Nickel Catalysis: Synergy between Method Development and Total Synthesis

Enantioselective Synthesis of anti-1,2-Diamines by Cu-Catalyzed Reductive Couplings of Azadienes with Aldimines and Ketimines

Here we report highly efficient and chemoselective azadiene-imine reductive couplings catalyzed by (Ph-BPE)Cu-H that afford anti-1,2-diamines. In all cases, reactions take place with either aldimine or ketimine electrophiles to deliver a single diastereomer of product in >95:5 er. The products' diamines are easily differentiable, facilitating downstream synthesis.

Brønsted Acid Enabled Nickel-Catalyzed Hydroalkenylation of Aldehydes with Styrene and its Derivatives

A Brønsted acid enabled nickel-catalyzed hydroalkenylation of aldehydes and styrene derivatives has been developed. The Brønsted acid acts as a proton shuttle to transfer a proton from the alkene to the aldehyde, thereby leading to an economical and byproduct-free coupling. A series of synthetically useful allylic alcohols were obtained through one-step reactions from readily available styrene derivatives and aliphatic aldehydes in up to 88 % yield and with high linear selectivity.

2-Azadienes as Reagents for Preparing Chiral Amines: Synthesis of 1,2-Amino Tertiary Alcohols by Cu-Catalyzed Enantioselective Reductive Couplings with Ketones

We introduce a new strategy for synthesis of chiral amines: couplings of α-aminoalkyl nucleophiles generated by enantioselective migratory insertion of 2-azadienes to a Cu-H. In this report, we demonstrate its application in catalytic reductive coupling of 2-azadienes and ketones to furnish 1,2-amino tertiary alcohols with vicinal stereogenic centers.

Nickel catalyzed regio- and stereoselective arylation and methylation of allenamides via coupling reactions. An experimental and computational study

A novel Ni-catalyzed regio- and stereoselective condensation of boronic acids/Me₂Zn to allenamides is documented and its potential explored.

Two-step synthesis of chiral fused tricyclic scaffolds from phenols via desymmetrization on nickel

Tricyclic furan derivatives with multiple chiral centers are ubiquitous in natural products. Construction of such tricyclic scaffolds in a stereocontrolled, step-economic, and atom-economic manner is a key challenge. Here we show a nickel-catalyzed highly enantioselective synthesis of hydronaphtho[1,8-bc]furans with five contiguous chiral centers via desymmetrization of alkynyl-cyclohexadienone by oxidative cyclization and following formal [4 + 2] cycloaddition processes. Alkynyl-cyclohexadienone was synthesized in one step from easily accessible phenols. This reaction represents excellent chemo-selectivity, regio-selectivity, diastereo-selectivity, and enantio-selectivity (single diastereomer, up to 99% ee). An extraordinary regioselectivity in the formal [4 + 2] cycloaddition step with enones revealed the diverse reactivity of the nickelacycle intermediate. Desymmetrization of alkynyl-cyclohexadienones via oxidative cyclization on nickel was supported by the isolation of a nickelacycle from a stoichiometric reaction. Enantioenriched tricyclic products contain various functional groups such as C=O and C=C. The synthetic utility of these products was demonstrated by derivatization of these functional groups.

Tricyclic furanic compounds with multiple chiral centers are found in a variety of natural products. Here, the authors show a highly enantioselective nickel-catalyzed procedure to access tricyclic oxygen-containing scaffolds with five contiguous chiral centers.

K2CO3-Mediated Cyclization and Rearrangement of γ,δ-Alkynyl Oximes To Form Pyridols

A novel K₂CO₃-mediated cyclization and rearrangement of γ,δ-alkynyl oximes for the synthesis of pyridols is described. The process accomplishes an efficient [1,3] rearrangement of the O-vinyl oxime intermediate which is in situ generated from the intramolecular nucleophilic addition of γ,δ-alkynyl oximes. The reaction employs readily accessible starting materials, tolerates a wide range of functional groups, and gives a variety of synthetically challenging pyridols in good yields.

