308,149 research outputs found
Electron Microscopy Characterization of Ni-Cr-B-Si-C Laser Deposited Coatings
No full textDuring laser deposition of Ni-Cr-B-Si-C alloys with high amounts of Cr and B, various microstructures and phases can be generated from the same chemical composition that results in heterogeneous properties in the clad layer. In this study, the microstructure and phase constitution of a high-alloy Ni-Cr-B-Si-C coating deposited by laser cladding were analyzed by a combination of several microscopy characterization techniques including scanning electron microscopy in secondary and backscatter imaging modes, energy dispersive spectroscopy (EDS), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). The combination of EDS and EBSD allowed unequivocal identification of micron-sized precipitates as polycrystalline orthorhombic CrB, single crystal tetragonal Cr5B3, and single crystal hexagonal Cr7C3. In addition, TEM characterization showed various equilibrium and metastable Ni-B, Ni-Si, and Ni-Si-B eutectic products in the alloy matrix. The findings of this study can be used to explain the phase formation reactions and to tune the microstructure of Ni-Cr-B-Si-C coatings to obtain the desired properties.
Ni-catalyzed asymmetric C-P cross-coupling reaction via Ni(I)/Ni(III) two-electron pathway
No full textNickel demonstrates excellent performance in C-C or C-X cross-coupling reactions involving a SET process. However, the Ni(I)/Ni(III) two-electron pathway, which does not require light irradiation, is still rare, particularly in the asym-metric version. Here, we disclose a Ni(II)-catalyzed asymmetric C-P cross-coupling reaction via the Ni(I)/Ni(III) two-electron redox pathway. Combined experimental and computational research reveals that Ni(II) can be readily reduced to Ni(0) by secondary phosphine oxides, resulting in the formation of a Ni(I) active catalyst through comproportiona-tion. The discovery of the Ni(I)/Ni(III) two-electron mechanism may serve as a new paradigm of Ni catalysis, offering exciting opportunities in asymmetric reactions complementary to the traditional SET process
Radical Capture at Ni(II) Complexes: C-C, C-N, and C-O Bond Formation
No full textThe dinuclear b-diketiminato NiIItert-butoxide {[Me3NN]Ni}2(μ-OtBu)2 (2), synthesized from [Me3NN]Ni(2,4-lutidine) (1) and di-tert-butylperoxide, is a versatile precursor for the synthesis of a series of NiIIcomplexes [Me3NN]Ni-FG to illustrate C-C, C-N, and C-O bond formation at NiII via radicals. {[Me3NN]Ni}2(μ-OtBu)2 reacts with nitromethane, alkyl and aryl amines, acetophenone, benzamide, ammonia and phenols to deliver corresponding mono- or dinuclear [Me3NN]Ni-FG species (FG = O2NCH2, R-NH, ArNH, PhC(O)NH, PhC(O)CH2, NH2and OAr). Many of these NiII complexes are capable of capturing the benzylic radical PhCH(•)CH3 to deliver corresponding PhCH(FG)CH3 products featuring C-C, C-N or C-O bonds. DFT studies shed light on the mechanism of these transformations and suggest two competing pathways that depend on the nature of the functional groups. These radical capture reactions at [NiII]-FG complexes outline key C-C, C-N, and C-O bond forming steps and suggest new families of nickel radical relay catalysts.</p
Ni-Catalyzed Electrochemical C(sp2)−C(sp3) Cross-Coupling Reactions
No full textNickel (Ni) catalyzed carbon-carbon (C−C) cross-coupling has been considerably developed in last decades and has demonstrated unique reactivities compared to palladium. However, existing Ni catalyzed cross-coupling reactions, despite success in organic synthesis, are still subject to the use of air-sensitive nucleophiles (i.e. Grignard and organozinc reagents), or catalysts (i.e. Ni0 pre-catalysts), significantly limiting their academic and industrial adoption. Herein, we report that, through electrochemical voltammetry screening and optimization, the redox neutral C(sp2)‒C(sp3) cross-coupling can be accomplished in an undivided cell configuration using bench-stable aryl halide or β-bromostyrene (electrophiles) and benzylic trifluoroborate (nucleophiles) reactants, non-precious, bench stable catalysts consisting of NiCl2•glyme pre-catalyst and polypyridine ligands under ambient conditions. The broad reaction scope and good yields of the Ni-catalyzed electrochemical coupling reaction were confirmed by 48 examples of aryl/β-styrenyl chloride/bromide and benzylic trifluoroborates. Its potential applications were demonstrated by late-stage functionalization of pharmaceuticals and natural amino acid modification. Furthermore, this electrochemical C−C cross-coupling reaction was demonstrated at gram-scale in a flow-cell electrolyzer for practical industrial adoption. Finally, an array of chemical and electrochemical studies mechanistically indicates that electrochemical C−C cross-coupling reaction proceeds through an unconventional radical trans-metalation mechanism
