Efficient Rh(I)-Catalyzed Direct Arylation and Alkenylation of Arene C−H Bonds via Decarbonylation of Benzoic and Cinnamic Anhydrides

Transition-Metal-Catalyzed C-C Bond Formation from C-C Activation

ConspectusC-C single bonds are ubiquitous in organic compounds. The activation and subsequent functionalization of C-C single bonds provide a unique opportunity to synthesize conventionally inaccessible molecules through the rearrangement of carbon skeletons, often with a favorable atom and step economy. However, the C-C bonds are thermodynamically and kinetically inert. Consequently, the activation of C-C bonds is particularly attractive yet challenging in the field of organic chemistry. In the past decade, we sought to develop efficient strategies to carry out transition-metal-catalyzed diverse C-C cleavage/C-C forming reactions and to obtain some insights into the intrinsic reactivities of different C-C bonds. With our efforts, readily available alcohols, carboxylic acids, and ketones served as suitable substrates for the catalytic C-C coupling reactions, which are reviewed in this Account. In 2009, we observed a Ni-catalyzed cross coupling of aryl nitriles with arylboronic esters through C-CN cleavage. Encouraged by these results, we are interested in transition-metal-catalyzed C-C bond activation. Due to their broad availability, we then turned our attention to C-C cleavage of carboxylic acids. Rhodium-catalyzed decarbonylative coupling of carboxylic acids with (hetero)arenes was then achieved through oxidative addition of in situ formed, more reactive mixed anhydrides to Rh(I) without the need for oxidants that are commonly required for the decarboxylative coupling of carboxylic acids. Subsequently, the decarbonylation of more challenging unstrained aryl ketones was realized under Rh catalysis assisted by N-containing directing groups. Following this work, a group exchange of aryl ketones with carboxylic acids was achieved through 2-fold C-C bond cleavage. By employing the chelation strategy, Rh-catalyzed C-C bond activation of secondary benzyl alcohols was also accomplished through β-carbon elimination of the rhodium alcoholate intermediates. The competing oxidation of secondary alcohols to ketones via β-hydrogen elimination of the same intermediates was suppressed as thermodynamically favorable five-membered rhodacycles are formed after β-carbon elimination. Different types of transformations of alcohols, including the Heck-type reaction with alkenes, cross coupling with arylsilanes, and Grignard-type addition with aldehydes or imines, have been achieved, showing the great potential of secondary alcohols in the formation of C-C bonds. These C-C bond-forming reactions are complementary to traditional cross couplings of aryl halides with organometallic reagents. However, these transformations produce small molecules as byproducts. To improve the atom economy, we then investigated C-C bond transformations of strained-ring cyclic compounds. Ni-catalyzed intermolecular cyclization of benzocyclobutenones with alkynes was recently achieved via the uncommon cleavage of the C1-C8 bond by employing a removable blocking strategy. Rh-catalyzed intramolecular annulation of benzocyclobutenols with alkynes was also achieved. In summary, our developments demonstrate the great potential of transition-metal-catalyzed C-C bond activation for the formation of new C-C bonds. To further expand the synthetic utility of C-C bond activation, more efforts are required to expand the substrate scope and to achieve earth-abundant metal-catalyzed transformations.

Rhodium-catalysed decarbonylative C(sp2)-H alkylation of indolines with alkyl carboxylic acids and carboxylic anhydrides under redox-neutral conditions

We developed a rhodium-catalysed decarbonylative C(sp²)-H alkylation method for indolines. This reaction facilitates the use of alkyl carboxylic acids and their anhydrides as a cheap, abundant and non-toxic alkyl source under redox-neutral conditions, featuring the introduction of a primary alkyl chain, which cannot be addressed by previous radical-mediated decarboxylative reaction. Through a mechanistic investigation, we revealed that an initially formed C-7 acylated indoline was transformed into the corresponding alkylated indoline via a decarbonylation process.

Rhodium-Catalyzed C(sp2)-H Alkoxycarbonylation/Acylation of Indolines with Anhydrides as a Carbonyl Source

We developed rhodium-catalyzed alkoxylcarbonylation/acylation of indolines using anhydrides as a safe and easy-to-handle carbonyl source. This catalytic process represents an additive- and CO-free carbonylation, establishing a simple and straightforward protocol for synthesizing C7-carbonylated indolines. Notably, this reaction provides a successful example of C–H acylation of indolines that results in the formation of α-branched ketones, which were difficult to prepare by previously reported analogous catalytic reactions.

