Intermolecular C–H Bond Activation of Alkanes and Arenes by NCN Pincer Iridium(III) Acetate Complexes Containing Bis(oxazolinyl)phenyl Ligands

Carbon–hydrogen bond activation of arenes by the pincer Ir­(III) acetate complex (phebox)­Ir­(OAc)₂(H₂O) (<b>1</b>; phebox = bis­(oxazolinyl)­phenyl) proceeded at 100 °C to give the corresponding aryl Ir­(III) complexes (phebox)­Ir­(Ar)­(OAc) in high yields. Reactions of the monosubstituted arenes anisole, toluene, chlorobenzene, acetophenone, and nitrobenzene resulted in the formation of meta- and para-substituted aryl complexes in high yields. Competitive experiments between two monosubstituted arenes exhibited relative reaction rates in the order C₆H₅OMe > C₆H₅NO₂ > C₆H₅COMe > C₆H₅Cl > C₆H₅Me. The kinetic isotope effects of C–H bond activation of benzene and nitrobenzene were determined to be 2.9 ± 0.1 and 2.0 ± 0.4, respectively. The Ir complex <b>1</b> underwent catalytic borylation of arenes with bis­(pinacolato)­diboron (B₂(pin)₂) or pinacolborane (HB­(pin)), giving the corresponding borylated products. The Ir complex <b>1</b> was also reactive toward alkane C–H bond activation, demonstrated by the reactions with <i>n</i>-heptane and <i>n</i>-octane at 160 °C that cleanly afforded the corresponding alkyl complexes (phebox)­Ir­(<i>n</i>-C_<i>n</i>H_2<i>n</i>+1)­(OAc) (<i>n</i> = 7, 8, respectively)

Intermolecular C–H Bond Activation of Alkanes and Arenes by NCN Pincer Iridium(III) Acetate Complexes Containing Bis(oxazolinyl)phenyl Ligands

Carbon–hydrogen bond activation of arenes by the pincer Ir­(III) acetate complex (phebox)­Ir­(OAc)₂(H₂O) (<b>1</b>; phebox = bis­(oxazolinyl)­phenyl) proceeded at 100 °C to give the corresponding aryl Ir­(III) complexes (phebox)­Ir­(Ar)­(OAc) in high yields. Reactions of the monosubstituted arenes anisole, toluene, chlorobenzene, acetophenone, and nitrobenzene resulted in the formation of meta- and para-substituted aryl complexes in high yields. Competitive experiments between two monosubstituted arenes exhibited relative reaction rates in the order C₆H₅OMe > C₆H₅NO₂ > C₆H₅COMe > C₆H₅Cl > C₆H₅Me. The kinetic isotope effects of C–H bond activation of benzene and nitrobenzene were determined to be 2.9 ± 0.1 and 2.0 ± 0.4, respectively. The Ir complex <b>1</b> underwent catalytic borylation of arenes with bis­(pinacolato)­diboron (B₂(pin)₂) or pinacolborane (HB­(pin)), giving the corresponding borylated products. The Ir complex <b>1</b> was also reactive toward alkane C–H bond activation, demonstrated by the reactions with <i>n</i>-heptane and <i>n</i>-octane at 160 °C that cleanly afforded the corresponding alkyl complexes (phebox)­Ir­(<i>n</i>-C_<i>n</i>H_2<i>n</i>+1)­(OAc) (<i>n</i> = 7, 8, respectively)

NCN-Pincer Cobalt Complexes Containing Bis(oxazolinyl)phenyl Ligands

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-<i>R</i>)Br (<b>1a</b>, R = Me₂; <b>1b</b>, R = <i>i</i>Pr) resulted in the formation of the tricarbonyl Co­(I) complex (phebox-R)­Co­(CO)₃ (<b>2a</b>, R = Me₂; <b>2b</b>, R = <i>i</i>Pr), in which NC-bidentate coordination of the phebox ligand was observed. Complexes <b>2</b> underwent oxidative addition of I₂ to give the Co­(III) aqua complex (phebox-<i>R</i>)­CoI₂(H₂O) (<b>4a</b>, R = Me₂; <b>4b</b>, R = <i>i</i>Pr) by a change in the coordination geometry to the NCN-tridentate mode. Ligand exchange reactions of H₂O or I ligand with CN<i>t</i>Bu or AgOAc smoothly proceeded to give the isocyanide complex (phebox-dm)­CoI₂(CN<i>t</i>Bu) (<b>5</b>) or the acetate complex (phebox-dm)­Co­(κ₁-OAc)­(κ₂-OAc) (<b>6</b>), respectively

