Merging allylic carbon–hydrogen and selective carbon–carbon bond activation

Migratory functionalization of unactivated alkyl bromides for construction of all-carbon quaternary centers via transposed tert-C-radicals

Despite remarkable recent advances in transition-metal-catalyzed C(sp³)−C cross-coupling reactions, there remain challenging bond formations. One class of such reactions include the formation of tertiary-C(sp³)−C bonds, presumably due to unfavorable steric interactions and competing isomerizations of tertiary alkyl metal intermediates. Reported herein is a Ni-catalyzed migratory 3,3-difluoroallylation of unactivated alkyl bromides at remote tertiary centers. This approach enables the facile construction of otherwise difficult to prepare all-carbon quaternary centers. Key to the success of this transformation is an unusual remote functionalization via chain walking to the most sterically hindered tertiary C(sp³) center of the substrate. Preliminary mechanistic and radical trapping studies with primary alkyl bromides suggest a unique mode of tertiary C-radical generation through chain-walking followed by Ni–C bond homolysis. This strategy is complementary to the existing coupling protocols with tert-alkyl organometallic or -alkyl halide reagents, and it enables the expedient formation of quaternary centers from easily available starting materials.

Formation of tertiary C(sp³)-C bonds is a formidable challenge due to steric interactions and low barriers for isomerization of intermediates. Here, the authors show a Ni-catalyzed migratory 3,3-difluoroallylation of unactivated alkyl bromides at remote tertiary carbon centers.

Enantioselective Nickel-Catalyzed Migratory Hydrocyanation of Nonconjugated Dienes

Metal-catalyzed chain-walking reactions have recently emerged as a powerful strategy to functionalize remote positions in organic molecules. However, a chain-walking protocol for nonconjugated dienes remains scarcely reported, and developments are currently ongoing. In this Communication, a nickel-catalyzed asymmetric hydrocyanation of nonconjugated dienes involving a chain-walking process is demonstrated. The reaction exhibits excellent regio- and chemoselectivity, and a wide range of substrates were tolerated, delivering the products in high yields and enantioselectivities. Deuterium-labeling experiments support the chain-walking process, which involves an iterative β-H elimination and reinsertion processes. Gram-scale synthesis, regioconvergent experiments, and downstream transformations gave further insights into the high potential of this transformation.

Visible-Light Photoredox-Catalyzed Remote Difunctionalizing Carboxylation of Unactivated Alkenes with CO2

Remote difunctionalization of unactivated alkenes is challenging but a highly attractive tactic to install two functional groups across long distances. Reported herein is the first remote difunctionalization of alkenes with CO₂ . This visible-light photoredox catalysis strategy provides a facile method to synthesize a series of carboxylic acids bearing valuable fluorine- or phosphorus-containing functional groups. Moreover, this versatile protocol shows mild reaction conditions, broad substrate scope, and good functional-group tolerance. Based on DFT calculations, a radical adds to an unactivated alkene to smoothly form a new carbon radical, followed by a 1,5-hydrogen atom-transfer process, the rate-limiting step, generating a more stable benzylic radical. The reduction of the benzylic radicals by an Ir^II species generates the corresponding benzylic carbanions as the key intermediates, which further undergo nucleophilic attack with CO₂ to generate carboxylates.

Chemoselective Cross-Coupling of gem-Borazirconocene Alkanes with Aryl Halides

The direct and chemoselective conversion of the carbon-metal bond of gem-dimetallic reagents enables rapid and sequential formation of multiple carbon-carbon and carbon-heteroatom bonds, thus representing a powerful method for efficiently increasing structural complexity. Herein, we report a visible-light-induced, nickel-catalyzed, chemoselective cross-coupling reaction between gem-borazirconocene alkanes and diverse aryl halides, affording a wide range of alkyl Bpin derivatives in high yields with excellent regioselectivity. This practical method features attractively simple reaction conditions and a broad substrate scope. Additionally, we systematically investigated a Bpin-directed chain walking process underlying the regioselectivity of alkylzirconocenes, thus uncovering the mechanism of the remote functionalization of internal olefins achieved with our method. Finally, DFT calculations indicate that the high regioselectivity of this reaction originates from the directing effect of the Bpin group.

