Cobalt-Catalyzed Csp3 −Csp3 Homocoupling

Flexible porous organic polymers constructed using C(sp3)-C(sp3) coupling reactions and their high methane-storage capacity

Carbon-carbon coupling is a basic design principle for the synthesis of porous organic polymers, which are widely used in gas adsorption/separation, photocatalysis, energy storage, etc. However, the C(sp3)-C(sp3) coupling reaction to construct porous organic polymers remains an important yet elusive objective due to its low reactivity and unknown side reactions. Herein, we report that nickel bis(1,5-cyclooctadiene) (Ni(COD)2), which was a famous catalyst for C(sp2)-C(sp2) coupling reactions, enables highly efficient C(sp3)-C(sp3) homo-coupling reactions to construct porous linear crystalline polymers and flexible three-dimensional porous aromatic frameworks (PAFs) under mild reaction conditions. The resulting linear polymers generated with dibromomethyl arenes have good crystallinity and high melting points (T m = 286 °C) due to controllability of reaction sites. Furthermore, the PAFs (PAF-64, PAF-65 and PAF-66) stemmed from tri-/tetra-bromomethyl arenes show high surface area (S BET = 390 m2 g-1) and high methane-storage capacity (up to 313 cm3 cm-3) because of their flexible frameworks. This work sheds new light on the construction of novel porous polymers through C(sp3)-C(sp3) coupling reactions and the development of methane-storage materials.

Cobalt-Catalyzed Reductive Cross-Coupling To Construct Csp3-Csp3 Bonds via Csp3-S and Csp3-X Bonds Activation

A cross-electrophilic coupling of benzyl sulfonium salts with alkyl halides forming Csp3-Csp3 bonds is described by using a Co-based catalytic system. The activation of the stable Csp3-S bond of benzyl sulfonium salts under mild reaction conditions leads to various functionalized alkyl derivatives. Preliminary mechanistic studies suggest the involvement of alkyl radicals formed from both alkyl halides and benzyl sulfoniums through a single electron transfer.

Homogeneous Organic Electron Donors in Nickel-Catalyzed Reductive Transformations

Many contemporary organic transformations, such as Ni-catalyzed cross-electrophile coupling (XEC), require a reductant. Typically, heterogeneous reductants, such as Zn⁰ or Mn⁰, are used as the electron source in these reactions. Although heterogeneous reductants are highly practical for preparative-scale batch reactions, they can lead to complications in performing reactions on process scale and are not easily compatible with modern applications, such as flow chemistry. In principle, homogeneous organic reductants can address some of the challenges associated with heterogeneous reductants and also provide greater control of the reductant strength, which can lead to new reactivity. Nevertheless, homogeneous organic reductants have rarely been used in XEC. In this Perspective, we summarize recent progress in the use of homogeneous organic electron donors in Ni-catalyzed XEC and related reactions, discuss potential synthetic and mechanistic benefits, describe the limitations that inhibit their implementation, and outline challenges that need to be solved in order for homogeneous organic reductants to be widely utilized in synthetic chemistry. Although our focus is on XEC, our discussion of the strengths and weaknesses of different methods for introducing electrons is general to other reductive transformations.

A Homocoupling Approach to the Key Dione of CyMe4-BTPhen – Vital Ligands for Nuclear Clean-Up by the SANEX Process

CyMe4-BTPhen and CyMe4-BTBP are the principal ligand systems used in Europe for the separation of actinides from lanthanides as a part of the SANEX process for nuclear recycling and reprocessing. We present a new approach to the synthesis of the CyMe4-fragment beginning from readily available hydroxypivalic acid. It features a cobalt-catalysed homocoupling of a neopentyl bromide to provide the key bis-ester precursor, thereby avoiding the requirement for technically challenging low temperature LDA-mediated aldol chemistries.

