Rhodium(III)-Catalyzed C-H Bond Functionalization of 2-Pyridones with Alkynes: Switchable Alkenylation, Alkenylation/Directing Group Migration and Rollover Annulation

Pd-catalyzed ortho-/meta-C-H-annulation of biphenyl amines with enynes through non-rollover cyclometallation

Annulations through dual C-H activation represent a powerful tool to selectively assemble multi-cyclic scaffolds. We present herein a palladium-catalyzed ortho-/meta-C-H-annulation of biphenyl amines with 1,6-enynes. This regioselective non-rollover cyclometallation was achieved through meticulous tuning of electronic factors of both the partners. This method is applicable to a wide range of protected o-arylanilines and enynes, and results in the regioselective preparation of benzo[f]isoindolyl derivatives in high yields with good diastereoselectivity (with respect to two types of stereogenic elements). Certain essential control experiments and kinetic isotope effect (KIE) studies were undertaken to elucidate the reaction mechanism, while subsequent transformations and a scale-up reaction were performed to substantiate the sturdiness of the transformation.

Hydroxyl-Assisted and Co(III)-Catalyzed Redox-Neutral C-H Activation/Directing Group Migration of 2-Pyridones with Propargyl Alcohols: Synthesis of Tetrasubstituted Alkenes

This study provides a practical route to synthesize tetrasubstituted alkenes that involves Co(III)-catalyzed C-H bond activation and regioselective insertion of the alkyne, followed by chelation of the substrate hydroxyl to Co and migration of the pyridine group. Density functional theory studies revealed the origin of regioselectivity and elucidated the crucial role of the hydroxyl group for the migration of pyridine. The method can be conducted on a gram scale, is compatible with a wide range of substrates, and has a high functional group tolerance. To demonstrate its significance, the method was used for the late-stage modification of Fasudil. Furthermore, the synthetic significance of the method was demonstrated by the various derivatizations of the products, many of which exhibit intriguing fluorescence characteristics.

Ruthenium(ii) catalyzed C-3 site selective alkenylation of indole derivatives via C-H activation

An efficient synthetic method has been developed for C-3 site-selective alkenylation of indole derivatives under ruthenium(ii) catalysis with an ester as a directing group. Besides the presence of two potential C(sp2)-H sites available for functionalization in the substrates, exclusive C3 selectivity was achieved in a selective manner as only mono-functionalized products were formed. The high site selectivity is attributed to the formation of an uncommon six-membered metallacycle intermediate between the ruthenium catalyst and ester directing group, enabled by the selective alkenylation at the C3 position of indole derivatives. This protocol features high site selectivity, operational simplicity, broad substrate scope, and moderate to high yields.

Synthesis of Naphtho[1',2':4,5]furo[3,2-b]pyridinones via Ir(III)-Catalyzed C6/C5 Dual C-H Functionalization of N-Pyridyl-2-pyridones with Diazonaphthalen-2(1H)-ones

Presented herein is an unprecedented synthesis of naphtho[1',2':4,5]furo[3,2-b]pyridinones via Ir(III)-catalyzed C6/C5 dual C-H functionalization of N-pyridyl-2-pyridones with diazonaphthalen-2(1H)-ones. This protocol forms C-C and C-O bonds in one pot in which diazonaphthalen-2(1H)-ones serve as bifunctional reagents, providing both alkyl and aryloxy sources. To the best of our knowledge, this is the first example of an Ir(III)-catalyzed synthesis of the title compounds by using diazonaphthalen-2(1H)-ones as bifunctional substrates. Notably, this method features operational simplicity, good functional group tolerance, high efficiency, and high atom economy.

Pd-catalyzed regioselective rollover dual C-H annulation cascade: facile approach to phenanthrene derivatives

Annulations of unsaturated systems through C-H activation represent a powerful tool for producing multicyclic scaffolds. Having coordinating centers in both annulation partners (a dual coordination strategy) would afford remarkable selectivities in the outcomes. Along this concept, we report herein a Pd-catalyzed regioselective rollover cascade dual C-H annulation of o-arylphenols with alkynols for constructing phenanthrene scaffolds. Control, KIE and deuteration studies were conducted to determine the reaction mechanism, and downstream transformations and scaled-up reactions were carried out to assess the robustness of the transformation.

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.

