Nickel(0)-Catalyzed Formation of Oxaaluminacyclopentenes via an Oxanickelacyclopentene Key Intermediate: Me2AlOTf-Assisted Oxidative Cyclization of an Aldehyde and an Alkyne with Nickel(0)

Redox-Neutral Coupling of Allyl Alcohols with Trifluoromethyl Ketones via Synergistic Ni-Ti Bimetallic Catalysis

A redox-neutral coupling of allyl alcohols with trifluoromethyl ketones has been developed via Ni-Ti bimetallic catalysis. This innovative method allows for the efficient synthesis of various β-tertiary trifluoromethyl alcohol-substituted ketones with yields of up to 98%. The reaction is scalable and compatible with a wide range of substrates, including complex bioactive molecules. Mechanistic studies suggest that the rate-determining step involving β-H elimination and the presence of the Ti-based Lewis acid, as well as a hydroxyl group on the substrates, is crucial for driving the reactivity of this transformation.

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.

A mechanistic study on the regioselective Ni-catalyzed methylation–alkenylation of alkyne with AlMe3 and allylic alcohol

The recently reported Ni-catalyzed methylation–allylation of alkynes with allylic alcohols and AlMe3 reagents delivers valuable tetrasubstituted alkene units in a highly regioselective fashion.

Univariate classification of phosphine ligation state and reactivity in cross-coupling catalysis

[Figure: see text].

Regioselective cycloaddition of potassium alkynyltrifluoroborates with 3-azetidinones and 3-oxetanone by nickel-catalysed C–C bond activation

The nickel-catalysed (4+2) cycloaddtion of potassium alkynyltrifluoroborates and 3-azetidinones and 3-oxetanone gives only one regioisomer for all alkyne substituents.

The roles of Lewis acidic additives in organotransition metal catalysis

We present recent advances in prominent organotransition metal-catalysed reactions in which Lewis acid cocatalysts are employed to increase catalyst activity or selectivity. The roles of Lewis acids are discussed.

Mechanistic Study of Nickel-Catalyzed Reductive Coupling of Ynoates and Aldehydes

In this work, (1,5-hexadiene)Ni(SIPr) (SIPr = 1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene) is used in place of Ni(COD)₂/SIPr·HBF₄/KO^tBu (COD = 1,5-cyclooctadiene) as a more robust catalyst for regioselective reductive coupling of ynoates and aldehydes with triethylsilane. The catalytic reaction of ethyl 3-(trimethylsilyl)propiolate and methyl 4-formylbenzoate shows first-order dependence on aldehyde and catalyst concentrations, inverse first-order dependence on [ynoate], and no dependence on [silane]. The kinetics data, coupled with deuterium-labeling experiments, support a mechanism involving dissociation of the ynoate from a catalytically dormant nickelacyclopentadiene intermediate prior to turnover-limiting formation of a catalytically active nickeladihydrofuran.

Recent Methodologies That Exploit C-C Single-Bond Cleavage of Strained Ring Systems by Transition Metal Complexes

In this review, synthetic and mechanistic aspects of key methodologies that exploit C-C single-bond cleavage of strained ring systems are highlighted. The focus is on transition-metal-catalyzed processes that are triggered by C-C bond activation and β-carbon elimination, with the review concentrating on developments from mid-2009 to mid-2016.

Catalytic Transformation of Aldehydes with Nickel Complexes through η(2) Coordination and Oxidative Cyclization

Chemists no longer doubt the importance of a methodology that could activate and utilize aldehydes in organic syntheses since many products prepared from them support our daily life. Tremendous effort has been devoted to the development of these methods using main-group elements and transition metals. Thus, many organic chemists have used an activator-(aldehyde oxygen) interaction, namely, η(1) coordination, whereby a Lewis or Brønsted acid activates an aldehyde. In the field of coordination chemistry, η(2) coordination of aldehydes to transition metals by coordination of a carbon-oxygen double bond has been well-studied; this activation mode, however, is rarely found in transition-metal catalysis. In view of the distinctive reactivity of an η(2)-aldehyde complex, unprecedented reactions via this intermediate are a distinct possibility. In this Account, we summarize our recent results dealing with nickel(0)-catalyzed transformations of aldehydes via η(2)-aldehyde nickel and oxanickelacycle intermediates. The combination of electron-rich nickel(0) and strong electron-donating N-heterocyclic carbene (NHC) ligands adequately form η(2)-aldehyde complexes in which the aldehyde is highly activated by back-bonding. With Ni(0)/NHC catalysts, processes involving intramolecular hydroacylation of alkenes and homo/cross-dimerization of aldehydes (the Tishchenko reaction) have been developed, and both proceed via the simultaneous η(2) coordination of aldehydes and other π components (alkenes or aldehydes). The results of the mechanistic studies are consistent with a reaction pathway that proceeds via an oxanickelacycle intermediate generated by the oxidative cyclization with a nickel(0) complex. In addition, we have used the η(2)-aldehyde nickel complex as an effective activator for an organosilane in order to generate a silicate reactant. These reactions show 100% atom efficiency, generate no wastes, and are conducted under mild conditions.

