Sequential C–H and C–C Bond Cleavage: Divergent Constructions of Fused N-Heterocycles via Tunable Cascade

Rhodium(III)-Catalyzed Sequential Cyclization of Enaminones with 1,3-Dienes via C-H Activation for the Synthesis of Fluorenones

An efficient method for construction of various fluorenones has been achieved via Rh(III)-catalyzed C-H activation/[4 + 2] annulation/aromatization sequences of simple and readily available enaminones and 1,3-dienes. This protocol showed good substrate compatibility as an array of structurally and electronically diverse fluorenones prepared efficiently in moderate to good yields and preparative scale utility showing very good efficiency in the late-stage functionalization of complex valuable molecules.

Synthesis of CF3-Azafluorenes through the Cascade Reaction of 2H-Imidazoles with CF3-Ynones

A concise synthesis of trifluoromethyl (CF3)-substituted azafluorenes based on the reaction of 5-aryl-2H-imidazoles with CF3-ynones is reported. The reaction proceeds through C-H activation-initiated formal [3+2] cycloaddition to give spiro[imidazole-4,1'-indene] as a key intermediate, followed by its retro [3+2] cycloaddition, isomerization, and enamine nucleophilic addition. This synthesis of CF3-azafluorene derivatives via simultaneous formation of both the indene and the pyridine scaffolds through cascade C-H/C-C/C-N bond cleavage and C≡C bond formation has not been disclosed before. Moreover, the products thus obtained showed antiproliferative activity against cancer cell lines.

Access to Functionalized Amines and Medium N-Heterocycles via Amine-Enabled Remote C-H Alkynylation

By using weakly coordinating amines, we developed remote C-H alkynylation with precise control of reactivity and regioselectivity, enabling modification of complex drugs, natural products, and materials. The readily transformable alkyne-containing amine products would facilitate expedient delivery of molecular libraries of functionalized amines and medium N-heterocycles, which are previously elusive to access. Moreover, the introduced alkyne functionality could serve as a versatile handle to expand the diversity and synthetic application of this remote C-H functionalization.

Ru(ii)-catalyzed regioselective oxidative Heck reaction with internal olefins that tolerated strongly coordinating heterocycles

The oxidative Heck reaction of strongly coordinating heterocycles with internal olefins often led to elusive reactivity and regioselectivity. Herein, by judicious choice of X-type directing groups under Ru(ii) catalysis, we achieved the regioselective oxidative Heck reaction of strongly coordinating heterocycles with sterically demanding internal olefins. It was postulated that the "match/mismatch effect" of sterically demanding internal olefins as coupling partners and subsequent kinetically favoured Michael addition or oxidative aromatization act as driving forces to facilitate the desired reactivity and site-selectivity.

Tunable Rh(III)-Catalyzed C(sp2)-H Bond Functionalization of Aryl Imidates with Cyclic 1,3-Diones: Strategic Use of Directing Groups

A tunable Rh(III)-catalyzed C(sp2)-H bond functionalization of aryl imidates with cyclic 1,3-diones was developed. With suitable and straightforward reaction condition adjustments, the C-H bond functionalization of diverse aryl imidates with cyclic 1,3-diones occurred smoothly and precisely at room temperature. Accompanied by different directing group transformations, a series of corresponding aryl nitriles, hydrophenanthridin-1(2H)-ones, spiro isoindoles, or hydrophenanthridine-1,6(2H,5H)-diones were synthesized in good yields to provide a rational directing group utilization strategy for the Rh(III)-catalyzed C(sp2)-H bond activation. Control experiments and primary mechanistic studies revealed that solvent effects and functional group electronic effects might influence the reaction's selectivity.

Cu(II)/Base Synergistic Promoted [2 + 1 + 3] Cyclization of Cyclic Ketones with α,β-Unsaturated Aldehydes/Ketones and NH4I to Access Fused Pyridines

Herein, a practical three-component [2 + 1 + 3] cyclization of various cyclic ketones with α,β-unsaturated aldehydes/ketones and ammonium iodide (NH4I) to access highly functional fused pyridines has been developed. The features of this transformation include mild reaction conditions, readily available starting materials, and excellent chemoselectivity. This protocol is compatible with various functional groups, and the preliminary studies on the mechanism of the reaction are also provided.

