Cobalt-Catalyzed Desymmetric Isomerization of Exocyclic Olefins

Asymmetric Olefin Isomerization via Phase-Transfer-Catalyzed [1,3]-Hydrogen Transfer for Access to Axially Chiral Furan-Benzimidazoles

An efficient catalytic asymmetric olefin isomerization of axially chiral methylene dihydrofuran-benzimidazoles via kinetic resolution is reported. Under mild phase-transfer catalysis, axially chiral furan-benzimidazole compounds and recovered methylene dihydrofuran-benzimidazoles were obtained in high ee. The combination of the kinetic resolution and TBD-catalyzed isomerization of recovered dihydrofuran-benzimidazoles provided access to both enantiomers of furan-benzimidazoles. Deuterium-labeling experiments reveal intramolecular [1,3]-H transfer mechanism. The utility of this method was demonstrated by scale-up synthesis and functionalization of the products.

Regiodivergent Hydroamidation of Alkenes via Cobalt-Hydride Catalysis

Regiodivergent hydroamin(d)ation of alkenes presents a valuable strategy for the synthesis of diverse amines or amides from a common set of starting materials, yet achieving controlled regioselectivity remains a significant challenge. In this work, we present a cobalt-catalyzed regiodivergent hydroamidation of alkenes, enabling enantioselective ipso- and migratory hydroamidation of heterocyclic alkenes. The ability to finely tune various reaction parameters allows for a seamless switch in regioselectivity. Notably, ipso- and migratory selectivity are governed by the choice of cobalt catalyst anions. Mechanistic studies reveal a neutral Co-H species mediating ipso-hydroamidation and a cationic Co-H intermediate promoting migratory hydroamidation. This protocol exhibits a broad substrate scope, high functional group tolerance, and provides an efficient pathway for synthetizing structurally diverse amides.

Co(II)-catalyzed isomerization of enals using hydrogen atom transfer

To develop a process for the synthesis of an enal acetate, the so called C5-acetate that is important in the industrial preparation of vitamin A, we show here that Co(II)-based hydrogen atom transfer catalysts under H2 promote such isomerizations in high yield with low catalyst loading (0.1 mol%). D-labelling studies suggest the enal isomerization process and catalyst activation by H2 to be reversible.

Palladium-catalyzed enantioselective β-hydride elimination for the construction of remote stereocenters

The β-H elimination is a crucial elementary step in transition-metal catalysis, but controlling the stereochemistry of this process has been underdeveloped. The limited works reported so far have only focused on creating axial chirality in allenes, and no report has been able to build central chirality using asymmetric β-H elimination. In this study, we report a Trost ligand-enabled enantioselective desymmetric β-H elimination reaction from π-allyl-Pd. This transformation provides rapid access to cyclohexenes bearing a C4-remoted stereocenter, and total synthesis of (-)-oleuropeic acid and (-)-7-hydroxyterpineol is demonstrated. Computational studies have shown that the β-H elimination is the rate-determining step, and the non-covalent interactions between the amide moiety of the Trost ligand and the benzene and cyclohexane moieties of the substrate play a key role in stereocontrol during the β-H elimination.

Manganese(I)-Catalyzed Enantioselective Alkylation To Access P-Stereogenic Phosphines

This work introduces a novel Mn(I)-catalyzed enantioselective alkylation methodology that efficiently produces a wide array of P-chiral phosphines with outstanding yields and enantioselectivities. Notably, the exceptional reactivity of Mn(I) complexes in these reactions is demonstrated by their effective catalysis with both typically reactive alkyl iodides and bromides, as well as with less reactive alkyl chlorides. This approach broadens the accessibility to various P-chiral phosphines and simplifies the synthesis of chiral tridentate pincer phosphines to a concise 1-2 step process, contrary to conventional, labor-intensive multistep procedures. Importantly, the development significantly expands the applicability of earth-abundant Mn(I)-based complexes beyond their recently established roles in catalytic hydrogenative and conjugate addition reactions, emphasizing the catalytic potential of Mn(I) complexes as a viable alternative to noble metal chemistry and, in some cases, even surpassing their performance.

Catalytic asymmetric fragmentation of cyclopropanes

The stereoselective activation of alkanes constitutes a long-standing and grand challenge for chemistry. Although metal-containing enzymes oxidize alkanes with remarkable ease and selectivity, chemical approaches have largely been limited to transition metal-based catalytic carbon-hydrogen functionalizations. Alkanes can be protonated to form pentacoordinated carbonium ions and fragmented into smaller hydrocarbons in the presence of strong Brønsted acids. However, catalytic stereocontrol over such reactions has not previously been accomplished. We show here that strong and confined acids catalyze highly enantioselective fragmentations of a variety of cyclopropanes into the corresponding alkenes, expanding the boundaries of catalytic selective alkane activation. Computational studies suggest the involvement of the long-debated cycloproponium ions.

