FLUXIONAL PROCESSES IN ASYMMETRIC HYDROFORMYLATION CATALYSTS [HRHL-L(CO)(2)] CONTAINING C-2-SYMMETRICAL DIPHOSPHITE LIGANDS

Solution structures of hydridorhodium diphosphite dicarbonyl complexes [HRhL over arc L1-9(CO)(2)] have been studied. Diphosphites L over ar c L-1 to L over arc L-9 are based on C-2 symmetric (2R,3R)-butane-2,3- diol, (2R,SR)-diethyl tartrate, (2R,4R)-pentane-2,4-diol, and (2S,5S)- hexane-2,5-diol backbones substituted with 1,1'-biphenyl-2,2-diyl- or (S)-(-)-1,1'-binaphthyl-2,2'-diylphosphoroxy derivatives. Variable-tem perature (293-163 K) P-31 and H-1 NMR spectroscopy revealed fluxional behavior in the trigonal bipyramidal HRhL over arc L(CO)(2) complexes of L over arc L1-9 which could be frozen out at low temperatures. Depe nding on the length of the bridge between the two phosphorus atoms in the diphosphite ligands, equatorial-axial or bis-equatorial coordinati on takes place. Enthalpies of activation (Delta H double dagger) varyi ng between 36.7 and 62.5 kJ mol(-1) have been calculated for the phosp horus exchange observed in these hydridorhodium diphosphite dicarbonyl complexes. The enthalpies of activation increase with larger steric b ulkiness of the coordinated diphosphite ligands.