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New Chiral Ruthenium( II ) Catalysts Containing 2,6‐Bis(4′‐( R )‐phenyloxazolin‐2′‐yl)pyridine (Ph‐pybox) Ligands for Highly Enantioselective Transfer Hydrogenation of Ketones
Author(s) -
Cuervo Darío,
Gamasa M. Pilar,
Gimeno José
Publication year - 2004
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200305170
Subject(s) - chemistry , transfer hydrogenation , ruthenium , acetophenone , pyridine , medicinal chemistry , catalysis , stereochemistry , isomerization , enantioselective synthesis , dichloromethane , methanol , solvent , organic chemistry
Treatment of complex trans ‐[RuCl 2 (η 2 ‐C 2 H 4 ){κ 3 ‐ N , N , N ‐( R , R )‐Ph‐pybox}] [( R , R )‐Ph‐pybox = 2,6‐bis{4′‐( R )‐phenyloxazolin‐2′‐yl}pyridine] with phosphines or phosphites in dichloromethane at 50 °C leads to the formation of novel ruthenium( II )‐pybox complexes trans ‐[RuCl 2 (L){κ 3 ‐ N , N , N ‐( R , R )‐Ph‐pybox}] [L = PPh 3 ( 1 a ), PPh 2 Me ( 2 a ), PPh 2 (C 3 H 5 ) ( 3 a ), PPh 2 (C 4 H 7 ) ( 4 a ), PMe 3 ( 5 a ), P i Pr 3 ( 6 a ), P(OMe) 3 ( 7 a ) and P(OPh) 3 ( 8 a )]. Likewise, reaction of trans ‐[RuCl 2 (η 2 ‐C 2 H 4 ){κ 3 ‐ N , N , N ‐( R , R )‐Ph‐pybox}] with PPh 3 or P i Pr 3 in refluxing methanol leads to the complexes cis ‐[RuCl 2 (L)(κ 3 ‐ N , N , N ‐( R , R )‐Ph‐pybox] [L = PPh 3 ( 1 b ), P i Pr 3 ( 6 b )]. No trans–cis isomerisation of complexes 1 a – 8 a has been observed. Complexes 1 a – 8 a , 1 b , 6 b together with the analogous trans ‐[RuCl 2 {P(OMe) 3 }{κ 3 ‐ N , N , N ‐( S , S )‐ i Pr‐pybox}] ( 10 a ) and the previously reported trans ‐ and cis ‐[RuCl 2 (PPh 3 ){κ 3 ‐ N , N , N ‐( S , S )‐ i Pr‐pybox}] ( 9 a and 9 b , respectively) are active catalysts for the transfer hydrogenation of acetophenone in 2‐propanol in the presence of NaOH (ketone/cat/NaOH 500:1:6). cis ‐Ph‐pybox derivatives are the most active catalysts. In particular, cis complexes 1 b and 6 b led to almost quantitative conversions in less than 5 min with a high enantioselectivity (up to 95 %). A variety of aromatic ketones have also been reduced to the corresponding secondary alcohols with very high TOF and ee up to 94 %. The overall catalytic performance seems to be a subtle combination of the steric and/or electronic properties both the phosphines and the ketones. A high TOF (27 300 h −1 ) and excellent ee (94 %) have been found for the reduction of 3‐bromoacetophenone with catalyst 6 b . Reductions of alkyl ketones also proceed with high and rapid conversions but low enantioselectivities are achieved.