Constrained chiral catalysts

Some of the approaches previously used to perform reactions enantioselectively using heterogeneous catalysts, together with a few illustrative examples, are first briefly summarised. We then outline various advantages and opportunities that nowadays exist for the confinement of large and rather complicated chiral organometallic catalytic entities as well as reactant and product species inside the cavities of mesoporous silicas, the pore diameters of which fall in the range 25 to 100 Angstrom. The unique opportunities afforded by chiral chelate ligands based on bis-diphenyl-phosphinoferrocene (BPPFA) held within mesoporous silica are adumbrated. Examples of potentially useful reaction systems and general synthetic possibilities are discussed with the view to highlight the emerging potential of the mesoporous dimension in regio- and enantioselective catalysis. (C) 1999 Published by Elsevier Science B.V. All rights reserved.