Dynamic covalent materials exploiting neighboring-broup effects

Abstract

Neighboring group participation or anchimeric assistance is of fundamental importance to a number of chemical transformations. In this regard, a promising reaction is the reversible condensation between a series of selected nucleophiles and ortho substituted aryl aldehyde or ketone boronic acids (Figure 1a: Dynamic covalent adduct formation (a), and frequency-sweep plot of G' and G'' for a selected dynamic covalent polymer gel exhibiting canonical Maxwell model behavior (b)). Such a reaction is quantitative in aqueous media at mM concentrations and neutral pH. The stability of the adducts is associated with neighboring-group interactions between the electron-rich imine nitrogen and the electron deficient boronic acid group, which is in close proximity to the imine bond. We have investigated the formation of a series of dynamic covalent adducts derived from a family of ortho substituted aryl aldehydes and ketone boronic acids. An overview on the binding mode (i.e. stoichiometry), thermodynamics and kinetics associated with the formation of the adducts, both in organic and aqueous media, will be provided. Our kinetic and thermodynamic studies are based on techniques such as isothermal titration calorimetry and 2D EXSY. Additionally, we will provide details on our synthetic strategies leading to dynamic covalent polymers and networks (Figure 1b), which exploit neighboring-group interactions. Our results contribute to the fundamental understanding of the formation of dynamic covalent adducts and the role of specific intramolecular neighboring-group effects, which unveils new opportunities for the preparation of stimuli-responsive and programmable materials.