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

Nickel(II)/BINOL-catalyzed enantioselective C-H activation via desymmetrization and kinetic resolution

The field of nickel catalysis has witnessed remarkable growth in recent years. However, the use of nickel catalysts in enantioselective C-H activation remains a daunting challenge because of their variable oxidation states, intricate coordination chemistry, and unpredictable reactivity patterns. Herein, we report an enantioselective C-H activation reaction catalyzed by commercially available and air-stable nickel(II) catalyst. Readily available and simple (S)-BINOL is used as a chiral ligand. This operationally simple protocol enables the synthesis of planar chiral metallocenes in high yields with excellent enantioselectivity through desymmetrization and kinetic resolution. Air-stable planar chiral nickelacycle intermediates are first synthesized via enantioselective C-H nickelation and shown to be possible intermediates of the reaction. Deuterium-labeling studies, alongside the characterization and transformation of chiral nickel(II) species, suggest that C-H cleavage is the enantio-determining step. Moreover, the large-scale synthesis and diverse synthetic transformations underscore the practicality of this protocol.

Origins of Ligand-Controlled Stereoselective Polymerization of ortho-Methoxystyrene by Rare-Earth Catalysts: A Theoretical Perspective

The stereoselective polymerization of polar vinyl monomers has recently received much attention due to their excellent physicochemical properties. Over the past decade, breakthroughs have been achieved in this field by rare-earth catalysts. However, the mechanistic origins of those stereoselective polymerizations still remain unclear. Herein, stereoselective polymerization of ortho-methoxystyrene (oMOS) by several representative rare-earth catalysts bearing different ligands (i.e., η5-C5Me5, pyridinyl-methylene-fluorenyl, quinolyl-anilido, β-diketiminato) were systematically investigated by density functional theory (DFT) calculations. After achieving agreement between the calculations and experiments, we focused on discussing the role of ligands in controlling stereoselectivity. Our results reveal that the stereoregularity of oMOS polymerization is mainly controlled by the steric effect of the catalyst-monomer structures. Specifically, the type of ligand influences the orientation and configuration of the inserting monomer, thereby affecting the tacticity of the polymers. In the cases of η5-C5Me5-, pyridinyl-methylene-fluorenyl, and quinolyl-anilido-ligated yttrium catalysts, we observe consistent insertion directions and alternating insertion sides of oMOS monomers, leading to syndiotactic selectivity. The opposite insertion directions and the alternating insertion sides of oMOS monomers were observed in the case of the β-diketiminato yttrium catalyst, leading to isotactic selectivity. These findings reported here offer valuable insights into the role of ligands in controlling stereoselectivity in rare-earth catalyzed coordination polymerization of polar vinyl monomers, thus providing guidance for the rational design of new ligands for stereospecific polymerization of polar monomers in the future.

Mechanistic Insights into Rare-Earth-Catalyzed Alternating Copolymerization through C-H Polyaddition of Functionalized Organic Compounds to Unconjugated Dienes

Density functional theory (DFT) calculations have been conducted to elucidate the detailed mechanisms of yttrium-catalyzed C-H polyaddition of 1,4-dimethoxybenzene (DMB) to 1,4-divinylbenzene (DVB). It was computationally determined that DMB not only serves as a substrate but also performs a crucial role as a ligand, stabilizing the catalytically active species and promoting alkene insertion. Side pathways involving Cβ-H activation and C═C continuous insertion were excluded due to steric and electronic factors, respectively, explaining why the reaction occurred efficiently and selectively to give perfectly alternating DMB-DVB polymers. Interestingly, the theoretical prediction of the reactivity of N,N-dimethyl-1,4-phenylenediamine and 2,2'-biethyl-4,4'-bipyridine reveals significant differences in the coordination effects of these substrates, leading to distinct mechanisms, primarily influenced by their steric effects. These findings shed new light on the previously overlooked role of substrate ligand effects in rare-earth-catalyzed step-growth copolymerization reactions.