Synergy Effects in the Chemical Synthesis and Extensions of Multicomponent Reactions (MCRs)-The Low Energy Way to Ultra-Short Syntheses of Tailor-Made Molecules

Several novel methods, catalysts and reagents have been developed to improve organic synthesis. Synergistic effects between reactions, reagents and catalysts can lead to minor heats of reaction and occur as an inherent result of multicomponent reactions (MCRs) and their extensions. They enable syntheses to be performed at a low energy level and the number of synthesis steps to be drastically reduced in comparison with ‘classical’ two-component reactions, fulfilling the rules of Green Chemistry. The very high potential for variability, diversity and complexity of MCRs additionally generates an extremely diverse range of products, thus bringing us closer to the aim of being able to produce tailor-made and extremely low-cost materials, drugs and compound libraries.

Unsymmetrical N-Aryl-1-(pyridin-2-yl)methanimine Ligands in Organonickel(II) Complexes: More Than a Blend of 2,2'-Bipyridine and N,N-Diaryl-α-diimines?

The new organonickel complexes [(R-PyMA)Ni(Mes)X] [R-PyMA = N-aryl-1-(pyridin-2-yl)methanimine; aryl = phenyl, 2,6-Me₂-, 3,5-Me₂-, 2,4,6-Me₃-, 2,6-ⁱPr₂-, 3,5-(OMe)₂-, 2-NO₂-4-Me-, 4-NO₂-, 2-CF₃-, and 2-CF₃-6-F-phenyl; Mes = 2,4,6-trimethylphenyl; X = F, Cl, Br, or I] were obtained as approximate 1/1 cis and trans isomeric mixtures or pure cis isomers depending on the PyMA ligand and X. The [(R-PyMA)Ni(Mes)X] complexes with X = Br or Cl were directly synthesized from the precursors trans-[(PPh₃)₂Ni(Mes)X], while [(PyMA)Ni(Mes)X] derivatives with X = F or I were obtained from [(PyMA)Ni(Mes)Br] through X exchange reactions. Although density functional theory (DFT) calculations show a preference for the sterically favored cis isomers, both isomers could be observed in many cases; in three cases, even single crystals for X-ray diffraction could be obtained for the trans isomers. Possible intermediates for the isomerization were investigated by DFT calculations. All complexes were studied by multiple spectroscopic means, electrochemistry, and spectroelectrochemistry (for the reduction processes). The long-wavelength metal-to-ligand charge-transfer (MLCT) absorptions vary markedly with the R substituent of the ligand and the cathodic electrochemical potentials to a far smaller degree. Both are almost invariable upon variation of X. All of this is in line with Ni-based and π*-based lowest unoccupied molecular orbitals (LUMOs). In line with the unsymmetric character of the N_Py^N_methanimine ligand, electrochemistry and MLCT transitions seem to not correspond to the same type of π* LUMO, making these PyMA ligands more interesting than the symmetric heteroaromatic polypyridine ligands such as 2,2'-bipyridine (bpy; N_Py^N_Py) and N,N-diaryl-substituted aliphatic α-diimines (N_methanimine^N_methanimine) such as the diaza-1,3-butadienes (DAB). First attempts to use these complexes in Negishi-type cross-coupling reactions were successful.

A Synthesis of Substituted α-Allenols via Iron-Catalyzed Cross-Coupling of Propargyl Carboxylates with Grignard Reagents

α-Allenols are attractive and versatile compounds whose preparation can be a nontrivial task. In this Letter, we provide a method for the prompt synthesis of substituted α-allenols via a catalytic cross-coupling reaction which makes use of a nontoxic and cost-effective iron catalyst. The catalyst loading is typically as low as 1-5 mol %. The mild reaction conditions (-20 °C) and the short reaction time (15 min) allow for the presence of a variety of functional groups. Moreover, the reaction was shown to be scalable up to gram-scale and the propargyl substrates are readily accessible by a one-pot synthesis.