(Z)-Selective Isomerization of 1,1-Disubstituted Alkenes by Scandium-Catalyzed Allylic C-H Activation

The isomerization of 1,1-disubstituted alkenes through 1,3-hydrogen shift is an atom-efficient route for synthesizing trisubstituted alkenes, which are important moieties in many natural products, pharmaceuticals, and organic materials. However, this reaction often encounters regio- and stereoselectivity challenges, typically yielding E/Z-mixtures of the alkene products or thermodynamically favored (E)-alkenes. Herein, we report the (Z)-selective isomerization of 1,1-disubstituted alkenes to trisubstituted (Z)-alkenes via the regio- and stereospecific activation of an allylic C-H bond. The key to the success of this unprecedented transformation is the use of a sterically demanding half-sandwich scandium catalyst in combination with a bulky quinoline compound, 2-tert-butylquinoline. Deuterium-labeling experiments and density functional theory (DFT) calculations have revealed that 2-tert-butylquinoline not only facilitates the C═C bond transposition through hydrogen shuttling but also governs the regio- and stereoselectivity due to the steric hindrance of the tert-butyl group. This protocol enables the synthesis of diverse (Z)-configured acyclic trisubstituted alkenes and endocyclic trisubstituted alkenes from readily accessible 1,1-disubstituted alkenes. It offers an efficient and selective route for preparing a new family of synthetically challenging (Z)-trisubstituted alkenes with broad substrate scope, 100% atom efficiency, high regio- and stereoselectivity, and an unprecedented reaction mechanism.

tert-Butyl as a Functional Group: Non-Directed Catalytic Hydroxylation of Sterically Congested Primary C-H Bonds

The tert-butyl group is a common aliphatic motif extensively employed to implement steric congestion and conformational rigidity in organic and organometallic molecules. Because of the combination of a high bond dissociation energy (~100 kcal mol-1) and limited accessibility, in the absence of directing groups, neither radical nor organometallic approaches are effective for the chemical modification of tert-butyl C-H bonds. Herein we overcome these limits by employing a highly electrophilic manganese catalyst, [Mn(CF3bpeb)(OTf)2], that operates in the strong hydrogen bond donor solvent nonafluoro-tert-butyl alcohol (NFTBA) and catalytically activates hydrogen peroxide to generate a powerful manganese-oxo species that effectively oxidizes tert-butyl C-H bonds. Leveraging on the interplay of steric, electronic, medium and torsional effects, site-selective and product chemoselective hydroxylation of the tert-butyl group is accomplished with broad reaction scope, delivering primary alcohols as largely dominant products in preparative yields. Late-stage hydroxylation at tert-butyl sites is demonstrated on 6 densely functionalized molecules of pharmaceutical interest. This work uncovers a novel disconnection approach, harnessing tert-butyl as a potential functional group in strategic synthetic planning for complex molecular architectures.

Structural Design of Single-Atom Catalysts for Enhancing Petrochemical Catalytic Reaction Process

Petroleum, as the "lifeblood" of industrial development, is the important energy source and raw material. The selective transformation of petroleum into high-end chemicals is of great significance, but still exists enormous challenges. Single-atom catalysts (SACs) with 100% atom utilization and homogeneous active sites, promise a broad application in petrochemical processes. Herein, the research systematically summarizes the recent research progress of SACs in petrochemical catalytic reaction, proposes the role of structural design of SACs in enhancing catalytic performance, elucidates the catalytic reaction mechanisms of SACs in the conversion of petrochemical processes, and reveals the high activity origins of SACs at the atomic scale. Finally, the key challenges are summarized and an outlook on the design, identification of active sites, and the appropriate application of artificial intelligence technology is provided for achieving scale-up application of SACs in petrochemical process.

Chromium-Catalyzed Alkene Isomerization with Switchable Selectivity

We report a single additive-responsive chromium-catalyzed system for selectively producing either of two different internal alkene isomers. The chromium catalyst, in the presence of HBpin/LiOtBu, enables the isomerization of alkenes over multiple carbon atoms to give the most thermodynamically stable isomers. The same catalyst allows for the selective isomerization of terminal alkenes over one carbon atom without an additive, exhibiting efficient and controllable alkene transposition selectivity.

