Construction of All-Carbon Quaternary Stereocenters by Scandium-Catalyzed Intramolecular C-H Alkylation of Imidazoles with 1,1-Disubstituted Alkenes

Enantioselective ortho-C-H Addition of Aromatic Amines to Alkenes by Bulky Chiral Anilido-Oxazoline Scandium Complexes

The enantioselective C-H addition of anilines to alkenes represents an ideal protocol for the synthesis of chiral aromatic amines in terms of step- and atom-economy. However, this field remains predominantly unexplored. Herein, a series of newly designed bulky chiral anilido-oxazoline ligand precursors were synthesized, and the corresponding rare-earth metal alkyl complexes were obtained successfully. The resultant scandium complexes exhibit high regioselectivity for the ortho-C-H addition of tertiary anilines to unactivated alkenes, providing a wide range of chiral alkylated anilines in high yields (up to 98% yield) with excellent enantioselectivity (up to 98% ee). Moreover, the addition products can be easily converted into biorelevant derivatives and pharmacophore-containing skeletons.

Heteroatom-Assisted Regio- and Stereoselective Hydrosilylation of Unsymmetric Internal Alkynes by Scandium Catalyst

The catalytic hydrosilylation of alkynes with hydrosilanes is the most straightforward and atom-efficient method for the synthesis of silylalkenes. However, the hydrosilylation of unsymmetrical internal alkynes often encounters regio- and stereoselectivity challenges. Herein, we report the regio- and syn-stereoselective hydrosilylation of unsymmetrical internal alkynes bearing heteroatom functional groups with hydrosilanes by half-sandwich scandium catalyst. This protocol offers an atom-efficient route for the synthesis of a new family of heteroatom (O, S or N)-functionalized multisubstituted silylalkenes from a variety of internal homopropargyl thioethers, ethers and tertiary amines and hydrosilanes, featuring 100 % atom-efficiency, broad substrate scope, and excellent regio- and syn-stereoselectivity (>19 : 1 r.r. and >19 : 1 syn/anti). The mechanistic details have been elucidated by control experiments and isolation and examination of some key reaction intermediates. It was revealed that an interaction between the heteroatom (O, S or N) in the internal alkynes and the Sc center was critical for achieving the unprecedented high selectivity.

Photo-induced decarboxylative radical cascade cyclization of unactivated alkenes: access to CF- and CF2-substituted ring-fused imidazoles

A mild and effective visible-light-induced decarboxylative radical cascade reaction of olefin-containing imidazoles with α-fluorinated carboxylic acids as building blocks containing CF or ArCF2 moieties, has been developed to afford a series of monofluoromethylated or aryldifluoromethylated polycyclic imidazoles in medium to excellent yields with features of simple operation, available raw materials, and wide substrate scopes. In addition, the mechanistic experiments indicated that the methodology involved a radical pathway.

Mechanisms and Origins of Regioselectivity in Rare-Earth-Catalyzed C-H Functionalization of Anisoles and Thioanisoles

The direct catalytic C-H functionalization of aromatic compounds such as anisoles and thioanisoles is of great interest and significance. However, achieving precise regioselectivity remains a major challenge. In this study, we conducted comprehensive density functional theory calculations to explore the mechanisms of rare-earth-catalyzed regioselective C-H alkylation, borylation, and silylation of anisole and thioanisole. The results reveal that in cationic C-H alkylation systems, the alkene insertion step follows a substrate-assisted mechanism, in which an additional substrate molecule acts as a ligand to facilitate the transformation. In neutral C-H borylation and silylation systems, although mononuclear hydride species readily dimerize into binuclear hydride species due to thermodynamic stability, the catalytic process predominantly proceeds via a mononuclear pathway. Furthermore, the origins of regioselectivity were thoroughly elucidated. A detailed analysis of electronic and steric effects in related transition states reveals that, for anisole, regioselectivity is primarily governed by ring strain. Since α-C(sp3)-H activation involves the formation of a highly strained three-membered ring, the reaction preferentially occurs at the ortho-C(sp2)-H site, forming a less strained four-membered ring. In contrast, for thioanisole, electronic effects play a decisive role, driving C-H activation at the more negatively charged α-C(sp3) site due to stronger metal-carbon interactions.

