Epoxides as Alkylating Reagents for the Catellani Reaction

Facile enantioselective synthesis of multi-substituted norbornanes/norbornenes using a latent synthon strategy

The development of a new catalytic asymmetric synthetic methodology has been following virtually the same pattern in the past decades. Herein, we present a latent synthon strategy within this well-established research domain. By employing substrates containing latent groups, specifically "ON-alkene" in this investigation, a single optimization exercise yields the Diels-Alder adduct with excellent enantiomeric purity. This adduct serves as a universal intermediate, undergoing late-stage diversifications via robust and easily performed synthetic transformations. Consequently, a broad array of structurally diverse chiral norbornanes (NBAs) and norbornenes (NBEs) are obtained with consistent and high enantiomeric purities. Furthermore, our methodology allows for facile asymmetric preparation of chiral NBE ligands as well as the concise synthesis of (+)-gemmacin, ( + )-gemmacin B, and their structural analogs.

A switch strategy for the synthesis of C4-ethylamine indole and C7-aminoindoline via controllable carbon elimination

Controllable β-carbon elimination to extrude norbornene remains a long-standing challenge in palladium and norbornene chemistry. Herein, this manuscript describes a switchable synthesis of biologically active C4-ethylaminoindole and C7-aminoindoline scaffolds by controlling β-carbon elimination, utilizing aziridine as a C-H ethylamination reagent through a C-N bond cleavage reaction. Furthermore, the protecting groups of the product can be easily removed, offering an unusual method for the synthesis of dopamine receptor agonists.

Unlocking regioselective meta-alkylation with epoxides and oxetanes via dynamic kinetic catalyst control

Regioselective arene C-H bond alkylation is a powerful tool in synthetic chemistry, yet subject to many challenges. Herein, we report the meta-C-H bond alkylation of aromatics bearing N-directing groups using (hetero)aromatic epoxides as alkylating agents. This method results in complete regioselectivity on both the arene as well as the epoxide coupling partners, cleaving exclusively the benzylic C-O bond. Oxetanes, which are normally unreactive, also participate as alkylating reagents under the reaction conditions. Our mechanistic studies reveal an unexpected reversible epoxide ring opening process undergoing catalyst-controlled regioselection, as key for the observed high regioselectivities.

Construction of Indole-Fused Seven- and Eight-Membered Azaheterocycles via a Tandem Pd/NBE-Catalyzed Decarbonylation and Dual C-H Activation Sequence

Herein, we report the transformation of aromatic acids to indole-fused seven- and eight-membered azaheterocycles. Two C-C bonds are formed via the cleavage of one C-C bond and two C-H bonds. The incorporation of indole moieties into bioactive pharmaceuticals and natural products to construct a medium-sized polyfused heterocycle demonstrates the synthetic utility of the protocol.

One-Pot Three-Component Synthesis of Phenanthrenes via Palladium-Catalyzed Catellani and Retro-Diels-Alder Reactions

An efficient one-pot three-component palladium-catalyzed domino reaction of aryl iodide, 2-bromophenylboronic acid, and norbornadiene to produce phenanthrenes has been developed. Norbornadiene serves both as the activator of ortho-C-H bond and the source of ethylene via a retro-Diels-Alder reaction. The method features inexpensive and readily available substrates, a broad range of functional groups, and good yields.

Palladium-catalyzed and norbornene-mediated C-H amination and C-O alkenylation of aryl triflates

The C-H amination and C-O alkenylation of aryl triflates was achieved through Pd/norbornene (NBE) cooperative catalysis. By this strategy, various ortho-alkenyl tertiary anilines including those bearing functional groups were produced in good to excellent yields. This reaction represents a new conversion model for phenoxides. It expands the scope of Catellani-type reactions and the application of phenoxides in organic synthesis.