A Neutral PCNHCP Co(I)-Me Pincer Complex as a Catalyst for N-Allylic Isomerization with a Broad Substrate Scope

Earth-abundant-metal catalyzed double bond transposition offers a sustainable and atom-economical route toward the synthesis of internal alkenes. With an emphasis specifically on internal olefins and ethers, the isomerization of allylic amines has been particularly under represented in the literature. Herein, we report an efficient methodology for the selective isomerization of N-allylic organic compounds, including amines, amides, and imines. The reaction is catalyzed by a neutral PCNHCP cobalt(I) pincer complex and proceeds via a π-allyl mechanism. The isomerization occurs readily at 80-90 °C, and it is compatible with a wide variety of functional groups. The in situ formed enamines could additionally be used for a one-pot inverse-electron-demand Diels-Alder reaction to furnish a series of diversely substituted heterobiaryls, which is further discussed in this report.

Cobalt-Catalyzed Asymmetric Hydrogenation: Substrate Specificity and Mechanistic Variability

Asymmetric hydrogenation finds widespread application in academia and industry. And indeed, a number of processes have been implemented for the production of pharma and agro intermediates as well as flavors & fragrances. Although these processes are all based on the use of late transition metals as catalysts, there is an increasing interest in the use of base metal catalysis in view of their lower cost and the expected different substrate scope. Catalysts based on cobalt have already shown their potential in enantioselective hydrogenation chemistry. This review outlines the impressive progress made in recent years on cobalt-catalyzed asymmetric hydrogenation of different unsaturated substrates. We also illustrate the ligand dependent substrate specificity as well as the mechanistic variability in detail. This may well guide further catalyst development in this research area.

Experimental and Computational Studies on Cobalt(I)-Catalyzed Regioselective Allylic Alkylation Reactions

Here, we report the development of cobalt(I)-catalyzed regioselective allylic alkylation reactions of tertiary allyl carbonates with 1,3-dicarbonyl compounds. A family of well-defined tetrahedral cobalt(I) complexes bearing commercially available bidentate bis(phosphine) ligands [(P,P)Co(PPh3 )Cl] are synthesized and explored as catalysts in allylic alkylation reactions. The catalyst [(dppp)Co(PPh3 )Cl] (dppp=1,3-Bis(diphenylphosphino)propane) enables the alkylation of a large variety of tertiary allyl carbonates with high yields and excellent regioselectivity for the branched product. Remarkably, this methodology is selective for the activation of tertiary allyl carbonates even in the presence of secondary allyl carbonates. This contrasts with the selectivity observed in cobalt-catalyzed allylic alkylations enabled by visible light photocatalysis. Mechanistic insights by means of experimental and computational investigations support a Co(I)/Co(III) catalytic cycle.

Aminodealkenylation: Ozonolysis and copper catalysis convert C(sp3)-C(sp2) bonds to C(sp3)-N bonds

Great efforts have been directed toward alkene π bond amination. In contrast, analogous functionalization of the adjacent C(sp3)-C(sp2) σ bonds is much rarer. Here we report how ozonolysis and copper catalysis under mild reaction conditions enable alkene C(sp3)-C(sp2) σ bond-rupturing cross-coupling reactions for the construction of new C(sp3)-N bonds. We have used this unconventional transformation for late-stage modification of hormones, pharmaceutical reagents, peptides, and nucleosides. Furthermore, we have coupled abundantly available terpenes and terpenoids with nitrogen nucleophiles to access artificial terpenoid alkaloids and complex chiral amines. In addition, we applied a commodity chemical, α-methylstyrene, as a methylation reagent to prepare methylated nucleosides directly from canonical nucleosides in one synthetic step. Our mechanistic investigation implicates an unusual copper ion pair cooperative process.

C-H Bonds as Functional Groups: Simultaneous Generation of Multiple Stereocenters by Enantioselective Hydroxylation at Unactivated Tertiary C-H Bonds

Enantioselective C-H oxidation is a standing chemical challenge foreseen as a powerful tool to transform readily available organic molecules into precious oxygenated building blocks. Here, we describe a catalytic enantioselective hydroxylation of tertiary C-H bonds in cyclohexane scaffolds with H2O2, an evolved manganese catalyst that provides structural complementary to the substrate similarly to the lock-and-key recognition operating in enzymatic active sites. Theoretical calculations unveil that enantioselectivity is governed by the precise fitting of the substrate scaffold into the catalytic site, through a network of complementary weak non-covalent interactions. Stereoretentive C(sp3)-H hydroxylation results in a single-step generation of multiple stereogenic centers (up to 4) that can be orthogonally manipulated by conventional methods providing rapid access, from a single precursor to a variety of chiral scaffolds.

B(C6 F5 )3 -Catalyzed E-Selective Isomerization of Alkenes

Herein, we report the B(C₆ F₅ )₃ -catalyzed E-selective isomerization of alkenes. The transition-metal-free method is applicable across a diverse array of readily accessible substrates, giving access to a broad range of synthetically useful products containing versatile stereodefined internal alkenes. The reaction mechanism was investigated by using synthetic and computational methods.

