Rhodium-Catalyzed Hydroformylation of 1,3-Butadiene to Adipic Aldehyde: Revealing Selectivity and Rate-Determining Steps

The reaction mechanism of Rh-catalyzed
hydroformylation of 1,3-butadiene
with triptycene-derived bisphosphite ligands L was studied by in situ
NMR and IR experiments, kinetic measurements, and deuterioformylation.
Under CO pressure the η3-crotyl complex (κ2-L)­Rh­(η3-crotyl)­(CO) is a stable intermediate
that slowly liberates 3-pentenal when hydrogen is added. It also represents
the most stable intermediate under real hydroformylation conditions
and exists in high concentrations during the catalysis. The rate law
of the reaction is found to be second-order in syngas pressure and
independent of the butadiene concentration. This agrees with the total
barrier between the η3-crotyl complex and one of
the transition states of hydrogen addition or reductive elimination
being rate determining. The olefin insertion step is found to be partly
reversible depending on the pressure, which means that n/iso regioselectivity is not controlled by this
step alone.