Palladium-Catalyzed Carbene Insertion and Trapping with Carbon Nucleophiles

Versatile Reactivity of Metalloid-Substituted π-Allylpalladium Species

π-Allylpalladium complexes can not only serve as electrophilic allylating agents for a broad range of nucleophiles, but also nucleophilic allylating agents for electrophiles depending on their electronic environments. In contrast to these typical reactivities of π-allylpalladium complexes, silylated and borylated π-allylpalladium species show unique reactivities that can allow versatile transformations in addition to simple allylation. Herein, four different types of transformations that are in principle achieved via the inherently reactive silylated and borylated π-allylpalladium species as common intermediates are described. An appropriate selection of ligands of the silylated and borylated π-allylpalladium species can allow control over the reaction pathways.

Synthetic and Computational Studies on RhIII-Catalyzed Redox-Neutral Cascade of Carbenoid Functionalization and Dephosphonylative Annulation

A Rh(III)-catalyzed regioselective redox-neutral cascade process of carbenoid functionalization followed by dephosphonylative annulation of benzoic acids with α-diazo-β-keto phosphonate has been realized, which led to the direct synthesis of a privileged 3-substituted isocoumarin scaffold. To the best of our knowledge, this is the first report of a complete redox neutral method to synthesize isocoumarins using C-H functionalization strategy. In the catalytic cycle of this reaction, there are two possible pathways for the C-C coupling between ortho-positioned carbon atom of benzoic acid and the diazo carbon atom: (i) concerted 1,2-aryl shift and (ii) stepwise metal-carbene formation followed by migratory insertion. DFT study has predicted that the concerted pathway has lower activation energy as compared to the stepwise pathway by 1.5 kcal/mol.

Diverting β-Hydride Elimination of a π-Allyl PdII Carbene Complex for the Assembly of Disubstituted Indolines via a Highly Diastereoselective (4 + 1)-Cycloaddition

A Pd⁰-catalyzed formal (4 + 1)-cycloaddition approach to 2,3-disubstituted dihydroindoles is described. The diastereoselective formation of dihydroindoles that is highlighted by a carbene migratory insertion/reductive elimination sequence proceeding via a π-allyl Pd^II-species compliments existing methods of indoline assembly.

Catalytic three-component C-C bond forming dearomatization of bromoarenes with malonates and diazo compounds

A Pd-catalyzed dearomative three-component C-C bond formation of bromoarenes with diazo compounds and malonates was developed. Various bromoarenes ranging from benzenoids to azines and heteroles were transformed to the corresponding substituted alicyclic molecules. The key to this reaction is the generation of a benzyl-palladium intermediate, which reacts with malonates to form a Pd-O-enolate species. Strikingly, the present method enabled rapid access to multi-substituted alicycles through subsequent elaboration of dearomatized products.

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.

Development of Robust 17(R),18(S)-Epoxyeicosatetraenoic Acid (17,18-EEQ) Analogues as Potential Clinical Antiarrhythmic Agents

17(R),18(S)-Epoxyeicosatetraenoic acid (EEQ) is a cytochrome P450 metabolite of eicosapentaenoic acid (EPA) and a powerful negative chronotrope with low nanomolar activity in a neonatal rat cardiomyocyte (NRCM) arrhythmia model. Prior studies identified oxamide 2b as a soluble epoxide hydrolase (sEH) stable replacement but unsuitable for in vivo applications due to limited oral bioavailability and metabolic stability. These ADME limitations have been addressed in an improved generation of negative chronotropes, e.g., 4 and 16, which were evaluated as potential clinical candidates.

Palladium-catalyzed cascade reactions of alkene-tethered carbamoyl chlorides with N-tosyl hydrazones: synthesis of alkene-functionalized oxindoles

An efficient approach for the construction of alkene-functionalized oxindoles was developed via palladium-catalyzed cascade reactions of alkene-tethered carbamoyl chlorides with N-tosyl hydrazones.

Palladium-catalyzed heck-type cascade cyclization of (Z)-1-iodo-1,6-dienes with N-tosyl hydrazones

A new way to synthesize tetrahydropyridine derivatives bearing a terminal alkene due to highly regioselective β-H elimination of alkylpalladium species.

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.

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.

3-Aza π-allyl palladium derived from imino migration in palladium-carbene: MCRs toward multiple substituted indole skeleton

Palladium-catalyzed multi-component reactions (MCRs) between 2-iodoaniline, aryl isonitrile, N-tosylhydrazones and solvent were developed.

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.

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.

Palladium catalyzed N–H bond insertion and intramolecular cyclization cascade: the divergent synthesis of heterocyclics

A palladium-catalyzed one-pot procedure of N–H insertion and Heck cyclization/base promoted cyclization has been developed.

Diazo compounds and N-tosylhydrazones: novel cross-coupling partners in transition-metal-catalyzed reactions

Transition-metal-catalyzed carbene transformations and cross-couplings represent two major reaction types in organometallic chemistry and organic synthesis. However, for a long period of time, these two important areas have evolved separately, with essentially no overlap or integration. Thus, an intriguing question has emerged: can cross-coupling and metal carbene transformations be merged into a single reaction cycle? Such a combination could facilitate the development of novel carbon-carbon bond-forming methodologies. Although this concept was first explored about 10 years ago, rapid developments inthis area have been achieved recently. Palladium catalysts can be used to couple diazo compounds with a wide variety of organic halides. Under oxidative coupling conditions, diazo compounds can also react with arylboronic acids and terminal alkynes. Both of these coupling reactions form carbon-carbon double bonds. As the key step in these catalytic processes, Pd carbene migratory insertion plays a vital role in merging the elementary steps of Pd intermediates, leading to novel carbon-carbon bond formations. Because the diazo substrates can be generated in situ from N-tosylhydrazones in the presence of base, the N-tosylhydrazones can be used as reaction partners, making this type of cross-coupling reaction practical in organic synthesis. N-Tosylhydrazones are easily derived from the corresponding aldehydes or ketones. The Pd-catalyzed cross-coupling of N-tosylhydrazones is considered a complementary reaction to the classic Shapiro reaction for converting carbonyl functionalities into carbon-carbon double bonds. It can also serve as an alternative approach for the Pd-catalyzed cross-coupling of carbonyl compounds, which is usually achieved via triflates. The combination of carbene formation and cross-coupling in a single catalytic cycle is not limited to Pd-catalyzed reactions. Recent studies of Cu-, Rh-, Ni-, and Co-catalyzed cross-coupling reactions with diazo compounds or N-tosylhydrazones show that these transformations also work with other transition metals, demonstrating the generality of the diazo compounds as new cross-coupling partners in transition-metal-catalyzed coupling reactions.