Metal-Catalyzed Annulations through Activation and Cleavage of C−H Bonds

Electronic and Steric Tuning of a Prototypical Piano Stool Complex: Rh(III) Catalysis for C-H Functionalization

The history of transition metal catalysis is heavily steeped in ligand design, clearly demonstrating the importance of this approach. The intimate relationship between metal and ligand can profoundly affect the outcome of a reaction, often impacting selectivity, physical properties, and the lifetime of a catalyst. Importantly, this metal-ligand relationship can provide near limitless opportunities for reaction discovery. Over the past several years, transition-metal-catalyzed C-H bond functionalization reactions have been established as a critical foundation in organic chemistry that provides new bond forming strategies. Among the d-block elements, palladium is arguably one of the most popular metals to accomplish such transformations. One possible explanation for this achievement could be the broad set of phosphine and amine based ligands available in the chemist's toolbox compatible with palladium. In parallel, other metals have been investigated for C-H bond functionalization. Among them, pentamethylcyclopentadienyl (Cp*) Rh(III) complexes have emerged as a powerful mode of catalysis for such transformations providing a broad spectrum of reactivity. This approach possesses the advantage of often very low catalyst loading, and reactions are typically performed under mild conditions allowing broad functional group tolerance. Cp*Rh(III) is considered as a privileged catalyst and a plethora of reactions involving a C-H bond cleavage event have been developed. The search for alternative cyclopentadienyl based ligands has been eclipsed by the tremendous effort devoted to exploring the considerable scope of reactions catalyzed by Cp*Rh(III) complexes, despite the potential of this strategy for enabling reactivity. Thus, ligand modification efforts in Rh(III) catalysis have been an exception and research directed toward new rhodium catalysts has been sparse. Recently, chiral cyclopentadienyl ligands have appeared allowing enantioselective Rh(III)-catalyzed C-H functionalization reactions to be performed. Alongside chiral ligands, an equally important collection of achiral cyclopentadienyl-derived ligands have also emerged. The design of this new set of ligands for rhodium has already translated to significant success in solving inherent problems of reactivity and selectivity encountered throughout the development of new Rh(III)-catalyzed transformations. This Account describes the evolution of cyclopentadienyl ligand skeletons in Rh(III)-catalysis since the introduction of pentamethylcyclopentadienyl ligands to the present. Specific emphasis is placed on reactivity and synthetic applications achieved with the new ligands with the introduction of achiral mono-, di-, or pentasubstituted cyclopentadienyl ligands exhibiting a stunning effect on reactivity and selectivity. Furthermore, an underlying question when dealing with ligand modification strategies is to explain the reason one ligand outperforms another. Conjecture and speculation abound, but extensive characterization of their steric and electronic properties has been carried out and information about electronic and steric properties of the ligands all contribute to our understanding and give crucial pieces to solve the puzzle.

Rhodium-Catalyzed Cyclization of O,ω-Unsaturated Alkoxyamines: Formation of Oxygen-Containing Heterocycles

O,ω-Unsaturated N-tosyl alkoxyamines undergo unexpected Rh^III -catalyzed intramolecular cyclization by oxyamination to produce oxygen-containing heterocycles. Mechanistic studies show that an aziridine intermediate seems to be responsible for the formation of the heterocycles, possibly via a Rh^V species.

A lesson for site-selective C-H functionalization on 2-pyridones: radical, organometallic, directing group and steric controls

A 2-pyridone ring is a frequently occurring subunit in natural products, biologically active compounds, and pharmaceutical targets. Thus, the selective synthesis of substituted 2-pyridone derivatives through decoration and/or formation of pyridone rings has been one of the important longstanding subjects in organic synthetic chemistry. This minireview focuses on recent advances in site-selective C-H functionalization on 2-pyridone. The reported procedures are categorized according to the site selectivity that is achieved, and the substrate scope, limitations, mechanism, and controlling factors are briefly summarized.

Multiple pathways for C–H cleavage in cationic Cp*Rh(iii)-catalyzed C–H activation without carboxylate assistance: a computational study

Multiple pathways for C–H cleavage was uncovered in cationic Cp*Rh(iii)-catalyzed C–H functionalization with different heteroatom-containing species as possible proton acceptors.

Cp*Co(iii)-Catalyzed oxidative [5+2] annulation: regioselective synthesis of 2-aminobenzoxepines via C–H/O–H functionalization of 2-vinylphenols with ynamides

A Cp*Co(iii)-catalyzed [5+2] C–H annulation reaction of 2-vinylphenols with ynamides toward the regioselective synthesis of 2-aminobenzoxepines has been reported.

