Rate-Limiting Spin Crossover and Cp Ligand Involvement During Ir(III) Retro-Hydroformylation Catalysis

While catalytic hydroformylation is a well-established
reaction,
there are only a few reports of homogeneous catalyzed retro-hydroformylation
where carbon monoxide and dihydrogen are eliminated from an aldehyde
to generate an olefin. Our in-depth assessment of reaction pathways
using density functional theory and metadynamics calculations for
aldehyde retro-hydroformylation by cyclopentadienyl phosphine-type
IrIII catalysts has revealed two surprising and unique
aspects of catalysis: (1) Catalytic cycle turnover is determined by
the rate of singlet-triplet spin state crossover; and (2) during catalysis,
after C–H bond activation, a transient IrIII–H
intermediate undergoes intramolecular proton transfer to give a dearomatized
η4-Cp-H diene ligand. This IrI intermediate
provides the key coordination unsaturation to enable decarbonylation
and β-hydride elimination reaction steps. Overall, these mechanistic
insights set the stage for the design of novel retro-hydroformylation
molecular catalysts.