Iridium‐Catalyzed Reductive Carbon–Carbon Bond Cleavage Reaction on a Curved Pyridylcorannulene Skeleton

The cleavage of C? C bonds in π‐conjugated systems is an important method for controlling their shape and coplanarity. An efficient way for the cleavage of an aromatic C? C bond in a typical buckybowl corannulene skeleton is reported. The reaction of 2‐pyridylcorannulene with a catalytic amount of IrCl₃?n H₂O in ethylene glycol at 250 °C resulted in a structural transformation from the curved corannulene skeleton to a strain‐free flat benzo[ghi]fluoranthene skeleton through a site‐selective C? C cleavage reaction. This cleavage reaction was found to be driven by both the coordination of the 2‐pyridyl substituent to iridium and the relief of strain in the curved corannulene skeleton. This finding should facilitate the design of carbon nanomaterials based on C? C bond cleavage reactions.     

Unstrained CarbonCarbon Bond Cleavage

This Focus Review presents recent developments in the cleavage of C? C bonds in organic molecules. Significant progress in C? C activation, including the development of a variety of new synthetic strategies, has contributed to the development of this field over the past few decades. Transition‐metal‐mediated C? C bond cleavage has been shown to be a quite efficient process and several elegant metal‐free methods have also recently been developed. Strained rings have been widely used in C? C cleavage transformations; however, unstrained C? C activation has increasingly caught the attention of organic researchers, which inspired us to clarify the developments in this field.     

Differential Reduction of CO2 by Molybdenum and Vanadium Nitrogenases

The molybdenum and vanadium nitrogenases are two homologous enzymes with distinct structural and catalytic features. Previously, it was demonstrated that the V nitrogenase was nearly 700 times more active than its Mo counterpart in reducing CO to hydrocarbons. Herein, a similar discrepancy between the two nitrogenases in the reduction of CO₂ is reported, with the V nitrogenase being capable of reducing CO₂ to CO, CD₄, C₂D₄, and C₂D₆, and its Mo counterpart only capable of reducing CO₂ to CO. Furthermore, it is shown that the V nitrogenase may direct the formation of CD₄ in part via CO₂‐derived CO, but that it does not catalyze the formation of C₂D₄ and C₂D₆ along this route. The exciting observation of a V nitrogenase‐catalyzed C? C coupling with CO₂ as the origin of the building blocks adds another interesting reaction to the catalytic repertoire of this unique enzyme system. The differential activities of the V and Mo nitrogenases in CO₂ reduction provide an important framework for systematic investigations of this reaction in the future.     

Dehydrogenative Carbon–Carbon Bond Formation Using Alkynyloxy Moieties as Hydrogen‐Accepting Directing Groups

In the presence of a catalyst system consisting of Pd(OAc)₂, PCy₃, and Zn(OAc)₂, the reaction of alkynyl aryl ethers with bicycloalkenes, α,ß‐unsaturated esters, or heteroarenes results in the site‐selective cleavage of two C? H bonds followed by the formation of C? C bonds. In all cases, the alkynyloxy group acts as a directing group for the activation of an ortho C? H bond and as a hydrogen acceptor, thus rendering the use of additives such as an oxidant or base unnecessary.     

Metal‐Catalyzed Reductive Coupling Reactions of Organic Halides with Carbonyl‐Type Compounds

Metal‐catalyzed reductive coupling reactions of aryl halides and (pseudo)halides with carbonyl‐type compounds have undergone an impressive development within the last years. These methodologies have shown to be a powerful alternate strategy, practicality aside, to the use of stoichiometric, well‐defined, and, in some cases, air‐sensitive organometallic species. In this Minireview, the recent findings in this field are summarized, with particular emphasis on the mechanistic interpretation of the results and future aspects of this area of expertise.     

Coupling of Titanacyclopentadienes with a Cp Ligand and Elimination of One Substituent

Titanacyclopentadienes, prepared from [Cp₂TiBu₂] and either two equivalents of an alkyne or a diyne, were treated with PMe₃ (3 equiv) at 50 °C for 3 h and then with azobenzene at room temperature for 12 h to give 4,5,6‐trisubstituted indene derivatives with the loss of one substituent in good yields. This reaction contrasts sharply with our previously reported reaction for the formation of 4,5,6,7‐tetrasubstituted indene derivatives without the loss of substituents by the treatment of titanacyclopentadienes with azobenzene without PMe₃. ¹³C NMR spectroscopy of the product derived from a ¹³C‐enriched complex revealed that the five carbon atoms originating from a Cp ligand were arranged linearly in the trisubstituted indene derivatives, in contrast to the 4,5,6,7‐tetrasubsituted indene derivatives, in which the corresponding five carbon atoms are arranged in a ring.     

Catalytic Cleavage of Ether CO Bonds by Pincer Iridium Complexes

The development of efficient catalytic methods to cleave the relatively unreactive C? O bonds of ethers remains an important challenge in catalysis. Building on our group’s recent work, we report the dehydroaryloxylation of aryl alkyl ethers using pincer iridium catalysts. This method represents a rare fully atom‐economical method for ether C? O bond cleavage.     

