Schiff base catalysis, bimetallic

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. 

To incorporate a cationic rare earth metal into the outer cavity of a dinucleating Schiff base, we used Schiff base 2 derived from o-vanilUn (Fig. 5). We selected a-additions of a-isocyanoacetamides to aldehydes as a model reaction because a salen-Al catalyst gave only modest enantioselectivity and reactivity [16]. The working hypothesis for bimetallic Schiff base catalysis is shown in Fig. 6. We assumed that the bimetallic Schiff base complex would not only activate the aldehyde with one metal, but might also interact with the a-isocyanoacetamide through the other metal to effectively control the orientation of the two substrates in the enantio-discriminating step. 

Fig. 6 Working hypothesis of bimetallic Schiff base catalysis on a-additions of a-isocyanoace-tamides to aldehydes...

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