Three-Component Coupling of Arenes, Ethylene, and Alkynes Catalyzed by a Cationic Bis(phosphine) Cobalt Complex: Intercepting Metallacyclopentenes for C-H Functionalization

Ligand Effects in Carboxylic Ester- and Aldehyde-Assisted β-C-H Activation in Regiodivergent and Enantioselective Cycloisomerization-Hydroalkenylation and Cycloisomerization-Hydroarylation, and [2 + 2 + 2]-Cycloadditions of 1,6-Enynes

Herein, we report room temperature, atom-economic protocols for high regio- and enantioselective tandem cycloisomerization-hydroarylation and cycloisomerization-hydroalkenylation of 1,6-enynes leading to vicinal carba-functionalized pyrrolidines, tetrahydrofurans, and cyclopentanes. The latter steps in these processes involve carbonyl-coordination-assisted ortho-C-H activation of aromatic aldehydes and esters, and, a similar, yet rarely seen, β-C-H activation in the case of the acrylates. Synthetically useful enantioselective versions of such reactions are rare and are limited to the C2-H activation of indoles and pyrroles. A similar reaction is also observed with N-vinylphthalimide, which also has a carbonyl group suitable for C-H activation. A dibenzooxaphosphole ligand, (2S,2S',3S,3S')-MeO-BIBOP was uniquely identified as crucial to achieving the challenging regio- and enantioselectivity. This methodology gives access to substituted five-membered carbo- and heterocyclic compounds in good yields and excellent enantioselectivities under a low catalyst loading. A primary KIE of 3.5 is observed in an intermolecular competition experiment with methyl benzoate and d5-methyl benzoate, which indicates that the C-H cleavage is the turnover-limiting step of this process. Unlike the acrylates, which undergoes exclusive hydroalkenylation, a β, γ-unsaturated ester, methyl but-3-enoate, undergoes the highly enantioselective cycloisomerization-coupling sequence with a 1,6-enyne giving either a [2 + 2 + 2]-cycloaddition with (S, S)-BDPP or hydroalkenylation with (2S,2'S,3S,3'S)-MeO-BIBOP depending on the ligand employed. The (E)-configuration of the newly formed double bond at the terminal alkynyl carbon (of the starting enyne) in the hydroalkenylation product of β,γ-unsaturated ester suggests a more classical migratory insertion-β-hydride elimination route for the formation of this product.

Modular assembly of arenes, ethylene and heteroarenes for the synthesis of 1,2-arylheteroaryl ethanes

The 1,2-arylheteroaryl ethane motif stands as a privileged scaffold with promising implications in drug discovery. Conventional de novo syntheses of these molecules have relied heavily on pre-functionalized synthons, entailing harsh conditions and multi-step processes. Here, to address these limitations, we present a modular approach for the direct synthesis of 1,2-arylheteroaryl ethanes using feedstock chemicals, including ethylene, arenes and heteroarenes. We disclosed a photo triplet-energy-transfer-initiated radical cascade process, leveraging homolytic cleavage of C-S bonds in aryl sulfonium salts as the key step to access aryl radicals with excellent regioselectivity. This method allows for rapid structural diversification of bioactive molecules, showcasing excellent functional group tolerance and streamlining the synthesis of bioactive compounds and their derivatives. Furthermore, our approach can be extended to propylene, non-gaseous terminal alkenes and various other electrophilic radical precursors, including heteroaryl radicals, hydroxyl radicals, trifluoromethyl radicals and α-carbonyl alkyl radicals. This study highlights the significance of radical polarity matching in designing selective multi-component couplings.

Rhodium-catalyzed three-component C(sp3)/C(sp2)-H activation enabled by a two-fold directing group strategy

Rh-catalyzed three-component C(sp3)/C(sp2)-H activation has been achieved through a two-directing group strategy. This protocol provides a convenient and efficient pathway for the construction of diverse 8-alkyl quinoline derivatives in one-pot. Furthermore, mechanistic studies revealed that the first C-H amidation was significantly faster than the sequential C-H alkylation.

