Z-Selective Alkyne Semireduction by an Organic Photoreductant

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.