Regio- and Diastereoselective Annulation of α,β-Unsaturated Aldimines with Alkenes via Allylic C(sp3)-H Activation by Rare-Earth Catalysts

The [3 + 2] or [4 + 2] annulation of α,β-unsaturated aldimines with alkenes via β'- or γ-allylic C(sp3)-H activation is, in principle, an atom-efficient route for the synthesis of five- or six-membered-ring cycloalkylamines, which are important structural motifs in numerous natural products, bioactive molecules, and pharmaceuticals. However, such a transformation has remained undeveloped to date probably due to the lack of suitable catalysts. We report herein for the first time the regio- and diastereoselective [3 + 2] and [4 + 2] annulations of α,β-unsaturated imines with alkenes via allylic C(sp3)-H activation by half-sandwich rare-earth catalysts having different metal ion sizes. The reaction of α-methyl-substituted α,β-unsaturated aldimines with alkenes by a C5Me4SiMe3-ligated scandium catalyst took place in a trans-diastereoselective [3 + 2] annulation fashion via C(sp3)-H activation at the α-methyl group (β'-position), exclusively affording alkylidene-functionalized cyclopentylamines with excellent trans-diastereoselectivity. In contrast, the reaction of β-methyl-substituted α,β-unsaturated aldimines with alkenes by a C5Me5-ligated cerium catalyst proceeded in a cis-diastereoselective [4 + 2] annulation fashion via γ-allylic C(sp3)-H activation, selectively yielding multisubstituted 2-cyclohexenylamines with excellent cis-diastereoselectivity. The mechanistic details of these transformations have been elucidated by deuterium-labeling experiments, kinetic isotope effect studies, and the isolation and transformations of key reaction intermediates. This work offers an efficient and selective protocol for the synthesis of a new family of cycloalkylamine derivatives, featuring 100% atom efficiency, high regio- and diastereoselectivity, broad substrate scope, and an unprecedented reaction mechanism.

Divergent Synthesis of Multi-Substituted Aminotetralins via [4+2] Annulation of Aldimines with Alkenes by Rare-Earth-Catalyzed Benzylic C(sp3 )-H Activation

The search for efficient and selective methods for the divergent synthesis of multi-substituted aminotetralins is of much interest and importance. We report herein for the first time the diastereoselective [4+2] annulation of 2-methyl aromatic aldimines with alkenes via benzylic C(sp3 )-H activation by half-sandwich rare-earth catalysts, which constitutes an efficient route for the divergent synthesis of both trans and cis diastereoisomers of multi-substituted 1-aminotetralin derivatives from readily accessible aldimines and alkenes. The use of a scandium catalyst bearing a sterically demanding cyclopentadienyl ligand such as C5 Me4 SiMe3 or C5 Me5 exclusively afforded the trans-selective annulation products in the reaction of aldimines with styrenes and aliphatic alkenes. In contrast, the analogous yttrium catalyst, whose metal ion size is larger than that of scandium, yielded the cis-selective annulation products. This protocol features 100 % atom-efficiency, excellent diastereoselectivity, broad substrate scope, and good functional group compatibility. The reaction mechanisms have been elucidated by kinetic isotope effect (KIE) experiments and the isolation and transformations of some key reaction intermediates.

