Diazo compounds and palladium-aryl complexes: trapping the elusive carbene migratory insertion organometallic products

The reactions of Pd-aryl complexes with diazo compounds N₂CH-CHCHPh and N₂CHPh allowed us to isolate the organometallic products formed right after the migratory insertion of a non-stabilized CHR carbene into the Pd-aryl bond. η³-Allylic and η³-benzylic palladium complexes were formed respectively. This is compelling experimental evidence for the key step in the palladium-catalyzed cascade transformations of diazo derivatives leading to multiple C-C or C-X bond formation.

Recent Advances in Transition-Metal-Catalyzed Reactions of N-Tosylhydrazones

N-Tosylhydrazones are highly versatile precursors for in situ carbene formation and are frequently used in metal-catalyzed cross-coupling reactions. Due to their many applications in organic synthesis, including C–C, C–O, C–N, and C–S bond formation, N-tosylhydrazones have recently received much interest. They can be simply synthesized by reacting an aldehyde or ketone with N-tosylhydrazine to produce a solid N-tosylhydrazone, which is a ‘green’ precursor of diazo compounds. Using a suitable metal catalyst, N-tosylhydrazones show versatile substrate scope for the synthesis of substituted diaminopyrroles, chromenopyrazoles, alkenylpyrazoles, benzofuran thioethers, tetrahydropyridazines, sulfur-containing heterocycles, and benzofurans with potent biological activities and even regioselective N-functionalization reactions. Metal-catalyzed reactions of N-tosylhydrazones for the construction of bioactive heterocycles are still highly in demand. Hence, this review focuses on the recent synthetic application of N-tosylhydrazones influenced by different transition metals with notable features like simple workup procedures, gram-scale synthesis, broad substrate scope, multicomponent processes, cyclization, and carbon–heteroatom bond formation.

1 Introduction

2 Applications of N-Tosylhydrazones

3 Conclusion

Pd-Catalyzed alkynyl aryl iodide cyclization/alkylation with cyclobutanols

A palladium-catalyzed cis-selective carboalkylation of internal alkynes with cyclobutanols is reported, providing a useful and facile approach to alkyl-substituted olefins with moderate to good yields and excellent stereoselectivity.

Palladium-catalyzed Heck cyclization/allylation with homoallyl alcohols via retro-allylation

A palladium-catalyzed reaction of aryl iodide-tethered alkenes with homoallyl alcohols is reported, providing a convenient and efficient approach to C(sp3)–allylation products.

Palladium-Catalyzed C-H Allylation of Electron-Deficient Polyfluoroarenes with gem-Difluorinated Cyclopropanes

A palladium-catalyzed C-H allylation of electron-deficient polyfluoroarenes with gem-difluorinated cyclopropanes is reported. It provides a useful and facile approach to 2-fluoroallylic polyfluoroarenes in moderate to excellent yields with high Z-selectivity. In addition, this new approach has good functional group compatibility and broad substrate scope.

Palladium Nanoparticles Supported on Smopex-234® as Valuable Catalysts for the Synthesis of Heterocycles

Supported catalysts are important tools for developing green-economy-based processes. Palladium nanoparticles (NPs) that are immobilized on two fibers developed as metal scavengers (i.e., Smopex®-234 and Smopex®-111, 1% w/w) have been prepared and tested in copper-free cyclocarbonylative Sonogashira reactions. Their catalytic activity has been compared with that of a homogeneous catalyst (i.e., PdCl2(PPh3)2). Pd/Smopex®-234 showed high activity and selectivity in the synthesis of functionalized heterocycles, such as phthalans and isochromans, even when working with a very low amount of palladium (0.2–0.5 mol%). The extension of Pd/Smopex®-234 promoted cyclocarbonylative reactions to propargyl and homopropargyl amides afforded the corresponding isoindoline and dihydrobenzazepine derivatives. A preliminary test on Pd NPs leaching into the solution (1.7 × 10−3 mg) seems to indicate that, at the end of the reaction, almost all of the active metal is present on the fiber surface.

Palladium-catalyzed intramolecular diastereoselective dearomatization reaction of indoles with N-tosylhydrazones

A novel, highly diastereoselective palladium-catalyzed dearomative reaction of N-halobenzoyl o-haloaniline derivatives has been developed using functionalized N-tosylhydrazones as the coupling partners.

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.

Recent progress on donor and donor-donor carbenes

Donor and donor-donor carbenes are two important kinds of carbenes, which have experienced tremendous growth in the past two decades. This review provides a comprehensive overview of the recent development of donor and donor-donor carbene chemistry. The development of this chemistry offers efficient protocols to construct a wide variety of C-C and C-X bonds in organic synthesis. This review is organized based on the different types of carbene precursors, including diazo compounds, hydrazones, enynones, cycloheptatrienes and cyclopropenes. The typical transformations, the reaction mechanisms, as well as their subsequent applications in the synthesis of complex natural products and bioactive molecules are discussed. Due to the rapidly increasing interest in this area, we believe that this review will provide a timely and comprehensive discussion of recent progress in donor and donor-donor carbene chemistry.

