Supported reagents have been widely used in organic synthesis for some 25 years and their importance is likely to increase as a result of new environmental legislation and the drive towards clean technology. While many supported reagents are stoichiometric in reactions the successful development of genuinely catalytic materials greatly enhances their value especially in liquid phase, typically fine chemical syntheses. Achieving an understanding of the nature of these fascinating materials is also an important aspect of their development and is essential if their true potential is t o be realised. Solid acids are the most widely studied of supported reagents and their use as more environmentally acceptable replacements for conventional Bronsted and Lewis acids is likely t o become increasingly important. Clayzic is a good example of an environmentally friendly catalyst with particular value as an alternative t o the hazardous reagent aluminium chloride in Friedel-Crafts reactions. The structure and properties of this catalyst are, however, poorly understood. X-Ray diffraction studies show that thermal treatment of either clayzic or its base material K10 results in the loss of any montmorillonite crystallinity that remained after the acid treatment of tonsil 13 used to form K1O. Thermal treatment of clayzic also results in a steady increase in the surface area of the material. While this is also consistent with structural changes the increase is also likely t o be partly due to the dehydration allowing the non-polar adsorbate to enter more of the polar regions of the material. These polar regions can be identified as mesopores created by the acid treatment of the clay and in which the zinc ions largely reside. Spectroscopic titration of the acid sites in clayzic show these to be largely Lewis acid in character. Thus clayzic owes its remarkable Friedel-Crafts activity to the presence of high local concentrations of zinc ions in structural mesopores. Relative reaction rates for the clayzic catalysed benzylation of alkylbenzenes also reveal the importance of these highly polar mesopores. Considerable rate enhancements can be achieved by thermally activating the catalyst and this can be largely attributed to the dehydration of the catalyst enabling better partitioning of the alkylbenzenes into the mesopores. Clayzic can be considered as being a large pore molecular sieve but where the sieving of molecules is controlled more by molecular polar it ies/polarisi bi I i ties than by molecular shape. Michael Reaction (e.g. AcCH=CH, + EtN02 -Ac[CH,],CHMeNO,) J. CHEM. SOC. PERKIN TRANS. 2 1994 KCN-18-Crown-6 50 'C, 6 h Oxidation Addition (e.g Ph,CH, -ph2CO) [e.g. CHCI, + m -0,NC,H4CH0m -02NCsH4CH (0H)CC 131 (e.g. ArCH,CH=cH,-ArCH = CHMe) Condensat ion (e.g. EtCHO + MeNO, -EtCHOHCH2NO2) Alkylation (e.g PhOH + MeOH -PhOMe) 1124 200 400 600

Montmorillonite supported zinc and nickel chlorides are highly active and selective reagents for the catalysis of Friedel-Crafts alkylations.

A new heterogeneous catalyst based on a chemically modified mesoporous silica gel possessing immobilised cobalt ions is developed and successfully applied to the epoxidation of alkenes.

An entirely heterogeneous form of aluminium chloride that is highly active in the liquid-phase alkylation of aromatics using alkenes and chloroalkanes is prepared by treating porous support materials with AIC13 in solution.

Zinc bromide supported on mesoporous silica or acidactivated montmorillonite is a fast, efficient, selective and reusable catalyst for the para-bromination of activated and moderately deactivated aromatic substrates.