Nickel boryl complexes and nickel-catalyzed alkyne borylation

The first nickel bis-boryl complexes cis-[Ni(ⁱPr₂Im^Me)₂(Bcat)₂], cis-[Ni(ⁱPr₂Im^Me)₂(Bpin)₂] and cis-[Ni(ⁱPr₂Im^Me)₂(Beg)₂] are reported, which were prepared via the reaction of a source of [Ni(ⁱPr₂Im^Me)₂] with the diboron(4) compounds B₂cat₂, B₂pin₂ and B₂eg₂ (ⁱPr₂Im^Me = 1,3-di-iso-propyl-4,5-dimethylimidazolin-2-ylidene; B₂cat₂ = bis(catecholato)diboron; B₂pin₂ = bis(pinacolato)diboron; B₂eg₂ = bis(ethylene glycolato)diboron). X-ray diffraction and DFT calculations strongly suggest that a delocalized, multicenter bonding scheme dictates the bonding situation of the NiB₂ moiety in these square planar complexes, reminiscent of the bonding situation of “non-classical” H₂ complexes. [Ni(ⁱPr₂Im^Me)₂] also efficiently catalyzes the diboration of alkynes using B₂cat₂ as the boron source under mild conditions. In contrast to the known platinum-catalyzed diboration, the nickel system follows a different mechanistic pathway, which not only provides the 1,2-borylation product in excellent yields, but also provides an efficient approach to other products such as C–C coupled borylation products or rare tetra-borylated compounds. The mechanism of the nickel-catalyzed alkyne borylation was examined by means of stoichiometric reactions and DFT calculations. Oxidative addition of the diboron reagent to nickel is not dominant; the first steps of the catalytic cycle are coordination of the alkyne to [Ni(ⁱPr₂Im^Me)₂] and subsequent borylation at the coordinated and, thus, activated alkyne to yield complexes of the type [Ni(NHC)₂(η²-cis-(Bcat)(R)C C(R)(Bcat))], exemplified by the isolation and structural characterization of [Ni(ⁱPr₂Im^Me)₂(η²-cis-(Bcat)(Me)C C(Me)(Bcat))] and [Ni(ⁱPr₂Im^Me)₂(η²-cis-(Bcat)(H₇C₃)C C(C₃H₇)(Bcat))].

Nickel bis-boryl complexes cis-[Ni(NHC)₂(Bcat)₂], cis-[Ni(NHC)₂(Bpin)₂] and cis-[Ni(NHC)₂(Beg)₂] are presented and the nickel-catalyzed alkyne borylation is reported.     

Formation of nickeladihydropyran by oxidative addition of cyclopropyl ketone. Key intermediate in nickel-catalyzed cycloaddition

Cyclopropyl phenyl ketone underwent oxidative addition to Ni(PCy3) generated from Ni(cod)2 and PCy3 to give a nickeladihydropyran, which is a key intermediate for the Ni(0)-catalyzed homo- or heterocycloaddition to give cyclopentane compounds having two carbonyl substituents at the 1,3-position.     

A convenient reagent for aldehyde to alkyne homologation

A convenient reagent for the one-carbon homologation of an aldehyde to the corresponding alkyne is reported. This reagent allows this conversion to conveniently be carried out on a large scale under ambient conditions.     

Two-carbon ring expansion of cyclobutanone skeletons by nickel-catalyzed intermolecular alkyne insertion

The reaction of cyclobutanone with an alkyne in the presence of a nickel(0) catalyst formally achieves intermolecular alkyne insertion between the carbonyl carbon and the α-carbon of a cyclobutanone, providing a six-membered carbocyclic skeleton.     

Highly chemoselective nickel-catalyzed three-component cross-trimerization between two distinct terminal alkynes and an internal alkyne

The highly chemo-, regio-, and stereoselective three-component cross-trimerization reaction between triisopropylsilylacetylene, diarylacetylene, and a terminal alkyne was achieved by Ni(cod)(2)/P(p-CF(3)C(6)H(4))(3) catalyst at room temperature via selective C-H oxidative addition of the terminal silylacetylene. The reaction is applicable for various diarylacetylenes and terminal alkynes to yield the corresponding 1,3-diene-5-yne compounds.     

Rapid access to pyrazolo[3,4-c]pyridines via alkyne annulation: limitations of steric control in nickel-catalyzed alkyne insertions

Polyfunctionalized pyrazolo[3,4-c]pyridines were readily prepared by the annulation of alkynes with tert-butyl 4-iodopyrazolocarboximines. The reaction was found to be catalyzed by both NiBr2(PPh3)2/Zn or PdCl2(PhCN)2 to yield complex heterocycles in good to moderate yields. Annulation using nickel catalysis was found to be regio-random, implying that steric control in nickel-catalyzed alkyne insertion has limitations based on the character of the Ni-C bond in the pre-insertion complex.     

The structure of a model intermediate for the nickel-catalyzed cyclodimerization of allene

The stoichiometric reaction between allene, zerovalent nickel and a chelating diphosphane leads to the formation of a bis-methylenenickelacyclopentane derivative whose structure has been established by X-ray crystallography.     

The first silver-catalyzed three-component coupling of aldehyde, alkyne, and amine

[reaction: see text] Silver iodide catalyzed the three-component coupling of aldehyde, alkyne, and amines to generate propargylic amines with high efficiency in water. The silver-catalyzed reaction is especially effective for reactions involving aliphatic aldehyde. No additional cocatalyst or activator is required.     