Ni-Catalyzed Electrochemical C(sp2)−C(sp3) Cross-Coupling Reactions
No full textNickel (Ni) catalyzed carbon-carbon (C−C) cross-coupling has been considerably developed in last decades and has demonstrated unique reactivities compared to palladium. However, existing Ni catalyzed cross-coupling reactions, despite success in organic synthesis, are still subject to the use of air-sensitive nucleophiles (i.e. Grignard and organozinc reagents), or catalysts (i.e. Ni0 pre-catalysts), significantly limiting their academic and industrial adoption. Herein, we report that, through electrochemical voltammetry screening and optimization, the redox neutral C(sp2)‒C(sp3) cross-coupling can be accomplished in an undivided cell configuration using bench-stable aryl halide or β-bromostyrene (electrophiles) and benzylic trifluoroborate (nucleophiles) reactants, non-precious, bench stable catalysts consisting of NiCl2•glyme pre-catalyst and polypyridine ligands under ambient conditions. The broad reaction scope and good yields of the Ni-catalyzed electrochemical coupling reaction were confirmed by 48 examples of aryl/β-styrenyl chloride/bromide and benzylic trifluoroborates. Its potential applications were demonstrated by late-stage functionalization of pharmaceuticals and natural amino acid modification. Furthermore, this electrochemical C−C cross-coupling reaction was demonstrated at gram-scale in a flow-cell electrolyzer for practical industrial adoption. Finally, an array of chemical and electrochemical studies mechanistically indicates that electrochemical C−C cross-coupling reaction proceeds through an unconventional radical trans-metalation mechanism
Ni-Catalyzed Deamination Cross-Electrophile Coupling of Katritzky Salts with Halides via C–N Bond Activation
No full textThis work describes the first Ni-catalyzed cross-electrophile coupling of alkylpyridinium salts, derived from the corresponding amines, with aryl iodide, bromoalkyne or bromoalkyl coupling partners. C(sp)-C(sp3), C(sp2)-C(sp3) and C(sp3)-C(sp3) bond formation was achieved to afford a variety of synthetically useful arenes, alkynes and alkanes in good yields from2-33. The advantages of the methodology are showcased in the two-step synthesis of the key lactonic moiety of (+)-Compactin and (+)-Mevinolin from commercially available starting materials. A one-pot procedure without isolation of alkylpyridinium tetrafluoroborate salt was also demonstrated to be successful. This work represents a new strategy for the cross-coupling reaction of two electrophiles, and also provides a complementary and highly valuable vista for the current methodologies of alkyl arene/alkyne/alkane synthesis
Ni-Catalyzed 1,2-Acyl Migration Reactions Triggered by C–C Bond Activation of Ketones
No full textA Ni-catalyzed
1,2-acyl migration triggered by C–C bond
cleavage was developed. The process of 1,2-acyl migration followed
by olefin isomerization provides a convenient access to α,β-unsaturated
ketones, which are well-known building blocks in organic synthesis.
Experimental and computational studies show that the selective β-hydride
elimination and Ni-hydride reinsertion play an essential role in this
reaction
Ni-Catalyzed 1,2-Acyl Migration Reactions Triggered by C–C Bond Activation of Ketones
No full textA Ni-catalyzed
1,2-acyl migration triggered by C–C bond
cleavage was developed. The process of 1,2-acyl migration followed
by olefin isomerization provides a convenient access to α,β-unsaturated
ketones, which are well-known building blocks in organic synthesis.
Experimental and computational studies show that the selective β-hydride
elimination and Ni-hydride reinsertion play an essential role in this
reaction
Ni-Catalyzed 1,2-Acyl Migration Reactions Triggered by C–C Bond Activation of Ketones
No full textA Ni-catalyzed
1,2-acyl migration triggered by C–C bond
cleavage was developed. The process of 1,2-acyl migration followed
by olefin isomerization provides a convenient access to α,β-unsaturated
ketones, which are well-known building blocks in organic synthesis.
Experimental and computational studies show that the selective β-hydride
elimination and Ni-hydride reinsertion play an essential role in this
reaction
Catalytic hydrogenolysis of lignin C-O-C bonds over Ni/C catalyst
No full textCatalytic hydrogenolysis of lignin C-O-C bonds over Ni C catalys
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