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Role of the NH 2 Functionality and Solvent in Terdentate CNN Alkoxide Ruthenium Complexes for the Fast Transfer Hydrogenation of Ketones in 2‐Propanol
Author(s) -
Baratta Walter,
Ballico Maurizio,
Esposito Gennaro,
Rigo Pierluigi
Publication year - 2008
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.200701870
Subject(s) - alkoxide , ruthenium , hydride , chemistry , medicinal chemistry , protonation , propanol , acetone , alcohol , adduct , hydrogen bond , catalysis , hydrogen , methanol , molecule , organic chemistry , ion
The reaction of [RuCl(CNN)(dppb)] ( 1 ; HCNN=6‐(4‐methylphenyl)‐2‐pyridylmethylamine) with NaO i Pr in 2‐propanol/C 6 D 6 affords the alcohol adduct alkoxide [Ru(O i Pr)(CNN)(dppb)] ⋅ n i PrOH ( 5 ), containing the RuNH 2 linkage. The alkoxide [Ru(O i Pr)(CNN)(dppb)] ( 4 ) is formed by treatment of the hydride [Ru(H)(CNN)(dppb)] ( 2 ) with acetone in C 6 D 6 . Complex 5 in 2‐propanol/C 6 D 6 equilibrates quickly with hydride 2 and acetone with an exchange rate of (5.4±0.2) s −1 at 25 °C, higher than that found between 4 and 2 ((2.9±0.4) s −1 ). This fast process, involving a β‐hydrogen elimination versus ketone insertion into the RuH bond, occurs within a hydrogen‐bonding network favored by the RuNH 2 motif. The cationic alcohol complex [Ru(CNN)(dppb)( i PrOH)](BAr f 4 ) ( 6 ; Ar f =3,5‐C 6 H 3 (CF 3 ) 2 ), obtained from 1 , Na[BAr f 4 ], and 2‐propanol, reacts with NaO i Pr to afford 5 . Complex 5 reacts with either 4,4′‐difluorobenzophenone through hydride 2 or with 4,4′‐difluorobenzhydrol through protonation, affording the alkoxide [Ru{OCH(4‐C 6 H 4 F) 2 }(CNN)(dppb)] ( 7 ) in 90 and 85 % yield of the isolated product. The chiral CNN–ruthenium compound [RuCl(CNN){( S , S )‐Skewphos}] ( 8 ), obtained by the reaction of [RuCl 2 (PPh 3 ) 3 ] with ( S , S )‐Skewphos and orthometalation of HCNN in the presence of NEt 3 , is a highly active catalyst for the enantioselective transfer hydrogenation of methylaryl ketones (turnover frequencies (TOFs) of up to 1.4×10 6 h −1 at reflux were obtained) with up to 89 % ee . Also the ketone CF 3 CO(4‐C 6 H 4 F), containing the strong electron‐withdrawing CF 3 group, is reduced to the R alcohol with 64 % ee and a TOF of 1.5×10 4 h −1 . The chiral alkoxide [Ru(O i Pr)(CNN){( S , S )‐Skewphos}] ⋅ n i PrOH ( 9 ), obtained from 8 and NaO i Pr in the presence of 2‐propanol, reacts with CF 3 CO(4‐C 6 H 4 F) to afford a mixture of the diastereomer alkoxides [Ru{OCH(CF 3 )(4‐C 6 H 4 F)}(CNN){( S , S )‐Skewphos}] ( 10 / 11 ; 74 % yield) with 67 % de . This value is very close to the enantiomeric excess of the alcohol ( R )‐CF 3 CH(OH)(4‐C 6 H 4 F) formed in catalysis, thus suggesting that diastereoisomeric alkoxides with the RuNH 2 linkage are key species in the catalytic asymmetric transfer hydrogenation reaction.