Calix[4]arene based monophosphites, identification of three conformations and their use in the rhodium-catalysed hydroformylation of 1-octene

Eight monodentate phosphites (2-9) based on the calix[4]arene backbone were synthesised following two synthetic routes. Out of six conformations only three were actually formed under the applied reaction conditions. X-Ray analysis of two conformers (4 and 5) provided insight into the 3-dimensional structure of two of these conformations. The three conformations were characterised by H-1, C-13 and P-31 NMR spectroscopy. NMR experiments showed that several of the phosphites are flexible showing fluxional behaviour of the molecular backbone in solution, but no interconversion between the different conformers was observed. The conformation of the product in the phosphite synthesis is determined at the point where the phosphorus atom is linked to two hydroxyl groups of the calix[4]arene (phosphorus amidite). Such an intermediate phosphorus amidite (12) was isolated in the synthesis of 4 and 5. Phosphites 3-6, 8 and 9 were tested in rhodium-catalysed hydroformylation. Differences in rate can be correlated to the conformation of the ligand.