Mechanism of the Palladium-Catalyzed Synthesis of Münchnones: The Role of Ligands in N-Acyl Iminium Salt Carbonylation

The palladium-catalyzed carbonylative coupling of imines, acid chlorides, and dipolarophiles can provide efficient routes to prepare nitrogen-containing heterocycles. One challenge in developing this reaction, and in the creation of more active catalyst systems, is the lack of data on how this complex transformation proceeds. To address this, we report here the results of our mechanistic studies on this system, and in particular the formation of mesoionic münchnones. This includes the synthesis of key catalytic intermediates, model reactions, and kinetic studies that support the role of these compounds in catalysis. Together, these studies provide a clear picture of the impact of catalyst structure, ligands, and palladium nanoparticles on facilitating the carbonylation of in situ generated iminium salts, and suggest an avenue for the creation of more active catalyst systems.

Which Metals are Green for Catalysis? Comparison of the Toxicities of Ni, Cu, Fe, Pd, Pt, Rh, and Au Salts

Environmental profiles for the selected metals were compiled on the basis of available data on their biological activities. Analysis of the profiles suggests that the concept of toxic heavy metals and safe nontoxic alternatives based on lighter metals should be re-evaluated. Comparison of the toxicological data indicates that palladium, platinum, and gold compounds, often considered heavy and toxic, may in fact be not so dangerous, whereas complexes of nickel and copper, typically assumed to be green and sustainable alternatives, may possess significant toxicities, which is also greatly affected by the solubility in water and biological fluids. It appears that the development of new catalysts and novel applications should not rely on the existing assumptions concerning toxicity/nontoxicity. Overall, the available experimental data seem insufficient for accurate evaluation of biological activity of these metals and its modulation by the ligands. Without dedicated experimental measurements for particular metal/ligand frameworks, toxicity should not be used as a "selling point" when describing new catalysts.

Nickel-Catalyzed Intramolecular C-O Bond Formation: Synthesis of Cyclic Enol Ethers

An efficient and exceptionally mild intramolecular nickel-catalyzed carbon-oxygen bond-forming reaction between vinyl halides and primary, secondary, and tertiary alcohols has been achieved. Zinc powder was found to be an essential additive for obtaining high catalyst turnover and yields. This operationally simple method allows direct access to cyclic vinyl ethers in high yields in a single step.

Iron-Catalyzed Regioselective Transfer Hydrogenative Couplings of Unactivated Aldehydes with Simple Alkenes

An FeBr3 -catalyzed reductive coupling of various aldehydes with alkenes that proceeds through a direct hydride transfer pathway has been developed. With (i) PrOH as the hydrogen donor under mild conditions, previously challenging coupling reactions of unactivated alkyl and aryl aldehydes with simple alkenes, such as styrene derivatives and α-olefins, proceeded smoothly to furnish a diverse range of functionalized alcohols with complete linear regioselectivity.

Iron-catalyzed Cross-Coupling of Propargyl Carboxylates and Grignard Reagents: Synthesis of Substituted Allenes

Presented herein is a mild, facile, and efficient iron-catalyzed synthesis of substituted allenes from propargyl carboxylates and Grignard reagents. Only 1-5 mol % of the inexpensive and environmentally benign [Fe(acac)3 ] at -20 °C was sufficient to afford a broad range of substituted allenes in excellent yields. The method tolerates a variety of functional groups.

Synthesis of P-stereogenic diarylphosphinic amides by directed lithiation: transformation into tertiary phosphine oxides via methanolysis, aryne chemistry and complexation behaviour toward zinc(ii)

A general synthesis of P-stereogenic compoundsviaDoLi–electrophilic quenching of phosphinic amides and subsequent derivatizations is reported.

Nickel(0)-catalyzed intramolecular reductive coupling of alkenes and aldehydes or ketones with hydrosilanes

A nickel(0)-catalyzed reductive coupling of aldehydes and simple alkenes with hydrosilanes has been developed.