Precise Supramolecular Control of Selectivity in the
Rh-Catalyzed Hydroformylation of Terminal and Internal Alkenes
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Abstract
In
this study, we report a series of DIMPhos ligands <b>L1</b>–<b>L3</b>, bidentate phosphorus ligands equipped with
an integral anion binding site (the DIM pocket). Coordination studies
show that these ligands bind to a rhodium center in a bidentate fashion.
Experiments under hydroformylation conditions confirm the formation
of the mononuclear hydridobiscarbonyl rhodium complexes that are generally
assumed to be active in hydroformylation. The metal complexes formed
still strongly bind the anionic species in the binding site of the
ligand, without affecting the metal coordination sphere. These bifunctional
properties of DIMPhos are further demonstrated by the crystal structure
of the rhodium complex with acetate anion bound in the binding site
of the ligand. The catalytic studies demonstrate that substrate preorganization
by binding in the DIM pocket of the ligand results in unprecedented
selectivities in hydroformylation of terminal and internal alkenes
functionalized with an anionic group. Remarkably, the selectivity
controlling anionic group can be even 10 bonds away from the reactive
double bond, demonstrating the potential of this supramolecular approach.
Control experiments confirm the crucial role of the anion binding
for the selectivity. DFT studies on the decisive intermediates reveal
that the anion binding in the DIM pocket restricts the rotational
freedom of the reactive double bound. As a consequence, the pathway
to the undesired product is strongly hindered, whereas that for the
desired product is lowered in energy. Detailed kinetic studies, together
with the in situ spectroscopic measurements and isotope-labeling studies,
support this mode of operation and reveal that these supramolecular
systems follow enzymatic-type Michaelis–Menten kinetics, with
competitive product inhibition