Unraveling the Potential of Vinyl Ether as an Ethylene Surrogate in Heteroarene C─H Functionalization via the Spin-Center Shift

Despite the simplicity and abundance of ethylene, its practical application presents significant hurdles due to its nature as a highly flammable gas. Herein, a strategic use of easily handled vinyl ether is reported as a latent ethylene surrogate achieved via a spin-center shift (SCS) pathway, enabling the successful three-component reaction that bridges heteroarenes and various coupling partners, including sulfinates, thiols, and phosphine oxides. Through a photoredox catalytic process, α-oxy radicals are generated by combining various radicals with phenyl vinyl ether, which are subsequently added to N-heteroarenes. Subsequently, the radical-mediated SCS pathway serves as the driving force for C─O bond cleavage, effectively engaging the phenoxy group as a leaving group. In addition, by broadening the utility of the method, a valuable synthon is provided for efficient C─H vinylation of N-heteroarenes following sulfonyl group elimination. This approach not only enriches the toolbox of synthetic methodology but also provides a more streamlined alternative, circumventing the challenges associated with direct ethylene gas usage. The versatility of the method, particularly evident in late-stage functionalizations of medicinally relevant molecules and peptides, underscores its capability to produce invaluable three-component compounds and vinylated N-heteroarene derivatives.

Mechanism and origins of cobalt-catalyzed ligand-controlled regiodivergent C-H functionalization of aldehydes with enynes

The influence of the P-M-P bite angle in diphosphine ligands on selectivity has been observed in various catalytic reactions. A better understanding of the ligand bite angle concept is important for the rational design of efficient catalytic systems. In the present work, the mechanism of cobalt-catalyzed C-H functionalization of aldehydes with enynes and how the diphosphine ligands alter regioselectivity were investigated by density functional theory (DFT) calculations. The catalytic cycle is initiated by the oxidative cyclization of enynes rather than the oxidative addition of aldehydes. Regioselectivity arises from competing σ-bond metathesis and migratory insertion steps, in which the steric effects of diphosphine ligands are the dominant factors influencing the activation barriers. The calculations indicate that σ-bond metathesis is more challenging and its feasibility is highly dependent on the ligand bite angle. The improved mechanistic understanding will enable further design of transition-metal-catalyzed selective cyclization reactions.

Catalytic Asymmetric Synthesis of Zinc Metallacycles

Transition-metal-catalyzed reductive coupling reactions of alkynes and imines are attractive methods for the synthesis of chiral allylic amines. Mechanistically, these reactions involve oxidative cyclization of the alkyne and the imine to generate a metallacyclic intermediate, which then reacts with H2 or a H2 surrogate to form the product. As an alternative to this hydrogenolysis pathway, here we show that transmetalation to zinc can occur, forming a zinc metallacycle product. This organozinc product serves as a versatile nucleophile for carbon-carbon and carbon-heteroatom coupling reactions. Mechanistic studies based on isotopic labeling experiments and DFT calculations suggest that the key transmetalation step occurs between a Co(II) species and ZnCl2.

Cp2Ti(II) Mediated Rearrangement of Cyclopropyl Imines

Ti-catalyzed oxidative alkyne carboamination with alkenes and azo compounds can yield either α,β-unsaturated imines or cyclopropyl imines through a common azatitanacyclohexene intermediate. Herein, we report the synthesis of a model azatitanacyclohexene complex (3) through the ring-opening of a cyclopropyl imine with Cp2Ti(BTMSA) (BTMSA = bis(trimethylsilyl)acetylene). 3 readily undergoes thermal or reductant-catalyzed ring contraction to an azatitanacyclopentene (4), analogous to the proposed mechanism for forming α,β-unsaturated imines in the catalytic reaction. A cyclopropyl imine or an α,β-unsaturated imine could be liberated via the oxidation of 3 or 4 with azobenzene, respectively, further implicating the role of these metallacycles in the Ti-catalyzed carboamination reaction.