Pd(II)-Catalyzed Oxidative Naphthylation of 2-Pyridone through N-H/C-H Activation Using Diarylacetylene as an Uncommon Arylating Agent

A Pd(II)-catalyzed straightforward oxidative naphthylation of unmasked 2-pyridone derivatives is described using a twofold internal alkyne as a coupling partner. The reaction proceeds through N-H/C-H activation to provide polyarylated N-naphthyl 2-pyridones. An unusual oxidative annulation at the arene C-H bond of the diarylalkyne leads to the formation of polyarylated N-naphthyl 2-pyridones, where the 2-pyridone-attached phenyl ring of the naphthyl ring is polyaryl-substituted. Mechanistic studies and DFT calculations suggest a plausible mechanism based on N-H/C-H activation. The N-naphthyl 2-pyridone derivatives were studied to explore encouraging photophysical properties.

(SCp)Rhodium-Catalyzed Asymmetric Satoh-Miura Reaction for Building-up Axial Chirality: Counteranion-Directed Switching of Reaction Pathways

Satoh-Miura reaction is an important method for extending π-systems by forging multi-substituted benzene rings via double aryl C-H activation and annulation with alkynes. However, the development of highly enantioselective Satoh-Miura reaction remains rather challenging. Herein, we report an asymmetric Satoh-Miura reaction between 1-aryl benzo[h]isoquinolines and internal alkynes enabled by a SCpRh-catalyst. Judiciously choosing the counteranion of the Rh-catalyst is crucial for the desired reactivity over the competitive formation of azoniahelicenes. Detailed mechanistic studies support the proposal of counteranion-directed switching of reaction pathways in Rh-catalyzed asymmetric C-H activation.

Redox-neutral rhodium(III)-catalyzed divergent synthesis of tetrasubstituted 1,3-enynes and alkynylated benzofurans

With the assistance of the acetamido directing group (DG), a rhodium-catalyzed C-H alkenylation/DG migration cascade for the synthesis of tetrasubstituted 1,3-enynes from N-phenoxyacetamides and 1,3-diynes has been achieved in this work. Alternatively, a rhodium-catalyzed [3 + 2] annulation for the synthesis of alkynylated benzofurans from the same set of substrates has also been achieved by simply changing the reaction conditions. This work highlights the tunable divergent synthesis of valuable compounds triggered by C-H activation.

Hydride Relay Exchange Mechanism for the Heterocyclic C-H Arylation of Benzofuran and Benzothiophene Catalyzed by Pd Complexes

The mechanism and regioselectivity of the heterocyclic C-H arylation of benzofuran and benzothiophene catalyzed by Pd(OAc)₂ complexes were investigated using the density functional theory (DFT) method. The Pd(0)L₂(PhI) complex (L = HOAc) is proposed to be the catalytic species. Compared to the traditional Heck-type mechanism, concerted metalation-deprotonation (CMD) mechanism, and electrophilic aromatic substitution (S_EAr) mechanism for the C-H arylation, a new hydride relay exchange mechanism was proposed for the benzoheterocyclic C-H arylation catalyzed by Pd complexes, which consists of two redox processes between Pd(II) and Pd(0) species to complete the regioselective C-H activation. The calculated results indicate that the reaction along the hydride relay exchange mechanism is more favorable than those along other mechanisms, including the traditional Heck-type mechanism and the base-assisted anti-H elimination mechanism. This agrees well with the experimental results. Meanwhile, the origin for the regioselective C-H arylation was unveiled in which the α-C-H arylation products are major for the heterocyclic C-H arylation of benzofuran, but the β-C-H arylation products are major for that of benzothiophene. This study might provide a deep mechanistic understanding on the regioselective C-H activation and arylation of benzoheterocycle compounds catalyzed by transition-metal complexes.

Switching the Reactivity of the Nickel-Catalyzed Reaction of 2-Pyridones with Alkynes: Easy Access to Polyaryl/Polyalkyl Quinolinones

A Ni-catalyzed C6 followed by C5 cascade C-H activation/[2 + 2 + 2] annulation of 2-pyridone with alkynes has been achieved. A change in the reaction pathway was achieved by tuning the reaction conditions and incorporating a directing group. A wide variety of substrates and alkynes are amenable to this transformation. The key to success for this transformation is the use of sodium iodide as an additive. More importantly, we detected the five-membered metallacycle intermediate through HRMS wherein iodide is ligated to the metal.