Intramolecular Remote C-H Activation via Sequential 1,4-Palladium Migration To Access Fused Polycycles

Controllable 1,4-Palladium Aryl to Aryl Migration in Fused Systems─Application to the Synthesis of Azaborole Multihelicenes

Herein, we report the first 1,4-Pd aryl to aryl migration/Miyaura borylation tandem reaction in fused systems. The Pd shift occurred in the bay region of the dibenzo[g,p]chrysene building blocks, giving rise to a thermodynamically controlled mixture of 1,8- and 1,9-borylated compounds that allowed the preparation of regioisomeric azaborole multihelicenes from the same starting material. The outcome of this synthesis can be controlled by the choice of reaction conditions, allowing the migration process to be turned off in the absence of an acetate additive and the target multiheterohelicenes to be prepared in a regioselective manner. The target compounds show bright green fluorescence in dichloromethane with emission quantum yields (Φ) of up to 0.29, |glum| values up to 2.7 × 10-3, and green or green-yellow emission in the solid state, reaching Φ of 0.22. Single crystal X-ray diffraction analyses gave insight into their molecular structures and the packing arrangement. Evaluation of aromaticity in these multihelicenes revealed a nonaromatic character of the 2H-1,2-azaborole constituent rings.

Remote C─H Bond Activation via Enantioselective Carbopalladation and 1,4-Pd Migration Cascade Process

Carbopalladation-initiated cascade reaction involving 1,4-Pd migration is a straightforward and powerful approach to activate remote C─H bond, forging versatile fused polycyclic compounds containing fluorene fragment which are highly valuable synthetic targets. However, its asymmetric variants pose considerable challenges and have not been explored. Here the first asymmetric palladium-catalyzed tandem carbopalladation is reported, 1,4-Pd migration reaction of ortho-iodophenol-derived allyl ether under mild conditions, allowing the transformation of a wide range of substrates in good to excellent enantioselectivities, and providing a facile and straight forward access to tetracyclic dihydroindeno[1,2,3-de]chromene bearing a chiral fluorene skeleton. A good functional group tolerance, high stereoselectivity, as well as the good chiroptical properties (high fluorescence quantum yields, circular dichroism) of the products make this approach highly attractive. Moreover, density functional theory (DFT) calculations indicate that the protonation of five-membered palladacycle intermediate is more favorable rather than its direct reductive elimination process.

Tunable Construction of Multisubstituted 1,3-Dienes and Allenes via a 1,4-Palladium Migration/Carbene Insertion Cascade

Efficient palladium-catalyzed vinylic C-H alkenylation and allenylation of gem-disubstituted ethylenes with N-tosylhydrazones of aryl alkyl and diaryl ketones were achieved to access trisubstituted 1,3-dienes and tetrasubstituted allenes, respectively. An aryl to vinyl 1,4-palladium migration/carbene insertion/β-hydride elimination sequence proceeded to switch the chemo- and regioselectivities to give structurally diverse products. Use of 2-FC₆H₄OH additive enables enhancement of the reaction efficiency through accelerating the key 1,4-palladium migration process.

Tandem Reactions involving 1,4-Palladium Migrations

Transition-metal-catalyzed tandem reactions have become a mainstay in organic chemistry owing to their high atom- and step-economies. Metal-migration-based tandem reactions allow the engagement of simple starting materials for incorporating functional groups into certain positions and constructing complex scaffolds, which provide novel means that are complementary to traditional cross-coupling or C-H activation processes. In light of the broad utility of the 1,4-Pd migration reaction, this paper reviews its progress in the past two decades, summarizing the tandem process and classifying it based on insertion, elimination, transmetalation, and C-H bond activation. Special emphasis is placed on the driving force of Pd migration and different migration mechanisms. Moreover, this review also attempts to summarize common strategies for improving the regio- and site-selectivities of the migration process.

Synthesis of tetrasubstituted allenes via a 1,4-palladium migration/carbene insertion/β-H elimination sequence

A palladium-catalyzed synthesis of tetrasubstituted allenes from aryl bromides and aryl diazoacetates is developed. This transformation proceeded via an aryl to alkenyl 1,4-palladium migration/carbene insertion/β-H elimination sequence under mild reaction conditions.

Alkylidene Carbene from Silyl Vinyl Iodide Provides Mechanistic Insights on Trimethylenemethane Diyl-Mediated Tandem Cyclizations

We describe a method to generate alkylidene carbenes via tetramethylammonium-fluoride-induced desilylation of silyl vinyl iodides. The reversible carbene generation from an iodovinyl anion enabled us to unearth mechanistic aspects of the trimethylenemethane (TMM) diyl cyclization reaction that could not be explored via previous methods. We observed that a slow diyl-diylophile cycloaddition can induce the reversible formation of an alkylidene carbene from the TMM diyl intermediate via a retro-cyclopropanation at ambient temperature.