Rhodium-Catalyzed Functionalization and Annulation of N-Aryl Phthalazinediones with Allyl Alcohols

A direct ortho-Csp2-H acylalkylation of 2-aryl-2,3-dihydrophthalazine-1,4-diones with unsubstituted and substituted allyl alcohols is achieved in high yields through Rh(III)-catalyzed C-H bond activation process. The additional employment of Cu(OAc)2⋅2H2O as an oxidant detour the reaction towards [4+1] annulation, producing 13-(2-oxopropyl)-13H-indazolo[1,2-b]phthalazine-6,11-diones in moderate yields. Interestingly, Lawesson's reagent-mediated conditions accomplished intramolecular cyclization in ortho-(formylalkylated)-2,3-dihydrophthalazine-1,4-diones to produce diazepino[1,2-b]phthalazine-diones in moderate yields. Furthermore, allyl alcohol showcased distinct reactivity in presence of different additives to produce ortho-allylated, oxidative and non-oxidative [4+2] annulated products.

Ru(II)-Catalyzed Divergent C-H Alkynylation Cascade with Bifunctional α-Alcohol Haloalkynes

Native functionality directed the C-H activation cascade to enable rapid construction of molecular complexity, featuring step-economy and synthetic efficiency. Herein, by exploiting bifunctional α-alcohol haloalkynes, we developed Ru(II)-catalyzed carboxylic acid, amine, and amide assisted divergent C-H alkynylation and annulation cascade, affording polyfunctional heterocycles. Significantly, a bilateral aryl C-H polycyclization cascade of azobenzenes was achieved using the versatile haloalkynes.

Rh(III)-catalyzed selective mono- and dual-functionalization/cyclization of 1-aryl-5-aminopyrazoles with iodonium ylides

An efficient Rh(III)-catalyzed selective mono- and dual-C-H bond functionalization/cyclization with iodonium ylide as a single coupling partner was demonstrated, in which fused benzodiazepine skeletons were obtained in excellent yields. This method greatly improved an effective approach to dual C-H unsymmetrical functionalization.

Rapid Access to Fused Tetracyclic N-Heterocycles via Amino-to-Alkyl 1,5-Palladium Migration Coupled with Intramolecular C(sp3)-C(sp2) Coupling

An unprecedented route for the preparation of fused tetracyclic N-heterocycles is presented through the palladium-catalyzed cyclization of isocyanides with alkyne-tethered aryl iodides. In this transformation, a novel amino-to-alkyl 1,5-palladium migration/intramolecular C(sp3)-C(sp2) coupling sequence was observed first. More importantly, isocyanide exhibited three roles, serving simultaneously as a C1 synthon, a C1N1 synthon, and the donor of C(sp3) for C(sp3)-C(sp2) coupling, and the reaction was the sole successful example that achieved C(sp3)-H activation of isocyanide.

Synthesis of 3,4,5-trisubstituted phenols via Rh(III)-catalyzed alkenyl C-H activation assisted by phosphonium cations

An efficient method for the construction of various 3,4,5-trisubstituted phenol derivatives has been achieved via the Rh(III)-catalyzed coupling of phosphonium cations with internal alkynes. This protocol shows good substrate compatibility, as an array of structurally and electronically diverse phosphonium compounds react efficiently with up to 87% yield.

Palladium-Catalyzed C-H Allylation/Annulation Reaction of Amides and Allylic Alcohols: Regioselective Construction of Vinyl-Substituted 3,4-Dihydroisoquinolones

A palladium-catalyzed highly regioselective C-H allylation/annulation reaction of N-sulfonyl amides with secondary or tertiary allylic alcohols has been developed to construct 3,4-dihydroisoquinolones bearing a synthetically valuable vinyl substituent. This cascade cyclization approach of allylic alcohols involving C-H allylation has not been reported previously. The commercially available allylic alcohol substrates, the only by-product of water, and the used terminal oxidant of O₂ provide environmentally benign advantages.

