Constrained chiral catalysts

Some of the approaches previously used to perform reactions enantioselectiv ely using heterogeneous catalysts, together with a few illustrative example s, are first briefly summarised. We then outline various advantages and opp ortunities that nowadays exist for the confinement of large and rather comp licated chiral organometallic catalytic entities as well as reactant and pr oduct species inside the cavities of mesoporous silicas, the pore diameters of which fall in the range 25 to 100 Angstrom. The unique opportunities af forded by chiral chelate ligands based on bis-diphenyl-phosphinoferrocene ( BPPFA) held within mesoporous silica are adumbrated. Examples of potentiall y useful reaction systems and general synthetic possibilities are discussed with the view to highlight the emerging potential of the mesoporous dimens ion in regio- and enantioselective catalysis. (C) 1999 Published by Elsevie r Science B.V. All rights reserved.