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

Coordination of a Phosphine-Tethered Aminoborane to Group 10 Metals

While π-complexes of C=C bonds are ubiquitous in organometallic chemistry, analogous complexes of the isoelectronic but strongly polarized B=N double bond of aminoboranes are extremely scarce. To address this gap, a diphosphine-aminoborane ligand (PhDPBAiPr) is introduced and its coordination with group 10 metals is investigated. The B=N bond does not coordinate to the metal in Pt(0) and Pd(II) complexes. In contrast, side-on coordination of the B=N bond is observed in the Ni(0) complex (PhDPBAiPr)Ni(NCPh), and the X-ray crystal structure reveals B-N bond elongation compared to the free ligand. The choice of co-ligand strongly influences the presence or absence of side-on coordination at Ni(0) as evidenced by NMR spectroscopy. While the B=N π-complex is geometrically similar to C=C analogues, a bonding analysis reveals that the interaction of the B=N motif with the electron-rich Ni(0) center is best described as 3c4e hyperbond, in which Ni and N are competing for the empty orbital on B.

Cooperative H2 activation at a nickel(0)-olefin centre

Catalytic olefin hydrogenation is ubiquitous in organic synthesis. In most proposed homogeneous catalytic cycles, reactive M-H bonds are generated either by oxidative addition of H2 to a metal centre or by deprotonation of a non-classical metal dihydrogen (M-H2) intermediate. Here we provide evidence for an alternative H2-activation mechanism that instead involves direct ligand-to-ligand hydrogen transfer (LLHT) from a metal-bound H2 molecule to a metal-coordinated olefin. An unusual pincer ligand that features two phosphine ligands and a central olefin supports the formation of a non-classical Ni-H2 complex and the Ni(alkyl)(hydrido) product of LLHT, in rapid equilibrium with dissolved H2. The usefulness of this cooperative H2-activation mechanism for catalysis is demonstrated in the semihydrogenation of diphenylacetylene. Experimental and computational mechanistic investigations support the central role of LLHT for H2 activation and catalytic semihydrogenation. The product distribution obtained is largely determined by the competition between (E)-(Z) isomerization and catalyst degradation by self-hydrogenation.

Reactivity of Nickel Complexes Bearing P(C=X)P Ligands (X = O, N) Toward Diazoalkanes: Evidence for Phosphorus Ylide Intermediates

Nickel carbenes are attracting attention for the development of more sustainable catalysts, among others, for cyclopropanation. Intramolecular trapping of a nickel carbene intermediate with an olefin incorporated in a P(C=C)P Ni pincer complex had previously allowed the isolation of a nickelacyclobutane intermediate and a detailed characterization of its reactivity. Herein, we report the reactivity of related nickel pincer complexes bearing a ketone P(C=O)P or an imine P(C=N)P with diazoalkanes as the carbene precursor. The observed reactivity suggests, in both cases, the reaction of the transient nickel carbene with one of the phosphine arms to form phosphorus ylides that subsequently react with the unsaturated backbone. Density functional theory (DFT) calculations are used to shed light on the mechanisms of these reactions.

Ratcheting synthesis

Synthetic chemistry has traditionally relied on reactions between reactants of high chemical potential and transformations that proceed energetically downhill to either a global or local minimum (thermodynamic or kinetic control). Catalysts can be used to manipulate kinetic control, lowering activation energies to influence reaction outcomes. However, such chemistry is still constrained by the shape of one-dimensional reaction coordinates. Coupling synthesis to an orthogonal energy input can allow ratcheting of chemical reaction outcomes, reminiscent of the ways that molecular machines ratchet random thermal motion to bias conformational dynamics. This fundamentally distinct approach to synthesis allows multi-dimensional potential energy surfaces to be navigated, enabling reaction outcomes that cannot be achieved under conventional kinetic or thermodynamic control. In this Review, we discuss how ratcheted synthesis is ubiquitous throughout biology and consider how chemists might harness ratchet mechanisms to accelerate catalysis, drive chemical reactions uphill and programme complex reaction sequences.

Reactions of Nickel(0)-Olefin Pincer Complexes with Terminal Alkynes: Cooperative C-H Bond Activation and Alkyne Coupling

Metal-ligand cooperation can facilitate the activation of chemical bonds, opening reaction pathways of interest for catalyst development. In this context, olefins occupying the central position of a diphosphine pincer ligand (PC=CP) are emerging as reversible H atom acceptors, e.g., for H2 activation. Here, we report on the reactivity of nickel complexes of PC=CP ligands with a terminal alkyne, for which two competing pathways are observed. First, cooperative and reversible C-H bond activation generates a Ni(II) alkyl/alkynyl complex as the kinetic product. Second, in the absence of a bulky substituent on the olefin, two alkyne molecules are incorporated in the ligand structure to form a conjugated triene bound to Ni(0). The mechanisms of these processes are studied by density functional theory calculations supported by experimental observations.