The regioselective synthesis of 2-phosphinoylindoles via Rh(iii)-catalyzed C–H activation

A step-economical route for the synthesis of 2-phosphinoylindoles via the Rh(iii)-catalyzed redox-neutral annulation of N-nitrosoanilines with 1-alkynylphosphine oxides was developed.

Rh(iii)-Catalyzed regioselective C–H [4 + 2] C-annulation of vinyl enaminones with alkynes to form polysubstituted salicylaldehydes

Sterically congested polysubstituted salicylaldehydes are accessed through a Rh(iii)-catalyzed regioselective vinylic C–H [4 + 2] C-annulation reaction of vinyl enaminones with alkynes.

Facile construction of hydrogenated azepino[3,2,1-hi]indoles by Rh(iii)-catalyzed C–H activation/[5 + 2] annulation of N-cyanoacetylindolines with sulfoxonium ylides

Rh(iii)-catalyzed regio- and chemoselective [5 + 2] annulation has been realized, leading to efficient construction of seven-membered rings under mild conditions.

Pd(ii)-Catalyzed [3 + 2] spiroannulation of α-aryl-β-naphthols with alkynes via a C–H activation/dearomatization approach

A Pd(ii)-catalyzed [3 + 2] spiroannulation of α-aryl-β-naphthols with internal alkynes has been developed by relying on a C–H activation/arene dearomatization approach.

Rhodium-catalyzed triazole-directed C–H bond functionalization of arenes with diazo compounds

Rhodium(iii)-catalyzed alkylation reactions of arenes through triazole directed C–H activation that lead to a number of dialkylated and monoalkylated triazoles are described.

Rapid synthesis of polysubstituted phenanthridines from simple aliphatic/aromatic nitriles and iodo arenes via Pd(ii) catalyzed domino C–C/C–C/C–N bond formation

An efficient method has been developed for the synthesis of polysubstituted phenanthridiene via palladium catalyzed nucleophilic addition of aryl iodides to nitriles followed by dual C–H activation.

From Simple to Complex: Rhodium(III)-Catalyzed C-C Bond Cleavage and C-H Bond Functionalization for the Synthesis of 3a,8b-Dihydro-1H-cyclopenta[b]benzofuran-1-ones

A rhodium(III)-catalyzed strategy for the one-step synthesis of polysubstituted cis-3a,8b-dihydro-1H-cyclopenta[b]benzofuran-1-ones from simple 2'-hydroxychalcones and alkynes is developed. This novel transformation involves a sequential C-C bond cleavage and dehydrogenative annulation, leading to the product bearing a quaternary and a tertiary carbon center. ¹³C labeling experiments revealed that C-C bond cleavage takes place not only at the C-C(C═O) bond but also at the C≡C bond. This study provides an alternative strategy using C-C bond cleavage thus demonstrating the power of this strategy combined with C-H bond functionalization for assembling complex structures from simple starting materials.

Synthesis of Phenalenyl-Fused Pyrylium Cations: Divergent C-H Activation/Annulation Reaction Sequence of Naphthalene Aldehydes with Alkynes

Described herein is the synthesis of stable oxonium-doped polycyclic aromatic hydrocarbons (PAHs) by the rhodium-catalyzed C-H activation/annulations of naphthalene-type aldehydes with internal alkynes. This protocol provides four divergent reaction types, including two unexpected annulations with an oxygen transposition process, which lead to diverse types of phenalenyl-fused pyrylium cations comprising a four-, five-, or six-ring-fused π-conjugated core. The annulations exhibit an exquisite regioselectivity and a high tolerance of sensitive functional groups. These PAHs feature intriguing photophysical properties such as full-color tunable fluorescence emission, high quantum yield, and positively charged core, and can be reduced easily to the phenalenyl radicals.

Synthesis of [5,6]-Bicyclic Heterocycles with a Ring-Junction Nitrogen Atom: Rhodium(III)-Catalyzed C-H Functionalization of Alkenyl Azoles

The first syntheses of privileged [5,6]-bicyclic heterocycles, with ring-junction nitrogen atoms, by transition metal catalyzed C-H functionalization of C-alkenyl azoles is disclosed. Several reactions are applied to alkenyl imidazoles, pyrazoles, and triazoles to provide products with nitrogen incorporated at different sites. Alkyne and diazoketone coupling partners give azolopyridines with various substitution patterns. In addition, 1,4,2-dioxazolone coupling partners yield azolopyrimidines. Furthermore, the mechanisms for the reactions are discussed and the utility of the developed approach is demonstrated by iterative application of C-H functionalization for the rapid synthesis of a patented drug candidate.

