Chiral tridentate ligand

Asymmetric -methylation of a, -enones. Chiral bidendate ligands derived from L-prolinol can be used for asymmetric Michael additions to a, 3-enones with cuprates of the type CH,L CuMgBr (10, 266). The highest optical yield in conjugate addition to chalcone is observed when L is (S)-N-methylprolinol (88% ee). The tridentate chiral ligand 1 is equally effective for asymmetric 3-methylation of chalcone with CH,L CuLi and CuBr the chemical yield is 95%. Reduction of the amide carbonyl group of 1 results in practically total loss of chiral induction. 

On the other hand, the enantioselective 1,4-addition of carbanions such as enolates to linear enones is an interesting challenge, since relatively few efficient methods exist for these transformations. The Michael reaction of p-dicarbonyl compounds with a,p-unsaturated ketones can be catalysed by a number of transition-metal compounds. The asymmetric version of this reaction has been performed using chiral diol, diamine, and diphosphine ligands. In the past few years, bidentate and polydentate thioethers have begun to be considered as chiral ligands for this reaction. As an example, Christoffers et al. have developed the synthesis of several S/O-bidentate and S/O/S-tridentate thioether... 

Tridentate chiral amino diether ligand 26 was reported to be designed and prepared from phenylalanine by Tomioka and coworkers in 1985 (equation 11). Lithiated phenyldithiane... 

The advances that have taken place over the past five years in catalytic, enantioselective aldol addition reactions is evident in a number of important respects. The types of transition metals and their complexes that function competently as catalysts have been expanded considerably. Thus, in addition to B(III), Ag(I), Au(I), Sn(II), and La(III), chiral catalysts prepared from Cu(II), Ti(IV), Ln(III), Si(IV), Pt(II) and Pd(II) have been introduced. The expansion in the use of transition metals has taken place hand-in-hand with the design and synthesis of new bidentate and tridentate organic ligands based on nitrogen, oxygen, and phosphorus donors. Additionally, whereas the older methods primarily relied on the use of Lewis-acids for the activation of... 

Very recently, Togni reported a chiral C2-symmetric tridentate PCP ligand (47) [90] related to the previously developed triphosphine Pigiphos (46)... 

R,R)-4,6-Dibenzofiirandiyl-2,2 -bis(4-phenyloxazoline), DBFOX/Ph (5), is a novel tridentate bisoxazoline ligand developed by Kanemasa and coworkers that has been successfully used as a chiral Lewis acid in enantioselective Diels-Alder-reactions, nitrone cycloadditions and conjugate additions of radicals and thiols to 3-(2-alkenoyl)-2-oxazolidinones. Representative examples for cycloadditions using the Ni(C104)2-6H20 derived complex are shown below. 

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