Pd-catalyzed allylative dearomatisation using Grignard reagents

Pd-catalyzed allylative dearomatisation of naphthyl halides is shown to be feasible by employing Grignard reagents. The high reactivity of the nucleophile allows for fast reactions and low catalyst loading, while a plethora of successfully substituted compounds illustrate the broad scope. Five membered heteroaromatic compounds are also demonstrated to be reactive under similar conditions.

Dearomatisation of benzylic halides have been achieved using Grignard reagents resulting in good isolated yields in short reaction times.

Synthesis of Dibenzyls by Nickel-Catalyzed Homocoupling of Benzyl Alcohols

Dibenzyls are essential building blocks that are widely used in organic synthesis, and they are typically prepared by the homocoupling of halides, organometallics, and ethers. Herein, we report an approach to this class of compounds using alcohols, which are more stable and readily available. The reaction proceeds via nickel-catalyzed and dimethyl oxalate assisted dynamic kinetic homocoupling of benzyl alcohols. Both primary and secondary alcohols are tolerated.

Birch-Type Reduction of Arenes in 2-Propanol Catalyzed by Zero-Valent Iron and Platinum on Carbon

Catalytic arene reduction was effectively realized by heating in 2-propanol/water in the presence of Pt on carbon (Pt/C) and metallic Fe. 2-Propanol acted as a hydrogen source, obviating the need for flammable (and hence, dangerous and hard-to-handle) hydrogen gas, while metallic Fe acted as an essential co-catalyst to promote reduction. The chemical states of Pt and Fe in the reaction mixture were determined by X-ray absorption near-edge structure analysis, and the obtained results were used to suggest a plausible reaction mechanism, implying that catalytic reduction involved Pt- and Fe-mediated single-electron transfer and the dehydrogenation of 2-propanol.

Ni/Co-Catalyzed Homo-Coupling of Alkyl Tosylates

A direct reductive homo-coupling of alkyl tosylates has been developed by employing a combination of nickel and nucleophilic cobalt catalysts. A single-electron-transfer-type oxidative addition is a pivotal process in the well-established nickel-catalyzed coupling of alkyl halides. However, the method cannot be applied to the homo-coupling of ubiquitous alkyl tosylates due to the high-lying σ*(C-O) orbital of the tosylates. This paper describes a Ni/Co-catalyzed protocol for the activation of alkyl tosylates on the construction of alkyl dimers under mild conditions.

Cobalt-Catalyzed C(sp2)-C(sp3) Cross-Coupling Reactions of Diarylmanganese Reagents with Secondary Alkyl Iodides

A cobalt-catalyzed cross-coupling of diarylmanganese reagents with secondary alkyl iodides using the THF-soluble salt CoCl₂·2LiCl, which leads to the cross-coupling products in up to 92% yield, is reported. High diastereoselectivities can be reached in these cross-couplings (dr up to 99:1). Remarkably, rearrangement of secondary alkyl iodides to unbranched products was not observed in these C-C forming reactions.

Cobalt-Catalyzed Reductive Cross-Coupling Between Styryl and Benzyl Halides

A simple and efficient protocol for the direct reductive cross-coupling between alkenyl and benzyl halides using a Co/Mn system has been developed. This reaction proceeds smoothly in the presence of [CoBr₂ (PPh₃ )₂ ] as the catalyst, with NaI as an additive in acetonitrile with a broad scope of functionalized alkenyl and benzyl halides. Different functional groups are tolerated on both coupling partners, thus, significantly extending the general scope of transition-metal-catalyzed benzylation of alkenyl halides. Moderate to excellent yields were also obtained. From a mechanistic point of view, a radical chain mechanism was proposed. This reaction is stereospecific and some studies suggest the retention of the double-bond configuration.

A well-defined low-valent cobalt catalyst Co(PMe3)4 with dimethylzinc: a simple catalytic approach for the reductive dimerization of benzyl halides

We report a simple catalytic version of a cobalt-catalysed reductive homocoupling of benzyl halides by combining Co(PMe₃)₄ and Me₂Zn.