On the rate-determining step and the ligand electronic effects in rhodium catalysed hydrogenation of enamines and the hydroaminomethylation of alkenes

In the course of studies on the tandem hydroformylation-reductive amination (hydroaminomethylation), fluorinated mono-phosphines were found to be more active than their more electron-donating counterparts in the enamine hydrogenation step of the reaction; this is in contrast to the widely held view that alkene hydrogenation activity increases with ligand donor strength. DFT calculations comparing the reaction pathways for a simple alkene and a representative enamine show that the rate-determining step changes from the first insertion into a Rh-H bond for but-2-ene to the final reductive elimination step from the Rh-hydride-alkyl species in the enamine hydrogenations.