Phosphorylation of C(sp3)-H Bonds via 1,4-Palladium Migration

1,4-Palladium migration has emerged as a reliable method for directed C-H functionalization. In contrast to coupling with carbon nucleophiles, limited examples with heteroatom nucleophiles have been reported. Herein we report a palladium-catalyzed intermolecular C(sp³)-H phosphorylation reaction via 1,4-palladium migration, which is often difficult because of the strong coordination of phosphorus reagents to palladium catalysts. Phosphorylation of C(sp³)-H bonds is accomplished in good reaction yields with excellent regioselectivity. The judicious selection of the phosphine ligand proved to be the key to the success of this cascade process.

Intermolecular C-H silylation through cascade carbopalladation and vinylic to aryl 1,4-palladium migration

A palladium-catalyzed remote C-H silylation reaction has been developed through vinylic to aryl 1,4-palladium migration. By using alkyne-tethered aryl iodides as the starting materials and hexamethyldisilane as the silylating reagent, the reaction involves cascade intramolecular carbopalladation, 1,4-palladium migration, and silylation with hexamethyldisilane, and leads to the formation of exocyclic alkene-containing 5-silylisoquinolines as the final products.

Regioselective Synthesis of Substituted Carbazoles, Bicarbazoles, and Clausine C

Substituted carbazoles are efficiently constructed from 3-triflato-2-pyrones and alkynyl anilines. Multiple substituents are tolerated on the carbazole, and complete control of regiochemistry is observed. Complicated and sterically congested substitution patterns are produced. This strategy is also used to prepare substituted bicarbazoles and related biaryls. Finally, the method was showcased in a synthesis of the carbazole natural product clausine C.

Transition Metal-Catalyzed Intermolecular Cascade C-H Activation/Annulation Processes for the Synthesis of Polycycles

Polycycles are abundantly present in numerous advanced chemicals, functional materials, bioactive molecules and natural products. However, the strategies for the synthesis of polycycles are limited to classical reactions and transition metal-catalyzed cross-coupling reactions, requiring pre-functionalized starting materials and lengthy synthetic operations. The emergence of novel approaches shows great promise for the fields of organic/medicinal/materials chemistry. Among them, transition metal-catalyzed C-H activation followed by intermolecular annulation reactions prevail, due to their straightforward manner with high atom- and step-economy, providing rapid, concise and efficient methods for the construction of diverse polycycles. Several strategies have been developed for the synthesis of polycycles, relying on sequential multiple C-H activation/annulation, or combination of C-H activation/annulation and further interaction with a proximal group, or merger of C-H activation with a cycloaddition reaction, or in situ formation of the directing group. These are attractive, efficient, step- and atom-economic methods starting from commercially available materials. This Minireview will provide an introduction to transition metal-catalyzed C-H activation for the synthesis of polycycles, helping researchers to discover indirect connections and reveal hidden opportunities. It will also promote the discovery of novel synthetic strategies relying on C-H activation.

Copper-Catalyzed Tandem Radical Cyclization of 8-Ethynyl-1-naphthyl-amines for the Synthesis of 2H-Benzo[e][1,2]thiazine 1,1-Dioxides and its Fluorescence Properties

A copper-catalyzed radical cascade dehydrogenative cyclization of N-tosyl-8-ethynyl-1-naphthylamines under air is described herein for the synthesis of thioazafluoranthenes. The reaction proceeds smoothly with high efficiency and a broad reaction scope. The product is indeed a new fluorophore and its photophysical properties are also investigated. Based on the results, we are pleased to find that the Stokes shift of amino-linked thioazafluoranthenes in dilute tetrahydrofuran is determined to be 143 nm (4830 cm^-1).

Facile access to 2,2-diaryl 2H-chromenes through a palladium-catalyzed cascade reaction of ortho-vinyl bromobenzenes with N-tosylhydrazones

A palladium-catalyzed cascade reaction of ortho-vinyl bromobenzenes with N-tosylhydrazones has been developed, which provides a facile approach to 2,2-disubstituted 2H-chromenes. The migration of palladium from the aryl to vinyl position is crucial, as the in situ produced vinyl palladium intermediate could further react with diazo compounds to generate the reactive species for the sequential annulation.

Understanding the Chemoselectivity in Palladium-Catalyzed Three-Component Reaction of o-Bromobenzaldehyde, N-Tosylhydrazone, and Methanol

To understand the ligand-controlled palladium-catalyzed coupling of o-bromobenzaldehyde, N-tosylhydrazone, and methanol to give methyl 2-benzylbenzoic ester or methyl ether, we herein investigated the mechanisms which account for how C-C and C-O bonds are formed and why bidentate dppf/dppb ligands afford ester, whereas P(o-tolyl)₃ ligand gives ether. The ester chemoselectivity of the bidentate ligands is attributed to the strong electron-donating effect that disfavors the C,Br-reductive elimination of the coupling intermediate of o-bromobenzaldehyde and N-tosylhydrazone.

Palladium-catalyzed intermolecular C–H silylation initiated by aminopalladation

The intermolecular disilylation reaction of C,C-palladacycles obtained through aminopalladation has been developed.

Recent advances in transition metal migration involving reactions

In this review, various types of metal-migration involved methodologies, including palladium, rhodium, iron, cobalt, iridium, chromium, nickel, platinum, are summarized and demonstrated elaborately for giving a better access to this field.