Photoredox cobalt-catalyzed stereodivergent synthesis of 1,4-dienes

1,4-Dienes are important scaffolds widely used in natural products and medicinal compounds. Herein, we report a highly efficient method for the straightforward synthesis of 1,4-dienes via regio- and stereoselective reductive coupling of alkynes and allenes, catalyzed by visible-light photoredox cobalt. In contrast to the conventional E-alkene products, both (Z,E)- and (E,E)-1,4-dienes were synthesized with good regio- and stereoselectivities under mild conditions. In this photoredox reaction, Hünig’s base and water were utilized as hydrogen sources instead of the commonly used Hantzsch esters. The mechanistic and density functional theory studies indicate that the reaction undergoes protolysis of a cobaltacyclopentene intermediate and photocatalytic E→Z isomerization of (E,E)-1,4-dienes to (Z,E)-1,4-dienes via an energy transfer process.