Biphasic aqueous organometallic catalysis promoted by cyclodextrins: How to design the water-soluble phenylphosphane to avoid interaction with cyclodextrin

The ability of cyclodextrin to interact with meta-trisulfonated triphenylphosphane derivatives bearing one or two methyl (or methoxy) groups on the aromatic ring has been investigated by NMR and UV-vis spectroscopy. In the case of native P-cyclodextrin (beta-CD), the presence of one methyl or methoxy group in the ortho-position on each aromatic ring is necessary to hamper the formation of an inclusion complex between the beta-CD and meta-trisulfonated triphenylphosphane derivatives. In the case of methylated beta-CD, the formation of an inclusion complex is only observed when the meta-trisulfonated triphenylphosphane contains a methyl group in the para-position. The poor affinity of methylated beta-CD towards modified trisulfonated triphenylphosphanes was attributed to the steric hindrance generated by the methyl groups on the CD secondary face. The absence or presence of an interaction between phosphanes and methylated beta-CD was also confirmed by catalytic experiments. Thus, the phosphanes that do not interact with the methytated CD were the most efficient mass-transfer promoters in an aqueous biphasic palladium-catalyzed Tsuji-Trost reaction.