Catalytic enantioselective amination of alcohols by the use of borrowing hydrogen methodology: cooperative catalysis by iridium and a chiral phosphoric acid.
The catalytic asymmetric reduction of ketimines has been explored extensively for the synthesis of chiral amines, with reductants ranging from Hantzsch esters, silanes, and formic acid to H2 gas. Alternatively, the amination of alcohols by the use of borrowing hydrogen methodology has proven a highly atom economical and green method for the production of amines without an external reductant, as the alcohol substrate serves as the H2 donor. A catalytic enantioselective variant of this process for the synthesis of chiral amines, however, was not known. We have examined various transition-metal complexes supported by chiral ligands known for asymmetric hydrogenation reactions, in combination with chiral Brønsted acids, which proved essential for the formation of the imine intermediate and the transfer-hydrogenation step. Our studies led to an asymmetric amination of alcohols to provide access to a wide range of chiral amines with good to excellent enantioselectivity.
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543 (Singapore) http://zhaoyu.science.nus.edu.sg.
Yao
Y
Zhang
Ching-Si
CS
Lim
Derek Sui Boon
DS
Sim
Hui-Jie
HJ
Pan
Yu
Y
Zhao
amination; borrowing hydrogen methodology; enantioselectivity; hydrogenation; tandem catalysis