Cooperative intermolecular catalysis

Fig. 13.4 General concept of rare earth bimetallic asymmetric catalysis (a) cooperative intramolecular catalysis, and (b) cooperative intermolecular catalysis.

Fig. 13.9 [(salen)AI]20 Complex and (PyBOXJErCE complex for intermolecular cooperative dual catalysis. 

The development of catalytic asymmetric reactions is one of the major areas of research in the field of organic chemistry. So far, a number of chiral catalysts have been reported, and some of them have exhibited a much higher catalytic efficiency than enzymes, which are natural catalysts.111 Most of the synthetic asymmetric catalysts, however, show limited activity in terms of either enantioselectivity or chemical yields. The major difference between synthetic asymmetric catalysts and enzymes is that the former activate only one side of the substrate in an intermolecular reaction, whereas the latter can not only activate both sides of the substrate but can also control the orientation of the substrate. If this kind of synergistic cooperation can be realized in synthetic asymmetric catalysis, the concept will open up a new field in asymmetric synthesis, and a wide range of applications may well ensure. In this review we would like to discuss two types of asymmetric two-center catalysis promoted by complexes showing Lewis acidity and Bronsted basicity and/or Lewis acidity and Lewis basicity.121... 

The easiest way to construct active sites of artificial enzymes is self-assembly of the active sites as illustrated by the cartoon of 60. If the catalytic groups take highly productive positions in the self-assembled active site, cooperation among them can result in effective catalysis. It is difficult to induce the self-assembly in aqueous solutions by using hydrogen bonding or electrostatic interactions as intermolecular... 

By taking advantage of the (salen)AlCl complex s ability to activate imide and of the lanthanide-PyBOX complex s ability to activate cyanide, intermolecular heterobimetallic dual cooperative catalysis was investigated. The dual catalyst system, that is, the combination of bench-stable (S,S)-[(salen)Al]20 complex and (S,S)-ErCl3-PyBOX complex, afforded distinctly superior results compared with (salen)AlCl complex alone (Table 13.33) [105]. Reaction times (from 26-48 to 8-14 hours), the amount of TMSCN (from 2.5-4 equiv to 2 equiv), and total catalyst loading (f rom 10 to 15 mol% to 7 mol%) decreased significantly. Reaction proceeded... 

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