Effect of Ligand Backbone on the Selectivity and Stability of Rhodium Hydroformylation Catalysts Derived from Phospholane-Phosphites

A study
on how ligand backbone structure has an impact on selectivity,
rate, and catalyst stability of hydroformylation catalysts was prompted
by some longer-term stability issues being discovered for a phospholane-phosphite
with a [−CH2O−] backbone. A series of phospholane-phosphite
ligands were synthesized. Catalysts made in situ from
these ligands and [Rh­(acac)­(CO)2] were found to give iso-butanal selectivities up to 75% at temperatures between
75 and 105 °C: the latter being a benchmark for iso-selectivity in reactions conducted at industrially meaningful temperatures.
A racemic rhodium complex of a bidentate phospholane-phosphite from
a tropos-biphenol with an extended backbone showed
unusually high stability at high temperatures, combined with even
better iso-selectivity in propene hydroformylation
relative to the original complex. A related ligand with an electron-withdrawing
group maintained the unusually high stability and improved activity.
Characterization of the precatalysts of type [RhH­(CO)2(L)]
was accomplished using in situ HPIR spectroscopy
and backed up by density functional theory calculations (B3PW91-D3
level) and by NMR studies; the latter showed that the variation of
the backbone also had a pronounced impact on the precatalyst structure.
A key finding is that it is now possible to prepare phospholane-phosphite
ligands that deliver high iso-butanal selectivity
and that show no signs of degradation after several days even above
typical reaction temperatures. In one stability test, several kilograms
of aldehydes were produced with TOF and selectivity being consistent
over several days.