Recent advancements in dehydrogenative cross coupling reactions for CC bond formation

Advances in Rhodium-Catalyzed Oxidative Arene Alkenylation

ConspectusAlkyl and alkenyl arenes are of substantial value in both large-scale and fine chemical processes. Billions of pounds of alkyl and alkenyl arenes are produced annually. Historically, the dominant method for synthesis of alkyl arenes is acid-catalyzed arene alkylation, and alkenyl arenes are often synthesized in a subsequent dehydrogenation step. But these methods have limitations that result from the catalytic mechanism including (1) common polyalkylation, which requires an energy intensive transalkylation process, (2) quantitative selectivity for Markovnikov products for arene alkylation using α-olefins, (3) for substituted arenes, regioselectivity that is dictated by the electronic character of the arene substituents, (4) inability to form alkenyl arenes in a single process, and (5) commonly observed slow reactivity with electron-deficient arenes. Transition-metal-catalyzed aryl-carbon coupling reactions can produce alkyl or alkenyl arenes from aryl halides. However, these reactions often generate halogenated waste and typically require a stoichiometric amount of metal-containing transmetalation reagent. Transition-metal-catalyzed arene alkylation or alkenylation that involves arene C-H activation and olefin insertion into metal-aryl bonds provides a potential alternative method to prepare alkyl or alkenylation arenes. Such reactions can circumvent carbocationic intermediates and, as a result, can overcome some of the limitations mentioned above. In particular, controlling the regioselectivity of the insertion of α-olefins into metal-aryl bonds provides a strategy to selectively synthesize anti-Markovnikov products. But, previously reported catalysts often show limited longevity and low selectivity for anti-Markovnikov products.In this Account, we present recent developments in single-step arene alkenylation using Rh catalyst precursors. The reactions are successful for unactivated hydrocarbons and exhibit unique selectivity. The catalytic production of alkenyl arenes operates via Rh-mediated aromatic C-H activation, which likely occurs by a concerted metalation-deprotonation mechanism, olefin insertion into a Rh-aryl bond, β-hydride elimination from the resulting Rh-hydrocarbon product, and net dissociation of alkenyl arene with formation of a Rh hydride. Reaction of the Rh hydride with Cu(II) oxidant completes the catalytic cycle. Although Rh nanoparticles can be formed under some conditions, mechanistic studies have revealed that soluble Rh species are likely responsible for the catalysis. These Rh catalyst precursors achieve high turnovers with >10,000 catalytic turnovers observed in some cases. Under anaerobic conditions, Cu(II) carboxylates are used as the oxidant. In some cases, aerobic recycling of Cu(II) oxidant has been demonstrated. Hence, the Rh arene alkenylation catalysis bears some similarities to Pd-catalyzed olefin oxidation (i.e., the Wacker-Hoechst process). The Rh-catalyzed arene alkenylation is compatible with some electron-deficient arenes, and they are selective for anti-Markovnikov products when using substituted olefins. Finally, when using monosubstituted arenes, consistent with a metal-mediated C-H activation process, Rh-catalyzed alkenylation of substituted arenes shows selectivity for meta- and para-alkenylation products.

Aerobic Catalyzed Oxidative Cross-Coupling of N,N-Disubstituted Anilines and Aminonaphthalenes with Phenols and Naphthols

The cross-coupling of N,N-dialkyl aniline and aminonaphthalenes with phenols and naphthols using a Cr-salen catalyst under aerobic conditions was developed. Notably, air serves as an effective oxidant affording products in high selectivity. Initial mechanistic studies suggest an outer-sphere oxidation of the aniline/aminonaphthalene partner, followed by nucleophilic attack of the phenol/naphthol. Single products were observed in most cases, whereas mixtures of C-C and C-O coupled products arose from reactions involving aminonapthalene and sterically unencumbered phenols.

Electrochemical dehydrogenative cross-coupling of two anilines: facile synthesis of unsymmetrical biaryls

New ortho/para-selective dehydrogenative cross-coupling of aryl amines for producing biaryls and incorporating pharmacophores is depicted.

One pot synthesis of pyrrolo[3,2,1-de]phenanthridines from 7-phenylindoles via tandem C–H olefination/aza-Michael addition

An unprecedented one-pot C–H olefination/aza-Michael addition tandem process has been developed for the synthesis of pyrrolo[3,2,1-de]phenanthridines from 7-phenylindoles and alkenes using a [Cp*RhCl₂]₂/AgOAc/Me₄NOAc catalytic system.