Mechanistic Investigation of RhI-Catalyzed Cycloisomerization of Benzylallene-Internal Alkynes via C-H Activation

Treatment of the benzylallene-internal alkynes with [RhCl(CO)₂]₂ effected a cycloisomerization via a C_sp2-H bond activation to produce the tricyclo[9.4.0.0^3,8]pentadecapentaene skeleton. The reaction mechanism via formation of the rhodabicyclo[4.3.0] intermediates and σ-bond metathesis between the C_sp2-H bond on the benzene ring and the C_sp2-Rh^III bond was proposed. In addition, a plausible alternative mechanism for the previously reported cycloisomerization of the benzylallene-terminal alkynes could also be proposed.

Iridium(I)-Catalyzed Intramolecular Hydrocarbonation of Alkenes: Efficient Access to Cyclic Systems Bearing Quaternary Stereocenters

A catalytic, versatile and atom-economical C-H functionalization process that provides a wide variety of cyclic systems featuring methyl-substituted quaternary stereocenters is described. The method relies on the use of a cationic Ir^I -bisphosphine catalyst, which promotes a carboxamide-assisted activation of an olefinic C(sp² )-H bond followed by exo-cyclization to a tethered 1,1-disubstituted alkene. The extension of the method to aromatic and heteroaromatic C-H bonds, as well as developments on an enantioselective variant, are also described.

C-H Activation and Alkyne Annulation via Automatic or Intrinsic Directing Groups: Towards High Step Economy

Direct transformation of carbon-hydrogen bond (C-H) has emerged to be a trend for construction of molecules from building blocks with no or less prefunctionalization, leading high atom and step economy. Directing group (DG) strategy is widely used to achieve higher reactivity and selectivity, but additional steps are usually needed for installation and/or cleavage of DGs, limiting step economy of the overall transformation. To meet this challenge, we proposed a concept of automatic DG (DG^auto ), which is auto-installed and/or auto-cleavable. Multifunctional oxime and hydrazone DG^auto were designed for C-H activation and alkyne annulation to furnish diverse nitrogen-containing heterocycles. Imidazole was employed as an intrinsic DG (DGⁱⁿ ) to synthesize ring-fused and π-extended functional molecules. The alkyne group in the substrates can also be served as DGⁱⁿ for ortho-C-H activation to afford carbocycles. In this account, we intend to give a review of our progress in this area and brief introduction of other related advances on C-H functionalization using DG^auto or DGⁱⁿ strategies.

Manganese(I)-Catalyzed Regio- and Stereoselective 1,2-Diheteroarylation of Allenes: Combination of C-H Activation and Smiles Rearrangement

Heteroarenes are important structural motif in functional molecules. A Mn^I -catalyzed 1,2-diheteroarylation of allenes via a C-H activation/Smiles rearrangement cascade is presented. The reaction occurred under additive-free or even solvent-free conditions, which allowed the creation of two C-C and one C-N bonds in a single operation. A series of structurally diverse bicyclic or tricyclic compounds bearing an exocyclic double bond were constructed in good to excellent efficiency. The decarboxylative ring-opening of the products led to the facile synthesis of vicinal biheteroaryls. Synthetic applications were demonstrated and preliminary mechanistic studies were conducted.

Rh-catalyzed aerobic oxidative cyclization of anilines, alkynes, and CO

We describe a novel Rh-catalyzed C–H cyclization of a wide range of anilines with alkynes and CO. Particularly, simple primary anilines and readily prepared tertiary anilines can be easily converted to quinolin-2(1H)-ones via C–N bond cleavage.

Transition-metal-catalyzed oxidative C–H cyclization of anilines has been an attractive and powerful strategy for the efficient construction of N-heterocycles. However, primary and tertiary anilines are rarely employed in this strategy due to the relative instability with strong oxidants or the presence of three C–N bonds. We describe here a novel Rh-catalyzed C–H cyclization of a wide range of anilines with alkynes and CO, using an aerobic oxidative protocol. Particularly, the simple primary anilines and readily prepared tertiary anilines could be easily converted to quinolin-2(1H)-ones, which are high value-added, biologically significant N-heterocycles, via C–N bond cleavage.