Ruthenium(II)-catalyzed amide directed spiroannulation with naphthoquinones: access to spiro-isoindolinone frameworks

C-H activation-initiated spiroannulation reactions and their applications in the synthesis of spirocyclic compounds

Spirocyclic skeletons are prevalent in natural products, pharmaceuticals and organic functional materials. Meanwhile, transition-metal-catalyzed C-H activation reactions have demonstrated unparalleled advantages such as high efficiency, excellent atom-economy, good chemoselectivity and regioselectivity for the formation of target organic molecules. In recent years, C-H activation reactions have been creatively utilized in the synthesis of spirocyclic compounds. This review summarizes the most recent progress made in C-H activation-initiated spiroannulation reactions and their applications in the construction of structurally diverse and biologically valuable spirocyclic scaffolds by using alkynes, diazo compounds, maleimides, alkenes, quinones and cyclopropenones as the coupling partners.

Synthesis of Spiroisoindolinones via Ru(II)-Catalyzed Spiroannulation of N-Acyl Ketimines with Aryl Isocyanates/Isothiocyanates through Aromatic C-H Bond Activation

Herein, we disclose the first report on Ru(II)-catalyzed amidation/thioamidation of 3-hydroxy-3-arylisoindolinones with isocyanates/isothiocyanates, respectively. The reaction furnishes spiroisoindolinones via sequential C-H functionalization of ortho C-H bond followed by intramolecular cyclization in moderate to high yields (up to 94%). Moreover, the developed strategy is highly atom-economical and site-selective and shows diverse substrate generality. Also, synthesized spiroisoindolinones undergo several chemical transformations.

Enaminone-directed ruthenium(II)-catalyzed C-H activation and annulation of arenes with diazonaphthoquinones for polycyclic benzocoumarins

The weakly coordinating enaminone functionality has been leveraged for a C-H bond activation strategy under ruthenium catalysis and employed in the regioselective annulative coupling of arenes with diazonaphthoquinones, offering polycyclic benzocoumarins in very high yields. The enaminone motif plays a dual role and the protocol operates through a Ru(II)/Ru(IV) catalytic pathway which is amenable to the diversification of various pharmacophore-coupled substrates.

Synthesis of spiropyrans and arylquinones via Ru(II)-catalyzed condition-controlled coupling of 3-aryl-2H-benzoxazinones with benzoquinones

Ru(II)-catalyzed condition-controlled divergent coupling between 3-aryl-2H-benzoxazin-2-ones and benzoquinones has been realized under operationally simple conditions, affording a series of structurally stable spiropyrans and valuable arylquinones. The potential of this method is also demonstrated by scale-up synthesis and derivatization. Additionally, an unprecedented cycloruthenated complex has been identified as a key intermediate.

Mechanistic Insights Into the Rhodium-Catalyzed C-H Alkenylation/Directing Group Migration and [3+2] Annulation: A DFT Study

The mechanism of the rhodium-catalyzed C-H alkenylation/directing group migration and [3+2] annulation of N-aminocarbonylindoles with 1,3-diynes has been investigated with DFT calculations. On the basis of mechanistic studies, we mainly focus on the regioselectivity of 1,3-diyne inserting into the Rh-C bond and the N-aminocarbonyl directing group migration involved in the reactions. Our theoretical study uncovers that the directing group migration undergoes a stepwise β-N elimination and isocyanate reinsertion process. As studied in this work, this finding is also applicable to other relevant reactions. Additionally, the role of Na⁺ versus Cs⁺ involved in the [3+2] cyclization reaction is also probed.