Regio- and Stereoselective Hydroalkynylation of Internal Alkynes with Terminal Alkynes by Half-Sandwich Rare-Earth Catalysts

The regio- and stereoselective hydroalkynylation of internal alkynes with terminal alkynes is of great interest and importance as a straightforward route for synthesizing multisubstituted 1,3-enynes. However, this transformation often suffers from regio- and stereoselectivity issues when working with unsymmetrical internal alkynes. Herein, we report for the first time the regio- and syn-stereoselective hydroalkynylation of a variety of heteroatom-functionalized unsymmetrical internal alkynes including homopropargyl ethers, thioethers, and tertiary amines with terminal alkynes by half-sandwich rare-earth catalysts. This protocol provides an atom-efficient and straightforward route for the synthesis of a new family of heteroatom (O, S, or N)-functionalized 1,3-enynes, featuring 100% atom-efficiency, broad substrate scope, and high regio- and syn-stereoselectivity (>19:1 r.r. and >19:1 syn/anti). The mechanistic details have been elucidated by deuterium-labeling experiments, control experiments, and isolation and transformations of key reaction intermediates, revealing that the reaction proceeded through the C(sp)-H deprotonation of a terminal alkyne by a half-sandwich scandium alkyl species to form a catalytically active dimeric half-sandwich scandium tetraalkynyl species followed by heteroatom-assisted insertion of internal alkyne into the Sc-alkynyl bond and the subsequent protonolysis of the resulting Sc-alkenyl bond with another terminal alkyne molecule. The coordination of the heteroatom (O, S, or N) of internal alkynes to the catalyst metal center plays a critically important role in achieving a high level of reactivity and regio- and stereoselectivity. Remarkably, the catalytically active dimeric half-sandwich scandium tetraalkynyl species can be recovered and reused, constituting the first example of a recyclable catalyst system for the hydroalkynylation of internal alkynes.

Theoretical Insights into Rare-Earth-Catalyst-Controlled Diastereo- and Enantioselective [3 + 2] Annulation of Aromatic Aldimines with Styrenes

Rare-earth-catalyzed annulation reactions using alkenes via C-H activation offer an atom-efficient approach to constructing cyclic compounds. However, the mechanisms underlying these reactions remain poorly understood, limiting the rational design of related catalytic systems. Recently, Hou and Cong reported an unprecedented example of rare-earth-catalyst-controlled diastereodivergent asymmetric [3 + 2] annulation of aromatic aldimines with alkenes. To elucidate the mechanisms and the origins of diastereo- and enantioselectivity, density functional theory calculations were performed. The results revealed that the styrene insertion step determines the stereoselectivity. Styrene insertion follows a similar metal-styrene interaction pattern across different catalysts. Specifically, during cis-insertion, styrene interacts strongly with the metal center, exhibiting significant Sc···Ph interactions, whereas such interactions are absent during trans-insertion. Thus, when the catalyst is employed with a small ligand, stereoselectivity is primarily governed by electronic factors, favoring the cis-insertion mode. In contrast, for the more sterically hindered catalyst, the Sc···Ph interactions in cis-insertion are insufficient to overcome the steric effects, leading to a preference for the trans-insertion mode, which minimizes steric hindrance. These findings offer deeper insights into the origins of catalyst-controlled diastereo- and enantioselectivity and will also contribute to the rational design of stereospecific annulation reactions in rare-earth catalysis.

