Novel dinuclear and trinuclear ruthenium clusters derived from 2‐aryl‐substituted indenylphosphines via C─H bond cleavage

Treatment of Ru₃(CO)₁₂ with an equivalent of (2‐phenyl‐1H ‐inden‐3‐yl)dicyclohexylphosphine ( 1 ) and (2‐pyridyl‐1H ‐inden‐1‐yl)dicyclohexylphosphine ( 4 ) in refluxing heptane gave the novel trinuclear ruthenium clusters (μ₃‐η¹:η²:η⁵–2‐phenyl‐3‐Cy₂PC₉H₄)Ru₃(CO)₈ ( 1c ) and [μ₂‐η¹–2‐(pyridin‐2‐yl)‐3‐Cy₂PC₉H₆]Ru₃(CO)₉ ( 4a ), respectively, via C ─ H bond cleavage. (2‐Mesityl‐1H ‐inden‐3‐yl)dicyclohexylphosphine ( 2 ) reacted with Ru₃(CO)₁₂ in refluxing heptane to give the trinuclear ruthenium cluster [μ‐2‐mesityl‐(3‐Cy₂PC₉H₅)](μ₂‐CO)Ru₃(CO)₉ ( 2c ) via C ─ H bond cleavage and carbonyl insertion. 2‐(Anthracen‐9‐yl)‐1H –inden‐3‐yldicyclohexylphosphine ( 3 ) reacted with Ru₃(CO)₁₂ in refluxing heptane to give the dinuclear ruthenium cluster [μ₂‐η³:η³–2‐(anthracen‐9‐yl)‐3‐Cy₂PC₉H₆]Ru₂(CO)₅ ( 3a ). The structures of 1c , 2c , 3a and 4a were fully characterized using IR and NMR spectroscopy, elemental analysis and single‐crystal X‐ray diffraction. These results suggest that the 2‐aryl substituent on the indenyl ring has a pronounced effect on the reaction and coordination modes of Ru₃(CO)₁₂.     

The synthesis of 1,2-diarylindenes via DDQ-mediated dehydrogenative intramolecular cyclization

A direct DDQ-mediated dehydrogenative intramolecular cyclization of (Z)-1,2,3-triaryl substituted propylenes promoted by Cu(OAc)₂ was developed, providing 1,2-diarylindene derivatives in moderate to good yields (up to 92%) under mild conditions. This protocol provides a straightforward access to 1,2-diarylindenes via DDQ-mediated benzylic/allylic sp³ C–H bond activation.     

Rhodium(III) catalyzed synthesis of isoquinolone fused azabicycles through C–H activation of N-pivaloyloxy benzamides

Herein we describe an efficient one pot strategy toward highly functionalized isoquinolone fused azabicycles having great synthetic potential via C–H activation of N-pivaloyloxy benzamides under very mild conditions. The reaction is accomplished at room temperature within one hour in good to excellent yields and is found to be compatible with a range of diazabicyclic olefins and benzamides. The present strategy offers an easy route for the synthesis of biologically relevant compounds which possess multiple points for divergent synthesis. N–N bond cleavage of synthesized compounds may enable their significant role as effective precursors for the preparation of diaminocyclopentane fused isoquinolones.     

Chemical and thermal stability of N‐heterocyclic ionic liquids in catalytic C–H activation reactions

¹H‐NMR spectrum analyses are applied to study the chemical and thermal stability of selected N‐heterocyclic ionic liquids within the reaction system that can highly efficiently activate a C–H bond of methane and convert it into the C–O bond in methanol. Our results indicate that under such reaction conditions involving using a powerful Pt‐based catalyst and strong acidic solvent, the aromatic ring of an imidazolium cation becomes unstable generating an ammonium ion (NH₄⁺). Our results also suggest that the instability of the imidazolium ring is more chemically (participation in reactions) than thermally based. Modifications of the aromatic ring structure such as pyrazolium and triazolium cations can increase the chemical/thermal stability of ionic liquids under these reaction conditions. Copyright © 2014 John Wiley & Sons, Ltd.     

