Cnu

Combinatorialiy Optimized Ligand ^^ = PEG-grafted Polystyrene Resin

Scheme 32.15. Combinatorial^ optimized Schiffbase ligand for the asymmetric Strecker reaction (hydrocyanation of imines). AA, amino acid; TFAA, trifluoroacetic anhydride.

2. TFAA

R^CN

Hoveyda, Snapper, and coworkers applied their modular approach to Schiff basetype ligands for identifying catalysts for a variety of asymmetric transformations. Scheme 32.16 summarizes the novel catalytic systems for enantioselective additions of cyanide to imines (a) [99], for Cu-catalyzed conjugate addition of alkylzinc reagents to cyclic enones (b) [100], for Zr-catalyzed addition of diethylzinc to imines (c) [101], and for Cu-catalyzed allylic substitutions (d) [102].

N IT

CN Ph

OMe iPr H Q

NHBu

NHBu

Me OPO(OEt)2

NHBu

10 mol-% CuCN Et2Zn

NHBu

10 mol-% CuCN Et2Zn

Scheme 32.16. Novel chiral catalysts containing Schiff base ligands.

0.1 - 20 mol-% Zr(0/-Pr)4.H0/Pr Et2Zn n"Et n = 1-3 up to 98 % yield up to 98 % ee

H OMe

Et Me

Et Me

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