Manganese‐Catalyzed Dehydrogenative [4+2] Annulation of NH Imines and Alkynes by CH/NH Activation

Described herein is a manganese‐catalyzed dehydrogenative [4+2] annulation of N? H imines and alkynes, a reaction providing highly atom‐economical access to diverse isoquinolines. This transformation represents the first example of manganese‐catalyzed C? H activation of imines; the stoichiometric variant of the cyclomanganation was reported in 1971. The redox neutral reaction produces H₂ as the major byproduct and eliminates the need for any oxidants, external ligands, or additives, thus standing out from known isoquinoline synthesis by transition‐metal‐catalyzed C? H activation. Mechanistic studies revealed the five‐membered manganacycle and manganese hydride species as key reaction intermediates in the catalytic cycle.     

Decarbonylative CC Bond‐Forming Reactions of Saccharins by Nickel Catalysis: Homocoupling and Cycloaddition

Decarbonylation of saccharins by nickel catalysis enables two kinds of C?C bond‐forming reactions; homocoupling of saccharins to form biaryls and cycloaddition with alkynes to form benzosultams. The former represents the first reported nickel‐catalyzed decarbonylative C?C homocoupling reaction, whereas the latter constitutes a powerful method to pharmaceutically relevant benzosultams. The reactions proceed with good functional‐group tolerance and excellent regioselectivity.     

Synthesis of Silaphenalenes by Ruthenium‐Catalyzed Annulation between 1‐Naphthylsilanes and Internal Alkynes through CH Bond Cleavage

Ruthenium‐catalyzed annulation of 1‐naphthylsilanes with internal alkynes afforded silaphenalenes through cleavage of the C?H bond at the 8‐position of the naphthalene. [RuH₂(CO){P(p‐FC₆H₄)₃}₃] efficiently catalyzed the reaction. The use of 1‐naphthyldiphenylsilane as a substrate resulted in a better yield of the annulation product compared to the use of silanes with alkyl groups on the silicon atom. Internal alkynes with both aryl and alkyl groups were tolerated in this reaction.     

Iron‐Carbonyl‐Catalyzed Redox‐Neutral [4+2] Annulation of N−H Imines and Internal Alkynes by C−H Bond Activation

Stoichiometric C?H bond activation of arenes mediated by iron carbonyls was reported by Pauson as early as in 1965, yet the catalytic C?H transformations have not been developed. Herein, an iron‐catalyzed annulation of N?H imines and internal alkynes to furnish cis‐3,4‐dihydroisoquinolines is described, and represents the first iron‐carbonyl‐catalyzed C?H activation reaction of arenes. Remarkablely, this is also the first redox‐neutral [4+2] annulation of imines and alkynes proceeding by C?H activation. The reaction also features only cis stereoselectivity and excellent atom economy as neither base, nor external ligand, nor additive is required. Experimental and theoretical studies reveal an oxidative addition mechanism for C?H bond activation to afford a dinuclear ferracycle and a synergetic diiron‐promoted H‐transfer to the alkyne as the turnover‐determining step.     

Rhodium(I)‐Catalyzed Decarbonylative Spirocyclization through CC Bond Cleavage of Benzocyclobutenones: An Efficient Approach to Functionalized Spirocycles

The rhodium‐catalyzed formation of all‐carbon spirocenters involves a decarbonylative coupling of trisubstituted cyclic olefins and benzocyclobutenones through C? C activation. The metal–ligand combination [{Rh(CO)₂Cl}₂]/P(C₆F₅)₃ catalyzed this transformation most efficiently. A range of diverse spirocycles were synthesized in good to excellent yields and many sensitive functional groups were tolerated. A mechanistic study supports a hydrogen‐transfer process that occurs through a β‐H elimination/decarbonylation pathway.     

