Palladium-Catalyzed Cascade Cyclization of Alkene-Tethered Aryl Halides with o-Bromobenzoic Acids: Access to Diverse Fused Indolo[2,1-a]isoquinolines

Synthesis of Amide-Containing Indolo[2,1-a]isoquinoline Derivatives via Palladium-Catalyzed Cascade Reactions

A convenient palladium-catalyzed cascade cyclization reaction for the construction of amide-containing indolo[2,1-a]isoquinolines from 2-aryl-N-propenyl indoles with aromatic amines and chloroform is described. A variety of amide-containing indolo[2,1-a]isoquinoline compounds have been successfully synthesized in moderate to good yields in which chloroform is employed as the source of CO.

A Cascade C(sp3)-H Annulation Involving C(alkyl),C(alkyl)-Palladacycle Intermediates

C-H bond functionalization involving C,C-palladacycle intermediates provides a unique platform for developing novel reactions. However, the vast majority of studies have been limited to the transformations of C(aryl),C-palladacycles. In sharp contrast, catalytic reactions involving C(alkyl),C(alkyl)-palladacycles have rarely been reported. Herein, we disclose an unprecedented cascade C(sp3)-H annulation involving C(alkyl),C(alkyl)-palladacycles. In this protocol, alkene-tethered cycloalkenyl bromides undergo intramolecular Heck/C(sp3)-H activation to generate C(alkyl),C(alkyl)-palladacycles, which can be captured by α-bromoacrylic acids to afford tricyclic fused pyridinediones. In addition, this strategy can also be applied to indole-tethered cycloalkenyl bromides to construct pentacyclic fused pyridinediones via suquential Heck dearomatization/C(sp3)-H activation/decarboxylative cyclization. Notably, the removal of α-bromoacrylic acids in the reaction of alkene-tethered cycloalkenyl bromides can build an interesting tricyclic skeleton containing a four-membered ring. Preliminary mechanistic experiments indicate that five-membered C(alkyl),C(alkyl)-palladacycles serve as the key intermediates. Meanwhile, density functional theory (DFT) calculations have provided insights into the reaction pathway.

Palladium-Catalyzed Dearomative Heck/C(sp2)-H Activation/Decarboxylative Cyclization of C2-Tethered Indoles

Until now, palladium-catalyzed dearomative Heck reactions of indoles were largely limited to β-H elimination and nucleophilic capture of the transient alkyl-Pd(II) species. Herein, we disclose a novel palladium-catalyzed dearomative Heck/C(sp2)-H activation/decarboxylative cyclization of C2-tethered indoles. In this protocol, the alkyl-Pd(II) species formed by dearomatization of C2-tethered indoles is not terminated by common β-H elimination or nucleophilic capture, but rather generates C,C-palladacycle via C-H activation. The latter is intercepted by α-bromoacrylic acids to produce pentacyclic and heptacyclic fused indolines.

Electrochemical Selenized Reaction of N-Arylbicyclo[1.1.0]butane-1-carboxamides: Access to 3-(Arylselanyl)spiro[cyclobutane-1,3'-indolin]-2'-one Derivatives

A novel selenized reaction of N-arylbicyclo [1.1.0]butane-1-carboxamides with diselenide for the synthesis of polycyclic indoline derivatives is developed under electrochemical conditions. The synthesis is achieved by the bicyclo[1.1.0]butane strain-release reaction and intramolecular cyclization process. In addition, this approach features a wide range of substrates, good group tolerance, shorter reaction time, and mild conditions.

Palladium-Catalyzed Bicycloaromatization of o-(Alkynyl)styrenes with Alkynes: Economical Access to Chrysene Derivatives

Herein, a palladium-catalyzed bicycloaromatization of o-(alkynyl)styrenes with alkynes is reported. In this protocol, the 6-endo-dig cyclization of o-(alkynyl)styrenes is followed by deprotonation to complete the first cycloaromatization, and then, a regioselective alkyne insertion/C-H activation occurs to achieve the second cycloaromatization, resulting in atom- and step-economical syntheses of polysubstituted chrysenes. Notably, the products can be further used to construct π-extended arenes using the Scholl reaction.