NCN-Pincer Cobalt Complexes Containing Bis(oxazolinyl)phenyl Ligands

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-<i>R</i>)Br (<b>1a</b>, R = Me₂; <b>1b</b>, R = <i>i</i>Pr) resulted in the formation of the tricarbonyl Co­(I) complex (phebox-R)­Co­(CO)₃ (<b>2a</b>, R = Me₂; <b>2b</b>, R = <i>i</i>Pr), in which NC-bidentate coordination of the phebox ligand was observed. Complexes <b>2</b> underwent oxidative addition of I₂ to give the Co­(III) aqua complex (phebox-<i>R</i>)­CoI₂(H₂O) (<b>4a</b>, R = Me₂; <b>4b</b>, R = <i>i</i>Pr) by a change in the coordination geometry to the NCN-tridentate mode. Ligand exchange reactions of H₂O or I ligand with CN<i>t</i>Bu or AgOAc smoothly proceeded to give the isocyanide complex (phebox-dm)­CoI₂(CN<i>t</i>Bu) (<b>5</b>) or the acetate complex (phebox-dm)­Co­(κ₁-OAc)­(κ₂-OAc) (<b>6</b>), respectively

Change in Coordination of NCN Pincer Iron Complexes Containing Bis(oxazolinyl)phenyl Ligands

The coordination of bis­(oxazolinyl)­phenyl (phebox) ligands to an Fe center was investigated in the reaction of (phebox-R)­Fe­(CO)₂Br (<b>1a</b>: R = Me₂; <b>1b</b>: R = <i>i</i>-Pr) with phosphine and isocyanide compounds. Reaction of <b>1</b> with an excess amount of PMe₃ in toluene proceeded at 50 °C to give the corresponding cationic complexes [(phebox-R)­Fe­(CO)­(PMe₃)₂]Br [<b>2a</b>: R = Me₂ (79%); <b>2b</b>: R = <i>i</i>-Pr (83%)]. The molecular structures of <b>2a</b> and <b>2b</b> were confirmed by X-ray diffraction analysis that revealed the pseudo-octahedral geometry with NCN meridional coordination of the phebox ligand. In contrast, reaction of <b>1</b> with PMe₂Ph gave the neutral phosphine complexes (η²-phebox-R)­Fe­(CO)­(PMe₂Ph)₂Br [<b>3a</b>: R = Me₂ (87%); <b>3b</b>: R = <i>i</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 <b>3a</b> resulted in the formation of the corresponding cationic complexes [(phebox-Me₂)­Fe­(CO)­(PMe₂Ph)₂]Br (<b>4</b>) <i>via</i> change in coordination to the tridentate mode. The reaction of <b>1</b> with <i>tert</i>-butylisocyanide CN­(<i>t</i>-Bu) gave a mixture of neutral isocyanide complexes (phebox-Me₂)­Fe­(CO)­[CN­(<i>t</i>-Bu)]Br (<b>5</b>, <b>6</b>) in 57 and 10% yields, respectively, <i>via</i> exchange of one of the CO ligands. Subsequent reaction of <b>5</b> with CN­(<i>t</i>-Bu) resulted in formation of the cationic complex {(phebox-Me₂)­Fe­[CN­(<i>t</i>-Bu)]₃}Br (<b>7a</b>). Similarly, treatment of <b>1</b> with an excess amount of CN­(t-Bu) afforded {(phebox-R)­Fe­[CN­(<i>t</i>-Bu)]₃}Br [<b>7a</b>: R = Me₂ (83%); <b>7b</b>: R = <i>i</i>-Pr (69%)]

Asymmetric Three-Component Coupling Reaction of Alkyne, Enone, and Aldehyde Catalyzed by Chiral Phebox Ruthenium Catalysts

