Density Functional Investigations of the Rh-Catalyzed Hydroformylation of 1,3-Butadiene with Bisphosphite Ligands

The catalytic cycle of the Rh-catalyzed
monohydroformylation of
1,3-butadiene with a triptycene-derived bisphosphite ligand was investigated
with density functional theory, as it determines the selectivity of
the 1,4-bis-hydroformylation of 1,3-butadiene to adipic aldehyde,
a dream reaction of chemical industry. Out of the variety of possible
reactive pathways, two dominant ones were highlighted leading to the
monoaldehydes 3-pentenal and 4-pentenal, which experimentally also
are the main primary products. With catalysts like the one studied
here, which are highly n-selective and reactive for
1-alkene hydroformylation, 4-pentenal is known to be exclusively converted
to the bis-hydroformylation product adipic aldehyde. An η3-crotyl complex formed by iso-insertion of
an (η2-butadiene)­Rh­(H) species, not involved in hydroformylation
reactions of 1-alkenes and requiring a slightly smaller activation
barrier than the desired n-insertion, could be identified
as an important intermediate for the monohydroformylation of butadiene.
Once formed, this η3-crotyl species opens up an unproductive
exit channel within the catalytic reaction mechanism, which does not
lead to adipic aldehyde. Free energy profiles in solution were calculated
in order to find the intermediates and transition states that govern
turnover frequency (TOF) and selectivity: The Rh crotyl complex and
the reductive elimination transition state most likely limit the TOF,
while the prediction of the regioselectivity is more complicated and
depends on several steps.