Rhodium(III)-Catalyzed Redox-Neutral C–H Arylation via Rearomatization

Synthesis of meta-arylphenol derivatives via acid-promoted rearrangement of cyclohexadienones

A highly effective strategy for the synthesis of meta-arylphenol derivatives through the selective rearrangement of 4-alkyl-4-aryl-2,5-cyclohexadienones under metal-free conditions was developed, in which acid-promoted [1,2]-migration of the aryl group at C-4 occurred exclusively when the alkyl group at C-4 was a methyl group. Treatment of 4-methyl-4-aryl-2,5-cyclohexadienones with 37% HCl in Ac2O at room temperature provided polysubstituted meta-arylphenyl acetates in 75-94% yields. The application of this protocol in the synthesis of polycyclic aromatic compounds was also described.

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 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.

Synthetic and Computational Studies on RhIII-Catalyzed Redox-Neutral Cascade of Carbenoid Functionalization and Dephosphonylative Annulation

A Rh(III)-catalyzed regioselective redox-neutral cascade process of carbenoid functionalization followed by dephosphonylative annulation of benzoic acids with α-diazo-β-keto phosphonate has been realized, which led to the direct synthesis of a privileged 3-substituted isocoumarin scaffold. To the best of our knowledge, this is the first report of a complete redox neutral method to synthesize isocoumarins using C-H functionalization strategy. In the catalytic cycle of this reaction, there are two possible pathways for the C-C coupling between ortho-positioned carbon atom of benzoic acid and the diazo carbon atom: (i) concerted 1,2-aryl shift and (ii) stepwise metal-carbene formation followed by migratory insertion. DFT study has predicted that the concerted pathway has lower activation energy as compared to the stepwise pathway by 1.5 kcal/mol.

Hydroxyl group-directed, tartaric acid-catalyzed synthesis of meta-functionalized aryl ethers and phenols through domino conjugate addition/aromatization of para-quinols

A tartaric acid-catalyzed three-component reaction of para-quinols, organoboronic acids, and alcohols affords meta-alkenylated aryl alkyl ether in good yields. In the absence of alcohols, meta-functionalized phenols could be obtained selectively.

Access to [4,3,1]-Bridged Carbocycles via Rhodium(III)-Catalyzed C-H Activation of 2-Arylindoles and Annulation with Quinone Monoacetals

Reported herein is the Rh(III)-catalyzed annulation of N-unprotected 2-arylindoles with quinone monoacetals for the straightforward synthesis of [4,3,1]-bridged carbocycles with exclusive C(3) selectivity. Mechanistic studies, particularly deuterium-labeling experiments, suggest that the coupling likely proceeds via two-fold C-H activation with two-fold migratory insertion into the enone moieties.

Rh(iii)-Catalyzed straightforward arylation of 8-methyl/formylquinolines using diazo compounds

A straightforward Rh(iii)-catalyzed general strategy was developed for the introduction of naphthol/phenol moieties to 8-methyl and formylquinolines using diazonaphthalen-2(1H)-ones/quinone diazides.

Rhodium(iii)-catalyzed three-component cascade synthesis of 6H-benzo[c]chromenes through C-H activation

A three-component cascade reaction is described that provides concise access to 6H-benzo[c]chromenes via Rh(iii)-catalyzed annulation of aryl ketone O-acyloximes, quinones and acetone. Acetone acts as a co-solvent and as a reactant. This reaction shows high efficiency, atom- and step-economy, good substrate scope, excellent functional group compatibility and gives the products in good yields.

Overcoming naphthoquinone deactivation: rhodium-catalyzed C-5 selective C-H iodination as a gateway to functionalized derivatives

We report a Rh-catalyzed method for the C-5 selective C-H iodination of naphthoquinones and show that complementary C-2 selective processes can be achieved under related conditions. C-C bond forming derivatizations of the C-5 iodinated products provide a gateway to previously inaccessible A-ring analogues. The present study encompasses the first catalytic directed ortho-functionalizations of simple (non-bias) naphthoquinones. The strategic considerations outlined here are likely to be applicable to C-H functionalizations of other weakly coordinating and/or redox sensitive substrates.

Rhodium-catalyzed ortho C–H bond activation of arylamines for the synthesis of quinoline carboxylates

A diverse catalytic approach for the synthesis of quinoline derivatives via Rh catalyzed ortho C–H activation of aryl amines with alkynic esters is described.

Catalytic stereoselective cascade reactions of quinols with trifluoromethyl ketones: direct access to CF3-containing 1,3-dioxolanes

A general and efficient synthesis of CF₃-containing 1,3-dioxolanes using Et₃N-catalyzed ketalization/oxa-Michael cascade of quinols with trifluoromethyl ketones is described. The desired products were obtained in 68%–95% yields with acceptable diastereoselectivities.

