A novel approach for rhodium(iii)-catalyzed C-H functionalization of 2,2'-bipyridine derivatives with alkynes: a significant substituent effect

Transition Metal-Catalyzed Dual C-H Activation/Annulation Reactions Involving Internal Alkynes

Recently, transition metal-catalyzed ortho-C-H bond activation/annulations involving two internal alkyne molecules have been extensively used to synthesize highly substituted polycyclic aromatic scaffolds. Such reactions have emerged as a powerful atom and step-economical strategy for the assembly of multifunctional bioactive molecules. In this context, we focused on the recent achievements of dual C-H bond activation/annulations, as well as functionalization reactions involving diaryl/alkyl alkynes.

Pyridine C(sp2)-H bond functionalization under transition-metal and rare earth metal catalysis

Pyridine is a crucial heterocyclic scaffold that is widely found in organic chemistry, medicines, natural products, and functional materials. In spite of the discovery of several methods for the synthesis of functionalized pyridines or their integration into an organic molecule, new methodologies for the direct functionalization of pyridine scaffolds have been developed during the past two decades. In addition, transition-metal-catalyzed C-H functionalization and rare earth metal-catalyzed reactions have flourished over the past two decades in the development of functionalized organic molecules of concern. In this review, we discuss recent achievements in the transition-metal and rare earth metal-catalyzed C-H bond functionalization of pyridine and look into the mechanisms involved.

Three-Component Chemo-Selective Synthesis of N-(o-Alkenylaryl) Pyrazoles by Pyrazole Annulation and Rh-Catalyzed Chemo-Selective Aryl C-H Addition Cascade

By using readily available enaminones, aryl hydrazine hydrochlorides, and alkynes as starting materials, the chemo-selective three-component synthesis of atropisomeric N-(o-alkenylaryl) pyrazoles has been efficiently accessed with rhodium catalysis. Unlike Satoh-Miura reaction leading to the alkyne-based C-H benzannulation by using prior prepared N-phenyl pyrazoles and alkynes as substrates, this three-component protocol displays unprecedented selectivity of C-H alkenylation by blocking the second round metal alkenylation with the key protonation step in the presence of acids.

Three-component synthesis of N-naphthyl pyrazoles via Rh(III)-catalyzed cascade pyrazole annulation and Satoh-Miura benzannulation

The synthesis of N-naphthyl pyrazoles has been realized by the direct three-component reactions of enaminones, aryl hydrazine hydrochlorides and internal alkynes via Rh(III) catalysis. The synthetic reactions employing simple substrates lead to simultaneous construction of dual cyclic moieties, including a pyrazole ring and a phenyl ring, via sequential formation of two C-N and three C-C bonds.

Rhodium(iii)-catalyzed switchable C-H acylmethylation and annulation of 2,2'-bipyridine derivatives with sulfoxonium ylides

A novel protocol for Rh(iii)-catalyzed switchable C-H acylmethylation and annulation of 2,2'-bipyridine derivatives with sulfoxonium ylides is reported. This protocol provides a facile approach to synthesize structurally diverse acylmethylated 2,2'-bipyridine derivatives and acyl pyrido[2,3-a]indolizines with a broad range of functional group tolerance.

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

Cp*Rh(III)-catalyzed chelation-assisted direct C-H bond functionalization of 1-(2-pyridyl)-2-pyridones with internal alkynes that can be controlled to give three different products in good yields has been realized. Depending on the reaction conditions, solvents and additives, the reaction pathway can be switched between alkenylation, alkenylation/directing group migration and rollover annulation. These reaction manifolds allow divergent access to a variety of valuable C6-alkenylated 1-(2-pyridyl)-2-pyridones, (Z)-6-(1,2-diaryl-2-(pyridin-2-yl)vinyl)pyridin-2(1H)-ones and 10H-pyrido[1,2-a][1,8]naphthyridin-10-ones from the same starting materials. These protocols exhibit excellent regio- and stereoselectivity, broad substrate scope, and good tolerance of functional groups. A combination of experimental and computational approaches have been employed to uncover the key mechanistic features of these reactions.

Efficient Access to Isoquinolines via Rhodium-Catalyzed Oxidative Annulation of Pyridyl C–H Bonds Directed by Carbonyl with Internal Alkynes

An efficient synthesis of amino-substituted isoquinolines via Rh(III)-catalyzed oxidative [2+2+2] annulation of pyridines with alkynes has been developed, which is cooperatively enabled by a directing carbonyl and steric hindrance adjacent to the amino group of the pyridine, via a six-membered rhodacyclic intermediate without coordination with the pyridinic nitrogen. The establishment of this C–H activation strategy also paves the way for other oxidative transformations of heterocyclic C–H bonds.

Rollover Cyclometalation as a Valuable Tool for Regioselective C-H Bond Activation and Functionalization

Rollover cyclometalation constitutes a particular case of cyclometallation reaction. This reaction occurs when a chelated heterocyclic ligand loses its bidentate coordination mode and undergoes an internal rotation, after which a remote C-H bond is regioselectively activated, affording an uncommon cyclometalated complex, called "rollover cyclometalated complex". The key of the process is the internal rotation of the ligand, which occurs before the C-H bond activation and releases from coordination a donor atom. The new "rollover" ligand has peculiar properties, being a ligand with multiple personalities, no more a spectator in the reactivity of the complex. The main reason of this peculiarity is the presence of an uncoordinated donor atom (the one initially involved in the chelation), able to promote a series of reactions not available for classic cyclometalated complexes. The rollover reaction is highly regioselective, because the activated C-H bond is usually in a symmetric position with respect to the donor atom which detaches from the metal stating the rollover process. Due to this novel behavior, a series of potential applications have appeared in the literature, in fields such as catalysis, organic synthesis, and advanced materials.

Rh(iii)-Catalyzed switchable C–H monoalkenylation and dialkenylation of 2-(1H-pyrazol-1-yl)pyridine with alkenes via rollover cyclometalation

We have demonstrated Rh(iii)-catalyzed switchable C–H monoalkenylation and dialkenylation of 2-(1H-pyrazol-1-yl)pyridine with alkenes via rollover cyclometalation.

Palladium-Catalyzed C-2 and C-3 Dual C-H Functionalization of Indoles: Synthesis of Fluorinated Isocryptolepine Analogues

Here we report a protocol to synthesize diversiform fluorinated isocryptolepine analogues with potential biological activities in one step via directed C-2 and C-3 dual C-H functionalization of indoles. We also attempted to take into account fluorinated imidoyl chlorides as a novel kind of synthons in the directed C-H functionalization reactions. As a result, a variety of fluorinated isocryptolepine analogues were obtained in up to 96% yield. Moreover, we conducted control experiments to disclose the reaction mechanism.

Pd/NBE-catalyzed sequential carbamoylation/olefination of aryl iodides

We present a Pd/NBE-catalyzed sequential carbamoylation/olefination of aryl iodides under mild reaction conditions, which provide diverse 4-methylene-3,4-dihydro-1(2H)-isoquinolin-1-one analogues.