Palladium-Catalyzed Decarbonylative Borylation of Aryl Anhydrides

A palladium-catalyzed base-free decarbonylative borylation of aryl anhydrides has been developed. Catalyst system consisting of Pd(OAc)₂/dppb enables readily available aryl anhydrides to be employed as electrophiles for the synthesis of versatile arylboronate esters via O-C(O) bond activation and decarbonylation. This method is characterized by an excellent functional group tolerance and broad substrate scope, using bench stable aryl anhydrides as aryl electrophiles in C-B bond formation. Mechanistic studies and functionalization of late-stage pharmaceutical molecules are disclosed.

Recent advances in rhodium-catalyzed C(sp2)-H (hetero)arylation

Arylation is a common behaviour in organic synthesis for the construction of complex structures, especially the biaryls. Among those reported arylation procedures, transition-metal-catalyzed direct C(sp²)-H arylation has been rapidly developed in recent decades and has become a reliable alternative to traditional cross-coupling procedures using organometallic reagents. Great achievements in rhodium-catalyzed C(sp²)-H arylation have been witnessed during the last decade. Aryl halides, simple arenes, aryl boronic acids, arylsilanes, aryl aldehyde, aryl carboxylic acid, diazides, etc. were successfully utilized as arylating reagents under rhodium-catalyzed conditions. In this review, recent achievements in rhodium-catalyzed arylations through C(sp²)-H bond activation were summarized together with the mechanism discussions.

Metal-catalysed C-Het (F, O, S, N) and C-C bond arylation

The formation of C-aryl bonds has been the focus of intensive research over the last decades for the construction of complex molecules from simple, readily available feedstocks. Traditionally, these strategies involve the coupling of organohalides (I, Br, Cl) with organometallic reagents (Mg, Zn, B, Si, Sn,…) such as Kumada-Corriu, Negishi, Suzuki-Miyaura, Hiyama and Sonogashira cross-couplings. More recently, alternative methods have provided access to these products by reactions with less reactive C-Het (F, O, S, N) and C-C bonds. Compared to traditional methods, the direct cleavage and arylation of these chemical bonds, the essential link in accessible feedstocks, has become increasingly important from the viewpoint of step-economy and functional-group compatibility. This comprehensive review aims to outline the development and advances of this topic, which was organized into (1) C-F bond arylation, (2) C-O bond arylation, (3) C-S bond arylation, (4) C-N bond arylation, and (5) C-C bond arylation. Substantial attention has been paid to the strategies and mechanistic investigations. We hope that this review can trigger chemists to discover more efficient methodologies to access arylation products by cleavage of these C-Het and C-C bonds.

A facile method for Rh-catalyzed decarbonylative ortho-C–H alkylation of (hetero)arenes with alkyl carboxylic acids

A facile and effective method for Rh-catalyzed direct ortho-alkylation of C–H bonds in (hetero)arenes with commercially available carboxylic acids has been developed. This strategy was initiated by in situ conversion of carboxylic acids to anhydrides which, without isolation, underwent Rh-catalyzed direct decarbonylative cross-coupling of aryl carboxamides containing 8-aminoquinoline. The reaction proceeds with high regioselectivity and exhibits a broad substrate scope as well as functional group tolerance.

A facile and effective method for Rh-catalyzed direct ortho-alkylation of C–H bonds in (hetero)arenes with commercially available carboxylic acids has been developed.

Ruthenium-catalyzed meta-C-H bond alkylation of aryl 2-pyridyl ketones

The first example of meta-selective CAr-H bond functionalization of aryl 2-pyridyl ketones has been developed using [Ru(p-cymene)Cl2]2 as the catalyst and alkyl bromide as the coupling reagent. This development provides an efficient strategy for modifying the meta-position of aryl 2-pyridyl ketone skeletons, which are found in various functional molecules.

Rhodium(i)-catalyzed C6-selective C-H alkenylation and polyenylation of 2-pyridones with alkenyl and conjugated polyenyl carboxylic acids

A versatile Rh(i)-catalyzed C6-selective decarbonylative C-H alkenylation of 2-pyridones with readily available, and inexpensive alkenyl carboxylic acids has been developed. This directed dehydrogenative cross-coupling reaction affords 6-alkenylated 2-pyridones that would otherwise be difficult to access using conventional C-H functionalization protocols. The reaction occurs with high efficiency and is tolerant of a broad range of functional groups. A wide scope of alkenyl carboxylic acids, including challenging conjugated polyene carboxylic acids, are amenable to this transformation and no addition of external oxidant is required. Mechanistic studies revealed that (1) Boc2O acts as the activator for the in situ transformation of the carboxylic acids into anhydrides before oxidative addition by the Rh catalyst, (2) a decarbonylation step is involved in the catalytic cycle, and (3) the C-H bond cleavage is likely the turnover-limiting step.