Cross-dehydrogenative coupling involving benzylic and allylic C–H bonds

Benzylic and allylic C–H bonds are coupled with C(sp)–H, C(sp²)–H and C(sp³)–H bonds in a straightforward and high atom-economic manner.

Transition metal-free cross-dehydrogenative arylation of unactivated benzylic C–H bonds

The cross-dehydrogenative arylation of benzylic C–H bonds with arenes provides straightforward access to synthetically useful 1,1-diarylmethanes, from readily available starting materials.

tert-Butylhydroperoxide (TBHP) mediated oxidative cross-dehydrogenative coupling of quinoxalin-2(1H)-ones with 4-hydroxycoumarins, 4-hydroxy-6-methyl-2-pyrone and 2-hydroxy-1,4-naphthoquinone under metal-free conditions

We report an efficient and atom-economical method of C-3 functionalization of quinoxalin-2(1H)-ones with 4-hydroxycoumarins, 4-hydroxy-6-methyl-2-pyrone, and 2-hydroxy-1,4-naphthoquinone via the free radical cross-coupling pathway under metal-free conditions. tert-Butylhydroperoxide (TBHP) smoothly promotes the reaction furnishing the cross-dehydrogenative coupling (CDC) products in very good to excellent yields. The protocol neither uses any toxic reagents nor metal catalysts to carry out the reaction, and all the products have been obtained without column chromatography purification. Different radical trapping experiments with 2,2,6,6-tetramethylpiperidine-1-oxyl, butylated hydroxytoluene, and diphenyl ethylene confirm the involvement of radicals.

Catalytic enantioselective cross dehydrogenative coupling of sp3 C-H of heterocycles

C-C bond formation in heterocycles via the direct coupling of C-H bonds, under oxidative conditions, classified as cross dehydrogenative coupling (CDC), is without doubt one of the most atom efficient methods for the functionalization of these molecules. The most common is the coupling at the position alpha to the heteroatom, owing to the stabilization of forming carbocation by the heteroatom. The corresponding asymmetric versions, except for a few isolated reports, have been rather evasive for several years. Optically active heterocycles with the chiral center alpha to the heteroatom are widely present in natural products and pharmaceuticals, thus making them an attractive synthetic target. Persistent efforts towards the asymmetric CDC of heterocycles since the beginning of this decade have led to several developments in this challenging area. Particularly, in the last few years, considerable progress has been witnessed in this field. This review summarizes the progress made in the area of asymmetric cross dehydrogenative coupling of heterocycles in recent years. The review covers the catalytic asymmetric CDC of sp³ C-H bonds of the heterocycles with various coupling partners and illustrates the different catalytic systems employed.

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.

Transition metal catalysed direct selanylation of arenes and heteroarenes

Transition metal catalysed C-H functionalization has reached an exciting level of sophistication, and, today, it represents a paradigm shift from the standard logic of synthetic chemistry. The direct conversion of C-H bonds into C-heteroatoms remains, however, a critical challenge. Nowadays, there is a great demand in general synthetic chemistry in, for example, the materials science for the development of straightforward C-Se bond formation, in order to fulfil the practical requirements. In this sense, this review summarizes recent outstanding advances in the C-Se bond formation through transition metal-catalysed direct selanylation, providing new insights into their mechanistic aspects and disclosing effective synthetic routes with high atom economy. In addition, this review intends to show the growing opportunities to construct complex chemical scaffolds containing selenium atoms.

Direct Functionalization of C(sp2)-H Bond in Nonaromatic Azaheterocycles: Palladium-Catalyzed Cross-Dehydrogenative Coupling (CDC) of 2H-Imidazole 1-Oxides with Pyrroles and Thiophenes

The C(sp²)-H bond functionalization methodology was first applied to carry out the palladium-catalyzed oxidative C-H/C-H coupling reactions of 2H-imidazole 1-oxides with pyrroles and thiophenes. As a result, a number of novel 5-heteroarylated 2H-imidazole 1-oxides, which are of particular interest in the design of bioactive molecules and advanced materials, have been synthesized in yields up to 78%. The detailed H/D-exchange experiments have also been performed to elucidate some mechanistic features of this cross-dehydrogenative coupling process.

Copper-catalyzed highly diastereoselective cross-dehydrogenative coupling between 8-hydroxyisochromanes and 1,3-dicarbonyl compounds

A novel copper-catalyzed highly diastereoselective cross-dehydrogenative coupling reaction for the access of tricyclic chromanes from 8-hydroxyisochromanes and 1,3-dicarbonyl compounds.