Unveiling the Detailed Mechanism and Origins of Chemo-, Regio-, and Stereoselectivity of Rare-Earth Catalyzed Alternating Copolymerization of Polar and Nonpolar Olefins

The direct copolymerization of polar and nonpolar olefins is of great interest and significance, as it is the most atom-economical and straightforward strategy for the synthesis of functional polyolefin materials. Despite considerable efforts, the precise control of monomer-sequence and their regio- and stereochemistry is full of challenges, and the related mechanistic origins are still in their infancy to date. Herein, the mechanistic studies on the model reaction of Sc-catalyzed co-syndiospecific alternating copolymerization of anisylpropylene (AP) and styrene were performed by DFT calculations. The results suggest that the subtle balance between electronic and steric factors plays an important role during monomer insertions, and a new amino-dissociated mechanism was proposed for AP insertion at chain initiation. AP insertion follows the 2,1-si-insertion pattern, which is mainly controlled by steric factors caused by the restricted MeO···Sc interaction. As for styrene insertion, it prefers the 2,1-re-insertion manner and its regio- and stereoselectivities are influenced by steric repulsions between the inserting styrene and the polymer chain or the ligand. More interestingly, it is found that the alternating monomer-sequence is mainly determined by the "steric matching" principle, which is quantitatively expressed by the buried volume of the metal center of the preinserted species. The concept of steric pocket has been successfully applied to explain the different performances of several catalysts and other alternating copolymerization reactions. The insightful mechanistic findings and the quantitative steric pocket model present here are expected to promote rational design of new rare-earth catalysts for developing regio-, stereo-, and sequence-controlled copolymerization of specific polar and nonpolar olefins.

P(III)-Directed Asymmetric C-H Arylation toward Planar Chiral Ferrocenes by Palladium Catalysis

Planar chiral ferrocenyl phosphines have been employed as highly valuable ligands in metal-catalyzed asymmetric reactions. However, their preparation remains a formidable challenge due to the requirement for intricate, multistep synthetic sequences. In addressing this issue, we have developed a groundbreaking enantioselective C-H activation strategy facilitated by P(III) directing groups, enabling the efficient construction of planar chiral ferrocenyl phosphines in a single step. Our innovative approach entails the combination of a palladium catalyst, a parent ferrocenyl phosphine, and a chiral phosphoramidite ligand, leading to exceptional reactivity and enantioselectivity. Remarkably, these novel ligands exhibit remarkable efficacy in silver-catalyzed asymmetric 1,3-dipolar cycloadditions. We carried out a combination of experimental and computational studies to obtain a more comprehensive understanding of the reaction pathway and the factors contributing to enantioselectivity.

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.

Iridium-Catalyzed Remote Site-Switchable Hydroarylation of Alkenes Controlled by Ligands

An iridium-catalyzed remote site-switchable hydroarylation of alkenes was reported, delivering the products functionalized at the subterminal methylene and terminal methyl positions on an alkyl chain controlled by two different ligands, respectively, in good yields and with good to excellent site-selectivities. The catalytic system showed good functional group tolerance and a broad substrate scope, including unactivated and activated alkenes. More importantly, the regioconvergent transformations of mixtures of isomeric alkenes were also successfully realized. The results of the mechanistic studies demonstrate that the reaction undergoes a chain-walking process to give an [Ar-Ir-H] complex of terminal alkene. The subsequent processes proceed through the modified Chalk-Harrod-type mechanism via the migratory insertion of terminal alkene into the Ir-C bond followed by C-H reductive elimination to afford the hydrofunctionalization products site-selectively.

Diastereo- and Enantioselective Hydrophosphination of Cyclopropenes under Lanthanocene Catalysis

The asymmetric hydrophosphination of cyclopropenes with phosphines is of much interest and importance, but has remained hardly explored to date probably because of the lack of suitable catalysts. We report here the diastereo- and enantioselective hydrophosphination of 3,3-disubstituted cyclopropenes with phosphines by a chiral lanthanocene catalyst bearing the C2 -symmetric 5,6-dioxy-4,7-trans-dialkyl-substituted tetrahydroindenyl ligands. This protocol offers a selective and efficient route for the synthesis of a new family of chiral phosphinocyclopropane derivatives, featuring 100 % atom efficiency, good diastereo- and enantioselectivity, broad substrate scope, and no need for a directing group.

