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%)].
This paper presents the synthesis and characterization of (TMS-ethynyl)-substituted germa [4]pericyclyne composed of acetylene moieties and germanium atoms. Characterization was performed by X-ray crystallography, UVvis absorption, and photoluminescence spectroscopy. DFT calculations suggested interactions between the side chain alkynes and the ringforming acetylenes through germanium vertex atoms. Keywords: Element-blocks | Germanium | PericyclynesResearch on polyacetylene materials has attracted increasing attention with the growing importance of "element-blocks" (functional nano building blocks composed of heteroatoms) for advanced electronic and optical materials.1,2 Skipped polyynes ([R 2 XC¸C] n ) have received particular interest as a versatile "element-block" due to their easy modification and tunability of their characteristics by choosing appropriate linker atoms (X). Specifically, recent reports reveal that by using group 14 heteroatoms like silicon, with smaller ionization energies than those of carbon, one can achieve electron delocalization (although usually, the conjugation is interrupted at the alkyne units), resulting in effective enhancement of electric and optical properties.4 Therefore, skipped polyynes are potential candidates for preparing functional materials such as semiconductors, ceramic precursors, and hole-transporting materials. 5We have studied the synthesis and properties of both acyclic and cyclic germylene-ethynylene materials containing germanium (Scheme 1).6 Having a smaller ionization energy than silicon, germanium also forms stronger bonds to sp-carbon compared to tin. Additionally, cyclic skipped Ge-polyynes, called germa [N]pericyclynes, display characteristic emission properties compared to acyclic molecules, due to the fixed conformations and through-bond/through-space interactions. Therefore, element-block polymers composed of germapericyclynes, which are expected to impart functionality superior to that provided by monomeric pericyclyne units, are a promising platform for the fabrication of unique optical materials. Polymeric pericyclyne materials with linear, branched, dendric, and spirocyclic forms can be designed, with each pericyclyne unit connected via alkyne bridges. Thus, to enable electronic delocalization across the alkyne units, it is necessary for the vertex atoms to mediate conjugation between the alkyne spacers. The existence of alkynealkyne interactions through Ge atoms in pericyclyne ring systems has been suggested in our previous investigations.6 Thus, exo-ethynylated pericyclynes could be expected to show exoendo and exoexo alkynes interactions, and would be suitable candidate monomer units for polypericyclynes. However, their optical properties along with their molecular orbitals have not been studied yet. To explore the unique character of germa[N]pericyclynes as polymer precursors, we herein disclose the synthesis of an ethynylated germa[4]pericyclyne for preliminary research on poly(germapericyclynes), and provide a discussion of its properties.Star...
The combination of commercially available palladium on carbon (Pd/C) with 1,1,3,3‐tetramethyldisiloxane (TMDS) was found to be an effective catalyst system for the production of tertiary amines by deoxygenative silane‐reduction of carboxamides. Facile removal of both the metallic species and silane residues can be achieved by simple extraction, and the recovered Pd/C was reusable. Other palladium complexes and salts, Pd2(dba)3(CHCl3) [dba = dibenzylideneacetone], Pd(OAc)2, and PdCl2, were also effective for the reduction of tertiary carboxamides with TMDS. Detailed mechanistic studies of the present Pd/C–TMDS system revealed that the catalytically active species is the soluble palladium colloids leached from the palladium particles on the carbon support by treating with TMDS, and carboxamides, amine products, and TMDS act as a stabilizer for the generated palladium species in solution.
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