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

Abstract

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.