Palladium-catalyzed dearomative 1,4-difunctionalization of naphthalenes

Electrophile-Controlled Regiodivergent Palladium-Catalyzed Imidoylative Spirocyclization of Cyclic Alkenes

An intermolecular controllable Pd-catalyzed spirocyclization of isocyano cycloalkenes has been developed, offering efficient and selective approaches toward spirocyclic hydropyrrole scaffolds. 2-Azaspiro-1,7-dienes could be obtained through a "chain-walking" process with aryl/vinyl iodides as electrophiles, while the normal Heck product 2-azaspiro-1,6-dienes were selectively generated when aryl triflates were used as the coupling partner of isocyanides. Mechanistic studies suggested that the counteranion of the Pd(II) intermediate played a crucial role in the regioselectivity control. Dihydropyrrole-fused 5,6,7-membered spirocycles were switchably accessed under mild conditions with wide functional group tolerance.

Radical-Dearomative Generation of Cyclohexadienyl Pd(II) toward the 3D Transformation of Nonactivated Phenyl Rings

Traditional palladium-catalyzed dearomatization of (hetero)arenes takes place via an ionic pathway and usually requires elevated temperatures to overcome the energy barrier of the dearomative insertion step. Herein, a combination of the radical and two-electron pathways is disclosed, which enables room temperature dearomative 3D transformations of nonactivated phenyl rings with Pd(0) as the catalyst. Experimental results together with density functional theory (DFT) calculations indicate a versatile π-allyl Pd(II) species, cyclohexadienyl Pd(II), possibly is involved in the dearomatization. This species is generated by combining the cyclohexadienyl radical and Pd(I). The cyclohexadienyl Pd(II) provides chemoselective (carboamination and trieneylation), regioselective (1,2-carboamination), and diastereoselective (carbonyl-group directed face selectivity) conversions.

Regioselective Dearomative Amidoximation of Nonactivated Arenes Enabled by Photohomolytic Cleavage of N-nitrosamides

Dearomative spirocyclization reactions represent a promising means to convert arenes into three-dimensional architectures; however, controlling the regioselectivity of radical dearomatization with nonactivated arenes to afford the spirocyclizative 1,2-difunctionalization other than its kinetically preferred 1,4-difunctionalization is exceptionally challenging. Here we disclose a novel strategy for dearomative 1,2- or 1,4-amidoximation of (hetero)arenes enabled by direct visible-light-induced homolysis of N-NO bonds of nitrosamides, giving rise to various highly regioselective amidoximated spirocycles that previously have been inaccessible or required elaborate synthetic efforts. The mechanism and origins of the observed regioselectivities were investigated by control experiments and density functional theory calculations.

Asymmetric dearomative cyclopropanation of naphthalenes to construct polycyclic compounds

Catalytic asymmetric dearomatization (CADA) reactions is an important synthetic method for constructing enantioenriched complex cyclic systems from simple aromatic feedstocks. However, the CADA reactions of nonactivated arenes, such as naphthalenes and benzenes, have been far less explored than those of electronically activated arenes, such as phenols, naphthols and indoles. Herein, we disclose an asymmetric dearomative cyclopropanation of naphthalenes for the rapid construction of polycyclic compounds. With chiral dirhodium carboxylate as a catalyst, the dearomative cyclopropanation proceeded smoothly under mild conditions and afforded benzonorcaradiene-containing tetracycles in good yield and high enantioselectivity (up to 99% ee). Three stereogenic centers, including two all-carbon quaternary centers, were created in the dearomatization reaction. Moreover, a variety of functional groups are well-tolerated in the reaction. The products could be readily converted into other complex polycycles while maintaining the high ee value.

Palladium-Catalyzed Three-Component Cross-Coupling of Conjugated Dienes with Indoles Using Ethynylbenziodazolones as Electrophilic Alkynylating Reagents

A palladium-catalyzed regioselective 1,2-alkynyl-carbonalization of conjugated dienes with ethynylbenziodazolone (EBZ) and indoles has been developed for the first time. Various molecules containing alkenyl, alkynyl, and indole groups were readily obtained. Moreover, the resulting products can be applied to various derivatizations. This protocol uses EBZ as an electrophilic alkynylating reagent, avoiding the byproduct of dimerization of alkynes.