Catalytic asymmetric three-component coupling reactions of terminal alkynes, α,β-unsaturated ketones, and aldehydes were studied. The chiral ruthenium complexes containing bis­(oxa­zolinyl)­phenyl ligands were found to serve as efficient catalysts for a tandem reaction based on conjugate addition of terminal alkynes to α,β-unsaturated ketones and subsequent aldol reaction with aldehydes, giving β-hydroxyketone derivatives containing α-propargyl groups in high yields with moderate to good enantioselectivities. This method can produce various functional molecules from commercially available substrates in a one-pot procedure. The absolute configuration of the major product was determined by X-ray analysis. The control experiments suggested that a ruthenium enolate species generated in situ by conjugate addition could be involved as an intermediate for the aldol coupling with an aldehyde

Asymmetric Three-Component Coupling Reaction of Alkyne, Enone, and Aldehyde Catalyzed by Chiral Phebox Ruthenium Catalysts

Catalytic asymmetric three-component coupling reactions of terminal alkynes, α,β-unsaturated ketones, and aldehydes were studied. The chiral ruthenium complexes containing bis­(oxa­zolinyl)­phenyl ligands were found to serve as efficient catalysts for a tandem reaction based on conjugate addition of terminal alkynes to α,β-unsaturated ketones and subsequent aldol reaction with aldehydes, giving β-hydroxyketone derivatives containing α-propargyl groups in high yields with moderate to good enantioselectivities. This method can produce various functional molecules from commercially available substrates in a one-pot procedure. The absolute configuration of the major product was determined by X-ray analysis. The control experiments suggested that a ruthenium enolate species generated in situ by conjugate addition could be involved as an intermediate for the aldol coupling with an aldehyde

Preparation, Characterization, and Catalytic Reactions of NCN Pincer Iron Complexes Containing Stannyl, Silyl, Methyl, and Phenyl Ligands

Preparation and reactivity of chiral and achiral NCN pincer Fe complexes containing bis­(oxazolinyl)­phenyl (abbreviated as phebox) ligands with SnMe₃, SiMe₃, Me, and Ph ligands were investigated. Irradiation of (phebox)­SnMe₃ (<b>2</b>) with 1 equiv of Fe­(CO)₅ led to oxidative addition to give NCN pincer stannyl complex (phebox)­Fe­(CO)₂(SnMe₃) (<b>3</b>). Similarly, oxidative addition of (phebox)­SiMe₃ (<b>4</b>) with Fe­(CO)₅ resulted in the formation of silyl complex (phebox)­Fe­(CO)₂SiMe₃ (<b>5</b>). Me and Ph complexes (phebox)­Fe­(CO)₂R (<b>7</b>, R = Me; <b>8</b>, R = Ph) were synthesized by transmetalation of the bromide complex (phebox)­Fe­(CO)₂Br (<b>1</b>) with ZnMe₂ and ZnPh₂, respectively. These phebox Fe complexes served as catalysts for hydrosilylation of a ketone and C–H silylation of <i>N</i>-methylindole

Synthesis of NHC-Oxazoline Pincer Complexes of Rh and Ru and Their Catalytic Activity for Hydrogenation and Conjugate Reduction

We describe the preparation and catalytic reactions of new CCN pincer Rh and Ru complexes containing NCH-oxazoline hybrid ligands. Oxazolinyl-phenyl-imidazolium derivatives (<b>3</b>) were suitable ligand precursors for the CCN pincer scaffold. C–H bond activation of <b>3</b> with RhCl₃·3H₂O in the presence of NEt₃ yielded the desired CCN pincer Rh complexes <b>5</b> in 13–27% yields. The related CCN pincer Ru complexes <b>8</b>–<b>10</b> were synthesized in good yields by C–H bond activation of <i>p</i>-cymene Ru complexes <b>7</b> in the presence of NaOAc in DMF. The chiral complexes <b>8</b> and <b>9</b> had two diastereomers according to the coordination of CO and OAc ligands. The CCN Rh complexes showed catalytic activity for conjugate reduction of ethyl β-methylcinnamate with hydrosilane, with moderate enantioselectivity. The CCN Ru complexes were found to be active in the hydrogenation of aromatic ketones. In particular, hydrogenation of 9-acetylanthracene took place at not only the CO bond but also the anthracene ring. The Ru complexes were also used as catalysts in the transfer hydrogenation of 9-acetylanthracene with 2-propanol; again, both the CO bond and the anthracene ring were hydrogenated