Rh(III)-catalyzed [4 + 1] cyclization of aryl substituted pyrazoles with cyclopropanols via C-H activation

A rhodium-catalyzed formal [4 + 1]-cyclization reaction of aryl substituted pyrazoles with cyclopropanols via C-H bond activation/cyclization processes to selectively construct a series of carbonyl functionalized pyrazolo[5,1-a]isoindoles is described. The reaction features good functional group compatibility and a broad substrate scope with respect to both cyclization components with up to 84% yields. Mechanistic studies indicated that the C-H cleavage might be the rate-determining step in this transformation.

Ligand-assisted olefin-switched divergent oxidative Heck cascade with molecular oxygen enabled by self-assembled imines

Divergent oxidative Heck reaction has proven to be reliable for the rapid construction of molecular complexity, while olefins switched the outcome that remained underexplored.

Metal-Free Atom-Economical Synthesis of Tetra-Substituted Imidazoles via Flavin-Iodine Catalyzed Aerobic Cross-Dehydrogenative Coupling of Amidines and Chalcones

Herein, we demonstrated the oxidative cross-dehydrogenative coupling between amidines and chalcones catalyzed by flavin and iodine. The riboflavin-iodine catalytic system played multiple roles in substrate- and O₂-activation, enabling the facile and atom-economical synthesis of tetra-substituted imidazoles in good yields (60-87%). This metal-free reaction consumed only 1 equiv of molecular oxygen and generated 2 equiv of environmentally benign H₂O as the only byproduct.

Rhodium(III)-Catalyzed Sequential Cyclization of N-Boc Hydrazones with Propargylic Monofluoroalkynes via C-H Activation/C-F Cleavage for the Synthesis of Spiro[cyclobutane-1,9'-indeno[1,2-a]indenes]

An effective rhodium(III) catalysis for the construction of valuable tetracyclic compounds is described herein. This domino process involving the C-H activation/[3 + 2] annulation/intramolecular Friedel-Crafts reaction sequences of simple and readily available N-Boc hydrazones and propargylic monofluoroalkynes afforded fused tetracyclic spiro[cyclobutane-1,9'-indeno[1,2-a]indenes] in moderate to good yields, featuring three C-C bond formation. Moreover, control experiments indicated that the C-H activation might be involved in the rate-determining step.

Cobalt-catalyzed highly diastereoselective [3 + 2] carboannulation reactions: facile access to substituted indane derivatives

Efficient oxidative [3 + 2] annulation reaction involving aryl hydrazones and heterobicyclic alkenes has been realized with inexpensive and earth-abundant cobalt salts under aerobic conditions. The reaction proceeds via directing-group-assisted C-H activation and exo-selective migratory insertion, followed by the intramolecular nucleophilic attack of the alkylcobalt(III) species onto the imine with high anti-diastereoselectivity to provide complex indane derivatives. The generation of three contiguous stereogenic centers within the indanyl unit and the avoidance of the use of stoichiometric amounts of metal oxidants make this transformation more valuable and appealing.

Rhodium(III)-catalyzed oxidative annulation of isoquinolones with allyl alcohols: synthesis of isoindolo[2,1-b]isoquinolin-5(7H)-ones

An efficient rhodium(III)-catalyzed direct C-H oxidative annulation of isoquinolones with allyl alcohols as C1 synthons has been successfully developed. This protocol enables the straightforward synthesis of structurally diverse isoindolo[2,1-b]isoquinolin-5(7H)-ones with high atom economy, tolerates a broad spectrum of functionalities, and is applicable to one-pot operation from readily available N-methoxybenzamides.

Modular construction of functionalized anilines via switchable C–H and N-alkylations of traceless N-nitroso anilines with olefins

Switchable C–H or N-alkylations of N-nitroso anilines with olefins.

Electrochemically induced Markovnikov-type selective hydro/deuterophosphonylation of electron-rich alkenes

An electrochemically induced Markovnikov-type selective hydro/deuterophosphonylation of electron-rich alkenes with P(O)H compounds to generate various organophosphorus compounds has been achieved.

