Catalytic Regio‐ and Enantioselective Protonation for the Synthesis of Chiral Allenes: Synergistic Effect of the Counterion and Water

A highly enantioselective tandem Pudovik addition/[1,2]‐phospha‐Brook rearrangement of α‐alkynylketoamides with diarylphosphine oxides was achieved with a N,N′‐dioxide/ScIII complex as the catalyst. This protocol features broad substrate scope, high regio‐ and enantioselectivity, and good functional‐group compatibility, providing a straightforward route to various trisubstituted allenes with a diarylphosphinate functionality in good yields with high enantioselectivities (up to 97 % yield, 96 % ee). Control experiments and theoretical calculations revealed that a synergistic effect of the counterion and water was critical for the regio‐ and enantioselective protonation after [1,2]‐phospha‐Brook rearrangement. The synthetic utility of this methodology was demonstrated by the conversion of products into complex bridged polycyclic architectures through intramolecular dearomatizing arene/allene cycloaddition. Pudovik addition/[1,2]‐phospha‐Brook rearrangement mediated by a chiral ScIII/N,N′‐dioxide complex led to axially chiral trisubstituted allenes with a diarylphosphinate moiety. A synergistic effect of the counterion and water enabled regio‐ and enantioselective protonation after [1,2]‐phospha‐Brook rearrangement. Intramolecular dearomatization of N‐Ph‐substituted products gave bridged polycyclic architectures through stereospecific Himbert reaction.