Resonance-Assisted Hydrogen Bonding as a Driving Force in Synthesis and a Synthon in the Design of Materials

Resonance-assisted hydrogen bonding (RAHB), a concept introduced by Gilli and co-workers in 1989, concerns a kind of intramolecular H-bonding strengthened by a conjugated π-system, usually in 6-, 8-, or 10-membered rings. This Review highlights the involvement of RAHB as a driving force in the synthesis of organic, coordination, and organometallic compounds, as a handy tool in the activation of covalent bonds, and in starting moieties for synthetic transformations. The unique roles of RAHB in molecular recognition and switches, E/Z isomeric resolution, racemization and epimerization of amino acids and chiral amino alcohols, solvatochromism, liquid-crystalline compounds, and in synthons for crystal engineering and polymer materials are also discussed. The Review can provide practical guidance for synthetic chemists that are interested in exploring and further developing RAHB-assisted synthesis and design of materials.

Understanding the interplay of weak forces in [3,3]-sigmatropic rearrangement for stereospecific synthesis of diamines

Chiral diamines are important building blocks for constructing stereoselective catalysts, including transition metal based catalysts and organocatalysts that facilitate oxidation, reduction, hydrolysis, and C-C bond forming reactions. These molecules are also critical components in the synthesis of drugs, including antiviral agents such as Tamiflu and Relenza and anticancer agents such as oxaliplatin and nutlin-3. The diaza-Cope rearrangement reaction provides one of the most versatile methods for rapidly generating a wide variety of chiral diamines stereospecifically and under mild conditions. Weak forces such as hydrogen bonding, electronic, steric, oxyanionic, and conjugation effects can drive this equilibrium process to completion. In this Account, we examine the effect of these individual weak forces on the value of the equilibrium constant for the diaza-Cope rearrangement reaction using both computational and experimental methods. The availability of a wide variety of aldehydes and diamines allows for the facile synthesis of the diimines needed to study the weak forces. Furthermore, because the reaction generally takes place cleanly at ambient temperature, we can easily measure equilibrium constants for rearrangement of the diimines. We use the Hammett equation to further examine the electronic and oxyanionic effects. In addition, computations and experiments provide us with new insights into the origin and extent of stereospecificity for this rearrangement reaction. The diaza-Cope rearrangement, with its unusual interplay between weak forces and the equilibrium constant of the reaction, provides a rare opportunity to study the effects of the fundamental weak forces on a chemical reaction. Among these many weak forces that affect the diaza-Cope rearrangement, the anion effect is the strongest (10.9 kcal/mol) followed by the resonance-assisted hydrogen-bond effect (7.1 kcal/mol), the steric effect (5.7 kcal/mol), the conjugation effect (5.5 kcal/mol), and the electronic effect (3.2 kcal/mol). Based on both computation and experimental data, the effects of these weak forces are additive. Understanding the interplay of the weak forces in the [3,3]-sigmatropic reaction is interesting in its own right and also provides valuable insights for the synthesis of chiral diamine based drugs and catalysts in excellent yield and enantiopurity.