Japanese chemists have combined a clay with a metal complex to make an effective catalyst for a selective organic reaction. The clay-complex can be easily removed from the reaction mixture by centrifugation. 

Catalytic enantioselective synthesis of chiral compounds is one of the most attractive topics in organic chemistry, says Kenso Soai, at Tokyo University of Science, who led the research. Scientists are trying to develop efficient chiral heterogeneous catalysts because they are more easily separated from reaction mixtures than homogeneous catalysts. While there have been a few reports of heterogeneous chiral clay catalysts, says Soai, the enantioselectivities have been low. 

Soai used a synthetic clay called hectorite doped with a chiral ruthenium complex to catalyse the addition of an alkylzinc reagent to an aldehyde. The reaction produced almost exclusively one enantiomer of an alkanol. 

The reaction is autocatalytic, explains Soai, which means that the chiral product acts as a chiral catalyst for its own production. This suppresses the formation of the other enantiomer and amplifies the enantiomeric excess of the reaction. The doped clay acts as a heterogeneous chiral inducer of the reaction and shows comparable yield and enantioselectivity compared with the metal complex alone. 

Soai plans to continue investigating the roles of inorganic materials in organic reactions. 'The orientation of the intercalated chiral metal complex is well characterised,' says Soai. 'It should be useful for modelling the interaction between the chiral complex and the substrates.' 

Joanne Thomson