Unraveling the origin of regioselectivity in rhodium diphosphine catalyzedhydroformylation. A DFT QM/MM study

The origin of regio selectivity in rhodium diphosphine catalyzed hydroformi lation was investigated by means of hybrid QM/MM calculations using the IMO MM method. The roles of the diphosphine bite angle and of the nonbonding in teractions were analyzed in detail by considering rhodium systems containin g xantphos-type ligands, for which a correlation between the natural bite a ngle and regioselectivity has been recently reported. From the pentacoordin ated equatorial-equatorial HRh(CO)(alkene)(diphosphine) key intermediate, e ight possible reaction paths were defined and characterized through their r espective transition states (TS). We succeeded in reproducing the experimen tally observed trends for the studied diphosphines. By performing additiona l calculations on model systems, in which the steric effects induced by the phenyl substituents of xantphos ligands were canceled, we were able to sep arate, identify, and evaluate the different contributions to regioselectivi ty. These additional calculations showed that regioselectivity is governed by the nonbonding interactions between the diphenylphosphino substituents a nd the substrate, whereas the effects directly associated to the bite angle , what we call orbital effects, seem to have a smaller influence.