Pyridine ligands enantioselective reduction

RuCl2 P(C6H5)3 3 is used as a precursor in combination with a pyridine-containing derivative of 2-amino-2 -hydroxy-l,T-binaphthyl [272], and an amino bisoxazo-line ligand, AMBOX (16) [273], effecting the enantioselective reduction of several aromatic ketones with up to 98% e.e. The enantioselectivity is decreased by increasing the bulldness of alkyl groups. 

Chiral BDSfOL ligands can be applied to add the Grignard reagent in an enantioselective fashion to the 2-position of pyridine A-oxide. Reductive workup then yields the more stable tetrahydropyridine (eq 38). Although, the enantioselectivites are moderate, further enrichment by crystallization is possible. 

Moderate to good enantioselectivities were obtained for nearly all examples, but the products from 83a-c could be recrystallized to higher enantiomeric purity. Addition of iodine was critical for catalysis as was the use of a ligand with electron-poor para-fluorophenyl groups on the phosphorous atom. Substitution at the 3 position of the pyridine ring was described as being difficult for both the quinolines and pyridine systems. The resulting hydrazine derivatives could be easily converted to piperdines by reduction with Raney nickel or under Birch conditions. 

New 4-substituted phenyl(bisoxazoline) ligands (PHEBOX ligands) have been com-plexed with rhodium and examined as enantioselective catalysts of the reductive aldol of acrylates and aldehydes.160 The results have been compared with the corresponding pyridine-centred (PYBOX) ligand complexes. 

Rhodium borohydride complexes described by the general formula [(chiral carboxam-ide)(pyridine)2Cl2(BH4)Rh] have been studied as enantioselective catalysts for the reduction of methyl 3-phenyl-2-butenoate with hydrogen89. Although enantiomeric excesses of up to ca. 60% have been observed using (R)- or (.S )-/V-(l-phenethyl)fonnamide as a chiral ligand, this class of catalysts has not been developed further. 

Other procedures for carbonyl hydrosilylation of aldehydes and ketones are using [bis(imino)pyridine]iron dinitrogen and dialkyl complexes as precatalysts. Only 0.1-1.0 mol% catalyst are required to achieve this transformation. The reductants are either phenylsilane or diphenylsilane in this case. A number of enantioselective versions of the hydrosilylation reaction is described. This includes the application of 1,2-bis[(25, 55)-2,5-dimethylphospholano]benzene [(S,5)-Me-DuPhos] (Scheme 4-328) as chiral ligand, iron(II) acetate as a precatalyst and polymethylhydrosiloxane as hydride source. A large variety of ketones can be transformed into the corresponding alcohols in excellent yield and up to 99% enantiomeric excess. Catalytic ketone hydrosilylation is also achieved with the dialkyliron complexes (S,S)-... 

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