Asymmetric transfer hydrogenation of ketones catalyzed by amino acid derived rhodium complexes: on the origin of enantioselectivity and enantioswitchability.
Amino acid based thioamides, hydroxamic acids, and hydrazides have been evaluated as ligands in the rhodium-catalyzed asymmetric transfer hydrogenation of ketones in 2-propanol. Catalysts containing thioamide ligands derived from L-valine were found to selectively generate the product with an R configuration (95 % ee), whereas the corresponding L-valine-based hydroxamic acids or hydrazides facilitated the formation of the (S)-alcohols (97 and 91 % ee, respectively). The catalytic reduction was examined by performing a structure-activity correlation investigation with differently functionalized or substituted ligands and the results obtained indicate that the major difference between the thioamide and hydroxamic acid based catalysts is the coordination mode of the ligands. Kinetic experiments were performed and the rate constants for the reduction reactions were determined by using rhodium-arene catalysts derived from amino acid thioamide and hydroxamic acid ligands. The data obtained show that the thioamide-based catalyst systems demonstrate a pseudo-first-order dependence on the substrate, whereas pseudo-zero-order dependence was observed for the hydroxamic acid containing catalysts. Furthermore, the kinetic experiments revealed that the rate-limiting steps of the two catalytic systems differ. From the data obtained in the structure-activity correlation investigation and along with the kinetic investigation it was concluded that the enantioswitchable nature of the catalysts studied originates from different ligand coordination, which affects the rate-limiting step of the catalytic reduction reaction.
Ahlford
Katrin
K
Department of Organic Chemistry, Stockholm University, The Arrhenius Laboratory, 10691 Stockholm, Sweden.
Ekström
Jesper
J
Zaitsev
Alexey B
AB
Ryberg
Per
P
Eriksson
Lars
L
Adolfsson
Hans
H
D000596:Amino Acids / Q000737:chemistry*; D002384:Catalysis; D018360:Crystallography, X-Ray; D006865:Hydrogenation; D006877:Hydroxamic Acids / Q000737:chemistry; D007659:Ketones / Q000737:chemistry*; D007700:Kinetics; D008968:Molecular Conformation; D012238:Rhodium / Q000737:chemistry*; D013237:Stereoisomerism; D013329:Structure-Activity Relationship; D013854:Thioamides / Q000737:chemistry
D000596:Amino Acids; D006877:Hydroxamic Acids; D007659:Ketones; D013854:Thioamides; D012238:Rhodium