Catalytic molecular gels for click-aldol reactions

Abstract

Thus, a gel can be defined as a two-component colloidal dispersion which has a continuous structure with macroscopic dimensions. Gels are composed mainly with a small amount of solid agent (minor component) named as gelator that is able to form a three dimensional network (3D) that can immobilise a large volume of solvent (major component). This network provides viscoelastic properties and the formation of a gel can be tested easily by the so-called “tube inversion test” where a gel is considered if it remains immobilized upon turning up side down the recipient. Molecular gels have attracted an enormous amount of interest by their potential applications proposed up to now in fields of great social impact like biomedicine or materials science. Molecular gels are nanostructured soft materials formed by low-molecular-weight organic compounds by the sole action of non-covalent intermolecular interactions (solvophobic, ionic, H-bonding, π-π stacking, van der Waals, metal coordination, etc.). These molecular interactions are reversible and can be modulated by stimuli. Amazingly, very simple and easy to synthesize molecules are capable of selfassembly into nanoscopic aggregates that form a 3D network that immobilises a large amount of solvent. Molecular gels are hierarchically built-up from the molecular to supramolecular level.The molecules are organised into 1D objects that bundle into fibrillar architectures, and after physical cross-linking, will form a network that percolates the solvent. The gel is formed in a super-saturated solution, and depending on the solubility constant and assembly mechanism, a given amount of free gelator could remain in solution.4 In many cases gels are formed by dissolution of gelator in hot solvent and subsequent cooling to a given temperature. The supramolecular approach to the field of catalysis has attracted the attention of many researchers since the pioneering work of Pedersen or Lehn in that field.5 Self-assembly is a ubiquitous phenomenon for the bottom-up construction of supramolecular catalysts that aim to emulate the efficiency and selectivity of natural enzymes due to the construction of a suprastructure capable of performing a task that the individual components cannot achieve independently.