Magnetically Induced Binary Ferrocene with Oxidized Iron

Ferrocene is perhaps the most popular and well-studied organometallic molecule, but our understanding of its structure and electronic properties has not changed for more than 70 years. In particular, all previous attempts of chemically oxidizing pure ferrocene by binding directly to the iron center have been unsuccessful, and no significant change in structure or magnetism has been reported. Using a metal organic framework host material, we were able to fundamentally change the electronic and magnetic structure of ferrocene to take on a never-before observed physically stretched/bent high-spin Fe(II) state, which readily accepts O2 from air, chemically oxidizing the iron from Fe(II) to Fe(III). We also show that the binding of oxygen is reversible through temperature swing experiments. Our analysis is based on combining Mößbauer spectroscopy, extended X-ray absorption fine structure, in situ infrared, SQUID, thermal gravimetric analysis, and energy dispersive X-ray fluorescence spectroscopy measurements with ab initio modeling.

Aryl C-H bond functionalization with diphenyldiazomethane induced by rare-earth metal alkyl complexes

The first examples of regioselective aryl ortho-C-H functionalization with diphenyldiazomethane for the construction of Caryl-Nhydrazinato bonds were accomplished via the activation of C-H bonds and the subsequent reaction of diphenyldiazomethane with the RE-Caryl bond. The reactions of rare-earth metal monoalkyl complexes LRE(CH2SiMe3)(THF)2 (L = 2,5-[(2-pyrrolyl)CPh2]2(N-Me-pyrrole)) supported by a neutral N-methylpyrrole anchored dipyrrolyl ligand with 2 equiv. of Ph2CN2 gave irreversibly unprecedented hydrazonato-functionalized imino rare-earth metal complexes LRE(Ph2CNNC6H4-(o-CNHPh) (RE = Y (2a), Lu (2a')) in good yields involving a rather complex process including the interaction of a diazo unit with a RE-Calkyl bond, a β-H elimination, a N-N cleavage, 1,4-hydrogen transfer and the subsequent C-N coupling with another diphenyldiazomethane. More important is that regioselective aryl C-H bond functionalization with diphenyldiazomethane to construct the Caryl-Nhydrazinato bonds can be easily achieved by three-component reactions of rare-earth metal monoalkyl complexes, a wide range of substituted imines (including aldimines, ketimines or analogous 2-phenylpyridine) and diphenyldiazomethane, affording various hydrazonato-functionalized phenyl, thienyl imino or pyridyl rare-earth metal complexes 2b-2j at room temperature. A further study indicated that the substituents on the phenyl ring have a great effect on the reaction pathway and governed the Caryl-Nhydrazinato bond construction. Moreover, the experimental studies show that the formation of the Caryl-Nhydrazinato bonds is thermodynamically facile, which could be realized at room temperature easily.

Enantioselective Synthesis of 1-Aminoindanes via [3 + 2] Annulation of Aldimines with Alkenes by Scandium-Catalyzed C-H Activation

Multisubstituted chiral 1-aminoindanes are important components in many pharmaceuticals and bioactive molecules. Therefore, the development of efficient and selective methods for the synthesis of chiral 1-aminoindanes is of great interest and importance. In principle, the asymmetric [3 + 2] annulation of aldimines with alkenes through C-H activation is the most atom-efficient and straightforward route for the construction of chiral 1-aminoindanes, but such a transformation has remained undeveloped to date probably due to the lack of suitable catalysts. Herein, we report for the first time the enantioselective [3 + 2] annulation of a wide range of aromatic aldimines and alkenes via ortho-C(sp2)-H activation by chiral half-sandwich scandium catalysts, which provides a straightforward route for the synthesis of multisubstituted chiral 1-aminoindanes. This protocol features 100% atom-efficiency, broad functional group compatibility, and high regio-, diastereo-, and enantioselectivity (up to >19:1 dr and 99:1 er). Remarkably, by fine-tuning the sterics of the chiral ligand around the catalyst metal center, the diastereodivergent asymmetric [3 + 2] annulation of aldimines and styrenes has been achieved with a high level of diastereo- and enantioselectivity, offering an efficient method for the synthesis of both the trans and cis diastereomers of a novel class of chiral 1-aminoindane derivatives containing two contiguous stereocenters from the same set of starting materials. Moreover, the asymmetric [3 + 2] annulation of aldimines with aliphatic α-olefins, norbornene, and 1,3-dienes has also been achieved.