Benzo-Fused-Ring Toolbox Based on Palladium/Norbornene Cooperative Catalysis: Methodology Development and Applications in Natural Product Synthesis

ConspectusBenzo-fused skeletons are ubiquitous in agrochemicals, medicines, natural products, catalysts, and other organic function materials. The assembly of these skeletons in an efficient manner is an actively explored field in organic synthesis. Palladium/norbornene (Pd/NBE) cooperative catalysis is a powerful tool for the expeditious assembly of polysubstituted arenes through bis-functionalization of the ortho and ipso positions of aryl iodides in one operation. Owing to the efforts of Lautens, Catellani, and others, an array of Pd/NBE-promoted annulations for the syntheses of diversified benzo-fused rings have been developed. However, these methods have not been broadly applied in total synthesis yet.Our group is interested in efficient and practical total synthesis of biologically active molecules. In the past 7 years, we have been devoted to the development of new annulation strategies for the assembly of common benzo-fused skeletons through Pd/NBE-promoted reactions of aryl iodides with novel bifunctional reagents. In this Account, we summarize our laboratory's systematic efforts in this direction. First, readily available epoxides and aziridines were exploited as versatile bifunctional alkylating reagents, which enables quick assembly of a series of valuable benzo-fused heterocycles, including isochromans, dihydrobenzofurans, 1,3-cis-tetrahydroisoquinolines (THIQs), 1,3-trans-THIQs, etc. Second, a convergent access to 5-7-membered benzo-fused carbocycles (including indanes and tetrahydronaphthalenes) was developed by Pd/NBE-promoted annulation of aryl iodides with simple olefinic alcohol-containing alkylating reagents. Third, a Pd/NBE-promoted annulation between aryl iodides and cyclohexanone-containing amination reagents was developed for the construction of benzo-fused N-containing bridged scaffolds. Thus, we have established a practical and versatile toolbox for the quick assembly of diversified benzo-fused skeletons. These new annulation reactions are of high chemo-, regio-, and stereoselectivities with good step and atom economy. Moreover, they are able to rapidly increase molecular complexity from simple building blocks. Finally, their synthetic value has been demonstrated by immediate adoption in several efficient total syntheses of medicines and complex natural products. Compared to conventional synthetic logics, the Pd/NBE-promoted annulation toolbox allows the development of highly convergent strategies, which significantly improves the overall synthetic efficiency.We believe the results presented in this Account will have significant implications beyond our research. It can be envisaged that new Pd/NBE-promoted annulations as well as new applications in complex total synthesis will be revealed in the near future.

Chiral Indoline-2-carboxylic Acid Enables Highly Enantioselective Catellani-type Annulation with 4-(Bromomethyl)cyclohexanone

Chiral indoline-2-carboxylic acid has been identified to enable a highly enantioselective Catellani-type annulation of (hetero)aryl, alkenyl triflate and conjugated vinyl iodides with 4-(bromomethyl)cyclohexanone, directly assembling a diverse range of chiral all-carbon bridged ring systems. Control experiments and DFT calculations suggest that the coordinating orientation of the chiral amino acid to the arylpalladium(II) center allows for high levels of stereochemical control.

Application of Pd-Catalyzed C-H Alkylation Reaction in Total Syntheses of Twelve Amicoumacin-Type Natural Products

Enantioselective total syntheses of 12 amicoumacin-type natural products are accomplished with a palladium(II)-catalyzed C-H alkylation as the key step to furnish the 3,4-dihydroisocoumarin scaffold. The target chemicals are assembled in a convergent protocol by merging 3,4-dihydroisocoumarin derived amine part with categories of acid segments that are efficiently prepared by chemoselective catalytic oxidation of chiral 1,2-dihydroxyethylfuran-2(5H)-ones. Afterward, the cytotoxicity of amicoumacins on five cancer cell lines and one normal cell line is investigated.

A Concise Total Synthesis of (-)-Berkelic Acid

Reported here is a concise total synthesis of (-)-berkelic acid in eight linear steps. This synthesis features a Catellani reaction/oxa-Michael cascade for the construction of the isochroman scaffold, a one-pot deprotection/spiroacetalization operation for the formation of the tetracyclic core structure, and a late-stage Ni-catalyzed reductive coupling for the introduction of the lateral chain. Notably, four stereocenters are established from a single existing chiral center with excellent stereocontrol during the deprotection/spiroacetalization process. Stereocontrol of the intriguing deprotection/spiroacetalization process is supported by DFT calculations.