Aza-nickelacycle key intermediate in nickel(0)-catalyzed transformation reactions

Oxidative cyclization of alkynes and imines with nickel(0) is a key step in multicomponent coupling and cycloaddition reactions to give nitrogen-containing organic compounds.

Mechanistic origin of chemoselectivity in thiolate-catalyzed Tishchenko reactions

The thiolate-catalyzed Tishchenko reaction has shown high chemoselectivity for the formation of double aromatic-substituted esters. In the present study, the detailed reaction mechanism and, in particular, the origin of the observed high chemoselectivity, have been studied with DFT calculations. The catalytic cycle mainly consisted of three steps: 1,2-addition, hydride transfer, and acyl transfer steps. The calculation results reproduce the experimental observations that 4-chlorobenzaldehyde acts as the hydrogen donor (carbonyl part in the ester product), while 2-methoxybenzaldehyde acts as the hydrogen acceptor (alcohol part in the product). The two main factors are responsible for such chemoselectivity: 1) in the rate-determining hydride transfer step, the para-chloride substituent facilitates the hydride-donating process by weakening the steric hindrance, and 2) the ortho-methoxy substituent facilitates the hydride-accepting process by stabilizing the magnesium center (by compensating for the electron deficiency).

Combined experimental and theoretical study on the reductive cleavage of inert C-O bonds with silanes: ruling out a classical Ni(0)/Ni(II) catalytic couple and evidence for Ni(I) intermediates

A mechanistic and computational study on the reductive cleavage of C-OMe bonds catalyzed by Ni(COD)(2)/PCy(3) with silanes as reducing agents is reported herein. Specifically, we demonstrate that the mechanism for this transformation does not proceed via oxidative addition of the Ni(0) precatalyst into the C-OMe bond. In the absence of an external reducing agent, the in-situ-generated oxidative addition complexes rapidly undergo β-hydride elimination at room temperature, ultimately leading to either Ni(0)-carbonyl- or Ni(0)-aldehyde-bound complexes. Characterization of these complexes by X-ray crystallography unambiguously suggested a different mechanistic scenario when silanes are present in the reaction media. Isotopic-labeling experiments, kinetic isotope effects, and computational studies clearly reinforced this perception. Additionally, we also found that water has a deleterious effect by deactivating the Ni catalyst via formation of a new Ni-bridged hydroxo species that was characterized by X-ray crystallography. The order in each component was determined by plotting the initial rates of the C-OMe bond cleavage at varying concentrations. These data together with the in-situ-monitoring experiments by (1)H NMR, EPR, IR spectroscopy, and theoretical calculations provided a mechanistic picture that involves Ni(I) as the key reaction intermediates, which are generated via comproportionation of initially formed Ni(II) species. This study strongly supports that a classical Ni(0)/Ni(II) for C-OMe bond cleavage is not operating, thus opening up new perspectives to be implemented in other related C-O bond-cleavage reactions.

Mechanistic origin of cross-coupling selectivity in Ni-catalysed Tishchenko reactions

Mechanistic studies have been performed for the recently developed, Ni-catalysed selective cross-coupling reaction between aryl and alkyl aldehydes. A mono-carbonyl activation (MCA) mechanism (in which one of the carbonyl groups is activated by oxidative addition) was found to be the most favourable pathway, and the rate-determining step is oxidative addition. Analysing the origin of the observed cross-coupling selectivity, we found the most favourable carbonyl activation step requires both coordination of the aryl aldehyde and oxidative addition of the alkyl aldehyde. Therefore, the stronger π-accepting ability of the aryl aldehyde (relative to alkyl aldehyde) and the ease of oxidative addition of the alkyl aldehyde (relative to aryl aldehyde) are responsible for the cross-coupling selectivity.