Solvent-dependent chiral discrimination

Interesting, solvent-dependent chiral discrimination properties have been observed for chiral host 28 [48], In the absence of toluene, compound 28 forms a 1 2 crystalline complex with rac-cyanohydrin (30). When both 28 and rac-30 were dissolved in toluene, the crystalline product contained 28 and (+)-30 and toluene in 1 1 1 ratio. One recrystallization of the complex from toluene gave crystals which upon heating in vacuo gave (+)-30 at 100% ee and 24% yield. 

In sharp contrast, the late transition metals are more coordinative to soft carbon-carbon multiple bonds rather than hard oxygen. These binding modes are further classified into mono- and bi-dentate coordinations, depending on the ligands on the metal catalysts, or the substituent pattern in the Claisen diene systems and solvents employed. Bi-dentate coordination of the Claisen substrate is advantageous over the weak mono-dentate coordination of the Claisen rearrangement product, y,d-unsaturated carbonyl compounds, to release the metal complex allowing the catalytic cycle. Furthermore, enantiodiscrimination by chiral late transition metal complexes is based on the discrimination of two enantiotopic diene faces in the enantiomeric six-membered transition states. 

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