Schiff base ligand, dipeptide ligands

In 1991, the Inoue group developed the first example of asymmetric cyano-silylation of aldehydes catalysed by the aluminium complexes (Scheme 19.1)." With chiral acyclic dipeptide ester 1 or a-amino amide 2 containing a phenolic Schiff base as the ligand, the silylated cyanohydrins were afforded in 66-92% yields and 37-71% enantiomeric excess. 

Hoveyda is one of the pioneers in the use of combinatorial chemistry techniques to discover novel ligands for catalytic asymmetric processes. A brilliant illustration is the identification of small oligopeptides as ligands for Cu-catalyzed Sn2 displacement reactions (Scheme 14.17) [95, 96). The copper complexes prepare from Schiff base-derived dipeptides such as 100 catalyze asymmetric displacement reactions of a wide range of allylic phosphates. Significantly, the method can also be utilized to prepare alkenes incorporating quaternary allylic stereogenic centers (cf 101). This approach was ele-... 

The chemistry already described is reproduced by numerous ligands that have not specifically been addressed in the previous discussion. The V-salicylidenehydrazides (Scheme 4.18a) and related compounds provide a good example. The structure [76] of a typical complex, represented in Scheme 4.18b, is not very different from that proposed for the solution structures of dipeptide complexes (Scheme 4.17). Interestingly, other similar complexes, based on Schiff base-derived ligands, form dimeric [VO]2 core complexes (Scheme 4.1) via two long ( 2.4 A) VO bonds [2], The cyclic core is not necessary for dimer formation, and a dimer can form via a linear VOV bond [77], These complexes otherwise are not significantly different in their vanadium coordination from that depicted in Scheme 4.18b. 

The selected examples by Cole et al. [120] and Shimizu et al. [121] reported the parallel synthesis of a small library of solid supported dipeptide Schiff bases as ligands for the Ti-catalyzed enantioselective addition of trimethylsilyl cyanide to meso epoxides, and the determination of their catalytic activity on different substrates. The catalyzed addition reaction and the general structure of the dipeptide ligands are shown in Figure 7.15. 

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