A visible light mediated, metal and oxidant free highly efficient cross dehydrogenative coupling (CDC) reaction between quinoxalin-2(1H)-ones and ethers

The white light mediated, 3C alkylation of quinoxalin-2(1H)-ones via a cross dehydrogenative coupling (CDC) reaction with cyclic ethers using eosin Y as a photocatalyst is described. The reaction proceeds via a hydrogen abstraction and transfer process.

Cu(ii)- or Co(ii)-Catalyzed C(SP3)–H oxidation of N,N-dimethylaminoethanol: facile synthesis of methylene-bridged biindoles and 3-formylindoles selectively

Methylene-bridged biindoles and 3-formylindoles were synthesized with high conversion rate and good regioselectivity using a new carbon donor N,N-dimethylethanolamine, catalyzed by CuCl₂ and CoCl₂ respectively.

Arylhydrazines: novel and versatile electrophilic partners in cross-coupling reactions

Arylhydrazines are extremely valuable compounds in organic chemistry that are widely used for the synthesis of a variety of biologically active molecules such as indoles, indazoles, pyrazoles, aryltriazoles, β-lactams and quinazolines. These compounds have also been widely utilized as arylation agents in oxidative cross-coupling reactions. In this review, we will highlight the most important explorations and developments in the carbon-carbon and carbon-heteroatom (nitrogen, phosphorus, sulfur, and selenium) cross-coupling of arylhydrazines. The literature has been surveyed from 2001 to June 2018.

Direct Arylation of Benzyl Ethers with Organozinc Reagents

A novel C(sp³)-H bond arylation of benzyl ethers with Knochel-type arylzinc reagents has been developed. This transition-metal-catalyst-free reaction proceeds well under mild conditions in a simple and effective manner and enables the synthesis of a wide range of potentially biologically active benzyl ethers by using highly functionalized organozinc reagents as a carbon nucleophile.

Cu-Catalyzed Cross-Dehydrogenative ortho-Aminomethylation of Phenols

A highly selective Cu^II‐catalyzed cross‐dehydrogenative ortho‐aminomethylation of phenols with aniline derivatives is described. The corresponding C(sp²)−C(sp³) coupling products were obtained in moderate to excellent yields under mild reaction conditions and with a broad substrate scope. A radical mechanism is proposed.

Intramolecular hydride transfer onto arynes: redox-neutral and transition metal-free C(sp3)-H functionalization of amines

Transition metal-free intramolecular hydride transfer onto arynes is reported for the first time. This unique transformation is utilized in redox-neutral intermolecular α-functionalization reactions of different tertiary amines, generating C(sp3)-C(sp3/sp2/sp) bonds in a single synthetic operation. Deuterium labeling studies support initial cleavage of the α-C-H bond via intramolecular 1,5-hydride transfer onto the aryne, which leads to activation of a range of integrated pronucleophiles and ultimately affords a new approach to cross-dehydrogenative coupling reactions which utilize aryne intermediates.

Regioselective Sulfenylation of α'-CH3 or α'-CH2 Groups of α,β-Unsaturated Ketones with Heterocyclic Thiols

A rare regioselective sulfenylation of α'-CH₃ or α'-CH₂ bonds adjacent to α,β-unsaturated ketones using dimethyl sulfoxide as an oxidant and a substoichiometric amount of aq HI as an additive is described. This methodology employs a strong acid such as aq HI or iodine and exhibits a high regioselectivity without undergoing conjugate addition, which is difficult to achieve under the cross dehydrogenative coupling method.

Total Synthesis of Aurofusarin: Studies on the Atropisomeric Stability of Bis-Naphthoquinones

An efficient annulation involving pyrone addition to a quinone and Dieckmann condensation was developed for rapid assembly of a γ-naphthopyrone monomeric precursor to the bis-naphthoquinone natural product aurofusarin. Dimerization was achieved through PdII -catalyzed dehydrogenative coupling. Further studies employing asymmetric nucleophilic epoxidation indicate that the atropisomers of aurofusarin and derivatives are not configurationally stable at ambient temperature.

A Cross-Dehydrogenative Annulation Strategy towards Synthesis of Polyfluorinated Phenanthridinones with Copper

The first cross-dehydrogenative annulation of (hetero)aromatic amides with polyfluoro(hetero)arenes is presented. This operationally simple oxidative annulation process is mediated by inexpensive copper salt, accommodates a wide range of substrates with exquisite chemo- and regioselectivity profile, and produces demanding polyfluorinated phenanthridinones in high yields (up to 92 %). Using alkenyl amides under identical conditions, the synthesis of polyfluorinated 2-quinolones has also been accomplished. Given the importance of fluorinated heterocycles in the pharmaceutical industry and drug discovery, this work is highly significant.