Enantioselective Twofold C-H Annulation of Formamides and Alkynes without Built-in Chelating Groups

Nickel-Catalyzed Atroposelective C-H Alkylation Enabled by Bimetallic Catalysis with Air-Stable Heteroatom-Substituted Secondary Phosphine Oxide Preligands

The catalytic asymmetric construction of axially chiral C-N atropisomers remains a formidable challenge due to their low rotational barriers and is largely reliant on toxic, cost-intensive, and precious metal catalysts. In sharp contrast, we herein describe the first nickel-catalyzed atroposelective C-H alkylation for the construction of C-N axially chiral compounds with the aid of a chiral heteroatom-substituted secondary phosphine oxide (HASPO)-ligated Ni-Al bimetallic catalyst. A wide range of alkenes, including terminal and internal alkenes, were well compatible with the reaction, providing a variety of benzimidazole derivatives in high yields and enantioselectivities (up to 97:3 e.r.). The key to success was the identification of novel HASPOs as highly effective chiral preligands. Mechanistic studies revealed the catalyst mode of action, and in-depth data science analysis elucidated the key features of the responsible chiral preligands in controlling the enantioselectivity.

Intermolecular Carbophosphination of Alkynes with Phosphole Oxides via Ni-Al Bimetal-Catalyzed C-P Bond Activation

Intermolecular carbophosphination reaction of alkynes or alkenes with unreactive C-P bonds remains an elusive challenge. Herein, we used a Ni-Al bimetallic catalyst to realize an intermolecular carbophosphination reaction of alkynes with 5-membered phosphole oxides, providing a series of 7-membered phosphepines in up to 94 % yield.

Cu-mediated enantioselective C-H alkynylation of ferrocenes with chiral BINOL ligands

A wide range of Cu(II)-catalyzed C-H activation reactions have been realized since 2006, however, whether a C-H metalation mechanism similar to Pd(II)-catalyzed C-H activation reaction is operating remains an open question. To address this question and ultimately develop ligand accelerated Cu(II)-catalyzed C-H activation reactions, realizing the enantioselective version and investigating the mechanism is critically important. With a modified chiral BINOL ligand, we report the first example of Cu-mediated enantioselective C-H activation reaction for the construction of planar chiral ferrocenes with high yields and stereoinduction. The key to the success of this reaction is the discovery of a ligand acceleration effect with the BINOL-based diol ligand in the directed Cu-catalyzed C-H alkynylation of ferrocene derivatives bearing an oxazoline-aniline directing group. This transformation is compatible with terminal aryl and alkyl alkynes, which are incompatible with Pd-catalyzed C-H activation reactions. This finding provides an invaluable mechanistic information in determining whether Cu(II) cleaves C-H bonds via CMD pathway in analogous manner to Pd(II) catalysts.

Enantioselective Rh(I)-Catalyzed C-H Arylation of Ferroceneformaldehydes

As an important class of platform molecules, planar chiral ferrocene carbonyl compounds could be transformed into various functional groups offering facile synthesis of chiral ligands and catalysts. However, developing efficient and straightforward methods for accessing enantiopure planar chiral ferrocene carbonyl compounds, especially ferroceneformaldehydes, remains highly challenging. Herein, we report a rhodium(I)/phosphoramidite-catalyzed enantioselective C-H bond arylation of ferroceneformaldehydes. Readily available aryl halides such as aryl iodides, aryl bromides, and even aryl chlorides are suitable coupling partners in this transformation, leading to a series of planar chiral ferroceneformaldehydes in good yields and excellent enantioselectivity (up to 83% yield and >99% ee). The aldehyde group could be transformed into diverse functional groups smoothly, and enantiopure Ugi's amine and PPFA analogues could be synthesized efficiently. The latter was found to be a highly efficient ligand in Pd-catalyzed asymmetric allylic alkylation reactions. Mechanistic experiments supported the formation of imine intermediates as the key step during the reaction.

Catalytic Undirected Meta-Selective C-H Borylation of Metallocenes

Metallocenes are privileged backbones in the fields of synthetic chemistry, catalysis, polymer science, etc. Direct C-H functionalization is undoubtedly the simplest approach for tuning the properties of metallocenes. However, owing to the presence of multiple identical C(sp2 )-H sites, this protocol often suffers from low reactivity and selectivity issues, especially for the regioselective synthesis of 1,3-difunctionalized metallocenes. Herein, an efficient iridium-catalyzed meta-selective C-H borylation of metallocenes is reported. With no need of preinstalled directing groups, this approach enables a rapid synthesis of various boronic esters based on benzoferrocenes, ferrocenes, ruthenocene, and related half sandwich complex. A broad range of electron-deficient and -rich functional groups are all compatible with the process. Notably, C-H borylation of benzoferrocenes takes place exclusively at the benzene ring, which is likely ascribed to the shielding effect of pentamethylcyclopentadiene. The synthetic utility is further demonstrated by easy scalability to gram quantities, the conversion of boron to heteroatoms including N3 , SePh, and OAc, as well as diverse cross-coupling reactions.

