Synthesis and Catalytic Activity of Heterobimetallic Rare Earth-Zinc Ethyl BINOLate Analogues of Shibasaki's Catalysts

摘要

Shibasaki's heterobimetallic complexes [M_3(THF)_n]_- [(BINOLate)_3RE] (BINOLate = 1,1'-bi-2-naphtholate; RE = Ln~(III), Y~(III); M = Li~+, Na~+, K~+) are among the most general and highly enantioselective catalysts known. Their structures, however, have been limited to group I metals in the peripheral sites. We envisioned that the utility of this class of catalysts could be broadened by the synthesis of new members. Herein, we report the first synthesis of Shibasaki-type catalysts that incorporate divalent Zn~(2+) ions in the peripheral positions. The compounds (EtZn)_3(THF)_2(BINOLate)_(3-) RE(THF) (RE = La~(III), Pr~(III), Eu~(III)) are easily prepared from the corresponding tris(silylamide) precursors RE[N(SiMe_3)_2]_(3,) 3 equiv of (S)-BINOL, and 3 equiv of ZnEt_2 in 68-86% crystalline yields. The compounds are isostructural with known [Li~3(THF)~4]- [(BINOLate)_3RE(THF)] catalysts. We have demonstrated that the (EtZn)_3(THF)_2(BINOLate)_3RE(THF) complexes are catalytically active in the enantioselective addition of diethylzinc to benzaldehyde with moderate enantioselectivities.