Pd(II)-Catalyzed Annulation Reactions of Epoxides with Benzamides to Synthesize Isoquinolones

Epoxides as alkylating reagents are unprecedentedly applied in Pd(II)-catalyzed C-H alkylation and oxidative annulation of substituted benzamides to synthesize isoquinolones rather than isochromans, which is accomplished through alerting the previously reported reaction mechanism by the addition of oxidant and TEA. Under these conditions, various isoquinolones have been prepared with yields up to 92%. In addition, this methodology has been successfully employed in the total syntheses of rupreschstyril, siamine, and cassiarin A in an expedient fashion.

Synthesis of Chiral-Substituted 2-Aryl-ferrocenes by the Catellani Reaction

A palladium-catalyzed and norbornene-mediated methodology has been developed for the synthesis of chiral 2-aryl-ferroceneamides from chiral 2-iodo-N,N-diisopropylferrocencarboxamide, iodoarenes, and alkenes using a JohnPhos ligand and potassium carbonate as a base in dimethylformamide at 105 °C. The developed three-component coupling protocol allows the compatibility of electron-withdrawing fluoro, chloro, ester, and nitro and electron-donating methyl, methoxy, dimethoxy, benzyl ether-substituted iodo-benzenes, other iodoarenes, such as iodo-naphthalene, heteroarenes, such as iodothiophene, and terminating substrates, such as methyl, ethyl, tert-butyl acrylates, and substituted styrenes with 2-iodo-N,N-diisopropylferrocencarboxamide. Furthermore, the developed three-component Catellani method proceeded with the retention of the configuration of the planar chiral ferrocene, which depends on the role of the participating carbon-iodine bond in ferrocene. Consequently, the developed protocol enabled the formation of densely substituted chiral 2-aryl ferroceneamides, exhibiting good to excellent enantioselectivity. The conversion of an ester of the synthesized chiral 2-aryl ferroceneamides has also been carried out to further accommodate the easily expendable acid and alcohol functionalities.

Structurally Modified Norbornenes: A Key Factor to Modulate Reaction Selectivity in the Palladium/Norbornene Cooperative Catalysis

Palladium/norbornene (Pd/NBE) cooperative catalysis has received enormous attention and found numerous synthetic applications in the past two decades. Considering the critical roles that NBE plays in the catalytic cycle, the use of structurally modified NBEs (smNBEs), starting from 2015, has become an important approach to address limitations and modulate reaction selectivity in Pd/NBE catalysis. This Perspective highlights the development of three types of smNBEs: C1-substituted, C2-substituted, and C5-substituted or C5,C6-disubstituted NBEs, as well as their synthetic applications toward site-selective C-H functionalization. A focus is on the structure-activity relationship of smNBEs in these reactions, and rationales for using smNBEs in many cases have also been provided.

Modular and Stereoselective Synthesis of C-Aryl Glycosides via Catellani Reaction

In this work, we describe a Catellani-type C-H glycosylation to provide rapid access to various highly decorated α-C-(hetero)aryl glycosides in a modular and stereoselective manner (>90 examples). The termination step is flexible, which is demonstrated by ipso-Heck reaction, hydrogenation, Suzuki coupling, and Sonogashira coupling. Application of this methodology has been showcased by preparing glycoside-pharmacophore conjugates and a dapagliflozin analogue. Notably, the technology developed herein represents an unprecedented example of Catellani-type alkylation involving an S_N1 pathway.

DMAP and PivOH-promoted amination/allenization reaction

This report described the first DMAP and PivOH-promoted ortho-C-H amination and ipso-allenization reaction of iodobenzenes realized by Pd/norbornene cooperative catalysis. Based on control experiments and DFT calculations, we speculated that the three ligands have different functions and mechanism paths in the reaction.

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.

Pd/NBE-catalyzed sequential carbamoylation/olefination of aryl iodides

We present a Pd/NBE-catalyzed sequential carbamoylation/olefination of aryl iodides under mild reaction conditions, which provide diverse 4-methylene-3,4-dihydro-1(2H)-isoquinolin-1-one analogues.

Palladium-catalyzed tandem reaction of epoxynitriles with arylboronic acids in aqueous medium: divergent synthesis of furans and pyrroles

A palladium-catalyzed controllable divergent synthesis of furans and pyrroles through a cascade reaction of epoxynitriles with arylboronic acids has been developed in the aqueous phase.