Copper-catalyzed asymmetric C(sp2)-H arylation for the synthesis of P- and axially chiral phosphorus compounds

Transition metal-catalyzed C-H bond functionalization is an important method in organic synthesis, but the development of methods that are lower cost and have a less environmental impact is desirable. Here, a Cu-catalyzed asymmetric C(sp2)-H arylation is reported. With diaryliodonium salts as arylating reagents, a range of ortho-arylated P-chiral phosphonic diamides were obtained in moderate to excellent yields with high enantioselectivities (up to 92% ee). Meanwhile, enantioselective C-3 arylation of diarylphosphine oxide indoles was also realized under similar conditions to construct axial chirality.

Hydroxyl-Directed Rh(III)-Catalyzed C-H Functionalization: Access to Benzo[de]chromenes

A cascade oxidative annulation reaction of heterocyclic ketene aminals (HKAs) with internal alkynes catalyzed by [Cp*RhCl₂]₂ and oxidized by Cu(OAc)₂·H₂O was developed to efficiently synthesize highly functionalized benzo[de]chromene derivatives in good to excellent yields. The reaction proceeded by the sequential cleavage of C(sp²)-H/O-H and C(sp²)-H/C(sp²)-H bonds. These multicomponent cascade reactions were highly regioselective. In addition, all of the benzo[de]chromene products exhibited intense fluorescence emission in the solid state, and they demonstrated concentration-dependent quenching in the presence of Fe³⁺, indicating that these compounds could be used in the recognition of Fe³⁺.

Design and synthesis of a new family of planar and central chiral ferrocenyl phosphine ligands

A family of planar chiral indene-fused ferrocenes were prepared through an intramolecular asymmetric C-H arylation in excellent yields (up to 99%) with excellent enantioselectivities (up to 99% ee). They were thereafter successfully transformed to chiral ferrocenyl phosphines, featuring both planar and central chiralities, in good yields (up to 83%) and excellent diastereoselectivities (up to 99% de) through highly diastereoselective phosphination. This protocol offers a general method for planar and central chiral ferrocenyl phosphines. The potential applications of the newly developed ligands were demonstrated by a Pd-catalyzed enantioselective allylic alkylation reaction, in which high enantioselectivity (92% ee) and good yield (89%) were obtained.

Mechanism and Origin of Site Selectivity and Regioselectivity of Scandium-Catalyzed Benzylic C-H Alkylation of Tertiary Anilines with Alkenes

Benzylic C(sp³)-H alkylation of tertiary anilines with alkenes by an anilido-oxazoline-ligated scandium alkyl catalyst was recently reported with C-H site selectivity and alkene-dependent regioselectivity. Revealing the mechanism and origin of selectivity is undoubtedly of great importance for understanding experimental observations and developing new reactions. Herein, density functional theory (DFT) calculations have been carried out on the model reaction of Sc-catalyzed benzylic C(sp³)-H alkylation of N,N-dimethyl-o-toluidine with allylbenzene. The reaction generally undergoes the generation of active species, alkene insertion, and protonation steps. The difference of the distortion energy of the aniline moiety in transition states, which is related to the ring size of the forming metallacycles, accounts for the site selectivity of C-H activation. Benzylic C(sp³)-H activation possessing less strained five-membered metallacycle compared to the ortho-C(sp²)-H and α-methyl C(sp³)-H activation results in benzylic C(sp³)-H alkylation observed experimentally. Both steric and electronic factors are responsible for the 1,2-insertion regioselectivity for alkyl-substituted alkenes, while electronic factors control the 2,1-insertion manner for vinylsilanes. The analysis of original alkene substrates further strengthens the understanding of the alkene-dependent regioselectivity. These results help us to obtain the mechanistic understanding and are expected to be conducive to the development of new C-H functionalization reactions.

