Rh(III)-catalyzed selective C2 C-H acyloxylation of indoles

Herein, we present the first highly regio- and chemoselective C2 C-H acyloxylation of indole under rhodium catalysis and an N-quinolinyl auxiliary. This strategy accommodates a wide range of indoles and structurally diverse carboxylic acids with good reaction efficiencies to yield functionalized indoles. The utility of this logic was demonstrated by the concise synthesis of the functionalized 2-oxindole derivatives. Preliminary mechanistic studies indicate that catalyst turnover of RhIII-RhIV/V-RhII/III-RhIII might be involved in this catalytic C-H transformation.

Ruthenium(II)-Catalyzed Sterically Hindered C-H Acyloxylation to Synthesize Biaryl Isoquinoline Derivatives via Peresters

A novel C-H acyloxylation method of 1-(1-naphthalen-1-yl)isoquinoline derivatives with peresters in the presence of [Ru(p-cymene)Cl₂]₂ has been developed. The combination of ruthenium(II), AgBF₄, CoI₂, and 2,2,6,6-tetramethyl-1-piperidinyloxy is found to be an effective catalytic system to provide various biaryl compounds in satisfactory yields within minutes. Notably, steric hindrance is a very important determinant of the reaction.

Recent Advances in Alkenyl sp2 C-H and C-F Bond Functionalizations: Scope, Mechanism, and Applications

Alkenes and their derivatives are featured widely in a variety of natural products, pharmaceuticals, and advanced materials. Significant efforts have been made toward the development of new and practical methods to access this important class of compounds by selectively activating the alkenyl C(sp²)-H bonds in recent years. In this comprehensive review, we describe the state-of-the-art strategies for the direct functionalization of alkenyl sp² C-H and C-F bonds until June 2022. Moreover, metal-free, photoredox, and electrochemical strategies are also covered. For clarity, this review has been divided into two parts; the first part focuses on currently available alkenyl sp² C-H functionalization methods using different alkene derivatives as the starting materials, and the second part describes the alkenyl sp² C-F bond functionalization using easily accessible gem-difluoroalkenes as the starting material. This review includes the scope, limitations, mechanistic studies, stereoselective control (using directing groups as well as metal-migration strategies), and their applications to complex molecule synthesis where appropriate. Overall, this comprehensive review aims to document the considerable advancements, current status, and emerging work by critically summarizing the contributions of researchers working in this fascinating area and is expected to stimulate novel, innovative, and broadly applicable strategies for alkenyl sp² C-H and C-F bond functionalizations in the coming years.

The polychloromethylation/acyloxylation of 1,6-enynes with chloroalkanes and diacyl peroxides through dual-role designs

The novel polychloromethylation/acyloxylation of 1,6-enynes with chloroalkanes and diacyl peroxides through dual-role designs has been developed to prepare 2-pyrrolidinone derivatives with polychloromethyl units with the use of an inexpensive copper salt under mild conditions. This strategy includes two dual-role designs, not only improving atomic utilization but also allowing a cleaner process. The wide substrate scope and simple reaction conditions demonstrate the practicability of this protocol.

Ruthenaelectro-catalyzed C-H acyloxylation for late-stage tyrosine and oligopeptide diversification

Ruthenaelectro(ii/iv)-catalyzed intermolecular C-H acyloxylations of phenols have been developed by guidance of experimental, CV and computational insights. The use of electricity bypassed the need for stoichiometric chemical oxidants. The sustainable electrocatalysis strategy was characterized by ample scope, and its unique robustness enabled the late-stage C-H diversification of tyrosine-derived peptides.

Thioether-directed Rh(III)-catalyzed peri-selective acyloxylation of arenes

A thioether directed acyloxylation of arenes has been realized via Cp*Rh(III)-catalyzed C-H activation and subsequent coupling with carboxylic acids. This new method showed high functional group compatibility and broad substrate scope. Primary mechanistic studies have been conducted and a tentative reaction mechanism was proposed. It represents the first example of a thioether-directed Cp*Rh(III)-catalyzed C(sp²)-H acyloxylation reaction.