Regioselective Acceptorless Dehydrogenative Coupling of N‐Heterocycles toward Functionalized Quinolines,Phenanthrolines, and Indoles

A new strategy has been developed for the oxidant‐ and base‐free dehydrogenative coupling of N‐heterocycles at mild conditions. Under the action of an iridium catalyst, N‐heterocycles undergo multiple sp³ C? H activation steps, generating a nucleophilic enamine that reacts in situ with various electrophiles to give highly functionalized products. The dehydrogenative coupling can be cascaded with Friedel–Crafts addition, resulting in a double functionalization of the N‐heterocycles.     

Modular Synthesis of Phenanthro[9,10‐c]thiophenes by a Sequence of CH Activation,Suzuki Cross‐Coupling and Photocyclization Reactions

A total number of 15 different 3,4‐diarylthiophenes were synthesized, which bear a chlorine atom in ortho‐position of one of the aryl substituents. One aryl group was introduced by an oxidative cross‐coupling reaction, involving a C?H activation at C4(3) of the thiophene core. The other aryl group was in most cases introduced by a Suzuki cross‐coupling reaction, which succeeded the oxidative cross‐coupling step. Photocyclization reactions of the 3,4‐diarylthiophenes were performed in a solvent mixture of benzene and acetonitrile (50:50 v/v) at λ=254 nm and proceeded to the title compounds in yields of 60–82 %. The selectivity of the photocyclization was determined at the ortho‐chloro‐substituted aryl ring by the position of the chlorine substituent. At the other ring, a single regioisomer was observed for phenyl and para‐substituted phenyl groups. For 2‐naphthyl and ortho‐substituted phenyl rings a clear preference was observed in favor of a major regioisomer, while meta‐substitution in the phenyl ring led to a about 1:1 mixture of 5‐ and 7‐substituted phenanthro[9,10‐c]thiophenes. Mechanistically, the photocyclization is likely to occur as a photochemically allowed, conrotatory [(4n+2)π] process accompanied by elimination of HCl. It was shown for two phenanthro[9,10‐c]thiophene products that they can be readily brominated in positions C1 and C3 (74–77 %), which in turn allows for further functionalization at these positions, for example, in the course of halogen–metal exchange and polymerization reactions.     

Selective Carbon–Carbon Bond Cleavage for the Stereoselective Synthesis of Acyclic Systems

Most of the efforts of organic chemists have been directed to the development of creative strategies to build carbon–carbon and carbon–heteroatom bonds in a predictable and efficient manner. In this Review, we show an alternative approach where challenging molecular skeletons could be prepared through selective cleavage of carbon–carbon bonds. We demonstrate that it has the potential to be a general principle in organic synthesis for the regio‐, diastereo‐, and even enantioselective preparation of adducts despite the fact that C? C single bonds are among the least reactive functional groups. The development of such strategies may have an impact on synthesis design and can ultimately lead to new selective and efficient processes for the utilization of simple hydrocarbons.     

Unprecedented Transformation of a Directing Group Generated In Situ and Its Application in the One‐Pot Synthesis of 2‐Alkenyl Benzonitriles

An unprecedented protocol for the transformation of benzoyl azides into benzonitrile derivatives via iminophosphoranes generated in situ is described. The strategy was successfully applied to the de‐novo synthesis of 2‐alkenylated benzonitrile derivatives from benzoyl azides through ortho C?H activation/alkenylation followed by subsequent rearrangement. The salient features of this protocol involve incorporation of two important functionalities through cyanation and olefination in one pot under mild reaction conditions by using a less expensive Ru catalyst. The mechanism was established by isolating and characterising (using ³¹P NMR) an intermediate with two ortho functionalities, iminophosphorane and olefin, under specific reaction conditions.     

Linear Alkane CC Bond Chemistry Mediated by Metal Surfaces

Linear alkanes undergo different C?C bond chemistry (coupling or dissociation) thermally activated on anisotropic metal surfaces depending on the choice of the substrate material. Owing to the one‐dimensional geometrical constraint, selective dehydrogenation and C?C coupling (polymerization) of linear alkanes take place on Au(110) surfaces with missing‐row reconstruction. However, the case is dramatically different on Pt(110) surfaces, which exhibit similar reconstruction as Au(110). Instead of dehydrogenative polymerization, alkanes tend to dehydrogenative pyrolysis, resulting in hydrocarbon fragments. Density functional theory calculations reveal that dehydrogenation of alkanes on Au(110) surfaces is an endothermic process, but further C?C coupling between alkyl intermediates is exothermic. On the contrary, due to the much stronger C?Pt bonds, dehydrogenation on Pt(110) surfaces is energetically favorable, resulting in multiple hydrogen loss followed by C?C bond dissociation.     