Substrate Facilitating Roles in Rare-Earth-Catalyzed C-H Alkenylation of Pyridines with Allenes: Mechanism and Origins of Regio- and Stereoselectivity

Although considerable progress has been achieved in C-H functionalization by cationic rare-earth alkyl complexes, the potential facilitating roles of heteroatom-containing substrates during the catalytic cycle remain highly underestimated. Herein, theoretical studies on the model reaction of C(sp²)-H addition of pyridines to allenes by scandium catalyst were carefully carried out to reveal the detailed mechanism. A coordinating pyridine substrate as a ligand can effectively stabilize some key structures. An obvious facilitating role delivered by the coordinating pyridine was found for allene insertion, while the pyridine-free mechanism prefers to occur for C(sp²)-H activation processes. Importantly, the elusive role of heteroatom-containing substrates was systematically revealed for the C-H activation event by designing a metal/ligand combination of catalysts and substrates. We found that the pyridyl C(sp²)-H activation would be switched to the pyridine-coordinated mechanism in the cases of the designed Y and La catalysts. To date, this is the first time to realize the potential substrate-facilitating role in cationic rare-earth-catalyzed C-H activation processes. Moreover, theoretical predictions show that similar switchable mechanisms also work for other types of C-H bonds and other heteroatom-involved substrates by fine-adjusting the steric surroundings of catalysts. The two C-H activation mechanisms are mainly the result of the delicate balance between electronic and steric factors. In general, the catalytic system with less steric hindrance prefers to undergo the substrate-coordinated mechanism. In contrast, the substrate-free mechanism is favorable due to steric repulsion. These results are helpful for us to better understand the variant mechanisms in rare-earth-catalyzed C-H functionalization at the atomistic level and may help guide the rational design of new catalytic reactions. In addition, the origins of the regio- and stereoselectivity were discussed through geometric parameters and distortion/interaction analysis.

Advanced Application of Planar Chiral Heterocyclic Ferrocenes

This manuscript is reviewing the superior catalytic activity and selectivity of ferrocene ligands in a wide range of reactions: reduction of ketones, hydrogenation of olefins, hydroboration, cycloaddition, enantioselective synthesis of biaryls, Tsuji–Trost allylation. Moreover, the correlation between a ligand structure and its catalytic activity is discussed in this review.

Synthesis of allylanilines via scandium-catalysed benzylic C(sp3)-H alkenylation with alkynes

The ortho-selective benzylic C(sp³)-H alkenylation of 2-methyl tertiary anilines with internal alkynes has been achieved for the first time by using a half-sandwich scandium catalyst. This protocol provides a straightforward route for the synthesis of a new family of 2-allylaniline derivatives, featuring broad substrate scope, 100% atom-efficiency, high yields, and high chemo-, regio-, and stereoselectivity.

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.

Scandium-Catalyzed Benzylic C(sp3)-H Alkenylation of Tertiary Anilines with Alkynes

This work describes the chemo- and stereoselective benzylic C(sp³)-H alkenylation of tertiary ortho-methyl anilines with internal alkynes using a simple β-diketiminato scandium catalyst. This protocol offers an efficient method for the synthesis of a new family of tertiary ortho-allylanilines in high yields. The resultant alkenylation products facilely underwent further chemical transformation to other valuable anilines. A cationic scandium benzyl species was isolated from a stoichiometric reaction and confirmed to be the catalytic intermediate.

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.