Direct Synthesis of Protoberberine Alkaloids by Rh‐Catalyzed C−H Bond Activation as the Key Step

A one‐pot reaction of substituted benzaldehydes with alkyne–amines by a Rh‐catalyzed C?H activation and annulation to afford various natural and unnatural protoberberine alkaloids is reported. This reaction provides a convenient route for the generation of a compound library of protoberberine salts, which recently have attracted great attention because of their diverse biological activities. In addition, pyridinium salt derivatives can also be formed in good yields from α,β‐unsaturated aldehydes and amino–alkynes. This reaction proceeds with excellent regioselectivity and good functional group compatibility under mild reaction conditions by using O₂ as the oxidant.     

Manganese‐Catalyzed Dehydrogenative [4+2] Annulation of N?H Imines and Alkynes by C?H/N?H Activation

Described herein is a manganese‐catalyzed dehydrogenative [4+2] annulation of N H imines and alkynes, a reaction providing highly atom‐economical access to diverse isoquinolines. This transformation represents the first example of manganese‐catalyzed C H activation of imines; the stoichiometric variant of the cyclomanganation was reported in 1971. The redox neutral reaction produces H₂ as the major byproduct and eliminates the need for any oxidants, external ligands, or additives, thus standing out from known isoquinoline synthesis by transition‐metal‐catalyzed C H activation. Mechanistic studies revealed the five‐membered manganacycle and manganese hydride species as key reaction intermediates in the catalytic cycle.     

PIII‐Chelation‐Assisted Indole C7‐Arylation,Olefination, Methylation,and Acylation with Carboxylic Acids/Anhydrides by Rhodium Catalysis

Rhodium‐catalyzed C7‐selective decarbonylative arylation, olefination, and methylation of indoles with carboxylic acids or anhydrides by C?H and C?C bond activation have been developed. Furthermore, C7‐acylation products can also be generated selectively at a lower reaction temperature in the developed system. The key to the high reactivity and regioselectivity of this transformation is the appropriate choice of an indole N‐PtBu₂ chelation‐assisted group. This method has many advantages, including easy access and removal of the directing group, the use of cheap and widely available coupling agents, no requirement of an external ligand or oxidant, a broad substrate scope, high efficiency, and the formation of a sole regioisomer.     

Copper‐Catalyzed (2+1) Annulation of Acetophenones with Maleimides: Direct Synthesis of Cyclopropanes

A practical copper‐catalyzed direct oxidative cyclopropanation of electron‐deficient alkenes with acetophenone derivatives is reported. The dehydrogenative annulation involves a double C? H bond functionalization at the α‐position of the ketone using di‐tert‐butyl peroxide as oxidant. The broad scope of the reaction and excellent functional‐group tolerance is demonstrated for the stereoselective synthesis of fused cyclopropanes. The developed transformation revealed an unprecedented reactivity for copper‐catalyzed processes.     

PIII‐Chelation‐Assisted Indole C7‐Arylation,Olefination, Methylation,and Acylation with Carboxylic Acids/Anhydrides by Rhodium Catalysis

Rhodium‐catalyzed C7‐selective decarbonylative arylation, olefination, and methylation of indoles with carboxylic acids or anhydrides by C?H and C?C bond activation have been developed. Furthermore, C7‐acylation products can also be generated selectively at a lower reaction temperature in the developed system. The key to the high reactivity and regioselectivity of this transformation is the appropriate choice of an indole N‐PtBu₂ chelation‐assisted group. This method has many advantages, including easy access and removal of the directing group, the use of cheap and widely available coupling agents, no requirement of an external ligand or oxidant, a broad substrate scope, high efficiency, and the formation of a sole regioisomer.     