Ortho/Ipso Alkylborylation of Aryl Iodides

This work describes a method for the difunctionalization of aryl iodides to generate polysubstituted arenes via Pd catalysis. The reaction hinges on the unique interplay between norbornene and the metal catalyst to impart a guided ortho C-H alkylation event followed by a programmatic ipso borylation to provide a diverse array of substituted arene products. The utility of this transformation is demonstrated through the functionalization of the boronic ester to a variety of valuable functionalities.

Synthesis of Benzofused Dioxabicycle Scaffolds via a Catellani Strategy

Reported is a modular strategy for the preparation of the unique benzofused dioxabicycle scaffolds involving a Catellani reaction of aryl iodides, epoxides, and terminal alkynes and an oxa-cyclization. This is a mild, scalable, chemoselective, and atom-economical protocol, compatible with various functionalized aryl iodides, epoxides, and terminal alkynes. With the ability to build up the molecular complexity rapidly and efficiently from feedstock chemicals, this method will have wide applications in organic synthesis.

Redox-neutral ortho-C-H amination of pinacol arylborates via palladium(ii)/norbornene catalysis for aniline synthesis

A palladium(ii)/norbornene cooperative catalysis enabled redox-neutral ortho-C-H amination of pinacol aryl- or heteroarylborates for the synthesis of structurally diverse anilines is reported. This method is scalable, robust (tolerance of air and moisture), phosphine ligand-free, and compatible with a wide range of functionalities. These practical features make this reaction amenable for industry. A plethora of synthetically very useful halogenated anilines, which often cannot be prepared via other transition-metal-catalyzed aminations, are readily produced using this method. Particularly, the orthogonal reactivity between pinacol arylborates and aryl iodides is demonstrated. Preliminary deuterium-labeling studies reveal a redox-neutral ipso-protonation mechanism of this process, which will surely inspire the future development of this field. Overall, the exceptionally broad scope (47 examples) and reliability of this procedure, together with the wide availability of pinacol arylborates, make this chemistry a valuable addition to the existing methods for aniline synthesis.

Modular Dual-Tasked C-H Methylation via the Catellani Strategy

We report a dual-tasked methylation that is based on cooperative palladium/norbornene catalysis. Readily available (hetero)aryl halides (39 iodides and 4 bromides) and inexpensive MeOTs or trimethylphosphate are utilized as the substrates and methylating reagent, respectively. Six types of "ipso" terminations can modularly couple with this "ortho" C-H methylation to constitute a versatile methylation toolbox for preparing diversified methylated arenes. This toolbox features inexpensive methyl sources, excellent functional-group tolerance, simple reaction procedures, and scalability. Importantly, it can be uneventfully extended to isotope-labeled methylation by switching to the corresponding reagents CD₃OTs or ¹³CH₃OTs. Moreover, this toolbox can be applied to late-stage modification of biorelevant substrates with complete stereoretention. We believe these salient and practical features of our dual-tasked methylation toolbox will be welcomed by academic and industrial researchers.

Synthesis of fluorenyl alcohols via cooperative palladium/norbornene catalysis

Herein, we report a novel catalytic synthesis of substituted 9H-fluoren-9-ols starting from aryl iodides and secondary ortho-bromobenzyl alcohols in the presence of palladium/norbornene as a catalytic system. The present protocol exhibits high functional group tolerance, mild reaction conditions and moderate to good yields. This transformation is based on two sequential pathways: (i) Pd(ii)-mediated oxidation of the secondary alcohol to the corresponding ketone and (ii) Pd(0)/norbornene-catalyzed reaction of the in situ generated ortho-bromoacetophenone with the aryl iodide.

Palladium/Norbornene Cooperative Catalysis

Palladium/norbornene cooperative catalysis has emerged as a distinct approach to construct polyfunctionalized arenes from readily available starting materials. This Review provides a comprehensive overview of this field, including the early stoichiometric investigations, catalytic reaction developments, as well as the applications in the syntheses of bioactive compounds and polymers. The section of catalytic reactions is divided into two parts according to the reaction initiation mode: Pd(0)-initiated reactions and Pd(II)-initiated reactions.

