Issue Date: January 12, 2009
Enantiopure Epoxides And Stereoregular Polyethers
Cornell University chemists have created a chiral cobalt catalyst that displays an extraordinary ability for resolving racemic mixtures of epoxides and for producing stereoregular polyethers from chiral epoxides (J. Am. Chem. Soc. 2008, 130, 17658). Wataru Hirahata, Geoffrey W. Coates, and coworkers designed the bulky bimetallic catalyst with epoxides in mind. The twin cobalt centers each sport a chelating salen ligand (N,N′-ethylenebis[salicylideneimine]) and share a chiral binaphthol ligand. The binaphthol holds the metals in the optimal geometry for binding and opening aliphatic epoxide rings, Coates says. The researchers show that the cobalt catalyst, when combined with bis(triphenylphosphine)iminium acetate as a cocatalyst, has a preference for polymerizing (S)-epoxides (shown). This selectivity permits easy isolation of the (R)-epoxides. In addition, the racemic form of the catalyst polymerizes racemic mixtures of epoxides to form highly stereoregular (isotactic) polyethers in quantitative yields. The Cornell chiral cobalt catalyst represents "a significant breakthrough in enantioselective polymerizations," says Caltech chemistry professor John E. Bercaw, who has studied polymeric kinetic resolution of simple alkenes. Beyond making polyethers, Bercaw envisions using the catalyst to produce enantiopure epoxides, which are valuable starting materials for synthesizing pharmaceuticals.
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