Steric effects in enantioselective allylic alkylation catalysed by cationic (h3-allyl)palladium complexes bearing chiral pyridine-aziridine ligands

Enantiopure N,N′-bidentate ligands (N,N′)*, containing substituted aziridine and aza-aromatic rings, were prepared from imines derived from (S)-valinol or (R)-phenylglycinol and heteroaromatic aldehydes (2-pyridine-, 6-benzyl-2-pyridine-, 2- and 8-quinolinecarbaldehyde) by the addition of iPrMgCl and subsequent cyclisation of the 1,2-amino alcohol moiety to aziridine. Crystalline [(N,N′)*(η3-allyl)Pd][SbF6] complexes were prepared from these ligands and used as catalysts in the alkylation of sodium dimethyl malonate with 1,3-diphenyl-2-propenyl acetate and carbonate. By comparing the performances of two complexes bearing similar pyridine-aziridine ligands but with a different C2-aziridine substituent (Ph vs. iPr), a better enantioselectivity was provided by the Ph-substituted ligand (90 % vs. 41 % ee), whereas the presence of a C6-pyridine benzyl substituent caused inversion of the enantioselectivity. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)