Three-Component Chemo-Selective Synthesis of N-(o-Alkenylaryl) Pyrazoles by Pyrazole Annulation and Rh-Catalyzed Chemo-Selective Aryl C-H Addition Cascade

By using readily available enaminones, aryl hydrazine hydrochlorides, and alkynes as starting materials, the chemo-selective three-component synthesis of atropisomeric N-(o-alkenylaryl) pyrazoles has been efficiently accessed with rhodium catalysis. Unlike Satoh-Miura reaction leading to the alkyne-based C-H benzannulation by using prior prepared N-phenyl pyrazoles and alkynes as substrates, this three-component protocol displays unprecedented selectivity of C-H alkenylation by blocking the second round metal alkenylation with the key protonation step in the presence of acids.

Transition-Metal-Catalyzed C-H Bond Activation for the Formation of C-C Bonds in Complex Molecules

Site-predictable and chemoselective C-H bond functionalization reactions offer synthetically powerful strategies for the step-economic diversification of both feedstock and fine chemicals. Many transition-metal-catalyzed methods have emerged for the selective activation and functionalization of C-H bonds. However, challenges of regio- and chemoselectivity have emerged with application to highly complex molecules bearing significant functional group density and diversity. As molecular complexity increases within molecular structures the risks of catalyst intolerance and limited applicability grow with the number of functional groups and potentially Lewis basic heteroatoms. Given the abundance of C-H bonds within highly complex and already diversified molecules such as pharmaceuticals, natural products, and materials, design and selection of reaction conditions and tolerant catalysts has proved critical for successful direct functionalization. As such, innovations within transition-metal-catalyzed C-H bond functionalization for the direct formation of carbon-carbon bonds have been discovered and developed to overcome these challenges and limitations. This review highlights progress made for the direct metal-catalyzed C-C bond forming reactions including alkylation, methylation, arylation, and olefination of C-H bonds within complex targets.

A three component 1,3-difunctionalization of vinyl diazo esters enabled by a cobalt catalyzed C-H activation/carbene migratory insertion

We report herein, a modular, regioselective 1,3-oxyarylation of vinyl diazo esters via a Co-catalyzed C-H activation/carbene migratory insertion cascade. The transformation involves the formation of C-C and C-O bonds in a one-pot fashion and displays a broad substrate scope with respect to both, vinyl diazo esters as well as benzamides. The coupled products were subjected to hydrogenation to access elusive allyl alcohol scaffolds. Mechanistic investigations reveal interesting insights on the mode of transformation, involving C-H activation, carbene migratory insertion of the diazo compound followed by a radical addition as the key steps of the transformation.

Selective Reductive Coupling of Vinyl Azaarenes and Alkynes via Photoredox Cobalt Dual Catalysis

A visible light-induced Co-catalyzed highly regio- and stereoselective reductive coupling of vinyl azaarenes and alkynes has been developed. Notably, Hünig's base together with simple ethanol has been successfully applied as the hydrogen sources instead of commonly used Hantzsch esters in this catalytic photoredox reaction. This approach has considerable advantages for the straightforward synthesis of stereodefined multiple substituted alkenes bearing an azaarene motif, such as excellent regioselectivity (>20 : 1 for >30 examples) and stereoselectivity (>20 : 1 E/Z), broad substrate scope and good functional group compatibility under mild reaction conditions, which has been utilized in the concise synthesis of natural product monomorine I. A reasonable catalytic reaction pathway involving protolysis of the cobaltacyclopentene intermediate has been proposed based on the mechanistic studies.

