Nickel-Catalyzed Hydroarylation of in Situ Generated 1,3-Dienes with Arylboronic Acids Using a Secondary Homoallyl Carbonate as a Surrogate for the 1,3-Diene and Hydride Source

Understanding the Regiodivergence between Hydroarylation and Trifluoromethylarylation of 1,3-Dienes Using Anilines in HFIP

Conjugated dienes (1,3-dienes) are versatile and valuable chemical feedstocks that can be used as two-carbon or four-carbon synthons with vast applications across the chemical industry. However, the main challenge for their productive incorporation in synthetic routes is their chemo-, regio-, and stereoselective functionalization. Herein, we introduce a unified strategy for the 1,2-hydroarylation and 1,4-trifluoromethylarylation of 1,3-dienes using anilines in hexafluoroisopropanol. DFT calculations point toward a kinetically controlled process in both transformations, particularly in the trifluoromethylarylation, to explain the regiodivergent outcome. In addition, we perform an extensive program of functionalization and diversification of the products obtained, including hydrogenation, oxidation, cyclizations, or cross-coupling reactions, that allows access to a library of high-value species in a straightforward manner.

Mechanistic study of the Ni-catalyzed hydroalkylation of 1,3-dienes: The origins of regio- and enantioselectivities and a further rational design

The development of the catalytic regio- and enantioselective hydrofunctionalization of 1,3-dienes remains a challenge and requires deep insight into the reaction mechanisms. We herein thoroughly studied the reaction mechanism of the Ni-catalyzed hydroalkylation of 1,3-dienes with ketones by density functional theory (DFT) calculations. It reveals that the reaction is initiated by stepwise oxidative addition of EtO-H followed by 1,3-diene migratory insertion to generate the alkylnickel(II) intermediate, rather than the experimentally proposed ligand-to-ligand hydrogen transfer (LLHT) mechanism. In addition, we rationalized the role of t BuOK in the subsequent addition of enolate of ketone and transmetalation process. Based on the whole catalysis, the CC reductive elimination step, turns out to be the rate- and enantioselectivity-determining step. Furthermore, we disclosed the origins of the regio- and enantioselectivity of the product, and found that the 1,2-selectivity lies in the combination effects of the ligand-substrate electrostatic interactions, orbital interactions and Pauli repulsions, while the enantioselectivity mainly arises from substrate-ligand steric repulsions. Based on mechanistic study, new biaryl bisphosphine ligands affording higher enantioselectivity were designed, which will help to improve current catalytic systems and develop new transition-metal-catalyzed hydroalkylations.

Migratory allylic arylation of 1,n-enols enabled by nickel catalysis

Transition-metal-catalyzed allylic substitution reactions (Tsuji-Trost reactions) proceeding via a π-allyl metal intermediate have been demonstrated as a powerful tool in synthetic chemistry. Herein, we disclose an unprecedented π-allyl metal species migration, walking on the carbon chain involving 1,4-hydride shift as confirmed by deuterium labeling experiments. This migratory allylic arylation can be realized under dual catalysis of nickel and lanthanide triflate, a Lewis acid. Olefin migration has been observed to preferentially occur with the substrate of 1,n-enols (n ≥ 3). The robust nature of the allylic substitution strategy is reflected by a broad scope of substrates with the control of regio- and stereoselectivity. DFT studies suggest that π-allyl metal species migration consists of the sequential β-H elimination and migratory insertion, with diene not being allowed to release from the metal center before producing a new π-allyl nickel species.

Nickel-catalyzed arylative substitution of homoallylic alcohols

Direct coupling of unactivated alcohols remains a challenge in synthetic chemistry. Current approaches to cross-coupling of alcohol-derived electrophiles often involve activated alcohols such as tosylates or carbonates. We report the direct arylative substitution of homoallylic alcohols catalyzed by a nickel-bisphosphine complex as a facile method to generate allylic arenes. These reactions proceed via formation of an allylic alcohol intermediate. Subsequent allylic substitution with arylboroxine nucleophiles enables the formation of a variety of allylic arenes. The presence of p-methoxyphenylboronic acid is crucial to activate the allylic alcohol to achieve high product yields.

Nickel/Photoredox-Catalyzed Enantioselective Reductive Cross-Coupling between Vinyl Bromides and Benzyl Chlorides

A visible-light-promoted nickel/photoredox-catalyzed reductive cross-coupling reaction between vinyl bromides and benzyl chlorides is reported. A diverse array of enantioenriched allylic centers containing products could be achieved in good yields (up to 90%) and high enantioselectivities (up to 95% ee). The mechanistic studies show that this reductive cross-coupling involves a radical pathway.

Nickel-Catalyzed Hydroarylation Reaction: A Useful Tool in Organic Synthesis

This article describes the recent advances in the field of nickel-catalyzed hydroarylation reaction of alkenes, alkynes, and arenes. All reactions proceeded either through internal hydride transfer or in presence of...

