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/Sc III 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.