Diaryl Thiourea Catalysts Promote the Formation of Propargyl Enol Ethers in the Claisen Rearrangement Sequence

Claisen rearrangement sequences with propargyl alcohols, where the propargyl enol ether intermediate formation is catalyzed by acids, suffer from concomitant isomerization of the desired allenyl ketone products to the corresponding conjugated dienones. We report that diarylthiourea catalysts effectively promote the reaction sequence, but suppress the isomerization, providing the allenyl ketones in generally good yields and chemoselectivities. Density functional theory calculations of the reaction pathway and control experiments support the role of the thiourea as a Brønsted acid catalyst.