MECHANISM OF THE ASYMMETRIC ISOMERIZATION OF ALLYLAMINES TO ENAMINES CATALYZED BY 2,2'-BIS(DIPHENYLPHOSPHINO)-1,1'-BINAPHTHYL RHODIUM COMPLEXES

Cationic Rh complexes containing the 2,2 -bis(diphenylphosphino)-1,1 -binaphthyl (BINAP) ligand catalyze a highly enantioselective isomerization of diethylgeranylamine or-nerylamine to give (R)-or (S)-citronellal (E)-diethylenamine in >95% ee. A new, nitrogen-triggered mechanism is postulated for the double-bond-migration reaction on the basis of and 31P NMR studies, kinetic measurements, and deuterium-labeling experiments. The initial nitrogen-coordinated allylamine-Rh+ complex causes a four-centered hydride elimination from C(l) via dissociative mechanism to generate a transient iminium-RhH complex. Delivery of the hydrogen from Rh to C(3) gives the enamine η3-complex. The latter, having an aza-allyl structure, serves as the chain carrier in the catalytic cycle. The BINAP-Rh+ complexes differentiate efficiently the enantiotopic C(l) hydrogens of the allylamines through interaction with the adjacent nitrogen atoms. The overall 1,3-hydrogen shift occurs in a suprafacial manner from an s-trans-type conformer of the flexible substrates. The origin of the chiral recognition has been interpreted in terms of the chiral environments of the BINAP-based Rh+ complexes. © 1990, American Chemical Society. All rights reserved.