Heterobimetallic Schiff base catalyst

Lewis Acid/Lewis Acid Heterobimetallic Schiff Base Catalyst... 

Asymmetric addition of an oxindole enolate to nitroalkenes has been observed by Matsunaga, Shibasaki, and coworkers [25]. As illustrated in Scheme 9, chiral oxindole 35 was prepared with good diastereoselective and enantioselective control (30 1 dr, 97% ee) upon treatment of oxindole 32 with nitroalkene 33 in the presence of the bimetallic (Mn2) Schiff Base complex 34. Control experiments with various heterobimetallic complexes (Cu/Mn or Pd/Mn with organic catalyst 34) and a mononuclear complex of Mn with organic catalyst 34 led to decreased selectivity, highlighting the importance of the homodinuclear Mn2-34 complex for promoting the stereoselective transformation. 

A heterobimetallic Ga/cationic Yb/Schiff base complex as a Lewis acid/Lewis acid bifimctional catalyst catalyzes asymmetric a-addition of a-isocyanoacetamides to aryl, heteroaryl, alkenyl, and alkyl aldehydes (Scheme 6). The Schiff base derived from o-vanillin was suitable to utilize cationic rare earth metal triflates with good Lewis acidity in bimetallic Schiff base catalysis. 

As expected, suitable selection of a metal combination for each targeted reaction was important to achieve high stereoselectivity. For a sy -selective nitro-Maimich-type reaction, a heterobimetallic complex prepared from Cu(OAc)2, Sm(0-/Pr)3, and dinucleating Schiff base la was the best [12]. Other metal combinations resulted in much lower selectivity. Either Cu or Sm alone also resulted in poor reactivity and selectivity. Thus, both Cu and Sm are essential for high catalytic activity and selectivity. After optimization studies, the addition of achiral phenol source and the use of oxo-samarium alkoxide, Sm50(0-jPr)i3, with a well-ordered structure, gave the superior reactivity and stereoselectivity. Under the optimized reaction cmiditions, 1-10 mol% of the Cu/Sm catalyst promoted asymmetric... 

In this chapter, recent achievements in asymmetric cooperative catalysis exploiting the power of multimetallic catalysts are described. The three types of multidentate ligands, Schiff base ligands, amide-based ligands, and sugar-derived ligands, serve as a suitable platform for the construction of homobimetallic, heterobimetallic, and... 

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