A theory-driven synthesis of symmetric and unsymmetric 1,2-bis(diphenylphosphino)ethane analogues via radical difunctionalization of ethylene

1,2-Bis(diphenylphosphino)ethane (DPPE) and its synthetic analogues are important structural motifs in organic synthesis, particularly as diphosphine ligands with a C₂-alkyl-linker chain. Since DPPE is known to bind to many metal centers in a bidentate fashion to stabilize the corresponding metal complex via the chelation effect originating from its entropic advantage over monodentate ligands, it is often used in transition-metal-catalyzed transformations. Symmetric DPPE derivatives (Ar¹₂P-CH₂-CH₂-PAr¹₂) are well-known and readily prepared, but electronically and sterically unsymmetric DPPE (Ar¹₂P-CH₂-CH₂-PAr²₂; Ar¹≠Ar²) ligands have been less explored, mostly due to the difficulties associated with their preparation. Here we report a synthetic method for both symmetric and unsymmetric DPPEs via radical difunctionalization of ethylene, a fundamental C₂ unit, with two phosphine-centered radicals, which is guided by the computational analysis with the artificial force induced reaction (AFIR) method, a quantum chemical calculation-based automated reaction path search tool. The obtained unsymmetric DPPE ligands can coordinate to several transition-metal salts to form the corresponding complexes, one of which exhibits distinctly different characteristics than the corresponding symmetric DPPE-metal complex.

C-H Activation by Isolable Cationic Bis(phosphine) Cobalt(III) Metallacycles

Five- and six-coordinate cationic bis(phosphine) cobalt(III) metallacycle complexes were synthesized with the general structures, [(depe)Co(cycloneophyl)(L)(L')][BAr^F₄] (depe = 1,2-bis(diethylphosphino)ethane; cycloneophyl = [κ-C:C-(CH₂C(Me)₂)C₆H₄]; L/L' = pyridine, pivalonitrile, or the vacant site, BAr₄^F = B[(3,5-(CF₃)₂)C₆H₃]₄). Each of these compounds promoted facile directed C(sp²)-H activation with exclusive selectivity for ortho-alkylated products, consistent with the selectivity of reported cobalt-catalyzed arene-alkene-alkyne coupling reactions. The direct observation of C-H activation by cobalt(III) metallacycles provided experimental support for the intermediacy of these compounds in this class of catalytic C-H functionalization reaction. Deuterium labeling and kinetic studies provided insight into the nature of C-H bond cleavage and C-C bond reductive elimination from isolable cobalt(III) precursors.

Copper-Catalyzed One-Step Formation of Four C-N Bonds toward Polyfunctionalized Triazoles via Multicomponent Reaction

A novel four-component reaction of alkynes, amines, azides, and 2H-azirines has been developed for the first time by the efficient formation of four C-N bonds in one step under mild conditions, rapidly preparing polyfunctionalized triazoles with molecular diversity involving three different intermediates of copper-acetylide, copper-allenylidene, and copper-vinyl nitrene. Propargylic ester is disclosed as a "three-in-one" building block possessing triplicate cycloaddition and nucleophilic and electrophilic properties, which could enable such a four-component transformation by high yields, broad substrate scope, and functionalization.

C-H bond activation and sequential addition to two different coupling partners: a versatile approach to molecular complexity

Sequential multicomponent C-H bond addition is a powerful approach for the rapid, modular generation of molecular complexity in a single reaction. In this approach, C-H bonds are typically added across π-bonds or π-bond isosteres, followed by subsequent coupling to another type of functionality, thereby forming two σ-bonds in a single reaction sequence. Many sequential C-H bond addition reactions have been developed to date, including additions across both conjugated and isolated π-systems followed by coupling with reactants such as carbonyl compounds, cyanating reagents, aminating reagents, halogenating reagents, oxygenating reagents, and alkylating reagents. These atom-economical reactions transform ubiquitous C-H bonds under mild conditions to more complex structures with a high level of regiochemical and stereochemical control. Surprising connectivities and diverse mechanisms have been elucidated in the development of these reactions. Given the large number of possible combinations of coupling partners, there are enormous opportunities for the discovery of new sequential C-H bond addition reactions.