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.

Pd-Catalyzed Asymmetric Oxidative C-H/C-H Cross-Coupling Reaction between Ferrocenes and Azoles

The asymmetric C-H bond functionalization reaction is one of the most efficient and straightforward methods for the synthesis of optically active molecules. Herein, our work discovered a Pd-catalyzed asymmetric oxidative C-H/C-H cross-coupling reaction of ferrocenes with azoles such as oxazoles and thiazoles. Mono-N-protected amino acid as chiral ligands with palladium(II) has been demonstrated as an effective catalytic system in a weakly azine-directed asymmetric C-H bond functionalization reaction. This method offers a powerful strategy for constructing various substituted planar chiral ferrocenes via a dual C-H bond activation pathway in medium yields (up to 70%) with good enantioselectivity (up to 95.3:4.7 er) under mild conditions.

Palladium-Catalyzed [3 + 2] Annulation of Aromatic Amides with Maleimides through Dual C-H Activation

A palladium-catalyzed [3 + 2] annulation of substituted aromatic amides with maleimides providing tricyclic heterocyclic molecules in good to moderate yields through weak carbonyl chelation is reported. The reaction proceeds via a dual C-H bond activation where the first C-H activation takes place selectively at the benzylic position followed by a second C-H bond activation at the meta position to afford a five-membered cyclic ring. An external ligand Ac-Gly-OH has been used to succeed in this protocol. A plausible reaction mechanism has been proposed for the [3 + 2] annulation reaction.

Site-Selective C-H Alkenylation of N-Heteroarenes by Ligand-Directed Co/Al and Co/Mg Cooperative Catalysis

We report herein the design and development of Co/Al and Co/Mg bimetallic catalysts, supported by a phosphine/secondary phosphine oxide (PSPO) bifunctional ligand, for the site-selective C-H alkenylation of nitrogen-containing heteroarenes with alkynes. These catalysts enable the alkenylation of pyridine, pyridone, and imidazo[1,2-a]pyridine derivatives at the C-H site proximal to the Lewis basic nitrogen or oxygen atom, which represents a selectivity profile distinct from that of the previously developed cobalt-diphosphine/aluminum catalyst. The alkenylated products were obtained in moderate to good yields using various heterocycles and differently substituted internal alkynes. Kinetic isotope effect experiments suggest the irreversibility of the C-H activation step, the relevance of which to the rate-limiting step depends on the reaction conditions. Density functional theory calculations indicate that ligand-to-ligand hydrogen transfer is the common mechanism of C-H activation.

Phosphine Oxide-Promoted Rh(I)-Catalyzed C-H Cyclization of Benzimidazoles with Alkenes

Ligands play a critical role in promoting transition-metal-catalyzed C-H activation reactions. However, owing to high sensitivity of the reactivity of C-H activation to metal catalysts, the development of effective ligands has been a formidable challenge in the field. Rh(I)-catalyzed C-H cyclization of benzimidazoles with alkenes has been faced with low reactivity, often requiring very harsh conditions. To address this challenge, a phosphine oxide-enabled Rh(I)-Al bimetallic catalyst was developed for the reaction, significantly promoting the reactivity and allowing the reaction to run at 120 °C with up to 97% yield.

Ni-catalyzed benzylic β-C(sp3)-H bond activation of formamides

The development of transition metal-catalyzed β-C-H bond activation via highly-strained 4-membered metallacycles has been a formidable task. So far, only scarce examples have been reported to undergo β-C-H bond activation via 4-membered metallacycles, and all of them rely on precious metals. In contrast, earth-abundant and inexpensive 3d transition metal-catalyzed β-C-H bond activation via 4-membered metallacycles still remains an elusive challenge. Herein, we report a phosphine oxide-ligated Ni-Al bimetallic catalyst to activate secondary benzylic C(sp³)-H bonds of formamides via 4-membered nickelacycles, providing a series of α,β-unsaturated γ-lactams in up to 97% yield.

Enantioselective C2-H Alkylation of Pyridines with 1,3-Dienes via Ni-Al Bimetallic Catalysis

A chiral phosphine oxide-ligated Ni-Al bimetallic catalyst was used to realize an enantioselective C2-H alkylation of pyridines without the need of a C2-block. A wide range of pyridines, including unsubstituted pyridine, C3, C4, and C2-substituted pyridines, and even complex pyridine-containing bioactive molecules are well compatible with the reaction, providing up to 81% yield and up to 97% ee.