Rh(III)-Catalyzed Switchable [4 + 1] and [4 + 2] Annulation of N-Aryl Pyrazolones with Maleimides: An Access to Spiro Pyrazolo[1,2-a]indazole-pyrrolidine and Fused Pyrazolopyrrolo Cinnolines

A rhodium(III)-catalyzed controllable [4 + 1] and [4 + 2] annulation of N-aryl pyrazolones with maleimides as C1 and C2 synthon has been explored for the synthesis of spiro[pyrazolo[1,2-a]indazole-pyrrolidines] and fused pyrazolopyrrolo cinnolines. The product selectivity was achieved through time-dependent annulation. The [4 + 1] annulation reaction involves sequential Rh(III)-catalyzed C-H alkenylation of N-aryl pyrazolone, followed by an intramolecular spirocyclization via aza-Michael-type addition to afford spiro[pyrazolo[1,2-a]indazole-pyrrolidine]. However, prolonged reaction time converts in situ formed spiro[pyrazolo[1,2-a]indazole-pyrrolidine] into fused pyrazolopyrrolocinnoline. This unique product formation switch proceeds via strain-driven ring expansion through a 1,2-shift of the C-C bond.

Palladium-Catalyzed C-H Functionalization of Aryl Acetamides and Benzoquinones: Synthesis of Substituted Aryl Quinones

An efficient synthesis of aryl-substituted quinones via Pd(II)-catalyzed C-H functionalization of less expensive and abundant benzoquinones with aryl acetamides is demonstrated. An auxiliary ligand N,N-bidentate-directing group 8-aminoquinoline plays a crucial role in the success of the reaction. A broad range of substituted phenyl acetamides including commercially available drug molecules were examined and also found to be highly compatible with quinones. The aryl-substituted quinones were also easily converted into aryl-substituted hydroquinone derivatives. A plausible reaction mechanism was proposed to account for the selective distal C-H bond functionalization.

Chemoselective and Regioselective Synthesis of Spiroisoindolinone Indenes via an Intercepted Meyer-Schuster Rearrangement/Intramolecular Friedel-Crafts Alkylation Relay

A Brønsted acid-catalyzed reaction between isoindolinone-derived propargylic alcohols and external aromatic nucleophiles for the construction of spiroisoindolinone indenes is described. The reaction proceeds rapidly with a broad range of substrates to generate spiroindenes chemoselectively and regioselectively in moderate to high yields. Key to the success of this transformation is an intercepted Meyer-Schuster rearrangement/intramolecular Friedel-Crafts alkylation relay that offers a modular approach in the synthesis of target compounds.

Redox-neutral rhodium(III)-catalyzed chemo- and regiospecific [4 + 1] annulation between benzamides and alkenes for the synthesis of functionalized isoindolinones

Herein, using electron-deficient alkenes embedded with an oxidizing function/leaving group as a rare and nontraditional C₁ synthon, we have achieved the redox-neutral Rh(III)-catalyzed chemo- and regioselective [4 + 1] annulation of benzamides for the synthesis of functionalized isoindolinones. This method features broad substrate scope, good to excellent yields, excellent chemo- and regioselectivity, good tolerance of functional groups and mild external-oxidant-free conditions.

Ruthenium(II)-catalyzed C-H activation and (4+2) annulation of aromatic hydroxamic acid esters with allylic amides

A (4+2) annulation under Ru(II)-catalysis is reported using aromatic hydroxamic acid esters as the oxidizing directing group and allylic amides as unactivated olefin coupling partners, delivering a wide variety of aminomethyl isoquinolinones in good to excellent yields. This annulation is distinctive as allylic congeners typically result in allylation and not the annulation. Late-stage derivatization of a bioactive synthetic bile acid has been showcased.

Ru(II)-Catalyzed Switchable C-H Alkylation and Spirocyclization of 2-Arylquinoxalines with Maleimides via ortho-C-H Activation

A Ru(II)-catalyzed facile and controllable protocol for C-H alkylation and spirocyclization of 2-arylquinoxalines with maleimides has been achieved under ambient air in high yields. Sequential ortho-C-H activation and C-annulation results in the formation of diverse polyheterocycles containing spiro[indeno[1,2-b]quinoxaline-11,3'-pyrrolidine]-2',5'-diones, which are of potent interest in medicinal chemistry. Mechanistic investigations suggest a reversible cleavage of the ortho-C-H bond in the turnover-limiting step.