method. It was found that the exo adduct is the most kinetically and thermodynamically favored product for all of the DA reactions mentioned above. Moreover, the role of the HB in stabilizing the exo stereoisomer (and the corresponding TS) was much more pronounced for the reactions involving cationic species (cases b–d). In the end, a hypothetical synthetic method was proposed for obtaining exo-2-alkylbicyclo[2.2.1]heptanes. Our findings suggest that the HB can be employed as an effective interaction to control the stereoselectivity of DA reactions. [ABSTRACT FROM AUTHOR]

Ionic liquids, which are widely known as room temperature molten salts, have been the subject of much scientific debate for more than a decade. These compounds can be used as green solvents or even as additives in many reactions. Due to their noticeable conductivity, which is a function of the interaction energies existing between the ion pairs in ionic liquids, the synthesis and introduction of new families of these chemicals are of importance. The target of this paper is to design new carboxylate-based ionic liquids containing OH and CF3 groups. Our results indicate that the properties of these ILs are highly affected by the presence of OH and CF3 groups, in which OH stabilizes the negative charge of the anion via intramolecular hydrogen bonds and CF3 stabilizes it through the electron withdrawing effect. These groups decrease the binding energy between the anion and cation of the carboxylate-based ionic liquids and thus affect their physical properties such as melting point and density as well as their chemical properties. All calculations in this study were performed in the gas phase at the B3LYP/6-311++G(d,p) level. [ABSTRACT FROM AUTHOR]

Polyols were used as model ligands for Mg2+, Ca2+, and Zn2+ complexes to study the role of the hydrogen bond network on the metal binding affinity and modulation of successive pKa values using density functional theory. The results confirm that the acidity of polyols dramatically increases upon metal complexation in the order Zn2+ ＞ Mg2+ ＞ Ca2+. For example, the three H-site positions in the hydroxyl groups of the heptaol, bound to Zn2+, are 11.2, 29.9, and 30.9 pKa units (in methanol) more acidic than those of pure heptaol. This acidity enhancement leads to making polyols as good ligands toward complexation. For instance, the formation constants of the heptaol in the presence of Zn2+, Mg2+, and Ca2+ in methanol were 5.67 × 1071, 6.46 × 1066, and 1.26 × 1053 times lower than those in its third deprotonation state, respectively. The natural bond orbital (NBO), quantum theory of atoms in molecules (QTAIM), and reduced density gradient (RDG) analyses show that the intramolecular hydrogen bond network and increment of carbon chain length lead to the enhancement of metal binding affinity. These findings can be used for the manipulation of ligands and metal cations in designing metalloproteins. [ABSTRACT FROM AUTHOR]