Recent Advances in Transition-Metal-Catalyzed C-H Functionalization of Ferrocene Amides

During the past decades, in synthetic organic chemistry, directing-group-assisted C-H functionalization is found to be a key tool for the expedient and site-selective construction of C-C and hybrid bonds. Among C-H functionalization of ferrocene derivatives, the directed group strategy is undoubtedly the most commonly used method. Compared to the other directing groups, ferrocene amides can be synthesized easily and are now recognized as one of the most efficient devices for the selective functionalization of certain positions because its metal centre permits fine, tuneable and reversible coordination. The family of amide directing groups mainly comprises monodentate and bidentate directing groups, which are categorized on the basis of coordination sites. In this review, various C-H bond functionalization reactions of ferrocene using amide directing groups are broadly discussed.

Enantio- and Regioselective Ni-Catalyzed para-C-H Alkylation of Pyridines with Styrenes via Intermolecular Hydroarylation

Direct asymmetric functionalization of the pyridyl C-H bond represents a longstanding challenge in organic chemistry. We herein describe the first enantioselective para-C-H activation of pyridines through the use of a Ni-Al bimetallic catalyst system and N-heterocyclic carbene (NHC) ligand for intermolecular hydroarylation of styrenes. The reaction procceds in high to excellent enantioselectivities (up to 98.5:1.5 er) and high site-selectivities for both styrene and pyridine components (up to >98:2). Consequently, a broad range of enantioenriched 1,1-diarylalkanes containing pyridine moieties could be prepared in a single step with 100% atom economy. Computational studies supported a mechanism involving a ligand-to-ligand H-transfer (LLHT) and reductive elimination sequence, with LLHT being the rate- and enantioselectivity-determining step. DFT studies indicate that the π-π stacking interaction between the NHC aryl fragment and trans-styrenes is critical for high reactivity and enantiocontrol.

Electrochemically driven regioselective C-H phosphorylation of group 8 metallocenes

Metallocenes are privileged backbones for synthesis and catalysis. However, the direct dehydrogenative C−H functionalization of unsymmetric metallocenes suffers from reactivity and selectivity issues. Herein, we report an electrochemically driven regioselective C−H phosphorylation of group 8 metallocenes. Mechanistic investigations indicate this dehydrogenative cross coupling occurs through an electrophilic radical substitution of the metallocene with a phosphoryl radical, facilitated by the metallocene itself. This work not only offers an efficient and divergent synthesis of phosphorylated metallocenes, but also provides a guide to interpret the reactivity and regioselectivity for the C−H functionalization of unsymmetric metallocenes.

Metallocene-based phosphines are compounds with potential use in catalysis. Here, the authors report the electrochemical regioselective functionalization of group 8 metallocenes with phosphine oxides; over 60 examples of phosphorylated (benzo)ferrocenes and ruthenocenes can be accessed via this method without the need for a preinstalled directing group.

Recent advances in the synthesis of ferrocene derivatives via 3d transition metal-catalyzed C-H functionalization

In recent years, transition-metal-catalyzed C-H functionalization has gradually developed into a powerful tool for the synthesis of ferrocenes in an atom- and step-economic fashion. However, despite significant achievements, the vast majority of these C-H functionalizations required precious 4d or 5d transition metal catalysts. The use of inexpensive and sustainable 3d metals in the C-H functionalization of ferrocenes remains challenging, especially the development of asymmetric versions. Herein, we summarize the remarkable recent progress in the synthesis of ferrocenes by 3d transition metal-catalyzed C-H activation until December 2021.

Copper-catalyzed monoselective C-H amination of ferrocenes with alkylamines

A copper-catalyzed mono-selective C–H amination of ferrocenes assisted by 8-aminoquinoline is presented here. A range of amines, including bioactive molecules, were successfully installed to the ortho-position of ferrocene amides with high efficiency under mild conditions. A range of functionalized ferrocenes were compatible to give the aminated products in moderate to good yields. The gram-scale reaction was smoothly conducted and the directing group could be removed easily under basic conditions.

Stereoselective rhodium-catalyzed 2-C-H 1,3-dienylation of indoles: dual functions of the directing group

A rhodium-catalyzed intermolecular highly stereoselective 1,3-dienylation at the 2-position of indoles with non-terminal allenyl carbonates has been developed by using 2-pyrimidinyl or pyridinyl as the directing group. The reaction tolerates many functional groups affording the products in decent yields under mild conditions. In addition to C-H bond activation, the directing group also played a vital role in the determination of Z-stereoselectivity for the C-H functionalization reaction with 4-aryl-2,3-allenyl carbonates, which is confirmed by the E-selectivity observed with 4-alkyl-2,3-allenyl carbonates. DFT calculations have been conducted to reveal that π-π stacking involving the directing 2-pyrimidinyl or pyridinyl group is the origin of the observed stereoselectivity. Various synthetic transformations have also been demonstrated.

