Nickel-Catalyzed Alkyne Research Articles

Nickel boryl complexes and nickel-catalyzed alkyne borylation.

The first nickel bis-boryl complexes cis-[Ni( i Pr2ImMe)2(Bcat)2], cis-[Ni( i Pr2ImMe)2(Bpin)2] and cis-[Ni( i Pr2ImMe)2(Beg)2] are reported, which were prepared via the reaction of a source of [Ni( i Pr2ImMe)2] with the diboron(4) compounds B2cat2, B2pin2 and B2eg2 ( i Pr2ImMe = 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( i Pr2ImMe)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( i Pr2ImMe)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[double bond, length as m-dash]C(R)(Bcat))], exemplified by the isolation and structural characterization of [Ni( i Pr2ImMe)2(η2-cis-(Bcat)(Me)C[double bond, length as m-dash]C(Me)(Bcat))] and [Ni( i Pr2ImMe)2(η2-cis-(Bcat)(H7C3)C[double bond, length as m-dash]C(C3H7)(Bcat))].

Nickel-Catalyzed Alkyne Difunctionalization Using Two Distinct sp3-Hybridized Electrophiles

Key words nickel catalysis - multicomponent reaction - cross-electrophiles - fluoromethylation

Enantioselective Nickel-Catalyzed Alkyne-Azide Cycloaddition by Dynamic Kinetic Resolution.

The triazole heterocycle has been widely adopted as an isostere for the amide bond. Many native amides are α-chiral, being derived from amino acids. This makes α-N-chiral triazoles attractive building blocks. This report describes the first enantioselective triazole synthesis that proceeds via nickel-catalyzed alkyne-azide cycloaddition (NiAAC). This dynamic kinetic resolution is enabled by a spontaneous [3,3]-sigmatropic rearrangement of the allylic azide. The 1,4,5-trisubstituted triazole products, derived from internal alkynes, are complementary to those commonly obtained by the related CuAAC reaction. Initial mechanistic experiments indicate that the NiAAC reaction proceeds through a monometallic Ni complex, which is distinct from the CuAAC manifold.

Nickel-Catalyzed Alkyne Cyclotrimerization Assisted by a Hemilabile Acceptor Ligand: A Computational Study

π-coordinating units incorporated in the supporting ligand of an organometallic complex may open up specific reactive pathways. The diphosphine ketone supported nickel complex [(p-tolL1)Ni(BPI)] (p...

Ni-Catalyzed stereoselective difunctionalization of alkynes

We summarize the progress of the nickel-catalyzed alkyne difunctionalization reaction for the synthesis of tri- and tetrasubstituted olefins, with an emphasis on the strategy and control of stereochemistry.

Enhanced Catalytic Activity of Nickel Complexes of an Adaptive Diphosphine-Benzophenone Ligand in Alkyne Cyclotrimerization.

Adaptive ligands, which can adapt their coordination mode to the electronic structure of various catalytic intermediates, offer the potential to develop improved homogeneous catalysts in terms of activity and selectivity. 2,2′-Diphosphinobenzophenones have previously been shown to act as adaptive ligands, the central ketone moiety preferentially coordinating reduced metal centers. Herein, the utility of this scaffold in nickel-catalyzed alkyne cyclotrimerization is investigated. The complex [(p-tolL1)Ni(BPI)] (p-tolL1 = 2,2′-bis(di(para-tolyl)phosphino)-benzophenone; BPI = benzophenone imine) is an active catalyst in the [2 + 2 + 2] cyclotrimerization of terminal alkynes, selectively affording 1,2,4-substituted benzenes from terminal alkynes. In particular, this catalyst outperforms closely related bi- and tridentate phosphine-based Ni catalysts. This suggests a reaction pathway involving a hemilabile interaction of the C=O unit with the nickel center. This is further borne out by a comparative study of the observed resting states and DFT calculations.

Nickel-catalyzed alkyne annulation by anilines: versatile indole synthesis by C–H/N–H functionalization

Versatile nickel catalysts enabled the step-economical synthesis of decorated indoles through alkyne annulations with anilines bearing removable directing groups. The C-H/N-H activation strategy efficiently occurred in the absence of any metal oxidants and with excellent selectivities.

Nickel-Catalyzed Synthesis of Strained Phenacenes

This study reports a series of nickel-catalyzed alkyne cycloadditions on different phenylene compounds (angular [3]phenylene 1a and [4]phenylene 1b, C 3-symmetric [4]phenylene 1c). In all cases, products of single addition (2a-c) are isolated in moderate yield. Surprisingly, the second, respectively third addition reactions afford systematically the most sterically crowded and strained compounds (phenacenes). All isolated products, six of whom were characterized by X-ray crystallography, exhibit pronounced distortion owing to the 4,5-diphenylphenanthrene substructures. Interestingly, performing the cycloaddition on compounds 2a or 2c under the same reaction conditions yields less strained products (helicenes, with phenyl groups pointing to opposite sides of the aromatic core) instead of hindered 3a or 3c, respectively.

Activated Phenacenes from Phenylenes by Nickel‐Catalyzed Alkyne Cycloadditions

Activated Phenacenes from Phenylenes by Nickel‐Catalyzed Alkyne Cycloadditions

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.

Alkene-Directed, Nickel-Catalyzed Alkyne Coupling Reactions

In alkene-directed, nickel-catalyzed coupling reactions of 1,3-enynes with aldehydes and epoxides, the conjugated alkene dramatically enhances reactivity and uniformly directs regioselectivity, independent of the nature of the other alkyne substituent (aryl, alkyl (1 degrees , 2 degrees , 3 degrees )) or the degree of alkene substitution (mono-, di-, tri-, and tetrasubstituted). These observations are best explained by a temporary interaction between the alkene and the transition metal center during the regioselectivity-determining step. The highly substituted 1,3-diene products are useful in organic synthesis and, in conjunction with a Rh-catalyzed, site-selective hydrogenation, afford allylic and homoallylic alcohols that previously could not be prepared in high regioselectivity (or at all) with related Ni-catalyzed alkyne coupling reactions.