Recent Development in Palladium-Catalyzed Domino Reactions: Access to Materials and Biologically Important Carbo- and Heterocycles

Recent Advances in the Domino Annulation Reaction of Quinone Imines

Quinone imines are important derivatives of quinones with a wide range of applications in organic synthesis and the pharmaceutical industry. The attack of nucleophilic reagents on quinone imines tends to lead to aromatization of the quinone skeleton, resulting in both the high reactivity and the unique reactivity of quinone imines. The extreme value of quinone imines in the construction of nitrogen- or oxygen-containing heterocycles has attracted widespread attention, and remarkable advances have been reported recently. This review provides an overview of the application of quinone imines in the synthesis of cyclic compounds via the domino annulation reaction.

Rapid Assembly of Unsymmetrically Linked Bis-heterocycles via Palladium Domino Catalysis

A palladium-catalyzed domino reaction of alkene-tethered oxime esters is reported. This transformation uses an air-stable palladium precatalyst that initiates a Narasaka-Heck reaction, that is interrupted with a Pd(II)-promoted cyclization. Through this methodology, a novel class of unsymmetrical alkyl-linked bis-heterocycles were synthesized in yields up to 99%. The reaction is scalable up to 1.0 mmol, with simplified purification steps. This transformation expands the scope of accessible bis-heterocycles available through Narasaka-Heck reactions beyond C-H activation and direct anionic capture termination steps.

Palladium-Catalyzed Synthesis of Linked Bis-Heterocycles─Synthesis and Investigation of Photophysical Properties

A palladium-catalyzed domino C-N coupling/Cacchi reaction is reported. Design of photoluminescent bis-heterocycles, aided by density functional theory calculations, was performed with synthetic yields up to 98%. The photophysical properties of the products accessed via this strategy were part of a comprehensive study that led to broad emission spectra and quantum yields of up to 0.59. Mechanistic experiments confirmed bromoalkynes as competent intermediates, and a density functional theory investigation suggests a pathway involving initial oxidative addition into the cis C-Br bond of the gem-dihaloolefin.

Palladium-Catalyzed [3 + 2] Annulation of ortho-Substituted Iodoarenes with Maleimides via a Consecutive Double Heck-type Strategy

Herein, we report an efficient [3 + 2] annulation of ortho-substituted iodoarenes with maleimides via a palladium-catalyzed consecutive double Heck-type strategy, leading to fused tricyclic frameworks of pharmaceutical relevance. The protocol ensued through consecutive inter- and intramolecular Heck couplings effectively. This approach was compatible with a large variety of substrates and functional groups, and it was remarkably tolerated with unprotected maleimide.

Conceptual advances in nucleophilic organophosphine-promoted transformations

Catalysis by trivalent nucleophilic organophosphines has emerged as an essential tool in organic synthesis. Several new organic transformations promoted by phosphines substantiate and complement the existing synthetic chemistry tools. Mere design of the substrate and reagent combinations has introduced new modes of reactivity patterns, which are otherwise difficult to achieve. These design considerations have led to the rapid build-up of complex molecular entities and laid a solid foundation to synthesise bioactive natural products and pharmaceuticals. This article presents an overview of some of the conceptual advances, including our contributions to nucleophilic organophosphine chemistry. The scope, limitations, mechanistic insights, and applications of these metal-free transformations are discussed elaborately.

Synthesis of mono-, di- and tripalladated 1,3,5-benzenetristyryl complexes. CO insertion to give a dipalladated indenone

The tribrominated arenes 1,3,5-C₆(E-CHCHAr)₃Br₃ (Ar = Ph, (I), p-To (I')), add oxidatively to [Pd(dba)₂] ([Pd₂(dba)₃]·dba) in the presence of two equivalents of a phosphine (PPh₃ or PMe₂Ph) to form the monopalladated complexes trans-[Pd{C₆(E-CHCHAr)₃Br₂}Br(L)₂] (Ar = Ph, L = PPh₃ (1a), Ar = p-To, L = PPh₃ (1a'), Ar = Ph, L = PMe₂Ph (1b)), while the reaction in a 1 : 2 : 4 arene : Pd : PMe₂Ph molar ratio affords the dipalladated complex [{trans-PdBr(PMe₂Ph)₂}₂{μ₂-C₆(E-CHCHPh)₃Br}] (2b). Both I and I' add oxidatively to 3 equivalents of [Pd(dba)₂] in the presence of the chelating N-donor ligand tmeda (N,N,N',N'-tetramethylethylenediamine) to form the tripalladated complexes [{PdBr(tmeda)}₃{μ₃-C₆(E-CHCHAr)₃}] (Ar = Ph, (3c), p-To (3c')). Complex 3c reacts with PMe₃ to form [{trans-PdBr(PMe₃)₂}₃{μ₃-C₆(E-CHCHPh)₃}] (3d). Compound 3c also reacts with CO to give the novel dipalladated indenone [2-Ph-4,6-{PdBr(tmeda)}₂-5,7-(E-CHCHPh)₂-inden-1-one] (4). The crystal structures of 1a' and 1b were determined by X-ray diffraction studies.

