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Low Pressure Asymmetric Hydrogenation of Quinolines using an Annulated Planar Chiral N ‐Ferrocenyl NHC‐Iridium Complex
Author(s) -
John Joshni,
WilsonKonderka Cody,
Metallinos Costa
Publication year - 2015
Publication title -
advanced synthesis and catalysis
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.541
H-Index - 155
eISSN - 1615-4169
pISSN - 1615-4150
DOI - 10.1002/adsc.201500105
Subject(s) - chemistry , iridium , tetrafluoroborate , deprotonation , asymmetric hydrogenation , cationic polymerization , medicinal chemistry , phosphine , enantioselective synthesis , cyclooctadiene , ether , stereochemistry , organic chemistry , catalysis , ionic liquid , ion
Annulated planar chiral N ‐ferrocenylimidazolones, obtained by acid‐mediated cyclization of diphenylmethanol derivatives, may be reduced with diisobutylaluminium hydide (DIBAL‐H) to afford a series of surprisingly stable and isolable hemiaminal ether aminals. Two of these derivatives can be oxidized with triphenylcarbenium tetrafluoroborate to imidazolinium salt precursors of N‐heterocyclic carbenes (NHCs). Deprotonation of these salts in the presence of (cyclooctadiene)iridium chloride dimer {[Ir(COD)Cl] 2 } provides chiral coordination complexes bearing N ‐ferrocenyl NHCs with unique rigid tetracyclic frameworks. Cationic analogues of these complexes catalyze the asymmetric hydrogenation of 2‐substituted quinolines under very mild conditions (1 mol% complex, 1 mol% PPh 3 , 1–5 atm H 2 , toluene, 25 °C) in appreciable enantioselectivity (up to 90:10 er ). The sensitivity of the hydrogenation process to changes in the phosphine additive suggests that an outer‐sphere reaction mechanism may be involved, as proposed for a related achiral NHC‐Ir complex reported by Crabtree and co‐workers.