Asymmetric Allylic Alkylation of Acyclic β‑Keto Esters Catalyzed by Iridium

Key words

asymmetric allylic alkylation - β-keto esters - all-carbon stereocenters - N-aryl phosphoramidites - iridium

Significance

The asymmetric allylic alkylation belongs to one of the fundamental C–C bond-­formation reactions and a variety of nucleophiles are successfully employed. However, the formation of an all-carbon quaternary stereocenter is still a challenging case. Herein, the authors report an efficient allylic alkylation of acyclic β-keto esters catalyzed by an Ir–N-aryl-phosphoramidite catalyst. The products bearing vicinal quaternary and tertiary stereocenters are obtained in high yield, regio-, diastereo-, and almost perfect enantio­selectivity.

Comment

The substrate scope of the iridium-catalyzed diastereoselective allylic alkylation covers aryl, heteroaryl and alkenyl substituents on the allyl carbonate electrophile. Interestingly, the regio­selective outcome of the reaction (branched vs. linear) is influenced by the substituent’s ability to donate or withdraw electron density. In the case of 4-MeOAr, the branched product is formed almost exclusively. Variations on the β-keto ester moiety are also conducted and a variety of functional groups are tolerated at the α position (R²), including alkyl, allyl, propargyl, and heteroaryl groups.