Iron Dihydride Complex Stabilized by an All-Phosphorus-Based Pincer Ligand and Carbon Monoxide

PNP-pincer-stabilized iron carbonyl dihydride complexes are key intermediates in catalytic hydrogenation and dehydrogenation reactions; however, decomposition through these intermediates has been observed. This inspires the development of a PPP-pincer system that may show improved catalyst stability. In this work, bis[2-(diisopropylphosphino)phenyl]phosphine (or ^iPrPP^HP) is used to react with FeCl₂ under a carbon monoxide (CO) atmosphere to yield trans-(^iPrPP^HP)Fe(CO)Cl₂. A subsequent reaction with NaBH₄ produces syn/anti-(^iPrPP^HP)FeH(CO)Cl or cis,anti-(^iPrPP^HP)Fe(CO)H₂, depending on the amount of NaBH₄ employed. The cis-dihydride complex shows catalytic activity for the conversion of PhCHO to PhCH₂OH (under H₂) or PhCO₂CH₂Ph (under Ar). It also catalyzes the dehydrogenation of PhCH₂OH to PhCHO and PhCO₂CH₂Ph, albeit with limited turnover numbers. A more efficient catalytic process is the dehydrogenation of formic acid to carbon dioxide (CO₂), which can operate under additive-free conditions. Mechanistic investigation suggests that the cis-dihydride complex undergoes protonation with formic acid to release H₂ while forming anti-(^iPrPP^HP)FeH(CO)(OCHO)·HCO₂H, in which the CO ligand has shifted and the formate is hydrogen-bonded to formic acid. The hydrido formate complex loses CO₂ under ambient conditions, completing the catalytic cycle by reforming the cis-dihydride complex.

Synthesis of oxaboranes via nickel-catalyzed dearylative cyclocondensation

We report Ni-catalyzed dearylative cyclocondensation of aldehydes, alkynes, and triphenylborane. The reaction is initiated by oxidative cyclization of the aldehyde and alkyne coupling partners to generate an oxanickelacyclopentene which reacts with triphenylborane to form oxaboranes. This formal dearylative cyclocondensation reaction generates oxaboranes in moderate-to-high yields (47–99%) with high regioselectivities under mild reaction conditions. This approach represents a direct and modular synthesis of oxaboranes which are difficult to access using current methods. These oxaboranes are readily transformed into valuable building blocks for organic synthesis and an additional class of boron heterocycles. Selective homocoupling forms oxaboroles, oxidation generates aldol products, and reduction and arylation form substituted allylic alcohols.

Oxaboranes are prepared via a nickel-catalyzed dearylative cyclocondensation reaction in up to 99% yield and excellent regioselectivity. These oxaborane products can be further transformed into a variety of synthetically useful building blocks.

Assembly of unsymmetrical 1,3,5-triarylbenzenes via tandem reaction of β-arylethenesulfonyl fluorides and α-cyano-β-methylenones

A transition-metal-free tandem reaction of β-arylethenesulfonyl fluorides and α-cyano-β-methylenones has been revealed. In the presence of cesium carbonate, 2-arylethenesulfonyl fluorides react with α-cyano-β-methylenones through a tandem Diels-Alder cycloaddition/sulfur (VI) fluoride...

Regioselectively switchable alkyne cyclotrimerization catalyzed by the system of Ni(II)/bidentate P-ligand/Zn with ZnI2 as additive

A simple catalyst system of Ni(PPh3)2Cl2/dppb or dppm/Zn can effect an efficient and regioselectively controlled alkyne cyclotrimerization to form 1,2,4- (with no additive) or 1,3,5-regioisomers (with the use of ZnI2...

Polymer-Supported-Cobalt-Catalyzed Regioselective Cyclotrimerization of Aryl Alkynes

A convoluted poly(4-vinylpyridine) cobalt(II) (P4VP-CoCl₂) system was developed as a stable and reusable heterogeneous catalyst. The local structure near the Co atom was determined on the basis of experimental data and theoretical calculations. This immobilized cobalt catalyst showed high selectivity and catalytic activity in the [2 + 2 + 2] cyclotrimerization of terminal aryl alkynes. With 0.033 mol % P4VP-CoCl₂, the regioselective formation of 1,3,5-triarylbenzene was realized without 1,2,4-triarylbenzene formation. Further, a multigram-scale (11 g) reaction proceeded efficiently. In addition, the polymer-supported catalyst was successfully recovered and used three times. X-ray photoelectron spectroscopy analysis of the recovered catalyst suggested that cobalt was in the +2 oxidation state. The 1,3,5-triarylbenzene derivatives were applied to the synthesis of a molecular beam electron resist and a polycyclic aromatic hydrocarbon.

