Origin of the bite angle effect on rhodium diphosphine catalyzed hydroformylation

The bite angle effect on the rhodium diphosphine catalyzed hydroformylation was investigated in detail. A series of xantphos-type ligands with natural bite angles ranging from 102 degrees to 121 degrees was synthesized, and t he effect of the natural bite angle on coordination chemistry and catalytic performance was studied. X-ray crystal structure determinations of the com plexes (nixantphos)Rh(CO)H(PPh3) and (benzoxantphos)Rh(CO)H(PPh3) were obta ined. In contrast to the natural bite angle calculations, approximately the same diphosphine bite angles were observed in both crystal structures,The solution structures of the (diphosphine)Rh(CO)H(PPh3) and (diphosphine)Rh(C O)(2)H complexes were studied by IR and NMR spectroscopy. The spectroscopic studies showed that all (diphosphine)Rh( CO)(2)H complexes exhibit dynamic equilibria between diequatorial (ee) and equatorial-apical (a) isomers. Th e equilibrium compositions could not be correlated with the calculated natu ral bite angles. In the hydroformylation of 1-octene an increase in selecti vity for linear aldehyde formation and activity was observed with increasin g natural bite angle. For styrene the same trend in selectivity for the lin ear aldehyde was found. For the first; time CO dissociation rates of (dipho sphine)Rh(CO)(2)H complexes were determined using (CO)-C-13 labeling in rap id-scan high-pressure (HP) IR experiments. The observed CO dissociation rat es for three complexes proved to be orders of magnitude higher than the hyd roformylation rates and, contrary to the hydroformylation activity, did not reveal a correlation with the natural bite angle. These findings indicate that the bite angle effect on hydroformylation activity is dominated by the rates of reaction of the reactive, unsaturated (diphosphine)Rh(CO)R interm ediates with CO and alkene. The bite angle affects the selectivity in the s teps of alkene coordination and hydride migration; the structure of the sat urated (diphosphine)Rh(CO)(2)H complex has only some circumstantial relevan ce to the selectivity.