Iridium-Catalyzed Selective trans -Semihydrogenation of 1,3-Enynes with Ethanol: Access to ( E,E )-1,4-Diarylbutadienes.

A trans -semihydrogenation of 1,3-enynes with ethanol as the hydrogen source was developed using a new (PCN)Ir complex as the precatalyst and t BuNH 2 as the cocatalyst. This catalyst system provides an efficient and atom-economical access to unsymmetrical ( E , E )-1,4-diarylbutadienes with high yields and stereoselectivities. Monitoring the process revealed that a sequence of cis -semihydrogenation of the triple bond of 1,3-enynes (to form ( E , Z )-butadienes) and ( E , Z )-to-( E , E ) isomerization occurs to form ( E , E )-butadienes.