Enantioselective [3+2] Annulation of Aldimines with Alkynes by Scandium-Catalyzed C-H Activation

The enantioselective [3+2] annulation of readily accessible aldimines with alkynes via C-H activation is, in principle, a straightforward and atom-efficient route for synthesizing chiral 1-aminoindenes, which are important components in a wide array of natural products, bioactive molecules, and functional materials. However, such asymmetric transformation has remained undeveloped to date due to the lack of suitable chiral catalysts. Here, we report for the first time the enantioselective [3+2] annulation of aldimines with alkynes via C-H activation using chiral half-sandwich scandium catalysts. This protocol enabled the synthesis of diverse multi-substituted chiral 1-aminoindene derivatives with 100 % atom-efficiency, broad substrate scope, and high regio- and enantioselectivity. Density functional theory (DFT) analyses have revealed that a noncovalent C-H⋅⋅⋅π interaction between a tert-Bu substituent in the chiral cyclopentadienyl (Cp) ligand and the phenyl ring of an aromatic aldimine substrate played an important role in achieving a high level of enantioselectivity. This work not only offers an efficient and selective route for synthesizing a new family of chiral 1-aminoindene derivatives but also offers unprecedented insights into enantioselectivity control in chiral Cp-ligated metal catalysts.

Enantioselective Construction of Oxindoles Bearing a Quaternary Carbon via Ni-Al Bimetal-Catalyzed Formyl C-H Alkylation

Enantioselective transition metal-catalyzed C-H alkylation emerges as one of the most atom- and step-economical routes to chiral quaternary carbons, while big challenges still remain with acyl C-H alkylations. Herein, we use a Ni-Al bimetallic catalyst to facilitate a highly regioselective and highly enantioselective C-H alkylation of formamides with alkenes, constructing various oxindoles bearing a chiral quaternary carbon in up to 94 % yield and up to 95 % ee.

Silver-mediated radical cascade trifluoromethylthiolation/cyclization of benzimidazole derivatives with AgSCF3

A silver-mediated cascade trifluoromethylthiolation/cyclization of unactivated alkenes has been investigated. This strategy employs AgSCF3 as the trifluoromethylthiolating reagent to obtain a variety of useful trifluoromethylthiolated tricyclic imidazol derivatives in reasonable yields. Preliminary mechanistic studies indicate that the present reaction takes place via a radical process. This method is distinguished by its atom economy, wide functional group compatibility, operational simplicity and product diversity.

(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.

Biocatalytic Synthesis of Nitrile-Bearing All-Carbon Quaternary Stereocenters

The synthesis of all-carbon quaternary stereocenters containing nitriles is a very important and challenging subject in organic chemistry. We used a biocatalytic approach under mild conditions to obtain new derivatives of these scaffolds by oxidation of catechols by Myceliophthora thermophila laccase (Novozym 51003) to afford o-quinones and 1,4-addition of a series of carbon nucleophiles containing tertiary alkyle nitriles to these intermediates. Using this approach, α-cyano carbonyls bearing a quaternary stereocenter were also prepared. Finally, the yields for the prepared compounds were 72-94%.

Visible Light-Induced Radical Cascade Difluoromethylation/Cyclization of Unactivated Alkenes: Access to CF2H-Substituted Polycyclic Imidazoles

An efficient and mild protocol for the visible light-induced radical cascade difluoromethylation/cyclization of imidazoles with unactivated alkenes using easily accessible and bench-stable difluoromethyltriphenylphosphonium bromide as the precursor of the -CF2H group has been developed to afford CF2H-substituted polycyclic imidazoles in moderate to good yields. This strategy, along with the construction of Csp3-CF2H/C-C bonds, is distinguished by mild conditions, no requirement of additives, simple operation, and wide substrate scope. In addition, the mechanistic experiments have indicated that the difluoromethyl radical pathway is essential for the methodology.

A One-Pot Cascade Strategy toward Organocatalytic Enantioselective Construction of Fused Benzimidazoles

Herein, we describe an asymmetric assembly of ortho-aromatic diamines and formyl tethered Michael acceptors forming chiral fused benzimidazoles. A cinchona-alkaloid-derived bifunctional squaramide catalyst enables the methodology through on-site dihydrobenzimidazole formation followed by an aza-Michael addition/oxidation cascade. This protocol stands out for its excellent catalytic efficiency over the background reaction and its mild conditions, making it more practical. Various Michael acceptors, including enones, ester, and thioester, were successful substrates in this study. Additionally, this methodology has demonstrated scalability and successfully showcased postsynthetic transformations.