Substrate activation strategies in rhodium(III)-catalyzed selective functionalization of arenes

The possibility of developing new methods for the efficient construction of organic molecules via disconnections other than traditional functional group transformations has driven the interest in direct functionalization of C-H bonds. The ubiquity of C-H bonds makes such transformations attractive, but they also pose several challenges. The first is the reactivity and selectivity of C-H bonds. To achieve this, directing groups (DGs) are often installed that can enhance the effective concentration of the catalyst, leading to thermodynamically stable metallacyclic intermediates. However, the presence of a pendant directing group in the product is often undesirable and unnecessary. This may account for the limitation of applications of C-H functionalization reactions in more common and general uses. Thus, the development of removable or functionalizable directing groups is desirable. Another key problem is that the reactivity of the resulting M-C bond can be low, which may limit the scope of the coupling partners and hence limit the reaction patterns of C-H activation reactions. While the first Cp*Rh(III)-catalyzed C-H activation of arenes was reported only 7 years ago, significant progress has been made in this area in the past few years. We began our studies in this area in 2010, and we and others have demonstrated that diversified catalytic functionalization of arenes can be realized using Cp*Rh(III) complexes with high reactivity, stability, and functional group compatibility. This Account describes our efforts to solve some of these challenges using Rh(III) catalysis. We fulfilled our design and activation of the arene substrates by taking advantage of the nucleophilicity, electrophilicity, oxidizing potential, and properties of a participating ligand of the directing groups when the arenes are coupled with relatively reactive unsaturated partners such as alkenes and alkynes. These in situ funtionalizable roles of the DG allowed extensive chemical manipulation of the initial coupled product, especially in the construction of a diverse array of heterocycles. In the coupling of arenes with polar coupling partners, the polar Rh(III)-C(aryl) bond showed higher reactivity as both an organometallic reagent and a nucleophilic aryl source. The polar coupling partners were accordingly activated by virtue of umpolung, ring strain, and rearomatization. All of these transformations have been made possible by integration of the higher reactivity, stability, and compatibility of Rh(III)-C bonds into catalytic systems. We have demonstrated that to date some of these transformations can be achieved only under rhodium catalysis. In addition, by means of stoichiometric reactions, we have gained mechanistic insights into the interactions between the Rh-C bond and the other coupling partners, which have opened new avenues in future direct C-H functionalization reactions.

Carboxylic acids as traceless directing groups for the rhodium(III)-catalyzed decarboxylative C-H arylation of thiophenes

A rhodium(III)-catalyzed carboxylic acid directed decarboxylative C-H/C-H cross-coupling of carboxylic acids with thiophenes has been developed. With a slight adjustment of the reaction conditions based on the nature of the substrates, aryl carboxylic acids with a variety of substituents could serve as suitable coupling partners, and a broad variety of functional groups were tolerated. This method provides straightforward access to biaryl scaffolds with diverse substitution patterns, many of which have conventionally been synthesized through lengthy synthetic sequences. An illustrative example is the one-step gram-scale synthesis of a biologically active 3,5-substituted 2-arylthiophene by way of the current method.

Regioselective oxidative Pd-catalysed coupling of alkylboronic acids with pyridin-2-yl-substituted heterocycles

A total of 19 alkylated heterocycles (thiophenes, benzo-thiophenes, pyrroles, furans) were prepared (36–99% yield) from the respective pyridin-2-yl-substituted precursors employing alkylboronic acids as the alkylating reagents in an oxidative (Ag₂CO₃ and 2,6-dimethyl-1,4-benzoquinone as oxidants) Pd-catalysed coupling reaction.

Rh(iii)-catalyzed oxime ether-directed heteroarylation of arene through oxidative C–H/C–H cross-coupling

A rhodium(iii)-catalyzed dehydrogenative cross-coupling reaction has been developed for the synthesis of bi(hetero)aryl scaffolds by using oxime ethers as the directing group. This protocol features a relatively broad substrate scope and a good tolerance of functional groups.

Rh(iii)-catalyzed oxidative C–H bond arylation with hydroquinones: sustainable synthesis of dibenzo[b,d]pyran-6-ones and benzo[d]naphtho[1,2-b]pyran-6-ones

An efficient Rh(iii)-catalyzed C–H bond arylation with phenol derivatives was developed for the direct and sustainable synthesis of dibenzo[b,d]pyran-6-ones and benzo[d]naphtho[1,2-b]pyran-6-ones.

Cp*Rh(iii) and Cp*Ir(iii)-catalysed redox-neutral C–H arylation with quinone diazides: quick and facile synthesis of arylated phenols

Cp*Rh(iii)- and Cp*Ir(iii)-catalysed direct C–H arylation with quinone diazides provides a facile and redox-neutral access to arylated phenols.

Transition metal-catalyzed C–H bond functionalizations by the use of diverse directing groups

In this review, a summary of transition metal-catalyzed C–H activation by utilizing the functionalities as directing groups is presented.