Multicomponent Direct Assembly of N-Heterospirocycles Facilitated by Visible-Light-Driven Photocatalysis

N-heterospirocycles are interesting structural units found in both natural products and medicinal compounds but have relatively few reliable methods for their synthesis. Here, we enlist the photocatalytic generation of N-centered radicals to construct β-spirocyclic pyrrolidines from N-allylsulfonamides and alkenes. A variety of β-spirocyclic pyrrolidines have been constructed, including drug derivatives, in moderate to very good yields. Further derivatization of the products has also been demonstrated as has a viable scale-up procedure, making use of flow chemistry techniques.

Oxyfunctionalization of Benzylic C-H Bonds of Toluene Mediated by Covalently Anchored Co-Schiff Bases

Oxyfunctionalization of toluene to value-added benzaldehyde, benzyl alcohol and benzoic acid is of great significance. In this work, Co-Schiff bases were immobilized on commercial silica gel by covalent anchoring, and resulting catalysts were used to catalyze the oxidation of toluene in the presence of the cocatalyst N-hydroxyphthalimide (NHPI). The catalysts exhibited excellent textural and structural properties, reliable bonding and a predomination of the cobaltous ions. The catalyst synthesized by diethylamino salicylaldehyde (EASA) possessed a grafting density of 0.14 mmol/g and exhibited a toluene conversion of 37.5%, with predominant selectivities to benzaldehyde, benzyl alcohol and benzoic acid under solvent-free conditions. It is concluded that the effect of ligands on their catalytic performance might be related to their electron-donating or -withdrawing properties.

Switching between Hydrogenation and Olefin Transposition Catalysis via Silencing NH Cooperativity in Mn(I) Pincer Complexes

While Mn-catalyzed (de)hydrogenation of carbonyl derivatives has been well established, the reactivity of Mn hydrides with olefins remains very rare. Herein, we report a Mn(I) pincer complex that effectively promotes site-controlled transposition of olefins. This reactivity is shown to emerge once the N–H functionality within the Mn/NH bifunctional complex is suppressed by alkylation. While detrimental for carbonyl (de)hydrogenation, such masking of the cooperative N–H functionality allows for the highly efficient conversion of a wide range of allylarenes to higher-value 1-propenybenzenes in near-quantitative yield with excellent stereoselectivities. The reactivity toward a single positional isomerization was also retained for long-chain alkenes, resulting in the highly regioselective formation of 2-alkenes, which are less thermodynamically stable compared to other possible isomerization products. The detailed mechanistic analysis of the reaction between the activated Mn catalyst and olefins points to catalysis operating via a metal–alkyl mechanism—one of the three conventional transposition mechanisms previously unknown in Mn complexes.

Cobalt-Catalyzed Migration Isomerization of Dienes

A cobalt-catalyzed multipositional isomerization of conjugated dienes has been reported for the first time using an 8-oxazoline iminoquinoline ligand. This reaction is operationally simple and atom-economical using readily available starting materials with an E/Z mixture to access disubstituted 1,3-dienes with excellent yields and good E,E stereoselectivity. The mechanism via alkene insertion of cobalt hydride species and β-H elimination of a π-allyl cobalt intermediate is proposed on the basis of deuterium labeling and control experiments and density functional theory calculations.

Synthesis of Cyclic Fragrances via Transformations of Alkenes, Alkynes and Enynes: Strategies and Recent Progress

With increasing demand for customized commodities and the greater insight and understanding of olfaction, the synthesis of fragrances with diverse structures and odor characters has become a core task. Recent progress in organic synthesis and catalysis enables the rapid construction of carbocycles and heterocycles from readily available unsaturated molecular building blocks, with increased selectivity, atom economy, sustainability and product diversity. In this review, synthetic methods for creating cyclic fragrances, including both natural and synthetic ones, will be discussed, with a focus on the key transformations of alkenes, alkynes, dienes and enynes. Several strategies will be discussed, including cycloaddition, catalytic cyclization, ring-closing metathesis, intramolecular addition, and rearrangement reactions. Representative examples and the featured olfactory investigations will be highlighted, along with some perspectives on future developments in this area.

Desymmetrizing Isomerization of Alkene via Thiazolinyl Iminoquinoline Cobalt Catalysis

We report a cobalt-catalyzed desymmetrizing isomerization of exo-cyclic alkenes to generate chiral 1-methylcyclohexene derivatives with good yields and enantioselectivities. A novel chiral thiazolinyl iminoquinoline ligand and its cobalt complex were designed and synthesized to control the establishment of tertiary or quaternary carbon centers at a remote position. This protocol is operationally simple, and a model for the stereochemical outcome has been proposed.