Aromatic π-Components for Enantioselective Heck Reactions and Heck/Anion-Capture Domino Sequences

Olefin functionalization represents one of the most important synthetic transformations in organic synthesis. Over the past decades, palladium-catalyzed enantioselective Heck reactions, and Heck/anion-capture domino sequences through olefin carbopalladation followed by termination of the resulting alkyl-Pd species have been extensively developed. Extension of the coupling partners from classical olefins to other π-components would enable further advances and open new space in this field. Aromatics are important and easily available bulk chemicals. Dearomative transformation of endocyclic aromatic π-bonds via the Heck reaction pathway provides an efficient and straightforward route to structurally diverse alicyclic compounds. Nevertheless, major challenges for this transformation include aromaticity breaking and reactivity and selectivity issues.Recently, we have engaged in developing catalytic enantioselective dearomative Heck reactions and related domino reactions. A range of heteroarenes and naphthalenes have been employed as novel π-coupling partners in these reactions. Through dearomative migratory insertion of endocyclic aromatic C-C π-bonds followed by interception of the transient alkyl-Pd species, enantioselective Heck reactions, reductive Heck reactions, Heck/anion-capture difunctionalization reactions, and heteroarenyne cycloisomerization reactions have been established. Relying on β-H elimination of the alkyl-Pd intermediate, we realized enantioselective dearomative Heck reactions with a range of aromatic partners, including heterocyclic indoles, pyrroles, furans, benzofurans, and more challenging carbocyclic naphthalenes. In order to avoid the utilization of organohalide electrophiles, heteroarenyne cycloisomerization reaction was developed by merging intermolecular alkyne hydropalladation with intramolecular dearomative Heck reaction. Cycloisomerization of alkyne-tethered indoles delivered chiral indolines in excellent enantioselectivities with 100% atom economy. On the other hand, Heck/anion-capture domino sequences were established through nucleophilic trapping of the alkyl-Pd intermediate. When HCO₂Na was employed as a capturing reagent, the enantioselective dearomative reductive Heck reaction of indoles was realized. By employing other nucleophiles, including alkynes, N-sulfonylhydrazones, and organoboron reagents, we developed a series of dearomative difunctionalization reactions. Two vicinal stereocenters with excellent enantio- and diastereoselectivities were constructed in the corresponding Heck/Sonogashira, Heck/vinylation, and Heck/borylation reactions. Moreover, dearomative 1,4-diarylation of naphthalenes was developed through Heck/Suzuki domino reactions, in which competitive C-H arylation and the direct Suzuki reaction were almost fully inhibited in the presence of a spiro-phosphoramidite ligand.In this Account, we provide a panoramic view of our results since 2015 on enantioselective Heck reactions and related domino sequences by extending the coupling partners from classical olefins to aromatic systems. Investigations outlined in this Account established straightforward and efficient access to a variety of structurally diverse chiral heteropolycyclic molecules starting from simple and planar aromatic compounds.

Palladium-catalyzed dearomative 1,4-arylmethylenation of naphthalenes

An efficient palladium-catalyzed construction of E-exocyclic-double-bond-containing spirooxindoles through 1,4-arylmethylenation of naphthalenes has been developed. Aryl aldehyde-derived N‑tosylhydrazones were successfully applied as carbene precursors to capture the endocyclic π-allylpalladium intermediate, which...

Asymmetric Domino Heck Arylation and Alkylation of Nonconjugated Dienes: Double C-F···Sodium Attractive Noncovalent Interaction

Palladium catalyzes a domino Heck arylation and alkylation of nonconjugated cyclodienes to produce trans isomers of disubstituted cyclohexenes in exceptionally high enantiomeric ratios, reaching 100:1 to 200:1 in many cases. Importantly, the interactions of the two CF bonds of Josiphos and the sodium ion of malonates facilitates stereoselective allylic attack through DFT calculations and experiments. This is a new type of attractive noncovalent interactions found in organometallic catalysis.

Palladium-Catalyzed Cascade Dearomative Spirocyclization and C-H Annulation of Aromatic Halides with Alkynes

Described herein is a palladium-catalyzed intermolecular dearomative annulation of aryl halides with alkynes, which provides a rapid approach to a class of structurally unique spiroembedded polycyclic aromatic compounds. The cascade process is accomplished by a sequential alkyne migratory insertion, Heck-type dearomatization, and C-H bond annulation. Further optoelectronic study indicated this fused spirocyclic scaffold could be a potential host material for OLEDs, as exemplified by a fabricated red PhOLED device with a maximum external quantum efficiency of 23.0%.