Rhodium-catalyzed selective CH functionalization of NNN tridentate chelating compounds via a rollover pathway

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.

Rhodium(I)-Catalyzed Regioselective C–H Alkenylation of 2,2′-Bipyridine Derivatives with Alkynes

In this work, a Rh(I)-catalyzed C3-alkenylation of 2,2′-bipyri­dine-6-carboxamide with terminal and internal alkynes was developed, providing an efficient access to a broad range of alkenylated 2,2′-bipyridine derivatives with exclusive regioselectivity and stereoselectivity. Deuterium labeling experiments indicate that an irreversible rollover C–H activation process is involved in this reaction.

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.

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.

Strong Impact of Intramolecular Hydrogen Bonding on the Cathodic Path of [Re(3,3'-dihydroxy-2,2'-bipyridine)(CO)3Cl] and Catalytic Reduction of Carbon Dioxide

Herein, we present the cathodic paths of the Group-7 metal complex [Re(3,3'-DHBPY)(CO)₃Cl] (3,3'-DHBPY = 3,3'-dihydroxy-2,2'-bipyridine) producing a moderately active catalyst of electrochemical reduction of CO₂ to CO. The combined techniques of cyclic voltammetry and IR/UV-vis spectroelectrochemistry have revealed significant differences in the chemistry of the electrochemically reduced parent complex compared to the previously published Re/4,4'-DHBPY congener. The initial irreversible cathodic step in weakly coordinating THF is shifted toward much less negative electrode potentials, reflecting facile reductive deprotonation of one hydroxyl group and strong intramolecular hydrogen bonding, O-H···O^-. The latter process occurs spontaneously in basic dimethylformamide where Re/4,4'-DHBPY remains stable. The subsequent reduction of singly deprotonated [Re(3,3'-DHBPY-H⁺)(CO)₃Cl]^- under ambient conditions occurs at a cathodic potential close to that of the Re/4,4'-DHBPY-H⁺ derivative. However, for the stabilized 3,3'-DHBPY-H⁺ ligand, the latter process at the second cathodic wave is more complex and involves an overall transfer of three electrons. Rapid potential step electrolysis induces 1e^--reductive cleavage of the second O-H bond, triggering dissociation of the Cl^- ligand from [Re(3,3'-DHBPY-2H⁺)(CO)₃Cl]^2-. The ultimate product of the second cathodic step in THF was identified as 5-coordinate [Re(3,3'-DHBPY-2H⁺)(CO)₃]^3-, the equivalent of classical 2e^--reduced [Re(BPY)(CO)₃]^-. Each reductive deprotonation of the DHBPY ligand results in a redshift of the IR ν(CO) absorption of the tricarbonyl complexes by ca. 10 cm^-1, facilitating the product assignment based on comparison with the literature data for corresponding Re/BPY complexes. The Cl^- dissociation from [Re(3,3'-DHBPY-2H⁺)(CO)₃Cl]^2- was proven in strongly coordinating butyronitrile. The latter dianion is stable at 223 K, converting at 258 K to 6-coordinate [Re(3,3'-DHBPY-2H⁺)(CO)₃(PrCN)]^3-. Useful reference data were obtained with substituted parent [Re(3,3'-DHBPY)(CO)₃(PrCN)]⁺ that also smoothly deprotonates by the initial reduction to [Re(3,3'-DHBPY-H⁺)(CO)₃(PrCN)]. The latter complex ultimately converts at the second cathodic wave to [Re(3,3'-DHBPY-2H⁺)(CO)₃(PrCN)]^3- via a counterintuitive ETC step generating the 1e^- radical of the parent complex, viz., [Re(3,3'-DHBPY)(CO)₃(PrCN)]. The same alternative reduction path is also followed by [Re(3,3'-DHBPY-H⁺)(CO)₃Cl]^- at the onset of the second cathodic wave, where the ETC step results in the intermediate [Re(3,3'-DHBPY)(CO)₃Cl]^•- further reducible to [Re(3,3'-DHBPY-2H⁺)(CO)₃]^3- as the CO₂ catalyst.

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

We described a novel approach for the C-H functionalization of 2,2'-bipyridine derivatives with alkynes. DFT calculations and experimental data showed a significant substituent effect at the 6-position of 2,2'-bipyridine, which weakened the adjacent N-Rh bond and provided the possibility of subsequent rollover cyclometalation, C-H activation, and functionalization.

Rh(iii)-Catalyzed switchable C–H functionalization of 2-(1H-pyrazol-1-yl)pyridine with internal alkynes

We reported a Rh(iii)-catalyzed switchable C–H functionalization of 2-(1H-pyrazol-1-yl)pyridine with internal alkynes, which provided diversiform functionalized N,N-bidentate chelating compounds.

Rhodium(III)-Catalyzed Regioselective C3-H Acylmethylation of [2,2'-Bipyridine]-6-carboxamides with Sulfoxonium Ylides

A rhodium(III)-catalyzed C-H acylmethylation of tridentate [2,2'-bipyridine]-6-carboxamides was developed. A variety of [2,2'-bipyridine]-6-carboxamides could be monoalkylated exclusively at the C3 position with sulfoxonium ylides as carbene precursors, giving 3-alkylated products in high yields. This protocol proceeds through a rollover cyclometalation pathway, has a broad range scope of substrates, and exhibits excellent functional group tolerance.

Orchestrated catalytic double rollover annulation: rapid access to N-enriched cationic and neutral PAHs

Disclosed herein is a rhodium(iii)-catalyzed novel one-step back-to-back double rollover annulation on pyridine and pyrazine backbones leading to structurally and optoelectronically diverse class of nicely decorated multi-ring-fused, extensively π-conjugated, N-enriched PAH molecules by virtue of orchestrated quadruple C–H activation events.

Palladium-Catalyzed Site-Selective C-H Arylation of 2,2'-Bipyridine-6-carboxamides via a Rollover Cyclometalation Pathway

The first example of Pd(II)-catalyzed arylation of 2,2'-bipyridine-6-carboxamides via a rollover cyclometalation pathway is reported. A diverse range of (hetero)aryl iodides could be directly coupled with various 2,2'-bipyridine-6-carboxamides in exclusive site-selectivity, which offers a distinct entry to arylated 2,2'-bipyridine-6-carboxamides. A preliminary mechanistic study supports the Pd(II)-rollover cyclometalation pathway.

To "Rollover" or Not? Stereoelectronically Guided C-H Functionalization Pathways from Rhodium-Abnormal NHC Intermediates

Rollover C-H activation with transition-metal complexes has been found to be a difficult but viable pathway to functionalize potentially chelating molecules, which are otherwise reluctant to react further. However, selective rollover or nonrollover C-H activation pathway depends on the stereoelectronic demand of the associated organometallic intermediate(s). The presented work addresses the above issue on abnormal N-heterocyclic carbene (NHC) platform. Catalytic reactions of pyridine-imidazolium substrates with internal alkynes have been selectively guided toward either rollover or nonrollover C-H functionalization route via fulfilling the steric and electronic demands of the relevant rhodium(III)-abnormal NHC metallacyclic intermediates.

C–H activation-annulation on the N-heterocyclic carbene platform

This review highlights the initial development of a new C–H activation–annulation chemistry accessible on the metal–N-heterocyclic carbene platform.