Rh(I) coordination chemistry of hexacationic Dendriphos ligands and their application in hydroformylation catalysis

The coordination chemistry of hexacationic Dendriphos ligands L∗ with respect to Rh(I), as well as their application in Rh-catalyzed hydroformylation reactions, is described. Complexes of the type RhCl(CO)(L∗)2 were synthesized and characterized. The results show that Dendriphos ligands are weaker σ-donors and/or stronger π-acceptors compared to PPh3. The reaction of L∗ with [Rh(cod)2]BF4 in MeCN afforded monophosphine-Rh(I) complexes of the type Rh(cod)(MeCN)(L∗), which points to the tendency of these ligands to form coordinatively unsaturated metal complexes. The catalytic performance of Dendriphos ligands in the Rh-catalyzed hydroformylation appeared to be dominated by steric effects arising from the large dendritic shells of these ligands. Furthermore, the possibility of tuning the catalytic activity and selectivity of the catalytic species, by changing the six counteranions of the hexacationic Dendriphos   ligand, has been investigated. Changing the anions from BF4- to the chiral anions camphorsulphonate or Δ-Trisphat did not render the hydroformylation reaction of styrene enantioselective, albeit small changes in its regioselectivity were observed.

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