Transition-Metal-Catalyzed Directing Group Assisted (Hetero)aryl C-H Functionalization: Construction of C-C/C-Heteroatom Bonds

Transition-metal-catalyzed C-H functionalization is one of the fascinating scientific fronts in organic synthesis for the formation of conjugated arenes and has emerged as a benchmark to revolutionize the synthetic enterprise since past decades. In this realm, chelation-guided functionalization of C-H bonds using an exogenous directing group has received considerable attention recently for the expedient regioselective construction of C-C and C-heteroatom bonds as an efficient and sustainable alternative. This article outlines our contribution towards a wide variety of transformations that have been achieved by the directed C-H functionalization through the fine tuning of catalytic systems.

Rhoda-Electrocatalyzed Bimetallic C-H Oxygenation by Weak O-Coordination

Rhodium-electrocatalyzed arene C-H oxygenation by weakly O-coordinating amides and ketones have been established by bimetallic electrocatalysis. Likewise, diverse dihydrooxazinones were selectively accessed by the judicious choice of current, enabling twofold C-H functionalization. Detailed mechanistic studies by experiment, mass spectroscopy and cyclovoltammetric analysis provided support for an unprecedented electrooxidation-induced C-H activation by a bimetallic rhodium catalysis manifold.

Recent advances in chelation-assisted site- and stereoselective alkenyl C–H functionalization

This review highlights recent advances in vicinal- and geminal-group-directed alkenyl C–H functionalizations which proceeded by endo- and exo-cyclometallation.

Nitrate promoted mild and versatile Pd-catalysed C(sp2)-H oxidation with carboxylic acids

A nitrate-promoted Pd-catalysed mild cross-dehydrogenative C(sp²)-H bond oxidation of oximes or azobenzenes with diverse carboxylic acids has been developed. In contrast to the previous catalytic systems, this protocol features mild conditions (close to room temperature for most cases) and a broad substrate scope (up to 64 examples), thus constituting a versatile method to directly prepare diverse O-aryl esters. Moreover, the superiority of the nitrate additive in this mild transformation was further determined by experimental and computational evidence.

Rh(III)-Catalyzed C(sp3)-H Acetoxylation of 8-Methylquinolines

A mild and efficient Rh(III)-catalyzed aliphatic C-H acetoxylation directed by quinolines has been developed with widespread functional groups, including various halogens, which usually can provide precursors for further organic synthesis but easily results in selectivity issues in the Pd- and Ni-catalyzed reaction. Interestingly, Ac₂O plays an essential role in promoting the transformation.

Cobalt-Catalyzed C-H Acetoxylation of Phenols with Removable Monodentate Directing Groups: Access to Pyrocatechol Derivatives

An efficient cobalt-catalyzed C-H acetoxylation of phenols has been developed by using PIDA (phenyliodine diacetate) as a sole acetoxy source to synthesize pyrocatechol derivatives for the first time. The key feature of this method is the use of earth-abundant metal cobalt as the green and inexpensive catalyst for the acetoxylation of C(sp²)-H bonds under neutral reaction conditions. Furthermore, the gram-scale reaction and late-stage functionalization demonstrated the usefulness of this method.

Cross-dehydrogenative coupling reactions between arenes (C-H) and carboxylic acids (O-H): a straightforward and environmentally benign access to O-aryl esters

Transition-metal catalyzed cross-dehydrogenative-coupling reactions encompass highly versatile and atom economical methods for the construction of various carbon-carbon and carbon-heteroatom bonds by combining two C(X)-H (X = heteroatom) bonds. Along this line, direct acyloxylation of C-H bonds with carboxylic acids has emerged as a powerful and green approach for the synthesis of structurally diverse esters. In this focus-review we will describe recent progress in direct esterification of aromatic C-H bonds with special emphasis on the mechanistic features of the reactions. Literature has been surveyed until the end of February 2019.