Norbornene Derivatives-Controlled Palladium-Catalyzed Divergent Synthesis of Dibenzo[a,c]cycloheptenones and Fluorenones from Aryl Iodides and α-Oxocarboxylic Acids

A palladium-catalyzed divergent cascade decarboxylative annulation of aryl iodides and α-oxocarboxylic acids using norbornene (NBE) derivatives as a controlled switch is reported. When NBE is used as a mediator, fluorenones are synthesized with moderate to excellent yields via a Catellani reaction that involves sequential ortho-C-H arylation and ipso-decarboxylative acylation of aryl iodides. Employing oxanorbornadiene (ONBD) instead of NBE enables the assembly of dibenzo[a,c]cycloheptenones by a retro-Diels-Alder reaction rather than the release of an ONBD. Additionally, the synthetic utility of this method is demonstrated by the diversification of the products.

Metal- and additive-free radical-triggered nitration/cyclization to construct indolo[2,1-α]isoquinoline and benzimidazo[2,1-a]isoquinolin-6(5H)-one derivatives using t-BuONO as nitro reagents

An efficient metal- and additive-free nitro radical-triggered addition/cyclization of 2-aryl-N-acryloyl indoles/2-arylbenzimidazoles for the synthesis of nitro-substituted indolo[2,1-α]isoquinoline and benzimidazo[2,1-a]isoquinolin-6(5H)-one derivatives has been developed. The commercially available and low-cost t-BuONO was used as a nitro reagent. Due to mild reaction conditions, a variety of functional groups could be tolerated to give the corresponding products in moderate to good yields. Moreover, this nitration process could be scaled-up and the nitro group could be readily converted into the amino group, which may find applications in synthetic and medicinal chemistry.

Synthesis of spirooxindoles via formal acetylene insertion into a common palladacycle intermediate

A palladium-catalyzed spirocyclization reaction is reported, which is proposed to arise via insertion of an oxabicycle into a palladacycle, formed from carbocyclization and a C-H functionalization sequence. Mechanistic studies suggest the insertion is diastereoselective and a post-catalytic retro-Diels-Alder step furnishes an alkene, wherein the oxibicycle has served as an acetylene surrogate. Aryl iodides and carbamoyl chlorides were compatible as starting materials under the same reaction conditions, enabling the convergent and complementary synthesis of spirooxindoles, as well as other azacycles. These spirooxindoles allowed further transformations that were previously unaccessible.

Ru(II)-catalyzed oxidative coupling of sulfoxonium ylides with amines: efficient synthesis of α-ketoamides and indolo[2,1-a]isoquinolines

The first use of sulfoxonium ylides for the synthesis of α-ketoamides is described via a Ru(II)-catalyzed amidation reaction with amines. The same Ru(II)-catalyzed reaction of sulfoxonium ylides with 2-phenylindoles provided indolo[2,1-a]isoquinolines instead of α-ketoamides.

Palladium-Catalyzed Carbonylative Synthesis of Amide-Containing Indolo[2,1-a]isoquinolines from Alkene-Tethered Indoles and Nitroarenes

In this new procedure, amide-containing indolo[2,1-a]isoquinoline scaffolds were prepared by palladium-catalyzed carbonylative cyclization of alkene-tethered indoles with nitroarenes. By using Mo(CO)₆ as the CO source and reductant and nitroarenes as the nitrogen source, this reaction produced various amide-containing indolo[2,1-a]isoquinoline derivatives in good yields in general. Furthermore, the late-stage modifications of bioactive molecules using this protocol were demonstrated as well.

Visible-light-promoted radical cascade alkylation/cyclization: access to alkylated indolo/benzoimidazo[2,1-a]isoquinolin-6(5H)-ones

A new protocol is herein described for the direct generation of alkylated indolo/benzoimidazo[2,1-a]isoquinolin-6(5H)-one derivatives by using Hantzsch esters as alkylation radical precursors using a photoredox/K₂S₂O₈ system. This oxidative alkylation of active alkenes involves a radical cascade cyclization process and a sequence of Hantzsch ester single electron oxidation, C-C bond cleavage, alkylation, arylation and oxidative deprotonation.

Pd(II)-Catalyzed Synthesis of Polycyclic Heteroarenes via an Aminopalladation/C-H Activation/Dealkylation/Decarboxylative Cyclization Cascade

Until now, cascade reactions involving five-membered C,C-palladacycles rely heavily on Pd(0)-catalyzed C-H functionalization of aryl halides initiated by carbopalladation. Herein, we report a novel Pd(II)-catalyzed cascade decarboxylative cyclization of o-alkynylanilines initiated by aminopalladation. In this protocol, o-alkynylanilines undergo sequential anti aminopalladation, C-H activation, and dealkylation to form C,C-palladacycles, which are then trapped by o-bromobenzoic acids or 8-bromo-1-naphthoic acid to produce diverse polycyclic heteroarenes, such as dibenzo[a,c]carbazoles and multiple arene-fused cyclohepta[1,2-b]indoles.

