Enantioselective Allylic Substitution Catalyzed by Chiral [Bis(dihydrooxazole)]palladium Complexes: Catalyst structure and possible mechanism of enantioselection

Abstract

Abstract Allylpalladium complexes with chiral bis(dihydrooxazole) ligands were studied as catalysts for the enantioselective allylic substitution reaction of rac ‐1,3‐diphenylprop‐2‐enyl acetate ( rac ‐ 5 ) with the anion of dimethyl malonate ( Scheme 1 ). Using enantiomerically pure ( S , E )‐1‐(4‐tolyl)‐3‐phenylprop‐2‐enyl acatete (( S )‐ 25 ) as substrate, the reaction was shown to proceed by a clean ‘ syn ’ displacement of acetate by dimethyl malonate ( Scheme 6 ). The [Pd 11 (η 3 ‐allyl)] complex 18 and the analogous [Pd(η 3 ‐1,3‐diphenylallyl)] complex 20 , both containing the same bis(dihydrooxazole) ligand, were characterized by X‐ray structure analysis and by NMR spectroscopy in solution. The structural data reveal that steric interactions of the allyl system with the chiral ligand result in selective electronic activation of one of the allylic termini. The higher reactivity of one allylic terminus toward nucleophilic attack is reflected in a significantly longer PdC bond and a shift of the corresponding 13 C‐NMR resonance to higher frequency.