Substrate-selective aqueous organometallic catalysis. How size and chemical modification of cyclodextrin influence the substrate selectivity

Randomly hydroxypropylated and methylated cyclodextrins with different cavity size have been used as inverse phase transfer catalysts in a palladium catalyzed Tsuji-Trost reaction with water-insoluble alkylallylcarbonates and alkylallylurethanes as substrate. It has been shown that the molecular recognition ability of both α-CD and β-CD derivatives towards these substrates was responsible for an increase in the reaction rates and remarkable substrate selectivities between a linear and a branched structure. By contrast, the too wide cavity of γ-CD derivatives did not allow these carriers to be efficient in terms of substrate selectivity. Thus, the performances of a cyclodextrin carrier in this cleavage reaction strongly depended on the size of the cavity in which the substrate had to fill in as close as possible the available space.