Synthesis of functionalized benzimidazoles via oxidative tandem quartic C-H aminations and cleavage of C-N and C-C bonds

Via aerobic copper-catalyzed tandem quartic C-H aminations, we herein present an unprecedented approach for the synthesis of functionalized benzimidazoles from aniline derivatives and 2-substituted cyclic amines. The cyclic amines act as the CN building blocks and are involved in the annulation reaction by cleavage of inert α-C-N and β-C-C bonds. The synthetic protocol features high selectivity, no need for specific aminating agents, mild conditions, and the use of a naturally abundant [Cu]/O₂ catalyst system.

Metal-Free Cascade Formation of Intermolecular C-N Bonds Accessing Substituted Isoindolinones under Cathodic Reduction

An electrochemical protocol for the construction of substituted isoindolinones via reduction/amidation of 2-carboxybenzaldehydes and amines has been realized. Under metal-free and external-reductant-free electrolytic conditions, the reaction achieves the cascade formation of intermolecular C-N bonds and provides a series of isoindolinones in moderate to good yields. The deuterium-labeling experiment proves that the hydrogen in the methylene of the product is mainly provided by H₂O in the system.

Sequential C-H activation enabled expedient delivery of polyfunctional arenes

Modular construction of polyfunctional arenes from abundant feedstocks stands as an unremitting pursue in synthetic chemistry, accelerating the discovery of drugs and materials. Herein, using the multiple C-H activation strategy with versatile imidate esters, the expedient delivery of molecular libraries of densely functionalized sulfur-containing arenes was achieved, which enabled the concise construction of biologically active molecules, such as Bipenamol.

Copper-catalyzed, N-auxiliary group-controlled switchable transannulation/nitration initiated by nitro radicals: selective synthesis of pyridoquinazolones and 3-nitroindoles

Within the scope of nitration reactions, the efficiency of sensitive heteroaromatics such as indoles is often eroded by various competitive oxidative decomposition pathways.

Rh(iii)-Catalyzed [3 + 2]/[4 + 2] annulation of acetophenone oxime ethers with 3-acetoxy-1,4-enynes involving C–H activation

A novel, synthetically simple, selective rhodium(iii)-catalyzed [3 + 2]/[4 + 2] annulation cascade reaction to construct complex azafluorenone frameworks has been developed.

Recent Advances on Synthetic Methodology Merging C-H Functionalization and C-C Cleavage

The functionalization of C-H bonds has become a major thread of research in organic synthesis that can be assessed from different angles, for instance depending on the type of catalyst employed or the overall transformation that is carried out. This review compiles recent progress in synthetic methodology that merges the functionalization of C-H bonds along with the cleavage of C-C bonds, either in intra- or intermolecular fashion. The manuscript is organized in two main sections according to the type of substrate in which the cleavage of the C-C bond takes place, basically attending to the scission of strained or unstrained C-C bonds. Furthermore, the related research works have been grouped on the basis of the mechanistic aspects of the different transformations that are carried out, i.e.,: (a) classic transition metal catalysis where organometallic intermediates are involved; (b) processes occurring via radical intermediates generated through the use of radical initiators or photochemically; and (c) reactions that are catalyzed or mediated by suitable Lewis or Brønsted acid or bases, where molecular rearrangements take place. Thus, throughout the review a wide range of synthetic approaches show that the combination of C-H and C-C cleavage in single synthetic operations can serve as a platform to achieve complex molecular skeletons in a straightforward manner, among them interesting carbo- and heterocyclic scaffolds.

Directing Group Assisted Unsymmetrical Multiple Functionalization of Arene C-H Bonds

Multiple C-H bond functionalizations promptly install diverse groups on the molecular framework and consequently fabricate complex molecular entities. This review briefly surveys the conceptual development of directing group assisted unsymmetrical multiple functionalization of arene C(sp² )-H bonds, which is exceedingly appealing and highly important.

Weak coordinated nitrogen functionality enabled regioselective C–H alkynylation via Pd(ii)/mono-N-protected amino acid catalysis

By using mono-N-protected amino acid (MPAA) ligand under Pd(ii) catalysis, regioselective C–H alkynylation of weak coordinated nitrogen functionalities was developed with great efficiency, proceeding via 5, 6 and 7-membered palladacycle intermediates.