Research advances in palladium-catalysed intermolecular C-H annulation of aryl halides with various aromatic ring precursors

Transition-metal-catalysed C-H functionalization has emerged as a powerful approach for the transformation of organic molecules due to its high atom and step economy. Palladium-catalysed intermolecular C-H annulation of aryl halides, especially those involving annulation of a five-membered C,C-palladacycle with coupling reagents, have attracted considerable attention in the past decades. This review summarizes the progress on palladium-catalysed intermolecular C-H annulation of aryl halides with various aromatic ring precursors. Mechanistically, five-membered C,C-palladacycles as intermediates are involved in the majority of reactions.

Regioselective synthesis of spirocyclic pyrrolines via a palladium-catalyzed Narasaka-Heck/C-H activation/[4 + 2] annulation cascade reaction

A novel palladium-catalyzed spirocyclization through sequential Narasaka-Heck cyclization, C-H activation and [4 + 2] annulation has been developed. In this reaction, cheap and readily available 2-chlorobenzoic acid or ethyl phenylpropiolate was employed as the C2 insertion unit to react with γ,δ-unsaturated oxime ester. The key step in this transformation is the regioselective insertion of the C2 synthon into the spiro-palladacycle intermediate that is formed by the δ-C-H activation process, thereby efficiently assembling a series of spirocyclic pyrrolines with high regiocontrol. Furthermore, density functional theory (DFT) calculations and control experiments were performed to gain some insights into the reaction mechanism.

Microwave-accelerated and benzoyl peroxide (BPO)-initiated cyclization of 1,5-enynes having cyano groups with cyclic alkanes under metal-free conditions

A novel and efficient method for preparing exocyclic indan derivatives, with this method involving benzoyl peroxide (BPO)-initiated cyclization of 1,5-enynes having cyano groups with simple cyclic alkanes under microwave irradiation, has been developed. The presented approach showed advantages of simple conditions, an environmentally friendly protocol, good functional-group tolerance, and high yields of products.

Visible-light-induced acylation/cyclization of active alkenes: facile access to acylated isoquinolinones

Nitrogen heterocycles, especially polycyclic compounds, are significant skeletons in valuable molecules. Herein, we developed an efficient and practical visible-light-induced acylation/cyclization of active alkenes with acyl oxime derivatives for constructing acylated indolo/benzimidazo-[2,1,a]isoquinolin-6(5H) ones. This reaction was compatible with various functional groups and a series of fused indole/imidazole skeletons were prepared in up to 95% yield at room temperature.

Three-component synthesis of arylsulfonyl-substituted indolo[2,1-a]isoquinolinones and benzimidazo-[2,1-a]isoquinolin-6(5H)-ones by SO2 insertion and radical cascade cyclization

An efficient arylsulfonylation/cyclization of 2-aryl-N-methacryloyl indoles with potassium metabisulfite and aryldiazonium tetrafluoroborates was developed. A series of variously substituted arylsulfonyl indolo[2,1-a]isoquinolin-6(5H)-ones were formed in moderate to good yields via utilization of the nature abundant inorganic salt potassium metabisulfite as a SO₂ surrogate. Additionally, this three-component protocol can also be employed for the synthesis of arylsulfonyl-substituted benzimidazo-[2,1-a]isoquinolin-6(5H)-ones.

Synthesis of Ester-Substituted Indolo[2,1-a]isoquinolines via Photocatalyzed Alkoxycarbonylation/Cyclization Reactions

A direct alkoxycarbonylation/cyclization reaction is accomplished under visible light-induced photoredox catalysis. With this approach, a variety of ester-substituted indolo[2,1-a]isoquinolines are prepared in good to excellent yields. It is worth noting that this method not only can afford the synthesis of indolo[2,1-a]isoquinolines but also can provide an alternative route for generating complex target structures bearing carboxylic esters.

Cascade Cyclization for the Synthesis of Indolo[2,1-α]isoquinoline Derivatives via Visible-Light-Induced Halogen-Atom-Transfer (XAT) and Hydrogen-Atom-Transfer (HAT)

A transition metal-free photoredox cascade cyclization is herein reported. In this protocol, sustainable visible light was used as energy source and organic light-emitting molecule Eosin Y served as efficient photocatalyst....