Nickel(0)-catalysed linear-selective hydroarylation of 2-aminostyrenes with arylboronic acids by a bifunctional temporary directing group strategy

We report a nickel(0)-catalyzed linear-selective hydroarylation of 2-aminostyrenes with arylboronic acids using a bifunctional temporary directing group strategy. In the presence of a catalytic amount of commercially available 3,5-dibromosalicylaldehyde, an...

Ni-Catalyzed Regioselective Hydroarylation of 1-Aryl-1,3-Butadienes with Aryl Halides

An efficient nickel-catalyzed regioselective hydroarylation of 1,3-dienes with aryl halides and a silane has been developed, affording a range of allylic arenes in good to excellent yields under mild conditions. This method exhibits broad substrate scope, and excellent functional group tolerance. Late-stage modification of complex architectures was demonstrated.

Directed Markovnikov hydroarylation and hydroalkenylation of alkenes under nickel catalysis

We report a full account of our research on nickel-catalyzed Markovnikov-selective hydroarylation and hydroalkenylation of non-conjugated alkenes, which has yielded a toolkit of methods that proceed under mild conditions with alkenyl sulfonamide, ketone, and amide substrates. Regioselectivity is controlled through catalyst coordination to the native Lewis basic functional groups contained within these substrates. To maximize product yield, reaction conditions were fine-tuned for each substrate class, reflecting the different coordination properties of the directing functionality. Detailed kinetic and computational studies shed light on the mechanism of this family of transformations, pointing to transmetalation as the turnover-limiting step.

Native Lewis basic functional groups enable the nickel-catalyzed Markovnikov-selective hydroarylation and hydroalkenylation of unactivated alkenes with organoboron reagents.

A Computational Mechanistic Study of Cp*Co(III)-Catalyzed Three-Component C-H Bond Addition to Terpenes and Formaldehydes: Insights into the Origins of Regioselectivity

Transition metal-catalyzed three-component reactions of arenes, dienes, and carbonyls enable the convergent synthesis of homoallylic alcohols. Controlling regioselectivity is a central challenge for the difunctionalization of substituted 1,3-dienes in which multiple unbiased C═C bonds exist. Here, the mechanisms of Cp*Co(III)-catalyzed three-component C-H bond addition to terpenes and formaldehydes were investigated by density functional theory calculations. The reaction proceeds via sequential C(sp²)-H activation, migratory insertion, β-hydride elimination, hydride reinsertion, and C-C bond formation to yield the final product. The migratory insertion is the rate- and regioselectivity-determining step of the overall reaction. We employed an energy decomposition approach to quantitatively dissect the contributions of different types of interactions to regioselectivity. For the 2-alkyl substituted 1,3-dienes, the orbital interactions in the 3,4-insertion are intrinsically more favorable as compared to that in the 4,3-insertion, while the stronger steric effects between metallacycle and 1,3-diene override the intrinsic electronic preference. However, the steric effects failed to rationalize the unfavorable 1,2-insertion that is analogous to 4,3-insertion and even bears smaller steric effects. The donor-acceptor interaction analysis indicates that orbital interactions between σ_Co-C and π_C═C decreased significantly in the 1,2-insertion transition state, which leads to higher activation energy barriers. These insights into the dominant effects controlling regioselectivity will enable rational design of new catalysts for selective functionalization of dienes.

Vinylogous Elimination/C-H Functionalization/Allylation Cascade Reaction of Allenoate Adducts: Synthesis of Ring-Fused Dihydropyridinones

A palladium-catalyzed cascade reaction of β'-allenoate adducts with aryl/heteroaryl carboxamides through a vinylogous elimination/C-H functionalization/intramolecular allylation reaction sequence has been developed with high Z stereoselectivity. Various ring-fused dihydropyridinones bearing an α,β-unsaturated ester substituent are obtained. It is the first example of application of the allenoate adducts to C-H functionalization annulations as practical precursors of hard-to-get functionalized electron-deficient 1,3-butadienes. Using air as the terminal oxidant also shows a great advantage in environmental friendliness.

MnO2 as a terminal oxidant in Wacker oxidation of homoallyl alcohols and terminal olefins

Efficient and mild reaction conditions for Wacker-type oxidation of terminal olefins of less explored homoallyl alcohols to β-hydroxy-methyl ketones have been developed by using a Pd(ii) catalyst and MnO2 as a co-oxidant. The method involves mild reaction conditions and shows good functional group compatibility along with high regio- and chemoselectivity. While our earlier system of PdCl2/CrO3/HCl produced α,β-unsaturated ketones from homoallyl alcohols, the present method provided orthogonally the β-hydroxy-methyl ketones. No overoxidation or elimination of benzylic and/or β-hydroxy groups was observed. The method could be extended to the oxidation of simple terminal olefins as well, to methyl ketones, displaying its versatility. An application to the regioselective synthesis of gingerol is demonstrated.

Sterically enriched bulky 1,3-bis(N,N′-aralkyl)benzimidazolium based Pd-PEPPSI complexes for Buchwald–Hartwig amination reactions

A simple and efficient synthesis of a series of unexisting Pd-PEPPSI complexes is summarized. These complexes are exploited for their high catalytic activity towards Buchwald–Hartwig amination.