Total synthesis of aigialomycin D: surprising chemoselectivity dependence on alkyne structure in nickel-catalyzed cyclizations

The total synthesis of aigialomycin D was carried out using a nickel-catalyzed ynal macrocyclization as a key step. This key step allowed macrocycle assembly and formation of a disubstituted alkene and a secondary hydroxyl stereocenter in a single step, although the stereocenter was formed unselectively. An interesting side reaction involving five-membered-ring synthesis by an aldehyde/styrene cyclization was observed when macrocyclization of an alkynyl silane was attempted. A mechanistic basis for this surprising process is provided.     

Intramolecular PIFA-mediated alkyne amidation and carboxylation reaction

The hypervalent iodine reagent PIFA promotes the intramolecular electrophilic cyclization of easily accessible alkynylamides and alkynyl carboxylic acids, leading to the formation of pyrrolidinone and lactone skeletons, respectively, in a very efficient way. A synthetic study and a mechanistic proposal for these transformations are presented.     

Palladium- and nickel-catalyzed reaction of methylenecyclopropanes with trimethylsilyl cyanide1

Pd- and Ni-catalyzed reactions of methylenecyclopropanes with trimethylsilyl cyanide (1) give β-(cyanomethyl)allylsilanes and 1-(trimethylsilyl)methyl-1-cyclopropanecarbonitriles; the ratio of those depends on catalysts used and on the structure of substrates employed.     

Regioselective synthesis of indenols via nickel-catalyzed carbocyclization reaction

2-Halophenyl ketones 1a-e (1a, o-IC(6)H(4)COCH(3)) undergo carbocyclization with alkyl propiolates (2a, CH(3)(CH(2))(4)C[triple bond]CCO(2)CH(3); 2b, TMSC[triple bond]CCO(2)Et 2c, CH(3)C[triple bond]CCO(2)CH(3); 2d, CH(3)OCH(2)C[triple bond]CCO(2)CH(3); 2e, CH(3)(CH(2))(3)C[triple bond]CCO(2)CH(3); 2f, PhC[triple bond]CCO(2)CH(3); and 2g, (CH(3))(3)C[triple bond]CCO(2)CH(3)) in the presence of Ni(dppe)Br(2) and zinc powder in acetonitrile at 80 degrees C to afford the corresponding indenol derivatives 3a-m with remarkable regioselectivity in good to excellent yields. The nickel-catalyzed carbocyclization reaction was successfully extended to other simple disubstituted alkynes. Thus, the reaction of 2-halophenyl ketones 1a-e with disubstituted alkynes (2h, PhC[triple bond]CPh; 2i, CH(3)C(6)H(4)C[triple bond]CC(6)H(4)CH(3); 2j, CH(3)CH(2)C[triple bond]CCH(2)CH(3); 2k, PhC[triple bond]CCH(3); 2l, TMSC[triple bond]CCH(3); and 2m, PhC[triple bond]C(CH(2))(3)CH(3)) proceeded smoothly to afford the corresponding indenols 4a-t in good to excellent yields. For unsymmetrical alkynes 2k-m, the carbocyclization gave two regioisomers with regioselectivities ranging from 1:2 to 1:12 depending on the substituents on the alkyne and on the aromatic ring of halophenyl ketone. A possible mechanism for this nickel-catalyzed carbocyclization reaction is also proposed.     

Enantioselective total synthesis of (+)-testudinariol a using a new nickel-catalyzed allenyl aldehyde cyclization

An enantioselective total synthesis of (+)-testudinariol A was completed. A new nickel-catalyzed allenyl aldehyde cyclization was developed in the approach. In addition, an asymmetric anti aldol reaction and a two-directional oxocarbenium ion/vinyl silane condensation were employed as key steps.     

Addition of a cyclopropyl alkyne to tetramesityldigermene: evidence for a biradical intermediate

The reaction pathway for the addition of alkynes to tetramesityldigermene was examined using a cyclopropyl alkyne mechanistic probe. The results obtained indicate that the reaction is stepwise and involves biradical intermediates.     

Stereoselective coupling reaction of dimethylzinc and alkyne toward nickelacycles

Ni catalyst promotes the three-component coupling reaction of vinyloxacyclopropane, alkyne, and dimethylzinc to provide 2,5-heptadienyl alcohol in high yields. Vinylcyclopropane also participates in a similar three-component coupling reaction to afford dimethyl (α-heptadienyl)malonate with excellent E-stereoselectivity.     

Palladium and nickel-catalyzed reaction of grignard reagents with trichloroethylene. A simple synthesis of 1,1-dichloroalkenes.

The palladium (or nickel) catalyzed reaction of Grignard reagents with trichloroethylene affords 1,1-dichloroalkenes in good yield under mild conditions.     

Highly efficient synthesis of isoquinolines via nickel-catalyzed annulation of 2-iodobenzaldimines with alkynes: evidence for dual pathways of alkyne insertion

[reaction: see text] A wide range of substituted isoquinolines were synthesized via a highly efficient nickel-catalyzed annulation of the tert-butyl imines of 2-iodobenzaldehydes and various alkynes; examination of the regiochemistry of isoquinolines synthesized indicates that there are two different alkyne insertion pathways for the catalytic reactions.     

Iron-catalyzed three-component coupling of aldehyde, alkyne, and amine under neat conditions in air

Various iron-salts (and complexes) and especially iron(III) chloride catalyzed the three-component coupling of aldehyde, alkyne, and amine to generate propargylic amines with high efficiency under neat conditions in air. The iron-catalyzed reaction is particularly effective for reactions involving aliphatic aldehydes. The reaction is not sensitive to and occurs smoothly in water and in air. No additional co-catalyst or activator is required.