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 tBuNH2 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.