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.

Transition-Metal-Catalyzed Decarbonylative Coupling Reactions: Concepts, Classifications, and Applications

Transition metal-catalyzed decarbonylative coupling reactions have emerged as a powerful alternative to conventional cross-coupling protocols due to the advantages associated with the use of carbonyl-containing functionalities as coupling electrophiles instead of commonly used organohalides or sulfates. A wide variety of novel transformations based on this concept have been successfully achieved, including decarbonylative carbon-carbon and carbon-heteroatom bond forming reactions. In this Review, we summarize the recent progress in this field and present a comprehensive overview of metal-catalyzed decarbonylative coupling reactions with carbonyl derivatives.

Cross-coupling of aromatic esters and amides

Catalytic cross-coupling reactions of aromatic esters and amides have recently gained considerable attention from synthetic chemists as de novo and efficient synthetic methods to form C-C and C-heteroatom bonds. Esters and amides can be used as diversifiable groups in metal-catalyzed cross-coupling: in a decarbonylative manner, they can be utilized as leaving groups, whereas in a non-decarbonylative manner, they can form ketone derivatives. In this review, recent advances of this research topic are discussed.

Ligand-Controlled Chemoselective C(acyl)-O Bond vs C(aryl)-C Bond Activation of Aromatic Esters in Nickel Catalyzed C(sp2)-C(sp3) Cross-Couplings

A ligand-controlled and site-selective nickel catalyzed Suzuki-Miyaura cross-coupling reaction with aromatic esters and alkyl organoboron reagents as coupling partners was developed. This methodology provides a facile route for C(sp²)-C(sp³) bond formation in a straightforward fashion by successful suppression of the undesired β-hydride elimination process. By simply switching the phosphorus ligand, the ester substrates are converted into the alkylated arenes and ketone products, respectively. The utility of this newly developed protocol was demonstrated by its wide substrate scope, broad functional group tolerance and application in the synthesis of key intermediates for the synthesis of bioactive compounds. DFT studies on the oxidative addition step helped rationalizing this intriguing reaction chemoselectivity: whereas nickel complexes with bidentate ligands favor the C(aryl)-C bond cleavage in the oxidative addition step leading to the alkylated product via a decarbonylative process, nickel complexes with monodentate phosphorus ligands favor activation of the C(acyl)-O bond, which later generates the ketone product.

Synthesis of perinaphthenones through rhodium-catalyzed dehydrative annulation of 1-naphthoic acids with alkynes

A convenient method for the synthesis of perinaphthenones via rhodium-catalyzed dehydrative annulation of naphthoic acids with alkynes, which gave good to high yields of perinaphthenones, was developed.

Rhodium-catalyzed C–H functionalization with N-acylsaccharins

The first rhodium-catalyzed C–H functionalization with N-acylsaccharins by decarbonylation has been developed and afforded biaryls in 51–93% yields.

Pd(OAc)2-catalysed regioselective alkoxylation of aryl (β-carbolin-1-yl)methanones via β-carboline directed ortho-C(sp(2))-H activation of an aryl ring

Synthesis of (2-alkoxyphenyl)(9H-pyrido[3,4-b]indol-1-yl)methanone via Pd(OAc)2-catalyzed regioselective alkoxylation of aryl (β-carbolin-1-yl) methanones employing β-carboline directed ortho-C(sp(2))-H activation of an aryl ring under oxidative conditions is described.

Highly selective directed arylation reactions via back-to-back dehydrogenative C-H borylation/arylation reactions

Dimeric rhodium N-heterocyclic carbene complexes are demonstrated to be effective catalyst precursors for directed C-H borylation reactions at room temperature. The reactions are highly selective for mono-borylation and can be combined with a one-pot Suzuki-Miyaura coupling to give C-H arylation products with exclusive selectivity for mono-arylation without the requirement for steric blocking groups.

DFT studies on the directing group dependent arene–alkene cross-couplings: arene activation vs. alkene activation

Two competing reaction pathways for the dehydrogenative Heck couplings have been found. The properties of the directing groups on reactants will determine the dominance of these two pathways.