Zirconium-Catalyzed Atom-Economical Synthesis of 1,1-Diborylalkanes from Terminal and Internal Alkenes

A general and atom‐economical synthesis of 1,1‐diborylalkanes from alkenes and a borane without the need for an additional H₂ acceptor is reported for the first time. The key to our success is the use of an earth‐abundant zirconium‐based catalyst, which allows a balance of self‐contradictory reactivities (dehydrogenative boration and hydroboration) to be achieved. Our method avoids using an excess amount of another alkene as an H₂ acceptor, which was required in other reported systems. Furthermore, substrates such as simple long‐chain aliphatic alkenes that did not react before also underwent 1,1‐diboration in our system. Significantly, the unprecedented 1,1‐diboration of internal alkenes enabled the preparation of 1,1‐diborylalkanes.

Ru-catalyzed isomerization of ω-alkenylboronates towards stereoselective synthesis of vinylboronates with subsequent in situ functionalization

The stereoselective preparation of synthetically versatile vinylboronates from ω-alkenylboronates is achieved through a ruthenium-catalyzed isomerization reaction. A variety of di- and trisubstituted vinylboronates were conveniently produced and could be used as a new starting point for subsequent in situ remote functionalization through either a sequential Ru/Pd or Ru/Cu double catalytic system.

A regio- and stereoselective ruthenium-catalyzed isomerization of ω-alkenyl boronates into stereodefined di- and trisubstituted alkenylboronate derivatives is reported.

Ligand-Enabled Nickel-Catalyzed Redox-Relay Migratory Hydroarylation of Alkenes with Arylborons

A redox-relay migratory hydroarylation of isomeric mixtures of olefins with arylboronic acids catalyzed by nickel complexes bearing diamine ligands is described. A range of structurally diverse 1,1-diarylalkanes, including those containing a 1,1-diarylated quaternary carbon, were obtained in excellent yields and with high regioselectivity. Preliminary experimental evidence supports the proposed non-dissociated chainwalking of aryl-nickel(II)-hydride species along the alkyl chain of alkenes before selective reductive elimination at a benzylic position. A catalyst loading as low as 0.5 mol % proved to be sufficient in large-scale synthesis while retaining high reactivity, highlighting the practical value of this transformation.

Migratory Hydrogenation of Terminal Alkynes by Base/Cobalt Relay Catalysis

Migratory functionalization of alkenes has emerged as a powerful strategy to achieve functionalization at a distal position to the original reactive site on a hydrocarbon chain. However, an analogous protocol for alkyne substrates is yet to be developed. Herein, a base and cobalt relay catalytic process for the selective synthesis of (Z)-2-alkenes and conjugated E alkenes by migratory hydrogenation of terminal alkynes is disclosed. Mechanistic studies support a relay catalytic process involving a sequential base-catalyzed isomerization of terminal alkynes and cobalt-catalyzed hydrogenation of either 2-alkynes or conjugated diene intermediates. Notably, this practical non-noble metal catalytic system enables efficient control of the chemo-, regio-, and stereoselectivity of this transformation.

Solvent-free and room temperature microwave-assisted direct C7 allylation of indolines via sequential C–H and C–C activation

A Ru or Rh-catalyzed efficient and atom-economic C7 allylation of indolines with vinylcyclopropanes was developed via sequential C–H and C–C activation. A wide range of substrates were well tolerated to afford the corresponding allylated indolines in high yields and E/Z selectivities under microwave irradiation. The obtained allylated indolines could further undergo transformations to afford various value-added chemicals. Importantly, this reaction proceeded at room temperature under solvent-free conditions.

A Ru or Rh-catalyzed direct C7 allylation of indolines with vinylcyclopropanes via sequential C–H/C–C activation under microwave irradiation has been disclosed.

Regio- and Diastereoselective Copper-Catalyzed Carbometalation of Cyclopropenylsilanes

We therein report a regio- and diastereoselective copper-catalyzed carbometalation of enantioenriched readily available cyclopropenylsilanes as a synthetic tool to access a large family of stereodefined fully-(hexa)-substituted cyclopropanes.

Rhodium catalyzed C-C bond cleavage/coupling of 2-(azetidin-3-ylidene)acetates and analogs

The C-C bond cleavage/coupling of 2-(azetidin-3-ylidene)acetates with aryl boronic acids catalyzed by a rhodium complex was studied with a "conjugate addition/β-C cleavage/protonation" strategy.

