Scandium and Ytterbium Tris(perfluorobutanesulfonyl)methide Complexes: Extremely Efficient Lewis Acid Catalysts - Nishikido, Joji; Yamamoto, Fumihiko; Nakajima, Hitoshi; Mikami, Yutaka; Matsumoto, Yousuke; Mikami, Koichi (Synlett, 1999)