Thermodynamic and Spectroscopic Scales of Hydrogen‐Bond Basicity and Affinity

Abstract

Water, alcohols, phenols, hydrogen halides, carboxylic acids, primary and secondary amines and amides, thiols, 1-alkynes and so on constitute a special class of Lewis acids. They all contain an XH group in which the hydrogen atom is bonded to an electronegative atom X and bears a partial positive charge. Therefore, the hydrogen is attracted, by means of electrostatic forces, toward high electron density regions, mainly the lone pairs of Lewis bases B, and a hydrogen bond (HB) X H · · · B is formed. In addition to this electrostatic interaction, there is a charge transfer from the base to the antibonding orbital σ * of the X H sigma bond. The respective contributions of electrostatic forces and charge transfer are a matter of debate. A modern electrostatic model of the hydrogen bond provides a near-quantitative description of structure and properties for a wide range of typical hydrogen bonds [1]. In contrast, NBO analysis suggests that charge transfer of n→σ * type is a characteristic feature of hydrogen bonding [2,3]. In any case, the existence of even a small charge transfer allows the hydrogen-bonding interaction to be considered as Lewis acid/base in nature [4]. The hydrogen-bond donor (HBD) XH is the electron acceptor and the hydrogen-bond acceptor (HBA) B is the electron donor. In the HSAB concept, HBDs are classified as hard Lewis acids [4] from the simple observation that the hydrogen bond is stronger when B is fluorine, oxygen or nitrogen than when it is iodine, sulfur or phosphorus, respectively.The reaction 4.1 of formation of a hydrogen bond between the molecules XH and B:can be used for the construction of a Lewis basicity scale, if a reference HBD is chosen and if the equilibrium constant K of the reaction is measured for a series of bases B under the same conditions: physical state (gas or solution in a given solvent), temperature and pressure. Such a scale, as logK or ∆G, must be named a hydrogen-bond basicity scale.

A Catalytic Asymmetric Hydrolactonization

“…Recognizing these limitations and inspired by our recent success in activating unbiased olefins toward hydrofunctionlizations, we sought to design a suitable chiral organocatalyst capable of imparting stereocontrol on the process. Two critical factors were central to the design of such a catalyst: first, enabling substrate reactivity via activating the alkene; and second, providing an environment conducive to facial discrimination during cyclization, rendering the process stereoselective …”