Substrate-selective catalysis in an aqueous biphasic system with per(2,6-di-O-methyl)-beta-cyclodextrin

The biphasic palladium-catalysed cleavage of water-insoluble allylic substr ates in the presence of methylated cyclodextrins has been investigated with the aim of performing substrate-selective catalytic reactions. While no su bstrate selectivity was observed in control experiments in which acetonitri le was used as mass transfer promoter, the use of DMCyD led in some cases t o high substrate selectivities. For instance, a 97:3 product ratio was obse rved during the cleavage of a 50:50 mixture of N-dodecyl-O-allyl urethane a nd N,N-dihexyl-O-allyl urethane. The whole results demonstrate that the siz e-fit concept which postulates the highest reactivity for the best size-mat ched host-guest pair is limited to predict the substrate selectivity. (C) 2 001 Elsevier Science B.V. All rights reserved.