Cyclopentadiene synthesis strikes gold

A gold catalyst can convert vinyl allenes into various functionalized cyclopentadienes-including tricyclic structures and complexes with a quaternary carbon center-in high yields and under mild reaction conditions (Angew. Chem. Int. Ed. 2007, 46, 912). To develop the new regiospecific synthesis, Jun Hee Lee and F. Dean Toste of the University of California, Berkeley, built upon their previous success of forming carbon-carbon bonds with cationic triphenylphosphine Au(I) catalysts and AgSbF₆. In the example shown, they used 2 mol % of the gold catalyst to transform the substituted vinyl allene into the desired cyclopentadiene as a single regioisomer. Lee and Toste report a 97% yield for the cycloisomerization, which proceeded in one minute at 0 ^oC. Lower catalyst loading and lower reaction temperatures gave similar results. The researchers' ongoing work includes using the synthesis to make optically active metallocenes.