Construction 7-membered ring via Ni-Al bimetal-enabled C-H cyclization for synthesis of tricyclic imidazoles

The construction of 7-membered ring via direct C7-H cyclization of benzoimidazoles with alkenes would provide a more atom- and step-economical route to tricyclic imidazoles and derivatives that widely exist in a broad range of bioactive molecules. However, transition metal-catalyzed C-H cyclization for medium-ring synthesis has been limited to reactive C-H bonds, instead, the activation of unreactive C-H bonds towards medium synthesis still remains an elusive challenge. Herein, we report a direct construction of 7-membered rings via Ni-Al co-catalyzed unreactive C7-H cyclization of benzoimidazoles with alkenes, providing a series of tricyclic imidazoles in 40-98% yield and with up to 95:5 er.

Ni-Catalyzed Dual C-H Annulation of Benzimidazoles with Alkynes for Synthesis of π-Extended Heteroarenes

Transition metal catalyzed dual C-H activation and annulation with alkynes was an attractive protocol to construct polycyclic π-extended structures. However, most of them were dominated by noble metal catalysts. Disclosed herein was the study of base-metal Ni-catalysis for dual C-H annulation of N-aromatic imidazole, which produced a range of desired polycyclic aza-quinolines in 48-95% yields. The use of bifunctional phosphine oxide ligand proved to be critical for success.

2,3-Dihydroferroceno[3,4]pyrrolo[2,1-b]thiazol-5(8b)-ones: Synthesis, Structure and DFT Study on the Mechanism of Chemo- and Diastereoslective Annulations of (Sp)-2-Formylferrocenecarbonyl Fluoride and (Sp)-2-Formylferrocenecarboxylic Acid

By means of the annulation of easy-to-handle yet sufficiently reactive (S_p)-2-formylferrocenecar- bonyl fluoride with the hydrochlorides of cysteamine and the methyl esters of enentiomer cysteines conducted in dichloromethane at room temperature in the presence of pyridine, the first members of 2,3-dihydroferroceno[3,4]pyrrolo[2,1-b]thiazol-5(8bH)-ones with the elements of planar- and central chirality were prepared as single enantiomers. An atom economic procedure was also elaborated for the synthesis of these organometallic heterocycles directly exploring (S_p)-2-formylferrocenecarboxylic acid in situ activated by CDI and TFA, sequentially added to the reaction mixture. The relative and consequently, the absolute, configuration of the isolated diastereomers was determined by NMR measurements supported by DFT structural optimization. On the basis of the results of synthetic control experiments and a series of further DFT modelling studies, including energetic and MO analysis of the iminium intermediates, we propose a mechanism for the thiazolidine-forming annulations that proceed via primary N-acylation followed by proton-mediated cyclocondensation and subsequent diastereoselective sulfhydryl-attack on the resulting iminium center.

Enantioselective C-H Alkenylation of Ferrocenes with Alkynes by Half-Sandwich Scandium Catalyst

The enantioselective C-H alkenylation of ferrocenes with alkynes is, in principle, a straightforward and atom-efficient route for the construction of planar-chiral ferrocene scaffolds bearing alkene functionality but has remained scarcely explored to date. Here we report for the first time the highly enantioselective C-H alkenylation of quinoline- and pyridine-substituted ferrocenes with alkynes by a half-sandwich scandium catalyst. This protocol features broad substrate scope, high enantioselectivity, and 100% atom efficiency, selectively affording a new family of planar-chiral ferrocenes bearing N/alkene functionalities. The mechanistic details have been clarified by DFT analyses. The use of a quinoline/alkene-functionalized ferrocene product as a chiral ligand for asymmetric catalysis is also demonstrated.

Origins of Lewis acid acceleration in nickel-catalysed C–H, C–C and C–O bond cleavage

The effects of charge transfer, Pauli repulsion and electrostatics/polarization are identified as dominant factors for Lewis acid accelerations in Ni-catalyzed C–X (X = H, C and O) bond cleavages.

Cp*Ir(iii)/chiral carboxylic acid-catalyzed enantioselective C–H alkylation of ferrocene carboxamides with diazomalonates

An achiral Cp*Ir(iii)/chiral carboxylic acid-catalysed enantioselective C–H alkylation of ferrocene carboxamides with diazomalonates was achieved, providing planar chiral alkylated ferrocenes in up to 94 : 6 er.

Pd(ii)-Catalyzed enantioconvergent twofold C–H annulation to access atropisomeric aldehydes: a platform for diversity-oriented-synthesis

The first Pd(ii)-catalyzed atroposelective dual C–H annulative strategy for diverse synthesis of functionalized axially chiral biaryls was developed.

Iodoferrocene as a partner inN-arylation of amides

A general copper-promoted amidation of iodoferrocene is described toward original scaffolds that can be further functionalized by cross-coupling reactions.