Phosphoramidites: Novel modifying ligands in rhodium catalysed hydroformylation

The phosphoramidites 2–9 have been applied as modifying ligands in rhodium‐catalysed hydroformylation of oct‐1‐ene and styrene. Monodentate phosphoramidites 2–5 gave rise to catalysts with average rates of 1000 mol [mol Rh]−1 h−1 and maximum selectivities of 65% to the linear product nonanal for the hydroformylation of oct‐1‐ene (P 20 bar CO/H2, T 80°C, [oct‐1‐ene] ≈︁ 1 M). Styrene was hydroformylated (T 40°C, P 20 bar CO/H2) at rates up to 150 mol [mol Rh]−1 h−1 and a selectivity for the branched 2‐phenylpropanal of 93%. Using diphosphoramidite modified catalysts a maximum rate of 1200 mol.[mol Rh]−1.h−1 for oct‐1‐ene was observed for 9, with 91% regioselectivity for the linear product. At 40°C styrene was hydroformylated at rates up to 25 mol [mol Rh]−1 h−1 and with selectivities for the branched aldehyde of 89%. Characterization of the rhodium complexes showed that formation of rhodium hydride complexes is slow and incomplete under the conditions applied for the monophosphoramidites. The diphosphoramidite ligands all form bidentate hydrido‐rhodium complexes. The diphosphoramidite ligands 6–9 coordinate bis‐equatorially in the trigonal bipyramidal rhodium complexes, which accounts for the high regioselectivities.