<i>Z</i> ‐Selective Alkyne Semireduction by an Organic Photoreductant

Abstract

Abstract The semireduction of alkynes is an indispensable transformation for the synthesis of Z ‐alkenes. Although extensive studies with mostly transition metal‐based catalysts have enabled it to produce a broad scope of alkenes, a SciFinder TM search and a robustness screen of Lindlar's catalyst indicate that highly Lewis‐basic nitrogen‐ and sulfur‐containing motifs, which are prevalent in drugs, remain problematic. Herein, we report an alkyne semi‐reduction using an organic photoreductant that tolerates all 10 of the most common Lewis‐basic motifs in drugs. The reaction reduces terminal alkyl and aryl, dialkyl, alkyl aryl, and diaryl alkynes in high yields and Z‐ selectivities, while tolerating other reducible motifs like sulfones, nitriles, ketones, and electron‐poor arenes. Ultraviolet/visible (UV/Vis) and nuclear magnetic resonance (NMR) titration studies indicate that the reaction occurs by formation of a photoexcitable electron donor‐acceptor (EDA) complex. Furthermore, a time study indicated that the reductant enforces a Z ‐selective reaction, which is unusual for alkyne reductions via electron transfer.