Enantioselective palladium-catalyzed addition of 1,3-dicarbonyl compounds to an allene derivative

Enhancing atom economy of the metal‐catalyzed asymmetric allylic alkylation (AAA) shifts from the usual nucleophilic displacement of a leaving group to an addition of a pronucleophile to a double bond. Using 1‐alkoxyallenes as proelectrophiles, the palladium‐catalyzed AAA proceeds with 1,3‐dicarbonyl compounds as pronucleophiles with excellent regioselectivity and enantiomeric excess under optimized conditions. The pH of the medium proved crucial for reactivity/selectivity. By using the more acidic Meldrum's acids, the reactions required a co‐catalytic amount of Brønsted acid, such as trifluoroacetic acid. Single regioisomeric products of 82–99 % ee were obtained. On the other hand, the less acidic 1,3‐diketones failed to react under such conditions. The fact that a less acidic acid like benzoic acid sufficed, suggested the need for general base catalysis as well. Thus, a mixture of triethylamine and benzoic acid proved optimal (ee's 93–99). Employment of the (R,R)‐phenyl Trost ligand gave a product with S configuration. A model to rationalize the results has been developed.