CHEMICAL YEAR IN REVIEW 2009

PHASE-SWITCHING CATALYSIS By simply adding or removing carbon dioxide, chemists in Scotland devised a neat trick for reversibly shuttling a homogeneous catalyst between the organic and aqueous phases in a biphasic solvent system (CE Angew. Chem. Int. Ed. 2009, 48 , 1472). The phase-switchable catalyst designed by Simon L. Desset and David J. Cole-Hamilton of the University of St. Andrews adds flexibility to the often complicated techniques required to isolate products and recycle catalysts during homogeneous reactions. The secret to the switchability is a weakly basic amidine group, –N=C(CH 3 )N(CH 3 ) 2 , that the researchers added to the phenyl rings of triphenylphosphine. The rhodium catalyst made with the modified phosphine ligand is soluble in organic solvent. On bubbling CO 2 into an aqueous-organic reaction system containing the catalyst, the CO 2 reacts with water to form carbonic acid (H 2 CO 3 ). The transient acid protonates the amidine groups and renders the catalyst water...