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CNN Pincer Ruthenium Catalysts for Hydrogenation and Transfer Hydrogenation of Ketones: Experimental and Computational Studies
Author(s) -
Baratta Walter,
Baldino Salvatore,
Calhorda Maria José,
Costa Paulo J.,
Esposito Gennaro,
Herdtweck Eberhardt,
Magnolia Santo,
Mealli Carlo,
Messaoudi Abdelatif,
Mason Sax A.,
Veiros Luis F.
Publication year - 2014
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.201402229
Subject(s) - chemistry , alkoxide , hydride , transfer hydrogenation , ruthenium , medicinal chemistry , pyridine , pincer movement , alcohol , ketone , catalysis , amide , hydrogen , organic chemistry
Reaction of [RuCl(CNN)(dppb)] ( 1‐Cl ) (HCNN=2‐aminomethyl‐6‐(4‐methylphenyl)pyridine; dppb=Ph 2 P(CH 2 ) 4 PPh 2 ) with NaOCH 2 CF 3 leads to the amine‐alkoxide [Ru(CNN)(OCH 2 CF 3 )(dppb)] ( 1‐OCH 2 CF 3 ), whose neutron diffraction study reveals a short RuO ⋅⋅⋅ HN bond length. Treatment of 1‐Cl with NaOEt and EtOH affords the alkoxide [Ru(CNN)(OEt)(dppb)] ⋅ (EtOH) n ( 1‐OEt⋅ n EtOH ), which equilibrates with the hydride [RuH(CNN)(dppb)] ( 1‐H ) and acetaldehyde. Compound 1‐OEt⋅ n EtOH reacts reversibly with H 2 leading to 1‐H and EtOH through dihydrogen splitting. NMR spectroscopic studies on 1‐OEt⋅ n EtOH and 1‐H reveal hydrogen bond interactions and exchange processes. The chloride 1‐Cl catalyzes the hydrogenation (5 atm of H 2 ) of ketones to alcohols (turnover frequency (TOF) up to 6.5×10 4 h −1 , 40 °C). DFT calculations were performed on the reaction of [RuH(CNN′)(dmpb)] ( 2‐H ) (HCNN′=2‐aminomethyl‐6‐(phenyl)pyridine; dmpb=Me 2 P(CH 2 ) 4 PMe 2 ) with acetone and with one molecule of 2‐propanol, in alcohol, with the alkoxide complex being the most stable species. In the first step, the Ru‐hydride transfers one hydrogen atom to the carbon of the ketone, whereas the second hydrogen transfer from NH 2 is mediated by the alcohol and leads to the key “amide” intermediate. Regeneration of the hydride complex may occur by reaction with 2‐propanol or with H 2 ; both pathways have low barriers and are alcohol assisted.