Mechanistic insights into rare-earth-catalysed C-H alkylation of sulfides: sulfide facilitating alkene insertion and beyond

The catalytic C-H alkylation with alkenes is of much interest and importance, as it offers a 100% atom efficient route for C-C bond construction. In the past decade, great progress in rare-earth catalysed C-H alkylation of various heteroatom-containing substrates with alkenes has been made. However, whether or how a heteroatom-containing substrate would influence the coordination or insertion of an alkene at the catalyst metal center remained elusive. In this work, the mechanism of Sc-catalysed C-H alkylation of sulfides with alkenes and dienes has been carefully examined by DFT calculations, which revealed that the alkene insertion could proceed via a sulfide-facilitated mechanism. It has been found that a similar mechanism may also work for the C-H alkylation of other heteroatom-containing substrates such as pyridine and anisole. Moreover, the substrate-facilitated alkene insertion mechanism and a substrate-free one could be switched by fine-tuning the sterics of catalysts and substrates. This work provides new insights into the role of heteroatom-containing substrates in alkene-insertion-involved reactions, and may help guide designing new catalysis systems.

Coordination-assisted, transition-metal-catalyzed enantioselective desymmetric C–H functionalization

Recent advances in transition-metal-catalyzed enantioselective desymmetric C–H functionalization are summarized.

Recent progress in rare-earth metal-catalyzed sp2 and sp3 C–H functionalization to construct C–C and C–heteroelement bonds

The review presents rare-earth metal-catalyzed C(sp2/sp3)–H functionalization accessing C–C/C–heteroatom bonds and olefin (co)polymerization, highlighting substrate scope, mechanistic realization, and origin of site-, enantio-/diastereo-selectivity.

Regio- and Diastereoselective [3+2] Annulation of Aliphatic Aldimines with Alkenes by Scandium-Catalyzed β-C(sp3 )-H Activation

Here we report for the first time the regio- and diastereoselective [3+2] annulation of a wide range of aliphatic aldimines with alkenes via the activation of an unactivated β-C(sp³ )-H bond by half-sandwich scandium catalysts. This protocol offers a straightforward and atom-efficient route for the synthesis of a new family of multi-substituted aminocyclopentane derivatives from easily accessible aliphatic aldimines and alkenes. The annulation of aldimines with styrenes exclusively afforded the 5-aryl-trans-substituted 1-aminocyclopentane derivatives with excellent diastereoselectivity through the 2,1-insertion of a styrene unit. The annulation of aldimines with aliphatic alkenes selectively gave the 4-alkyl-trans-substituted 1-aminocyclopentane products in a 1,2-insertion fashion. A catalytic amount of an appropriate amine such as adamantylamine (AdNH₂ ) or dibenzylamine (Bn₂ NH) showed significant effects on the catalyst activity and stereoselectivity.

Modular Access to Spiro-dihydroquinolines via Scandium-Catalyzed Dearomative Annulation of Quinolines with Alkynes

The catalytic enantioselective construction of three-dimensional molecular architectures from planar aromatics such as quinolines is of great interest and importance from the viewpoint of both organic synthesis and drug discovery, but there still exist many challenges. Here, we report the scandium-catalyzed asymmetric dearomative spiro-annulation of quinolines with alkynes. This protocol offers an efficient and selective route for the synthesis of spiro-dihydroquinoline derivatives containing a quaternary carbon stereocenter with an unprotected N-H group from readily accessible quinolines and diverse alkynes, featuring high yields, high enantioselectivity, 100% atom-efficiency, and broad substrate scope. Experimental and density functional theory studies revealed that the reaction proceeded through the C-H activation of the 2-aryl substituent in a quinoline substrate by a scandium alkyl (or amido) species followed by alkyne insertion into the Sc-aryl bond and the subsequent dearomative 1,2-addition of the resulting scandium alkenyl species to the C═N unit in the quinoline moiety. This work opens a new avenue for the dearomatization of quinolines, leading to efficient and selective construction of spiro molecular architectures that were previously difficult to access by other means.

