A Comparison of (R,R)‐Me‐DUPHOS and (R,R)‐DUPHOS‐iPr Ligands in the Pd0‐Catalysed Asymmetric Allylic Alkylation Reaction: Stereochemical and Kinetic Considerations

Abstract

A relatively unexploited commercial ligand, (R,R)-DUPHOS-iPr (1b) was tested in the Pd0-catalysed asymmetric allylic alkyation reaction using rac-1,3-diphenylpropenyl acetate as substrate, malonate as nucleophile and a variety of Pd pre-catalysts under standard conditions. Excellent ee values (up to ≥ 98 %) could be obtained with 1b, but the conversions were generally inferior to those obtained using (R,R)-Me-DUPHOS (1a) under similar conditions. It was also observed that there was a switch in the absolute configuration of the malonate product to (R) on using 1b to form the catalyst. This was an indication of the complementary nature of these two ligands. In order to explain this, a rigorous detailed semi-emperical computational study was undertaken. DFT calculations of the Fukui Function (FF) were also conducted to confirm the most likely site (electrophilic) for nucleophilic attack on the active Pd-allyl complex. In the case of the Pd catalyst formed with 1b. This result indicated that stereochemical factors were more important than electronic factors in this reaction with 1b. Kinetic studies were also conducted to compare the activities of the catalysts 2a and 2b formed with (R,R)-Me-DUPHOS (1a) and (R,R)-DUPHOS-iPr (1b).(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)