Ru(ii)-Catalyzed C7-acyloxylation of indolines with carboxylic acids

Ruthenium(ii)-catalyzed site-selective C7-acyloxylation of indolines with carboxylic acids is presented. The substrate scope and functional group tolerance are important practical features. The kinetic isotope studies suggest that C-H bond activation may be the rate-determining step.

Cobalt catalyzed regioselective C–H methylation/acetoxylation of anilides: new routes for C–C and C–O bond formation

The first cobalt-catalyzed regioselective C–H methylation/acetoxylation reactions of anilides using peroxides and acetic acid respectively as attacking reagents have been developed herein.

Ruthenium(II)-Catalyzed Regioselective C-8 Hydroxylation of 1,2,3,4-Tetrahydroquinolines

Ru(II)-catalyzed chelation-assisted highly regioselective C8-hydroxylation of 1,2,3,4-tretrahydroquinolines has been developed. Various 1,2,3,4-tetrahydroquinolines underwent smooth C8-H hydroxylation with cheap and safe K₂S₂O₈ as the oxidant and oxygen source to furnish the corresponding products in good to excellent yields with high tolerance of the functional groups. The choice of a readily installable and removable N-pyrimidyl directing group is the key to catalysis. Mechanistic studies suggest the involvement of a six-membered ruthenacycle intermediate in the catalytic cycle. The method can also be extended to the direct hydroxylation of other (hetero)arene C-H bonds.

A comprehensive overview of directing groups applied in metal-catalysed C-H functionalisation chemistry

The present review is devoted to summarizing the recent advances (2015-2017) in the field of metal-catalysed group-directed C-H functionalisation. In order to clearly showcase the molecular diversity that can now be accessed by means of directed C-H functionalisation, the whole is organized following the directing groups installed on a substrate. Its aim is to be a comprehensive reference work, where a specific directing group can be easily found, together with the transformations which have been carried out with it. Hence, the primary format of this review is schemes accompanied with a concise explanatory text, in which the directing groups are ordered in sections according to their chemical structure. The schemes feature typical substrates used, the products obtained as well as the required reaction conditions. Importantly, each example is commented on with respect to the most important positive features and drawbacks, on aspects such as selectivity, substrate scope, reaction conditions, directing group removal, and greenness. The targeted readership are both experts in the field of C-H functionalisation chemistry (to provide a comprehensive overview of the progress made in the last years) and, even more so, all organic chemists who want to introduce the C-H functionalisation way of thinking for a design of straightforward, efficient and step-economic synthetic routes towards molecules of interest to them. Accordingly, this review should be of particular interest also for scientists from industrial R&D sector. Hence, the overall goal of this review is to promote the application of C-H functionalisation reactions outside the research groups dedicated to method development and establishing it as a valuable reaction archetype in contemporary R&D, comparable to the role cross-coupling reactions play to date.

Ruthenium(II)-Catalyzed Positional Selective C-H Oxygenation of N-Aryl-2-pyrimidines

Efficient Ru-catalyzed regioselective C-H oxygenation of N-aryl-2-pyrimidines is described with aryl carboxylic acids in the presence of AgSbF₆ as an additive and Ag₂CO₃ as an oxidant. The reaction can be extended to alkyl, heteroaryl, and α,β-unsaturated carboxylic acids. The regioselectivity, broad substrate scope, and functional group tolerance are the significant practical advantages.

Copper-catalyzed C–H acyloxylation of 2-phenylpyridine using oxygen as the oxidant

An efficient copper-catalyzed direct o-acyloxylation of 2-phenylpyridine with carboxylic acids using oxygen as the oxidant has been developed. Various acids including aromatic acids, cinnamic acids and aliphatic acids are effective acyloxylation reagents and provide the desired products in moderate to excellent yields. The reaction proceeds well under an oxygen atmosphere, making this method potentially practical.

A copper catalyzed direct o-acyloxylation of 2-phenylpyridine with various carboxylic acids using oxygen as oxidant has been developed.