Rhodium(III)‐Catalyzed Alkenylation–Annulation of closo‐Dodecaborate Anions through Double B−H Activation at Room Temperature

1,2,3‐Trisubstituted closo‐dodecaborates with B?O, B?N, and B?C bonds as well as a fused borane oxazole ring have been synthesized by rhodium‐catalyzed direct cage B?H alkenylation and annulation of ureido boranes in the first reported example of regioselective B?H bond functionalization of the [B₁₂H₁₂]^2? cage by transition‐metal catalysis. This reaction proceeded at room temperature under ambient conditions and exhibited excellent selectivity for efficient monoalkenylation with good functional‐group tolerance. The urea moiety enabled B?H activation by acting as a directing group, was incorporated in the oxazole ring in situ, and also avoided multiple alkenylation. A possible mechanism is proposed on the basis of the isolation of a rhodium agostic intermediate and control experiments.     

Asymmetric Alkenyl C−H Functionalization by CpxRhIII forms 2H‐Pyrrol‐2‐ones through [4+1]‐Annulation of Acryl Amides and Allenes

An efficient Cp^xRh^III‐catalyzed enantioselective alkenyl C?H functionalization/[4+1] annulation of acryl amides and allenes is reported. The described transformation provides straightforward access to enantioenriched α,β‐unsaturated‐γ‐lactams bearing a quaternary stereocenter. The reaction operates under mild conditions, displays a broad functional‐group tolerance, and provides 2H‐pyrrol‐2‐ones with excellent selectivity of up to 97:3 er. Such scaffolds are frequently found in natural products and synthetic bioactive compounds and are of significant synthetic value. It is noteworthy that the allene serves as a one‐carbon unit in the [4+1]‐annulation.     

Palladium‐Catalyzed Regioselective Aromatic Extension of Internal Alkynes through a Norbornene‐Controlled Reaction Sequence

A regioselective aromatic π‐extension reaction of internal alkynes is reported. The proposed method employs three easily available components, namely aryl halides, 2‐haloarylcarboxylic acids, and disubstituted acetylenes. The transformation is driven by a controlled reaction sequence of C?H activation, decarboxylation, and annulation to give poly(hetero)aromatic compounds in a site‐selective fashion. Unlike in previously reported palladium‐catalyzed three‐component annulations, alkyne carbopalladation is the last step of this tandem reaction.     

Tandem Regioselective Substitution and Palladium‐Catalyzed Ring Fusion Reaction for Core‐Expanded Boron Dipyrromethenes with Red‐Shifted Absorption and Intense Fluorescence

A selective method for the core‐extension of boron dipyrromethene (BODIPY) with two annulated indole rings with exclusive syn‐connectivity is reported. The method is based on a regioselective nucleophilic substitution reaction of 2,3,5,6‐tetrabromoBODIPY with aryl amines, followed by palladium‐catalyzed intramolecular C?C coupling ring fusion. The unsymmetrical core‐expanded BODIPY with annulated indole and benzofuran rings was also synthesized by stepwise and regioselective nucleophilic substitution and palladium‐catalyzed intramolecular C?C coupling reaction. The diindole‐annulated BODIPY was unambiguously characterized by single‐crystal X‐ray analysis. The optical properties of the present core‐expanded BODIPYs were studied, revealing clearly red‐shifted absorption and emission bands and enhanced absorption coefficients upon annulation.     

Divergent Syntheses of Fused β‐Naphthol and Indene Scaffolds by Rhodium‐Catalyzed Direct and Decarbonylative Alkyne–Benzocyclobutenone Couplings

A tunable rhodium‐catalyzed intramolecular alkyne insertion reaction proceeding through the C? C cleavage of benzocyclobutenones is described. Selective formation of either the direct or decarbonylative insertion product can be controlled by using different catalytic systems. A variety of fused β‐naphthol and indene scaffolds were obtained in good yields with high functional group tolerance. This work illustrates a divergent approach to synthesize fused‐ring systems by C? C activation/functionalization.     