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.

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.

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.

Rhodium-catalyzed atroposelective access to trisubstituted olefins via C-H bond olefination of diverse arenes

The atroposelective synthesis of axially chiral acyclic olefins remains a daunting challenge due to their relatively lower racemization barriers, especially for trisubstituted ones. In this work, atroposelective C-H olefination has been realized for synthesis of open-chain trisubstituted olefins via C-H activation of two classes of (hetero)arenes in the coupling with sterically hindered alkynes. The employment of phenyl N-methoxycarbamates as arene reagents afforded phenol-tethered olefins, with the carbamate being a traceless directing group. The olefination of N-methoxy-2-indolylcarboxamides afforded the corresponding chiral olefin by circumventing the redox-neutral [4 + 2] annulation. The reactions proceeded with excellent Z/E selectivity, chemoselectivity, regioselectivity, and enantioselectivity in both hydroarylation systems.

Synthesis of 1,2-Fused Benzimidazoles by Amine-Initiated [3 + 3] Annulations of β'-Acetoxy Allenoates with 1C,3N-Bisnucleophiles

The amine-catalyzed [3 + 3] annulations of β'-acetoxy allenoates with 1C,3N-bisnucleophiles have been established. Under the optimal reaction conditions, this operationally simple synthetic process works well with a wide substrate scope, delivering novel 1,2-fused benzimidazole derivatives in moderate to good yields. In addition, preliminary attempts on the asymmetric version of this reaction have been explored by using cinchona alkaloid-based tertiary amines.

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.

Biocatalytic stereoselective synthesis of pyrrolidine-2,3-diones containing all-carbon quaternary stereocenters

Highly functionalized pyrrolidine-2,3-diones can be synthesized efficiently and stereoselectively under mild conditions using a biocatalytic approach. The reaction led to the formation of new all-carbon quaternary stereocenters from Myceliophthora thermophila laccase (Novozym 51003) catalyzed oxidation of catechols to ortho-quinones and subsequent 1,4-addition with 3-hydroxy-1,5-dihydro-2H-pyrrol-2-ones. The reaction was conducted with various substituents on both reactants, resulting in 13 products in moderate to good yields (42-91%). The same 15 reactions were also tested with K₃Fe(CN)₆ as a catalyst, but here only one reaction resulted in a product (60% yield).

Catalyst-Controlled Nickel-Catalyzed Intramolecular endo-Selective C-H Cyclization of Benzimidazoles with Alkenes

Compared with the widely explored exo-selective C-H cyclization, transition metal-catalyzed endo-selective C-H cyclization of benzimidazoles with alkenes has been a formidable challenge. Previous efforts mainly rely on substrate-controlled methods, rendering the product complexity restricted. Herein we report a catalyst-controlled method to facilitate endo-cyclization, in which a bulky N-heterocyclic carbene ligand and ^tBuOK base-enabled Ni-Al bimetallic catalyst prove critical to the endo selectivity.

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.

Iridium-Catalyzed Branch-Selective and Enantioselective Hydroalkenylation of α-Olefins through C-H Cleavage of Enamides

Catalytic branch-selective hydrofunctionalization of feedstock α-olefins to form enantioenriched chiral compounds is a particularly attractive yet challenging transformation in asymmetric catalysis. Herein we report an iridium-catalyzed asymmetric hydroalkenylation of α-olefins through directed C-H cleavage of enamides. This atom-economical addition process is highly branch-selective and enantioselective, delivering trisubstituted alkenes with an allylic stereocenter. DFT calculations reveal the origin of regio- and enantioselectivity.