Decarboxylative cyclization of o-chlorobenzoic acids with C,C-palladacycles formed by an aminopalladation/dealkylation strategy to access dibenzo[a,c]carbazoles

A novel decarboxylative cyclization of o-chlorobenzoic acids with C,C-palladacycles formed by an aminopalladation/dealkylation strategy for the assembly of dibenzo[a,c]carbazoles has been reported.

Atmosphere-Controlled Palladium-Catalyzed Divergent Decarboxylative Cyclization of 2-Iodobiphenyls and α-Oxocarboxylic Acids

A novel palladium-catalyzed divergent decarboxylative cyclization of 2-iodobiphenyls and α-oxocarboxylic acids utilizing the atmosphere as a controlled switch is reported. Under the protection of a nitrogen atmosphere, tribenzotropones are synthesized by a [4 + 3] decarboxylative cyclization. Employing a palladium/O₂ system enables a [4 + 2] decarboxylative cyclization to assemble triphenylenes. Notably, preliminary mechanistic studies indicate that the formation of triphenylenes involves a double decarboxylation.

α-Bromoacrylic Acids as C1 Insertion Units for Palladium-Catalyzed Decarboxylative Synthesis of Diverse Dibenzofulvenes

Herein α-bromoacrylic acids have been employed as C1 insertion units to achieve the palladium-catalyzed [4 + 1] annulation of 2-iodobiphenyls, which provides an efficient platform for the construction of diverse dibenzofulvenes. This protocol enables the formation of double C(aryl)-C(vinyl) bonds via a C(vinyl)-Br bond cleavage and decarboxylation. It is particularly noteworthy that the method features a broad substrate scope, and various interesting frameworks, such as bridged ring, fused (hetero)aromatic ring, and divinylbenzene, can be successfully incorporated into the products.

A palladium-catalyzed Heck/[4 + 1] decarboxylative cyclization cascade to access diverse heteropolycycles by using α-bromoacrylic acids as C1 insertion units

A novel palladium-catalyzed Heck/[4 + 1] decarboxylative cyclization cascade of alkene-tethered aryl iodides by employing α-bromoacrylic acids as C1 insertion units is reported.

Pd-Catalyzed Domino Reactions Involving Alkenes To Access Substituted Indole Derivatives

Palladium-catalyzed domino reactions are advanced tools in achieving various nitrogen-containing heterocycles in an efficient and economical manner due to the reduced number of steps in the process. This review highlights recent advances in domino processes aimed at the synthesis of indole derivatives and polycyclic systems containing the indole nucleus in intra/intra- or intra/intermolecular reactions. In particular, we consider domino processes that involve a double bond in a step of the sequence, which allow the issue of regioselectivity in the cyclization to be faced and overcome. The different sections in this review focus on the synthesis of the indole nucleus and functionalization of the scaffold starting from different substrates that have been identified as activated starting materials, which involve a halogenated moiety or unactivated unsaturated systems. In the former case, the reaction is under Pd(0) catalysis, and in the second case a Pd(II) catalytic species is required and then an oxidant is necessary to reconvert the Pd(0) into the active Pd(II) species. On the other hand, the second method has the advantage that it uses easy available and inexpensive substrates.

1 Introduction

2 Indole Scaffold Synthesis

2.1 Activated Substrates

2.2 Unactivated Substrates

3 Functionalization of Indole Scaffold

3.1 Activated Substrates

3.2 Unactivated Substrates

4 Conclusions

Palladium-catalyzed domino Heck-disilylation and -borylation of alkene-tethered 2-(2-halophenyl)-1H-indoles: access to diverse disilylated and borylated indolo[2,1-a]isoquinolines

A chemoselective domino Heck-disilylation and -borylation reaction for generating various disilylated and borylated tetracyclic indolo[2,1-a]isoquinolines has been developed.

Pd-Catalyzed one-pot synthesis of vinylsilanes via a three-component tandem reaction

A palladium-catalyzed three-component tandem reaction for stereoselective assembly of various tri- or tetrasubstituted vinylsilanes is established.

C(sp3)–H activation-enabled cross-coupling of two aryl halides: an approach to 9,10-dihydrophenanthrenes

A palladium-catalyzed cross-coupling reaction of aryl halides with 2-chlorobenzoic acids has been developed through C(sp³)–H activation, which provides an innovative method for the synthesis of 9,10-dihydrophenanthren.