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 (1: 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.