As part of a continuing scientific challenge, a substantial effort during the past few decades has been devoted towards altering the selectivity of chemical transformations by arranging the potential reactants in a number of organized and confining media. Such systems, having features significantly different from those of isotropic solutions, include, for example, micelles, microemulsions, molecular aggregates, liquid crystals, and zeolites. Among these materials, stimuli-response gels constitute another important class of nanostructured and dynamic systems with high active surface areas and remarkable diffusion properties. Within this group, polymer gels have been traditionally used to obtain catalytic and reactive soft materials. Moreover, gels made of low-molecular-weight compounds represent a major novelty in this area as potential soft-vessels to carry out chemical reactions with control on product selectivity. In addition, the possibility of integrating switchable catalytic functions in both organo- and hydrogels shall accelerate the development of robust platforms for the 'bottom-up' tailor-fabrication of more sophisticated functional materials. The present critical review reports on the most important results published during the last decade regarding the use of 'smart' gels that has displayed promising properties as selective soft-nanoreactors and/or heterogeneous recyclable catalysts (152 references).

The effectiveness of neutral pH chitosan hydrogel beads (CSHB) as a green organocatalyst for a variety of C-C bond forming reactions (i.e. aldol reaction, Knoevenagel condensation, nitroaldol (Henry) reaction, Michael addition) has been comprehensively evaluated. Reaction rates, conversions and selectivities were studied as a function of a series of input variables including size, pH and reactive surface area of the beads, catalyst loading, temperature, molecular weight of the biopolymer, concentration, solvent system and molar ratio of reactants. Moreover, the catalytic biohydrogel beads were characterized by a variety of techniques including, among others, SEM, FT-IR, TGA and DSC.

An exemplar competition between gelation and crystallisation phenomena was examined with an unusual synergistic multicomponent (organo)gelator solution (MGS), which consists of a well-defined methanolic solution of (1R,2R)-1,2-diaminocyclohexane L-tartrate containing 2.4 equiv of concentrated hydrochloric acid. The optimal composition of the MGS was determined through meticulous solubility, gelation and structural studies, which support a transient gelation mechanism based on the kinetic self-assembly of the tartrate salt driven by hydrogen-bonding interactions, involving ammonium nitrogen donors and hydroxyl oxygen acceptors, and electrostatic interactions. The hydrochloric acid is involved in the solubilisation of the salt through an ionic dissociation-exchange process, which ends up with the formation-precipitation of (1R,2R)-1,2-diaminocyclohexane dihydrochloride. As a consequence, an irreversible destruction of the gel takes place, which indicates the metastable nature of this phase that cannot be accessed from the thermodynamically equilibrated state. Gelation of a variety of oxygenated and nitrogenated solvents with moderate polarity occurred efficiently using extremely low MGS concentrations at low temperatures, and the gel phase was confirmed by dynamic rheological measurements. Several features make the described MGS unique: (1) it is a multicomponent solution where each component and its stoichiometry plays a key role in the reproducible formation and stabilization of the gels; (2) it is formed by simple, small, and commercially available chiral building blocks (dissolved in a well-defined solvent system), which are easily amenable for further modifications; (3) the gelation phenomenon takes place efficiently at low temperature upon warming up the isotropic solution, conversely to the typical gel preparation protocol; (4) the formed organogels are not thermoreversible despite the non-covalent interactions that characterize the 3D-network. 45 non-covalent bonds, predominantly hydrogen-bonding, van der Waals, charge-transfer, dipole-dipole, π-π stacking, and coordination interactions, which usually lead to reversible gel-tosol phase transitions-. Furthermore, systems based on both types of connections are also known. 12,13 The solid-like appearance of 50 gel materials is the result of the entrapment of the liquid (major component) into the compartments of a solid 3D-matrix of a large surface area (minor component), typically through surface tension

Calcium alginate hydrogel beads have been found to promote the Henry reaction under mild conditions.