New chiral pincer iron complexes having bis-(oxazolinyl)phenyl ligands have been synthesized by oxidative addition of Fe 2 (CO) 9 to 2-bromo-substituted ligands. This chiral pincer iron complex can catalyze the asymmetric hydrosilylation of simple aromatic ketones to give the corresponding alcohols.
Preparation and reactivity of chiral and achiral NCN pincer Fe complexes containing bis(oxazolinyl)phenyl (abbreviated as phebox) ligands with SnMe 3 , SiMe 3 , Me, and Ph ligands were investigated. Irradiation of (phebox)SnMe 3 (2) with 1 equiv of Fe(CO) 5 led to oxidative addition to give NCN pincer stannyl complex (phebox)Fe(CO) 2 (SnMe 3 ) (3). Similarly, oxidative addition of (phebox)SiMe 3 (4) with Fe(CO) 5 resulted in the formation of silyl complex (phebox)Fe(CO) 2 SiMe 3 (5). Me and Ph complexes (phebox)Fe(CO) 2 R (7, R = Me; 8, R = Ph) were synthesized by transmetalation of the bromide complex (phebox)Fe(CO) 2 Br (1) with ZnMe 2 and ZnPh 2 , respectively. These phebox Fe complexes served as catalysts for hydrosilylation of a ketone and C−H silylation of N-methylindole.
Practical procedures for the production of aldehydes, lactols, and ethers are offered by the palladium‐catalyzed silane‐reduction of carboxylic esters and lactones. The partial reduction of carboxylic esters or lactones with 1,1,3,3‐tetramethyldisiloxane in the presence of commercially available palladium on carbon (Pd/C) and some copper compounds as a co‐catalyst smoothly proceeds under mild conditions to afford the silyl acetal derivatives in high yields, which are easily converted to the corresponding aldehydes or lactols by hydrolysis of silyloxy group. On the other hand, deoxygenated ethers are obtained with high selectivity by treatment of silyl acetals with catalytic amounts of Me3SiOTf at –78 °C or stoichiometric amounts of BF3•OEt2 at ambient temperature.
We describe the preparation and characterization
of new NCN-pincer
Co(III) complexes containing bis(oxazolinyl)phenyl (phebox) ligands
as auxiliary ligands. The reaction of Co2(CO)8 with the 2-bromo-substituted ligand precursor (phebox-R)Br (1a, R = Me2; 1b, R = iPr) resulted in the formation of the tricarbonyl Co(I)
complex (phebox-R)Co(CO)3 (2a, R = Me2; 2b, R = iPr), in which NC-bidentate
coordination of the phebox ligand was observed. Complexes 2 underwent oxidative addition of I2 to give the Co(III)
aqua complex (phebox-R)CoI2(H2O) (4a, R = Me2; 4b, R = iPr) by a change in the coordination geometry to the NCN-tridentate
mode. Ligand exchange reactions of H2O or I ligand with
CNtBu or AgOAc smoothly proceeded to give the isocyanide
complex (phebox-dm)CoI2(CNtBu) (5) or the acetate complex (phebox-dm)Co(κ1-OAc)(κ2-OAc) (6), respectively.
The coordination of bis(oxazolinyl)phenyl (phebox) ligands to an Fe center was investigated in the reaction of (phebox-R)Fe(CO) 2 Br (1a: R = Me 2 ; 1b: R = i-Pr) with phosphine and isocyanide compounds. Reaction of 1 with an excess amount of PMe 3 in toluene proceeded at 50 °C to give the corresponding cationic complexes [(phebox-R)Fe-(CO)(PMe 3 ) 2 ]Br [2a: R = Me 2 (79%); 2b: R = i-Pr (83%)]. The molecular structures of 2a and 2b were confirmed by Xray diffraction analysis that revealed the pseudo-octahedral geometry with NCN meridional coordination of the phebox ligand. In contrast, reaction of 1 with PMe 2 Ph gave the neutral phosphine complexes (η 2 -phebox-R)Fe(CO)(PMe 2 Ph) 2 Br [3a: R = Me 2 (87%); 3b: R = i-Pr (79%)], in which the phebox ligand was coordinated to Fe as an NC bidentate ligand with the oxazoline and phenyl groups. Subsequent reaction of the neutral phosphine complex 3a resulted in the formation of the corresponding cationic complexes [(phebox-Me 2 )Fe(CO)(PMe 2 Ph) 2 ]Br (4) via change in coordination to the tridentate mode. The reaction of 1 with tert-butylisocyanide CN(t-Bu) gave a mixture of neutral isocyanide complexes (phebox-Me 2 )Fe(CO)[CN(t-Bu)]Br (5, 6) in 57 and 10% yields, respectively, via exchange of one of the CO ligands. Subsequent reaction of 5 with CN(t-Bu) resulted in formation of the cationic complex {(phebox-Me 2 )Fe[CN(t-Bu)] 3 }Br (7a). Similarly, treatment of 1 with an excess amount of CN(t-Bu) afforded {(phebox-R)Fe[CN(t-Bu)] 3 }Br [7a: R = Me 2 (83%); 7b: R = i-Pr (69%)].
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