Synthesis of 1,4-enynes via nickel-catalyzed cross-coupling of allylic alcohols with alkynylzinc reagents

Synthesis of 1,4-enynes was performed via nickel-catalyzed cross-coupling of allylic alcohols with alkynylzinc reagents. The reaction features high regio- and E/Z-selectivity when aryl-substituted allylic alcohols were employed. The method also exhibits a wide scope of substrates and good compatibility of functional groups.

Regioselective Synthesis of Tetrasubstituted Benzenes via Co-Catalyzed Cycloaddition of Alkynyl Ketones and 2-Acetylpyridines

A Co(II)-catalyzed cycloaddition reaction of alkynyl ketones and 2-acetylpyridines using 2,2'-bipyridine as the ligand has been developed. These reactions have been realized through Co-catalyzed reductive coupling of two molecules of 2-acetylpyridine followed by regioselective insertion of the alkynone. It is the first example of regioselective cyclotrimerization of one molecule of alkyne and two molecules of monoketone to polysubstituted benzene derivatives in good to excellent yields. A mechanism involving the formation of a cobaltacyclopentane via homocoupling of 2-acetylpyridines is proposed, and it is supported by the DFT calculations.

Allylic alcohol synthesis by Ni-catalyzed direct and selective coupling of alkynes and methanol

Methanol is an abundant and renewable chemical raw material, but its use as a C1 source in C-C bond coupling reactions still constitutes a big challenge, and the known methods are limited to the use of expensive and noble metal catalysts such as Ru, Rh and Ir. We herein report nickel-catalyzed direct coupling of alkynes and methanol, providing direct access to valuable allylic alcohols in good yields and excellent chemo- and regioselectivity. The approach features a broad substrate scope and high atom-, step- and redox-economy. Moreover, this method was successfully extended to the synthesis of [5,6]-bicyclic hemiacetals through a cascade cyclization reaction of alkynones and methanol.

Hydrogen-Bonding Controlled Nickel-Catalyzed Regioselective Cyclotrimerization of Terminal Alkynes

Herein we report a hydrogen-bonding controlled nickel-catalyzed regioselective cyclotrimerization of terminal alkynes in moderate to excellent yields with high regioselectivities toward 1,3,5-trisubstituted benzenes. This method features a cheap catalyst, mild reaction conditions, and excellent functional group compatibility. The Ni-B(OH)₂ complex in situ generated from NiCl₂·DME and tetrahydroxydiboron might act as an active catalyst. After three consecutive cis-additions of terminal alkynes, internal migratory insertion cyclization, and β-boron elimination induced aromatization, 1,3,5-trisubstituted benzenes were selectively established.

Oxidative Addition of Aryl Halides to a Triphosphine Ni(0) Center to Form Pentacoordinate Ni(II) Aryl Species

Oxidative addition of aryl halides to Ni(0) is a ubiquitous elementary step in cross-coupling and related reactions, usually producing a square-planar Ni(II)-aryl intermediate. Here we show that a triphosphine ligand supports oxidative addition at a tris-ligated Ni(0) center to cleanly form stable five-coordinate Ni(II)-aryl compounds. Kinetic and computational studies support a concerted, two-electron mechanism rather than radical halogen abstraction. These results support the idea that oxidative addition to triphosphine Ni(0) species may be more generally involved in Ni/phosphine catalytic systems.

Stereospecific Synthesis of E-Alkenes through Anti-Markovnikov Hydroalkylation of Terminal Alkynes

We have developed a method for stereospecific synthesis of E-alkenes from terminal alkynes and alkyl iodides. The hydroalkylation reaction is enabled by a cooperative action of copper and nickel catalysts and proceeds with excellent anti-Markovnikov selectivity. We demonstrate the broad scope of the reaction, which can be accomplished in the presence of esters, nitriles, aryl bromides, ethers, alkyl chlorides, anilines, and a wide range of nitrogen-containing heteroaromatic compounds. Mechanistic studies provide evidence that the copper catalyst activates the alkyne by hydrocupration, which controls both the regio- and diastereoselectivity of the overall reaction. The nickel catalyst activates the alkyl iodide and promotes cross coupling with the alkenyl copper intermediate.