Practical Catalytic Cleavage of C(sp3 )-C(sp3 ) Bonds in Amines

The selective cleavage of thermodynamically stable C(sp3 )-C(sp3 ) single bonds is rare compared to their ubiquitous formation. Herein, we describe a general methodology for such transformations using homogeneous copper-based catalysts in the presence of air. The utility of this novel methodology is demonstrated for Cα -Cβ bond scission in >70 amines with excellent functional group tolerance. This transformation establishes tertiary amines as a general synthon for amides and provides valuable possibilities for their scalable functionalization in, for example, natural products and bioactive molecules.

Rapid Access to Highly Functionalized Alkyl Boronates by NiH-Catalyzed Remote Hydroarylation of Boron-Containing Alkenes

The direct and selective functionalization of relatively simple and readily accessible precursors to produce highly functionalized alkyl boronates is a synthetically useful process. Herein we report a NiH-catalyzed remote hydroarylation process that can, through a synergistic combination of chain walking and subsequent cross-coupling, introduce an aryl group at the adjacent carbon atom of alkyl boronates under mild conditions. By means of a preliminary experiment with moderate enantioselectivity, it was shown that an asymmetric version could also be realized.

Nickel-catalysed selective migratory hydrothiolation of alkenes and alkynes with thiols

Direct (utilize easily available and abundant precursors) and selective (both chemo- and regio-) aliphatic C–H functionalization is an attractive mean with which to streamline chemical synthesis. With many possible sites of reaction, traditional methods often need an adjacent polar directing group nearby to achieve high regio- and chemoselectivity and are often restricted to a single site of functionalization. Here we report a remote aliphatic C–H thiolation process with predictable and switchable regioselectivity through NiH-catalysed migratory hydrothiolation of two feedstock chemicals (alkenes/alkynes and thiols). This mild reaction avoids the preparation of electrophilic thiolation reagents and is highly selective to thiols over other nucleophilic groups, such as alcohols, acids, amines, and amides. Mechanistic studies show that the reaction occurs through the formation of an RS-Bpin intermediate, and THF as the solvent plays an important role in the regeneration of NiH species.

Construction of C–S bonds via C–H functionalization is an attractive route to organosulfur compounds. Here, the authors show the synergistic combination of NiH-catalysed alkene isomerization and subsequent thiolation to afford remote hydrothiolation products from alkenes (or alkynes) and thiols under mild conditions.

Highly E-Selective, Stereoconvergent Nickel-Catalyzed Suzuki-Miyaura Cross-Coupling of Alkenyl Ethers

An improved method for the nickel-catalyzed Suzuki–Miyaura cross-coupling of alkenyl ethers is reported. This stereoconvergent protocol allows for the utilization of a wide range of alkenyl ethers and aryl boronic esters for the synthesis of variously substituted styrene derivatives. An olefinic mixture with respect to the alkenyl ethers can be employed, thereby circumventing the stereodefined synthesis of starting materials. Preliminary mechanistic investigations indicate a nickel-catalyzed olefin isomerization following initial stereoretentive cross-coupling.

A Tandem Iridium-Catalyzed "Chain-Walking"/Cope Rearrangement Sequence

An iridium-catalyzed tandem olefin migration/Cope rearrangement of alkenyl ω-ene cyclopropanes is reported. By this means, a variety of complex annulenes are obtained as single diastereomers starting from cyclopropyl ester derived from simple 1,ω-dienes and alkenyldiazo compounds. Long-range olefin migration over up to 10 positions could be realized and coupled with an efficient Cope rearrangement to yield valuable scaffolds. Various functional groups are well-tolerated, giving rise to densely functionalized products. Furthermore, the present methodology could be successfully extended to yield bicyclic cycloheptenones starting from readily available alkenyl cyclopropanols via a Kulinkovich reaction.

Synthesis of cyano-substituted carbazoles via successive C-C/C-H cleavage

A highly efficient protocol for the synthesis of polysubstituted cyano carbazoles has been developed. This process is realized through the tandem reactions of Cs2CO3 promoted C-C σ-bond activation of α-cyano ketones followed by Fe-catalyzed selective C-H and/or C-C bond activations. Two of the sp2 C-H bonds are functionalized, and two of the C-C σ-bonds are cleaved involving a 1,2-acyl migration during the reaction process.