Catalytic enantioselective alkenylation-heteroarylation of olefins: stereoselective syntheses of 5-7 membered azacycles and oxacycles

Catalytic enantioselective domino alkenylation-heteroarylation of nonconjugated iododienes proceeded with excellent stereoselectivity and broad scope of substrates. The reaction enables stereoselective syntheses of substituted azacycles such as piperidine, pyrrolidine azepine and dihydropyrans carrying new quaternary stereocenters. Mechanistically, C-H bonds of heterocycles were activated by lithium alkoxides via reversible deprotonation, rather than conventional palladium(ii)-assisted metalation processes. Many types of heteroarenes can be used, including not only azoles (such as thiazoles, oxazoles, imidazoles and oxadiazoles), but also nonazoles (thiophene, furan and azine N-oxides).

The regioselective Wacker oxidation of internal allylamines: synthesis of functionalized and challenging β-amino ketones

A convenient and general protocol for the palladium-catalysed oxidation of internal allylamine derivatives to β-amino ketones is reported. The transformation occurs at room temperature and shows a wide substrate scope as well as high functional group and N-protecting group tolerance. We also describe potential applications of the method, e.g., the synthesis of bioactive molecules or simple transformations of selected β-amino ketones into other interesting building blocks.

The Application of Sulfonyl Hydrazides in Electrosynthesis: A Review of Recent Studies

Sulfonyl hydrazides are viewed as alternatives to sulfinic acids and their salts or sulfonyl halides, which are broadly used in organic synthesis or work as active pharmaceutical substances. Generally, sulfonyl hydrazides are considered good building blocks and show powerful value in a diverse range of reactions to construct C-S bonds or C-C bonds, and even C-N bonds as sulfur, carbon, or nitrogen sources, respectively. As a profound synthetic tool, the electrosynthesis method was recently used to achieve efficient and green applications of sulfonyl hydrazides. Interestingly, many unique and novel electrochemical syntheses using sulfonyl hydrazides as radical precursors have been developed, including cascade reactions, functionalization of heterocycles, as well as a continuous flow method combining with electrochemical synthesis since 2017. Accordingly, it is necessary to specifically summarize the recent developments of electrosynthesis with only sulfonyl hydrazides as radical precursors to more deeply understand and better design novel electrochemical synthesis reactions. Herein, electrosynthesis research using sulfonyl hydrazides as radical precursors since 2017 is reviewed in detail based on the chemical structures of products and reaction mechanisms.

Intermolecular Sonogashira Coupling and Intramolecular 5-Exo-dig Cycloisomerization Cascade: A One-Pot Pathway for Accessing (3-Benzylbenzofuran-2-yl)(phenyl)methanones

Herein, we present an efficient and straightforward strategy enabling access to 2,3-disubstituted benzo[b]furans. The whole synthetic process proceeds via a domino intermolecular Sonogashira coupling of 2-(2-bromophenoxy)-1-phenylethan-1-ones/alkyl 2-(2-bromophenoxy)acetates/2-(2-bromophenoxy)acetonitrile/1-(2-bromophenoxy)propan-2-one with terminal acetylenes followed by an intramolecular carbanion-yne cyclization in a 5-exo-dig manner and subsequent double-bond isomerization. Notably, two C-C bonds have been constructed in a one-pot manner and a wide variety of (3-benzylbenzofuran-2-yl)(phenyl)methanones were accomplished with good functional group tolerance.

