Rhodium‐Catalyzed Oxidative Coupling between Salicylaldehydes and Internal Alkynes with C?H Bond Cleavage To Produce 2,3‐Disubstituted Chromones

A direct oxidative coupling of salicylaldehydes with internal alkynes proceeds efficiently with cleavage of the aldehyde C? H bond to produce the corresponding chromone derivatives. A rhodium catalyst in combination with a cyclopentadiene ligand and a copper oxidant promote this straightforward annulation reaction. Solid‐state luminescence was observed for certain chromone products.     

Catalytic C?C,C?N,and C?O Ullmann‐Type Coupling Reactions

Copper‐catalyzed Ullmann condensations are key reactions for the formation of carbon–heteroatom and carbon–carbon bonds in organic synthesis. These reactions can lead to structural moieties that are prevalent in building blocks of active molecules in the life sciences and in many material precursors. An increasing number of publications have appeared concerning Ullmann‐type intermolecular reactions for the coupling of aryl and vinyl halides with N, O, and C nucleophiles, and this Minireview highlights recent and major developments in this topic since 2004.     

Rhodium(III)‐Catalyzed C?C and C?O Coupling of Quinoline N‐Oxides with Alkynes: Combination of C?H Activation with O‐Atom Transfer

[Cp*Rh^III]‐catalyzed C H activation of arenes assisted by an oxidizing N O or N N directing group has allowed the construction of a number of hetercycles. In contrast, a polar N O bond is well‐known to undergo O‐atom transfer (OAT) to alkynes. Despite the liability of N O bonds in both C H activation and OAT, these two important areas evolved separately. In this report, [Cp*Rh^III] catalysts integrate both areas in an efficient redox‐neutral coupling of quinoline N‐oxides with alkynes to afford α‐(8‐quinolyl)acetophenones. In this process the N O bond acts as both a directing group for C H activation and as an O‐atom donor.     

Catalytic Methylation of C?H Bonds Using CO2 and H2

Formation of C C bonds from CO₂ is a much sought after reaction in organic synthesis. To date, other than C H carboxylations using stoichiometric amounts of metals, base, or organometallic reagents, little is known about C C bond formation. In fact, to the best of our knowledge no catalytic methylation of C H bonds using CO₂ and H₂ has been reported. Described herein is the combination of CO₂ and H₂ for efficient methylation of carbon nucleophiles such as indoles, pyrroles, and electron‐rich arenes. Comparison experiments which employ paraformaldehyde show similar reactivity for the CO₂/H₂ system.     

Enantioselective ortho‐C?H Cross‐Coupling of Diarylmethylamines with Organoborons

The commonly used para‐nitrobenzenesulfonyl (nosyl) protecting group is employed to direct the C H activation of amines for the first time. An enantioselective ortho‐C H cross‐coupling between nosyl‐protected diarylmethylamines and arylboronic acid pinacol esters has been achieved utilizing chiral mono‐N‐protected amino acid (MPAA) ligands as a promoter.     

Oxindole Synthesis by Direct Coupling of C?H and C?H Centers

An sp ² /sp ³ get‐together : A novel and efficient method can be used to synthesize 3,3‐disubstitued oxindoles by the direct intramolecular oxidative coupling of an aryl C? H and a C? H center (see scheme; DMF=N,N‐dimethylformamide).

     

Ruthenium‐Catalyzed Hydroarylation of Methylenecyclopropanes through C?H Bond Cleavage: Scope and Mechanism

Intermolecular hydroarylation reactions of highly strained methylenecyclopropanes 2‐phenylmethylenecyclopropane ( 1 ), 2,2‐diphenylmethylenecyclopropane ( 2 ), methylenespiropentane ( 3 ), bicyclopropylidene ( 4 ), (dicyclopropylmethylene)cyclopropane ( 5 ), and benzhydrylidenecyclopropane ( 6 ) through C? H bond functionalization of 2‐phenylpyridine ( 7 a ) and other arenes with directing groups were studied. The reaction was very sensitive to the substitution on the methylenecyclopropanes. Although these transformations involved (cyclopropylcarbinyl)–metal intermediates, substrates 1 and 4 furnished anti‐Markovnikov hydroarylation products with complete conservation of all cyclopropane rings in 11–93 % yield, whereas starting materials 3 and 5 were inert toward hydroarylation. Methylenecyclopropane 6 formed the products of formal hydroarylation reactions of the longest distal C? C bond in the methylenecyclopropane moiety in high yield, and hydrocarbon 2 afforded mixtures of hydroarylated products in low yields with a predominance of compounds that retained the cyclopropane unit. As byproducts, Diels–Alder cycloadducts and self‐reorganization products were obtained in several cases from substrates 1 – 3 and 5 . The structures of the most important new products have been unambiguously determined by X‐ray diffraction analyses. On the basis of the results of hydroarylation experiments with isotopically labeled 7 a ‐[D₅], a plausible mechanistic rationale and a catalytic cycle for these unusual ruthenium‐catalyzed hydroarylation reactions have been proposed. Arene‐tethered ruthenium–phosphane complex 53 , either isolated from the reaction mixture or independently prepared, did not show any catalytic activity.     

Amide‐Directed Tandem C?C/C?N Bond Formation through C?H Activation

The transformation of C? H bonds into other chemical bonds is of great significance in synthetic chemistry. C? H bond‐activation processes provide a straightforward and atom‐economic strategy for the construction of complex structures; as such, they have attracted widespread interest over the past decade. As a prevalent directing group in the field of C? H activation, the amide group not only offers excellent regiodirecting ability, but is also a potential C? N bond precursor. As a consequence, a variety of nitrogen‐containing heterocycles have been obtained by using these reactions. This Focus Review addresses the recent research into the amide‐directed tandem C? C/C? N bond‐formation process through C? H activation. The large body of research in this field over the past three years has established it as one of the most‐important topics in organic chemistry.     

Rhodium(I)‐Catalyzed Cycloisomerization of Benzylallene‐Alkynes through C?H Activation

The efficient Rh^I‐catalyzed cycloisomerization of benzylallene‐alkynes produced the tricyclo[9.4.0.0^3,8]pentadecapentaene skeleton through a C H bond activation in good yields. A plausible reaction mechanism proceeds via oxidative addition of the acetylenic C H bond to Rh^I, an ene‐type cyclization to the vinylidenecarbene–Rh^I intermediate, and an electrophilic aromatic substitution with the vinylidenecarbene species. It was proposed based on deuteration and competition experiments.     

Synthesis of Triborylalkenes from Terminal Alkynes by Iridium‐Catalyzed Tandem C?H Borylation and Diboration

A two‐step reaction to convert terminal alkynes into triborylalkenes is reported. In the first step, the terminal alkyne and pinacolborane (HBpin) are converted into an alkynylboronate, which is catalyzed by an iridium complex supported by a SiNN pincer ligand. In the second step, treatment of the reaction mixture with CO generates a new catalyst which mediates dehydrogenative diboration of alkynylboronate with pinacolborane. The mechanism of the diboration remains unclear but it does not proceed via intermediacy of hydroboration products or via B₂pin₂.