Deprotonation of Coordinated Phosphanes in a Rhenium Complex: CC Coupling with Diimine Coligands

The reaction of fac‐[Re(bipy)(CO)₃(PMe₃)][OTf] (bipy=2,2′‐bipyridine) with KN(SiMe₃)₂ affords two neutral products: cis,trans‐[Re(bipy)(CO)₂(CN)(PMe₃)], and a thermally unstable compound, which features a new C?C bond between a P‐bonded methylene group (from methyl group deprotonation) and the C6 position of bipy. The solid‐state structures of more stable 1,2‐bis[(2,6‐diisopropylphenyl)imino]acenaphthene analogs, resulting from the deprotonation of PMe₃, PPhMe₂, and PPh₂Me ligands, are determined by X‐ray diffraction.     

Copper‐Catalyzed Aerobic Oxidative Inert CC and CN Bond Cleavage: A New Strategy for the Synthesis of Tertiary Amides

A copper‐catalyzed aerobic oxidative amidation reaction of inert C?C bonds with tertiary amines has been developed for the synthesis of tertiary amides, which are significant units in many natural products, pharmaceuticals, and fine chemicals. This method combines C?C bond activation, C?N bond cleavage, and C?H bond oxygenation in a one‐pot protocol, using molecular oxygen as the sole oxidant without any additional ligands.     

Water‐Promoted Generation of a Diazairida Homobarrelene by CC Coupling Between an Iridacyclic Alkylidene and Acetonitrile

The stable cationic iridacyclopentenylidene [Tp^Me2Ir(?CHC(Me)?C(Me)C H₂(NCMe)]PF₆ ( A ; Tp^Me2=hydrotris(3,5‐dimethylpyrazolyl)borate) has been obtained by α‐hydride abstraction from the iridacyclopent‐2‐ene [Tp^Me2Ir(CH₂C(Me)?C(Me)C H₂)(NCMe)]. Complex A exhibits Brønsted–Lowry acidity at the Ir? CH₂ and proximal (relative to Ir? CH₂) methyl sites. The coordination of an extra molecule of acetonitrile to the iridium center initiates the reversible isomerization of the chelating carbon chain of A to the monodentate butadienyl ligand of complex [Tp^Me2Ir(CH?C(Me)C(Me)?CH₂)(NCMe)₂]PF₆, which is capable to engage in a water‐promoted C? C coupling with the MeCN co‐ligands. The product is an aesthetically appealing bicyclic structure that resembles the hydrocarbon barrelene.     

Metal‐Free CC Coupling Reactions with Tetraguanidino‐Functionalized Pyridines and Light

Herein we report on metal‐free C?C coupling reactions mediated by the pyridine derivative 2,3,6,7‐tetrakis(tetramethylguanidino)pyridine under the action of visible light. The rate‐determining step is the homolytic N?C bond cleavage of the initially formed N‐alkyl pyridinium ion upon excitation with visible light. The released alkyl radicals subsequently dimerize to the C?C coupling product. 2,3,6,7‐Tetrakis(tetramethylguanidino)pyridine, which is a strong electron donor (E_1/2(CH₂Cl₂)=?0.76 V vs. ferrocene) is oxidized to the dication. For alkyl=benzyl and allyl, relatively high first‐order rate constants of 0.23±0.03 and 0.13±0.03 s^?1 were determined. Regeneration of neutral 2,3,6,7‐tetrakis(tetramethylguanidino)‐pyridine by reduction allows to drive the process in a cycle.     

Ruthenium‐Catalyzed CC Bond Cleavage in Lignin Model Substrates

Ruthenium–triphos complexes exhibited unprecedented catalytic activity and selectivity in the redox‐neutral C? C bond cleavage of the β‐O‐4 lignin linkage of 1,3‐dilignol model compounds. A mechanistic pathway involving a dehydrogenation‐initiated retro‐aldol reaction for the C? C bond cleavage was proposed in line with experimental data and DFT calculations.     

Palladium‐Catalyzed Synthesis of Phenanthridine/Benzoxazine‐Fused Quinazolinones by Intramolecular CH Bond Activation

A highly efficient synthesis of phenanthridine/benzoxazine‐fused quinazolinones by ligand‐free palladium‐catalyzed intramolecular C?H bond activation under mild conditions has been developed. The C?C coupling provides the corresponding N‐fused polycyclic heterocycles in good to excellent yields and with wide functional group tolerance.     

Catalytic Reduction of CN−, CO,and CO2 by Nitrogenase Cofactors in Lanthanide‐Driven Reactions

Nitrogenase cofactors can be extracted into an organic solvent to catalyze the reduction of cyanide (CN^?), carbon monoxide (CO), and carbon dioxide (CO₂) without using adenosine triphosphate (ATP), when samarium(II) iodide (SmI₂) and 2,6‐lutidinium triflate (Lut‐H) are employed as a reductant and a proton source, respectively. Driven by SmI₂, the cofactors catalytically reduce CN^? or CO to C₁–C₄ hydrocarbons, and CO₂ to CO and C₁–C₃ hydrocarbons. The C? C coupling from CO₂ indicates a unique Fischer–Tropsch‐like reaction with an atypical carbonaceous substrate, whereas the catalytic turnover of CN^?, CO, and CO₂ by isolated cofactors suggests the possibility to develop nitrogenase‐based electrocatalysts for the production of hydrocarbons from these carbon‐containing compounds.