Rhodium-Catalyzed Dehydrogenative Annulation of N-Arylmethanimines with Vinylene Carbonate for Synthesizing Quinolines

Here we report a novel Rh-catalyzed C-H/C-H alkenylation of N-arylmethanimines with vinylene carbonate acting as a vinylene unit. Forty examples of C3,C4-nonsubstituted quinolines were achieved from commercially available starting materials. This identified process features an exceedingly simple system, a lower loading of catalyst, and the capacity for postfunctionalization with bioactive molecules.

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.

CpxM(iii)-catalyzed enantioselective C-H functionalization through migratory insertion of metal-carbenes/nitrenes

Cp^xM(iii)-catalyzed enantioselective C-H functionalization reactions have progressed rapidly using either chiral cyclopentadienyl ligands or appropriate chiral carboxylic acids. In this context, highly reactive carbene and nitrene precursors can serve as effective C-H coupling partners, providing a straightforward and efficient approach to access chiral molecules. In this review, we highlight the developments in Cp^xM(iii)-catalyzed enantioselective C-H functionalization reactions through migratory insertion of metal-carbenes/nitrenes by employing chiral Cp^xM(iii) complexes or achiral Cp^xM(iii) complexes combined with chiral carboxylic acids.

Asymmetric synthesis of chiral organosilicon compounds via transition metal-catalyzed stereoselective C-H activation and silylation

This feature article details the progress of transition metal-catalyzed stereoselective sp² and sp³ C-H activation and silylation in the synthesis of chiral organosilicon compounds, and the asymmetric C-H silylation includes intramolecular cyclizing silylation and intermolecular silylation. The silylating reagents include monohydrosilanes, dihydrosilanes, silacylcobutanes and disilanes. In general, catalytic systems include a transition metal salt as the catalyst and a chiral ligand. No external chiral ligand is required in some cases where the chiral substrates act as the source of chirality. Many kinds of silylated compounds with central, axial, planar, or helical chirality have been constructed via C-H activation by asymmetric rhodium, iridium or palladium catalysis. Some pharmacophores and material building blocks were successfully introduced into the target molecules. Some silylated products proved to be useful in medicinal chemistry, synthetic organic chemistry, and materials science. Besides reaction development, mechanisms for stereoselective C-H activation and silylation are also discussed.

On the application of 3d metals for C-H activation toward bioactive compounds: The key step for the synthesis of silver bullets

Several valuable biologically active molecules can be obtained through C-H activation processes. However, the use of expensive and not readily accessible catalysts complicates the process of pharmacological application of these compounds. A plausible way to overcome this issue is developing and using cheaper, more accessible, and equally effective catalysts. First-row transition (3d) metals have shown to be important catalysts in this matter. This review summarizes the use of 3d metal catalysts in C-H activation processes to obtain potentially (or proved) biologically active compounds.

Oxygen-Linked Cyclopentadienyl Rhodium(III) Complexes-Catalyzed Asymmetric C-H Arylation of Benzo[h]quinolines with 1-Diazonaphthoquinones

Chiral cyclopentadienyl rhodium (CpRh) complex-catalyzed asymmetric C-H functionalization reactions have witnessed a significant progress in organic synthesis. In sharp contrast, the reported chiral Cp ligands are limited to C-linked Cp and are often synthetically challenging. To address these issues, we have developed a novel class of tunable chiral cyclopentadienyl ligands bearing oxygen linkers, which were efficient catalysts for C-H arylation of benzo[h]quinolines with 1-diazonaphthoquinones, affording axially chiral heterobiaryls in excellent yields and enantioselectivity (up to 99 % yield, 98.5:1.5 er). Mechanistic studies suggest that the reaction is likely to proceed by electrophilic C-H activation, and followed by coupling of the cyclometalated rhodium(III) complex with 1-diazonaphthoquinones.