Mechanism of the Asymmetric Isomerization of Allylamines to enamines Catalyzed by 2,2′-Bis(diphenylphosphino)-1,1′-binaphthyl-Rhodium Complexes

Article abstract of DOI:10.1021/ja00168a040

Cationic Rh complexes containing the 2,2′-bis(diphenylphosphino)-1,1′-binahthyl lgand 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 ¹H and ³¹P NMR stdies, kinetic measurements, and deuterium-labeling experiments. The initial nitrogen-coordinated allylamine-Rh⁺ complex causes a four-centered hydride elimination from C(1) via dissociative mechanism to generate a transien iminium-RhH complex. Delivery of the hydrogen from Rh to C(3) gives the enamine η³complex. The latter, having an aza-allyl structure, serves as the chain carrier in the catalytic cycle. The BINAP-Rh⁺ complexes differetiate efficiently the enantotopic C(1) 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 n terms of the chiral environments of the BINAP-based Rh⁺ complexes.