Brian R. Linton

CALORIMETRIC INVESTIGATION OF GUANIDINIUM-CARBOXYLATE INTERACTIONS

Abstract Isothermal titration calorimetry was utilized to study the association between a series of guanidinium derivatives and tetrabutylammonium acetate. This technique provides a measure of association strength, stoichiometry of binding, as well as thermodynamic parameters of association from a single experiment. Guanidinium derivatives which can form bidentate linear hydrogen bonds with acetate show significant, exothermic binding in DMSO (Ka = 5600 M −1 ; Δ H = −3.6 kcal/mol), while derivatives which lack this bidentate linear hydrogen bonding interaction result in complexes where association is weaker (Ka ≈ 100 M −1 ) and enthalpically neutral or endothermic. Additionally calorimetry permits the complete assessment of the multiple binding equilibria when derivatives complex two equivalents of guest.

Nitronate anion recognition and modulation of ambident reactivity by hydrogen-bonding receptors.

Nitronate anions were shown to form complexes in DMSO with hydrogen-bonding receptors such as 1,3-dimethylthiourea 1 (K(a)= 120M(-1)) and bicyclic guanidinium 2 (K(a) = 3200M(-1)). A ditopic bis-thiourea exhibited increased association with substrates, that contained either two nitronates (K(a)= 7000M(-1)) or a combination of nitronate and carboxylate (K(a)=7200M(-1)). Complexation of nitronate resulted in a change in the ambident reactivity during alkylation with p-nitrobenzyl bromide. The predominant reaction pathway was shifted from oxygen alkylation to carbon alkylation as receptor binding strength increased. Kinetic analysis indicated an overall inhibition of nitronate reactivity, and this suggests that greater suppression of the oxygen pathway allows carbon alkylation to predominate.

Host-guest chemistry: Combinatorial receptors

A combinatorial approach to receptor design provides an expedient method to discover the most effective host-guest complexes from within a library. Recent advances focus on generation of larger libraries, facile detection, combinatorial catalysis and the formation of dynamic receptor libraries.

Controlling Hydrogen Bonding: From Molecular Recognition to Organogelation

Hydrogen bonding has been studied extensively in solution, in the solid state and using theoretical methods. These studies complement each other and contribute to the progress made in understanding the behavior of this intermolecular interaction and its implications in chemistry, biology and physics. Extensive progress has been made in employing hydrogen bonding in fields such as self-assembly, host-guest recognition and crystal engineering. Recently, various hydrogen-bonding groups have been used in the design of organogelators, molecules that gel various solvents. In this chapter, we will outline the important work that has laid the foundations for the use of different hydrogen-bonding motifs in small host recognition studies and how this can be exploited in the design of self-assembling and gelating structures.

Curtin–Hammett and Steric Effects in HOBt Acylation Regiochemistry

While hydroxybenzotriazole is commonly used in a variety of bond-forming reactions, its acylation has been shown to produce a regiochemical (O vs N) mixture with complex kinetic behavior. Increased steric bulk on the electrophile favors formation of the oxygen-acylated product. Upon standing as a solid, the mixture can isomerize completely to the nitrogen adduct. An equilibrium ratio of regioisomers can be re-established in solution by adding either nucleophilic or electrophilic reagents, suggesting that the composition of the mixture is not significant to subsequent reactivity. Solvents can affect this regiochemical equilibrium through a Curtin-Hammett effect, where the shift in the tautomeric equilibrium of HOBt in polar solvents biases the reaction toward the oxygen adduct.

Electrochemical studies on molecular recognition of anions : complex formation between xylylenyl bis-iminoimidazolinium and dicarboxylates in nitrobenzene and water

Abstract This paper reports studies of complex formation between the synthetic dicarboxylate receptor para -xylylenylbis-(iminoimidazolinium) ( 1 2+ ) and dicarboxylates in electrolyte-containing nitrobenzene and water using cyclic voltammetry resulting from charge transfer across a liquid|liquid interface. A series of dicarboxylate—tetrabutylammonium salts was prepared and investigated systematically. The receptor 1 2+ shows modest selectivity in binding glutarate in nitrobenzene over longer dicarboxylates. The interactions between 1 2+ and glutarate are very weak in the aqueous phase, and the association constant for the binding reaction is 190 ± 100 M −1 . The association constants between 1 2+ and glutarate in electrolyte-containing nitrobenzene saturated with an aqueous phase and in electrolyte-containing nitrobenzene that has not been saturated with an aqueous phase are 7.46 ± 0.39 × 10 4 and 2.12 ± 0.28 × 10 5 M −1 , respectively. 1 2+ transfers across the water|nitrobenzene interface reversibly. However, it does not facilitate the transfer of shorter chain dicarboxylates, as crown ethers do for alkali metals. The failure to observe facilitated transfer is not due to a lack of complexation between glutarate and 1 2+ . Rather it arises because at potentials sufficiently negative (water phase vs. nitrobenzene phase) to drive the anionic dicarboxylate into the receptor-enriched nitrobenzene, the cationic receptor has already been driven in the opposite direction, depleting it from the organic side of the interface.