Selective Ortho Thiolation Enabled by Tuning the Ancillary Ligand in Palladium/Norbornene Catalysis

Site-selective introduction of a sulfur group into aromatic compounds is essential and useful in organic, material, and pharmaceutical chemistry. A palladium/norbornene-catalyzed chemoselective ortho thiolation of aryl halides was reported. The selectivity of reductive elimination for C(Ar)-SR bond formation was well controlled by tuning the ancillary ligand in the aryl-NBE palladacycle Pd(IV) intermediate. The reaction showcased good substrate scope: both S-alkyl and S-aryl thiosulfonates were compatible.

NBE-Controlled Palladium-Catalyzed Interannular Selective C-H Silylation: Access to Divergent Silicon-Containing 1,1'-Biaryl-2-Acetamides

A novel palladium-catalyzed interannular selective C-H silylation of 1,1'-biaryl-2-acetamides is described. The combination of palladium catalyst with copper oxidant enables meta- or ortho-selective C-H silylation by employing hexamethyldisilane as a trimethylsilyl source, which relies on the control of NBE derivatives as a switch, thus providing straightforward access to divergent silicon-containing 1,1'-biaryl-2-acetamides.

Redox-Neutral ortho Functionalization of Aryl Boroxines via Palladium/Norbornene Cooperative Catalysis

Palladium/norbornene (Pd/NBE) cooperative catalysis, also known as the Catellani reaction, has become an increasingly useful method for site-selective arene functionalization; however, certain constraints still exist due to its intrinsic mechanistic pathway. Herein we report a redox-neutral ortho functionalization of aryl boroxines via Pd/NBE catalysis. An electrophile, such as carboxylic acid anhydrides or O-benzoyl hydroxylamines, is coupled at the boroxine ortho position, and a proton as the second electrophile is introduced at the ipso position. This reaction does not require extra oxidants or reductants, and avoids stoichiometric bases or acids, thereby tolerating a wide range of functional groups. In particular, orthogonal chemoselectivity between aryl iodide and boroxine moieties is demonstrated, which could be used to control reaction sequences. Finally, a deuterium labelling study supports the ipso-protonation pathway. This unique mechanistic feature could inspire the development of a new class of Pd/NBE-catalyzed transformations.

Palladium/Norbornene-Catalyzed Indenone Synthesis from Simple Aryl Iodides: Concise Syntheses of Pauciflorol F and Acredinone A

To show the synthetic utility of palladium/norbornene (Pd/NBE) cooperative catalysis, here we report concise syntheses of indenone-based natural products, pauciflorol F and acredinone A, which are enabled by direct annulation between aryl iodides and unsaturated carboxylic acid anhydrides. Compared to the previous indenone-preparation approaches, this method allows simple aryl iodides to be used as substrates with complete control of the regioselectivity. The total synthesis of acredinone A features two different Pd/NBE-catalyzed ortho acylation reactions for constructing penta-substituted arene cores, including the development of a new ortho acylation/ipso borylation.

Stereoselective Multicomponent Reactions in the Synthesis or Transformations of Epoxides and Aziridines

Small ring heterocycles, such as epoxides and aziridines, are present in several natural products and are also highly versatile building blocks, frequently involved in the synthesis of numerous bioactive products and pharmaceuticals. Because of the potential for increased efficiency and selectivity, along with the advantages of environmentally benign synthetic procedures, multicomponent reactions (MCRs) have been explored in the synthesis and ring opening of these heterocyclic units. In this review, the recent advances in MCRs involving the synthesis and applications of epoxides and aziridines to the preparation of other heterocycles are discussed emphasizing the stereoselectivity of the reactions.

A novel and efficient synthesis of phenanthrene derivatives via palladium/norbornadiene-catalyzed domino one-pot reaction

Herein we report a novel palladium-catalyzed reaction that results in phenanthrene derivatives using aryl iodides, ortho-bromobenzoyl chlorides and norbornadiene in one pot. This dramatic transformation undergoes ortho-C-H activation, decarbonylation and subsequent a retro-Diels-Alder process. Pleasantly, this protocol has a wider substrate range, shorter reaction times and higher yields of products than previously reported methods.