Hydroformylation of Alkenes in a Planetary Ball Mill: From Additive-Controlled Reactivity to Supramolecular Control of Regioselectivity.

The Rh-catalyzed hydroformylation of aromatic-substituted alkenes is performed in a planetary ball mill under CO/H2 pressure. The dispersion of the substrate molecules and the Rh-catalyst into the grinding jar is ensured by saccharides: methyl-α-d-glucopyranoside, acyclic dextrins, or cyclodextrins (CDs, cyclic oligosaccharides). The reaction affords the exclusive formation of aldehydes whatever the saccharide. Acyclic saccharides disperse the components within the solid mixture leading to high conversions of alkenes. However, they showed typical selectivity for α-aldehyde products. If CDs are the dispersing additive, the steric hindrance exerted by the CDs on the primary coordination sphere of the metal modifies the selectivity so that the β-aldehydes were also formed in non-negligible proportions. Such through-space control via hydrophobic effects over reactivity and regioselectivity reveals the potential of such solventless process for catalysis in solid state.