Palladium-Catalyzed Three-Component Dearomatization/Sulfonylation Cascade

The diastereoselective synthesis of sulfonylated indolines is reported. A palladium-catalyzed dearomative sulfination of (aza)indole-tethered aryl iodides generates reactive benzylic sulfinates. These intermediates react with electrophiles in a one-pot, two-step process to generate sulfonylated products in good yields and excellent diastereoselectivity. This three-component sequence demonstrates good scalability and can be applied toward the synthesis of sulfonamides. Additionally, further derivatizations of aryl iodide containing products furnish spiro- and alkynylated indoline products.

Synthesis of dihydroisoquinolinone-4-methylboronic esters via domino Heck/borylation using a structurally characterized palladacycle as a catalyst

Synthesis of dihydroisoquinolinone-4-methylboronic esters from N-allylcarboxamides and B₂(Pin)₂via domino Heck/borylation approach is reported. A quinoxaline-based NHC-palladacycle [Pd(C∧C:)PPh₃Cl], which has been structurally characterized, is used as a catalyst. The scope of the substrate with a wide range of substituents is explored. In addition to the synthesis of title compounds, a few examples of methylboronic esters of indoline and benzofuran motifs have also been prepared using the same protocol.

Synthesis of dihydroisoquinolinone-4-methylboronic esters from N-allylcarboxamides and B₂(Pin)₂via domino Heck/borylation has been achieved by using a quinoxaline-based NHC-palladacycle [Pd(C∧C:)PPh₃Cl].

Accessing Unsymmetrically Linked Heterocycles through Stereoselective Palladium-Catalyzed Domino Cyclization

A palladium-catalyzed strategy is presented to synthesize unsymmetrically linked heterocycles within stereoselective tetrasubstituted olefins. This reaction is proposed to occur via a vinyl-Pd^II intermediate capable of initiating the cyclization of various alkyne-tethered nucleophiles. Products are formed in up to 96 % yield and excellent stereoselectivities are obtained using low catalyst loadings. This transformation was scalable up to 1 mmol and mechanistic studies suggest a syn-carbopalladation of the carbamoyl chloride followed by Pd^II -catalyzed cyclization of alkyne-tethered nucleophiles.

Recent advances in transition metal-catalyzed heteroannulative difunctionalization of alkenes via C-H activation for the synthesis of heterocycles

Heterocyclic compounds are the fundamental structural motifs distributed in natural products, pharmaceuticals and biologically active compounds. Thus, there is increasing interest in the development of novel synthetic strategies for the...

Sequential Pd0 - and PdII -Catalyzed Cyclizations: Enantioselective Heck and Nucleopalladation Reactions

An enantioselective consecutive cyclization/coupling process, catalyzed by palladium is reported. Stereoinduction arises from an enantioselective carbopalladation, generating an intermediate which promotes a nucleopalladation step. The dual cyclization sequence was compatible with a variety of alkyne-tethered oxygen- and nitrogen-centered nucleophiles, and a variety of alkenyl-tethered aryl iodides, to forge numerous bisheterocycles in good yields and high regio- and enantioselectivities.

Quadruple Role of Pd Catalyst in Domino Reaction Involving Aryl to Alkyl 1,5-Pd Migration to Access 1,9-Bridged Triptycenes

A Pd-catalyzed domino reaction of 1,8,13-tribromo-9-methoxytriptycenes is reported. Under conventional Suzuki coupling conditions, the triptycenes underwent multiple transformations to give 1,9-bridged triptycenes. Based on mechanistic investigations, a single Pd catalyst functions as Pd⁰ , Pd^II and Pd^IV species to catalyze four distinct processes: (1) aryl to alkyl 1,5-Pd migration, (2) intramolecular arylation, (3) homocoupling of phenylboronic acid and (4) Suzuki coupling. DFT calculations revealed that 1,5-Pd migration likely proceeds via both concerted Pd^II and stepwise Pd^IV routes. Asymmetric synthesis of the chiral triptycenes, as well as optical resolution, and further transformation are also reported.