Transition-metal-catalyzed C-H bond alkylation using olefins: recent advances and mechanistic aspects

Transition metal catalysis has contributed immensely to C-C bond formation reactions over the last few decades, and alkylation is no exception. The superiority of such methodologies over traditional alkylation is evident from minimal reaction steps, shorter reaction times, and atom economy while also allowing control over regio- and stereo-selectivity. In particular, hydrocarbonation of alkenes has grabbed increased attention due its fundamental ability to effectively and selectively synthesise a wide range of industrially and pharmaceutically relevant moieties. This review attempts to provide a scientific viewpoint and a systematic analysis of the recent developments in transition-metal-catalyzed alkylation of various C-H bonds using simple and activated olefins. The key features and mechanistic studies involved in these transformations are described briefly.

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.

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.

Metal-catalyzed asymmetric heteroarylation of alkenes: diverse activation mechanisms

This review summarizes the state-of-the-art in transition metal-catalyzed asymmetric alkylation of heteroarenes using alkenes (covering literature from 2000 to late 2021). Based on elementary reactions on metals for substrate activation, these reactions are broadly classified in several categories: (A) concerted oxidative addition of heteroaryl C-H bonds on rhodium(I) and iridium(I), (B) ligand-to-ligand hydrogen transfer (LLHT) on low-valent 3d metal complexes of nickel and cobalt, (C) different ways for deprotonation of heteroaryl C-H bonds by late transition metal complexes, especially palladium, including electrophilic aromatic substitution and a related mechanism, base-assisted intramolecular electrophilic substitution, concerted and nonconcerted metalation deprotonation, (D) σ-bond metathesis by d⁰ early transition metal complexes, (E) electrophilic activation of olefins by Pd(II), Pt(II) and Au(I), and (F) metal hydride insertion of aryl olefins and dienes. The demand to achieve enantiocontrol in the heteroarylation reactions has also driven innovation in chiral ancillary ligands, exemplified by extremely bulky, chiral N-heterocyclic carbenes for nickel catalysts, bulky monodentate oxazolines for Wacker-type reactions and chiral cyclopentadienyl ligands for half-sandwich complexes of scandium.

Visible light-mediated synthesis of α-alkoxy/hydroxy diarylacetaldehydes from terminal alkynes

A visible light-mediated approach enabling the use of alcohols as nucleophiles in a one-step synthesis of α-alkoxy/hydroxy diarylacetaldehydes is reported.

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.

Recent advances in catalytic enantioselective direct C–H bond functionalization of electron-deficient N-containing heteroarenes

Catalytic enantioselective direct C–H bond functionalization of electron-deficient N-containing heteroarenes represents one of the most straightforward and powerful protocols to construct diverse enantioenriched highly functionalized N-heteroarenes.

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.

Visible-Light-Induced Radical Cascade Reaction of 1-Allyl-2-ethynylbenzoimidazoles with Thiosulfonates to Assemble Thiosulfonylated Pyrrolo[1,2-a]benzimidazoles

A visible-light-induced radical domino reaction of 1-allyl-2-ethynylbenzoimidazoles with thiosulfonates was developed, which generated the thiosulfonylated pyrrolo[1,2-a]benzimidazoles in moderate to good yields. This reaction proceeded under transition-metal-free conditions with good functional group tolerance and high regioselectivity. The possible pathway involved thiosulfonates were activated through the energy transfer route promoted by photocatalysis.

Cp*Co(III)-Catalyzed Enantioselective Hydroarylation of Unactivated Terminal Alkenes via C-H Activation

Enantioselective hydroarylation of unactivated terminal akenes constitutes a prominent challenge in organic chemistry. Herein, we reported a Cp*Co(III)-catalyzed asymmetric hydroarylation of unactivated aliphatic terminal alkenes assisted by a new type of tailor-made amino acid ligands. Critical to the chiral induction was the engaging of a novel noncovalent interaction (NCI), which has seldomly been disclosed in the C-H activation area, arising from the molecular recognition among the organocobalt(III) intermediate, the coordinated alkene, and the well-designed chiral ligand. A broad range of C2-alkylated indoles were obtained in high yields and excellent enantioselectivities. DFT calculations revealed the reaction mechanism and elucidated the origins of chiral induction in the stereodetermining alkene insertion step.