Rh(III)‐Catalyzed Denitrogenative [4+2] Annulation of Benzamides and 3‐Diazoindolin‐2‐imines: Expedient Access to Indolo[2,3‐c] isoquinolin‐5‐ones

An effective and pragmatic strategy for the synthesis of structurally diverse indolo[2,3‐c]isoquinolin‐5‐ones has been developed via a Rh(III)‐catalyzed C?H activation and [4+2] annulation reaction of N‐methoxybenzamides and 3‐diazoindolin‐2‐imines. The reaction involves the efficient formation of two new (one C?C and one C?N) bonds under operationally simple conditions and has the benefits of a broad substrate scope.     

Divergent Reactivity in Palladium‐Catalyzed Annulation with Diarylamines and α,β‐Unsaturated Acids: Direct Access to Substituted 2‐Quinolinones and Indoles

A palladium‐catalyzed C?H activation strategy has been successfully employed for exclusive synthesis of a variety of 3‐substituted indoles. A [3+3] annulation for synthesizing substituted 2‐quinolinones was recently developed by reaction of α,β‐unsaturated carboxylic acids with diarylamines under acidic conditions. In the present work, an analogous [3+2] annulation is achieved from the same set of starting materials under basic conditions to generate 1,3‐disubstituted indoles exclusively. Mechanistic studies revealed an ortho‐palladation–π‐coordination–β‐migratory insertion–β‐hydride elimination reaction sequence to be operative under the reaction conditions.     

RhIII‐Catalyzed C?H Activation: A Versatile Route towards Various Polycyclic Pyridinium Salts

An efficient and convenient method for the synthesis of highly substituted polycyclic pyridinium salts from the reaction of various 2‐aryl‐pyridines and 2‐aryl‐sp²‐nitrogen‐atom‐containing heterocycles with alkynes through rhodium(III)‐catalyzed C? H activation and annulation under an O₂ atmosphere is described. A possible mechanism that involves the chelation‐assisted C? H activation of the 2‐aryl‐pyridine substrate, insertion of the alkyne, and reductive elimination is proposed. This mechanism was supported by the isolation of a five‐membered rhodacycle ( I′ ). In addition, kinetic isotope studies were performed to understand the intimate reaction mechanism.     

Prediction on the Origin of Selectivities in Base‐controlled Switchable NHC‐catalyzed Transformations

The base‐controlled mechanisms for N‐heterocyclic carbene (NHC)‐catalyzed divergent [3+3] and [3+2] annulation reactions were examined by using the DFT method. The reaction initiates with the complexation of NHC and enal to give the Breslow intermediate, which diverges afterward. Then, the azomethine imine can either react with the Breslow intermediate to give the six‐membered ring product or the β‐carbon protonation occurs for forming the enolate intermediate controlled by different bases. The formed enolate intermediate reacts with azomethine imine to afford the five‐membered ring product. The calculated results show that only the base K₂CO₃ can facilitate the structural transformation between homoenolate and enolate to switch the chemoselectivity; therefore, the [3+3] annulation happens preferentially in the presence of base DBU while the other situation occurs with K₂CO₃ as base. The NCI analysis results reveal that the stereoselectivity is predominately determined by the π???π, C?H???O, and C?H???N interactions. The obtained mechanistic insights should provide valuable clues for the rational design of these kinds of divergent reactions.     

Iridium‐Catalyzed Annulation Reactions of Thiophenes with Carboxylic Acids: Direct Evidence for a Heck‐type Pathway

The functionalization of thiophenes is a fundamental and important reaction. Herein, we disclose iridium‐catalyzed one‐pot annulation reactions of (benzo)thiophenes with (hetero)aromatic or α,β‐unsaturated carboxylic acids, which afford thiophene‐fused coumarin‐type frameworks. Dearomatization reactions of 2‐substituted thiophenes with α,β‐unsaturated carboxylic acids deliver various thiophene‐containing spirocyclic products. The occurrence of two interconnected reactions provides direct evidence for a Heck‐type pathway. The mechanistic scenario described herein is distinctly different from the S_EAr and concerted metalation–protodemetalation (CMD) pathways encountered in the well‐described oxidative C?H/C?H cross‐coupling reactions of thiophenes with other heteroarenes.