Sequential Insertion of Alkynes, Alkenes, and CO into the Pd–C Bond of ortho-Palladated Primary Phenethylamines: from η3-Allyl Complexes and Enlarged Palladacycles to Functionalized Arylalkylamines

The eight-membered metallacycles arising from the insertion of 1 equiv of alkyne into the Pd–C bond of ortho-metalated homoveratrylamine and phentermine can further react with alkenes to give two different types of mononuclear complexes depending on the nature of the olefin. When terminal alkenes (styrene and ethyl acrylate) are used, a mixture of the anti/syn η³-allyl Pd(II) complexes are isolated, which evolve slowly to the syn isomers by heating the mixtures appropriately. These η³-allyl Pd(II) complexes do not react with CO or weak bases, but when they are treated with a strong base, such as KO^tBu, they afford Pd(0) and the functionalized starting phenethylamines containing a 1,3-butadienyl substituent in an ortho position. When 2-norbornene was used instead of terminal alkenes, the strained olefin inserts into the alkenyl Pd(II) complex to afford a 10-membered norbornyl palladium(II) complex, in which the new C,N-chelate ligand is coordinated to the metal through an additional double bond, occupying three coordination positions. The reactivity of these norbornyl complexes depends on the substituents on the inserted alkenyl fragment, and thus they can further react with (1) KO^tBu, to give Pd(0) and a tetrahydroisoquinoline nucleus containing a tricyclo[3.2.1]octyl ring, or (2) CO and TlOTf, to afford Pd(0) and amino acid derivatives or the corresponding lactones arising from an intramolecular Michael addition of the CO₂H group to the α,β-unsaturated ester moiety. Crystal structures of every type of compound have been determined by X-ray diffraction studies.

Palladium-catalyzed cascade cyclizations involving C–C and C–X bond formation: strategic applications in natural product synthesis

This tutorial review highlights the use of palladium-catalyzed cascade cyclizations in natural product synthesis, focusing on cascades that construct multiple rings and form both C–C and C–X (X = O, N) bonds in a single synthetic operation.

Regioselective synthesis of fused oxa-heterocycles via iodine-mediated annulation of cyclic 1,3-dicarbonyl compounds with propargylic alcohols

The synthesis of structurally diverse fused oxa-heterocycles is established through the iodine-mediated cascade annulation of cyclic 1,3-dicarbonyl compounds with propargylic alcohols.

Tunable synthesis of benzothiophene fused pyranone and thiochromen fused furan derivatives via a domino process

A K₂CO₃-promoted tunable domino reaction between thioisatins and α-bromoketones was developed for the synthesis of benzothiophene fused pyranone and thiochromen fused furan derivatives via adjusting MgSO₄.

Palladium(II)-catalyzed tandem cyclization of 2-ethynylaniline tethered cinnamyl acetate for the synthesis of indenoindoles

A palladium(ii)-catalyzed cyclization of 2-ethynylaniline tethered cinnamyl acetate involving aminopalladation/alkene insertion/β-acetoxy elimination cascade processes was established. The new protocol provides efficient access to indenoindoles in moderate to good yields under mild conditions.

Cascade polymerizations: recent developments in the formation of polymer repeat units by cascade reactions

Traditionally, most polymerizations rely on simple reactions such as alkene addition, ring-opening, and condensation because they are robust, highly efficient, and selective. These reactions, however, generally only yield a single new C-C or C-O bond during each propagation step. In recent years, novel macromolecules have been prepared with propagation steps that involve cascade reactions, enabling various combinations of bond making and breaking steps to form more complex repeat units. These polymerizations are often challenging, given the requirements for high conversion and selectivity in controlled polymerizations, yet they provide polymers with unique chemical structures and significantly broaden the scope of how polymers can be made. In this perspective, we summarize the recent developments in cascade polymerizations, primarily focusing on single-component cascades (rather than multi-component polymerizations). Polymerization performance, monomer scope, and mechanisms are discussed for polymerizations utilizing radical, ionic, and metathesis-based mechanisms.

A practical ortho-acylation of aryl iodides enabled by moisture-insensitive activated esters via palladium/norbornene catalysis

Herein reported is a practical Catellani-type ortho-acylation of aryl iodides enabled by employing moisture-insensitive esters as the electrophile via C(O)–O bond cleavage.

Catalytic enantioselective domino Michael/transannular aldol reaction under bifunctional catalysis

Chiral bifunctional tertiary amine/squaramides catalyze the enantioselective Michael/transannular aldol reaction on medium-sized cyclic ketoenones leading to bicycle[5.4.0]undecanes.

Pd-catalyzed cascade reactions involving skipped dienes: from double carbopalladation to remote C-C cleavage

We report two ligand-controlled cascade reactions relying on the intramolecular carbopalladation of skipped dienes. The use of a bulky monodentate phosphine ligand affords [4,5]-spirocycles via sequential double carbopalladation, however bidentate phosphines promote a remote β-C-elimination process which does not rely on the use of strained or sterically hindered substrates.