Palladium/copper-catalyzed decarbonylative heteroarylation of amides via C–N bond activation

A novel strategy for the synthesis of 2-arylated oxazole derivatives via palladium/copper-catalyzed decarbonylative heteroarylation of amides via C–N bond activation by ground-state destabilization is reported.

Transition-Metal-Catalyzed Cross-Couplings through Carbene Migratory Insertion

Transition-metal-catalyzed cross-coupling reactions have been well-established as indispensable tools in modern organic synthesis. One of the major research goals in cross-coupling area is expanding the scope of the coupling partners. In the past decade, diazo compounds (or their precursors N-tosylhydrazones) have emerged as nucleophilic cross-coupling partners in C-C single bond or C═C double bond formations in transition-metal-catalyzed reactions. This type of coupling reaction involves the following general steps. First, the organometallic species is generated by various processes, including oxidative addition, transmetalation, cyclization, C-C bond cleavage, and C-H bond activation. Subsequently, the organometallic species reacts with the diazo substrate to generate metal carbene intermediate, which undergoes rapid migratory insertion to form a C-C bond. The new organometallic species generated from migratory insertion may undergo various transformations. This type of carbene-based coupling has proven to be general: various transition metals including Pd, Cu, Rh, Ni, Co, and Ir are effective catalysts; the scope of the reaction has also been extended to substrates other than diazo compounds; and various cascade processes have also been devised based on the carbene migratory insertion. This review will summarize the achievements made in this field since 2001.

N-Tosylhydrazones: versatile synthons in the construction of cyclic compounds

N-Tosylhydrazones have been extensively explored as versatile building blocks in the construction of various cyclic compounds.

Pd-catalyzed cascade reactions between o-iodo-N-alkenylanilines and tosylhydrazones: novel approaches to the synthesis of polysubstituted indoles and 1,4-dihydroquinolines

Pd-catalyzed reactions between tosylhydrazones and o-iodo-N-alkenylanilines provide indoles or dihydroquinolines through carbene migratory insertion/substituent-controlled cyclization cascades.

Access to steroidal pyridazines via modified thiohydrazides

Cytotoxic effects against breast cancer cells.

The Pd-catalyzed synthesis of benzofused carbo- and heterocycles through carbene migratory insertion/carbopalladation cascades with tosylhydrazones

Indenes and benzofurans are synthesized from tosylhydrazones and o-substituted aryliodides through a Pd-catalyzed carbene migratory insertion/intramolecular carbopalladation cascade reaction.

Access to polysubstituted indoles or benzothiophenes via palladium-catalyzed cross-coupling of furfural tosylhydrazones with 2-iodoanilines or 2-iodothiophenols

Palladium-catalyzed cross-coupling of furfural tosylhydrazones with 2-iodoanilines or 2-iodothiophenols produces polysubstituted indoles or benzothiophenes, respectively.

A simple copper-catalyzed two-step one-pot synthesis of indolo[1,2-a]quinazoline

A convenient CuI/L-proline-catalyzed, two-step one-pot method has been developed for the preparation of indolo[1,2-a]quinazoline derivatives using a sequential Ullmann-type C–C and C–N coupling. This protocol provides an operationally simple and rapid strategy for preparing indolo[1,2-a]quinazoline derivatives and displays good functional group tolerance. All the starting materials are commercial available or can be easily prepared.

Palladium-catalyzed cross-coupling reaction of diazo compounds and vinyl boronic acids: an approach to 1,3-diene compounds

A palladium-catalyzed oxidative cross-coupling of vinyl boronic acids and cyclic α-diazocarbonyl compounds has been reported. The reaction constitutes an efficient method for the synthesis of 1,3-diene compounds bearing a ring structure. Mechanistically, the reaction involves migratory insertion of palladium carbene as the key step.

Palladium-catalyzed/norbornene-mediated ortho-amination/N-tosylhydrazone insertion reaction: an approach to the synthesis of ortho-aminated vinylarenes

ortho-Aminated vinylarene derivatives were obtained via a reaction of aryl iodides, N-benzoyloxyamines, and N-tosylhydrazones. This approach involves a palladium-catalyzed, norbornene-mediated ortho-amination/N-tosylhydrazone insertion reaction. In this transformation, one C-N bond and one C-C bond are formed and an amine group is introduced at the ortho position successfully.

Palladium-catalyzed three-component reaction of N-tosylhydrazone, norbornene and aryl halide

A palladium-catalyzed three-component reaction that involves an intermolecular Heck-type reaction and alkyl palladium carbene migratory insertion has been demonstrated.