Phosphine containing dendrimers for highly regioselective rhodium catalysed hydroformylation of alkenes: a positive ‘dendritic effect’

Diphenylphosphine functionalised polyhedral oligomeric silsesquioxane (POSS) dendrimers are used as ligands for the rhodium catalysed hydroformylation of oct-1-ene showing unexpectedly high regioselectivity to the linear aldehyde nonan-1-al (l ∶ b = 14 ∶ 1). Comparative studies with the small molecule analogues, bis-diphenylphosphinopentane, Me2Si[CH2CH2PPh2]2 and Si[CH2CH2PPh2]4, clearly confirm the unusual selectivity of these bidentate dendritic ligands and show that a ‘positive dendritic effect’ occurs. This high selectivity is obtained with only one structure within the 1st and 2nd generation dendrimer (spacer of five atoms between the phosphorus atoms, and carbon–silicon linkage) whilst other frameworks (spacer of three and seven atoms between the P atoms, or carbon–oxygen–silicon linkage) lead to lower selectivity.