Chelation‐Assisted Nickel‐Catalyzed Oxidative Annulation via Double C−H Activation/Alkyne Insertion Reaction

A nickel/NHC system for regioselective oxidative annulation by double C?H bond activation and concomitant alkyne insertion is described. The catalytic reaction requires a bidentate directing group, such as an 8‐aminoquinoline, embedded in the substrate. Various 5,6,7,8‐tetrasubstituted‐N‐(quinolin‐8‐yl)‐1‐naphthamides can be prepared as well as phenanthrene and benzo[h]quinoline amide derivatives. Diarylalkynes, dialkylalkynes, and arylalkylalkynes can be used in the system. A Ni⁰/Ni^II catalytic cycle is proposed as the main catalytic cycle. The alkyne plays a double role as a two‐component coupling partner and as a hydrogen acceptor.     

Photocatalytic α‐Tertiary Amine Synthesis via C−H Alkylation of Unmasked Primary Amines

A practical, catalytic entry to α,α,α‐trisubstituted (α‐tertiary) primary amines by C?H functionalisation has long been recognised as a critical gap in the synthetic toolbox. We report a simple and scalable solution to this problem that does not require any in situ protection of the amino group and proceeds with 100 % atom‐economy. Our strategy, which uses an organic photocatalyst in combination with azide ion as a hydrogen atom transfer (HAT) catalyst, provides a direct synthesis of α‐tertiary amines, or their corresponding γ‐lactams. We anticipate that this methodology will inspire new retrosynthetic disconnections for substituted amine derivatives in organic synthesis, and particularly for challenging α‐tertiary primary amines.     

Pd‐catalyzed direct CH cyanation of picolinamides via bidentate chelation assistance

A bidentate‐chelation assistant palladium‐catalyzed direct C‐H cyanation of picolinamides with TMSCN is described. The reaction of various derivatives gave the corresponding cyanated products in moderate to good yields under mild conditions. In addition, the cyanated product could transform into some valuable functional groups in good yields. Copyright © 2016 John Wiley & Sons, Ltd.     

Aromatic C–H bond activation revealed by infrared multiphoton dissociation spectroscopy

Metal‐oxide cations are models of catalyst mediating the C–H bond activation of organic substrates. One of the most powerful reagents suggested in the gas phase is based on CuO⁺. Here, we describe the activation of the aromatic C–H bonds of phenanthroline in its complex with CuO⁺. The reaction sequence starts with a hydrogen atom abstraction by the oxygen atom from the 2‐position of the phenanthroline ring, followed by OH migration to the ring. Using infrared multiphoton spectroscopy, it is shown that the reaction can be energetically facilitated by additional coordination of a water ligand to the copper ion. As the reaction is intramolecular, a spectroscopic characterization of the product is mandatory in order to unambiguously address the reaction mechanism. Copyright © 2012 John Wiley & Sons, Ltd.     

Synthesis of Thiophene‐annulated Naphthalene Diimide‐based Small‐Molecular Acceptors via Two‐step C−H Activation

Thiophene‐annulated naphthalene diimide (NTI)‐based molecules have recently emerged as an important class of n‐type electronic materials. However, their synthesis has predominantly been achieved by Stille or Suzuki coupling reactions despite the presence of a potential C?H bond in NTI. Additionally, the synthesis of NTI or more generally mono‐functionalization of naphthalene diimide (NDI) starts with a cumbersome bromination that results in a low yield, is unselective, and requires tedious purification. We herein thus address these issues via a two‐step C?H activation: a rhodium‐catalyzed direct C?H iodinization is first presented for NDI, followed by establishing an efficient direct arylation protocol for NTI with high yield and robustness. Coupling of up to four NTI units on a benzene or pyrene core is demonstrated along with other aryl bromide substrates. All the herein reported NTI‐based small molecules showed n‐type semiconductor behavior under air.     

Synthesis of phenanthridine and phenanthridinone derivatives based on Pd‐catalyzed C–H activation

This review article provides an overview of the most recent and exciting developments in palladium‐catalyzed C–H activation and mechanistic aspects of these catalytic reactions as the fast‐growing field for the synthesis of phenanthridine derivatives.     

Polystyrene‐supported Pd(II) complex‐catalysed carboacylation of 2‐arylpyridines with alcohols via C─H bond activation under solvent‐free conditions

Polystyrene‐supported N ,N ‐dimethylethylenediamine Pd(II) complex C was used as an efficient catalyst for the synthesis of aromatic ketones via ortho ‐acylation of sp² C─H bonds of 2‐arylpyridines with alcohols as effective coupling partners. The alcohols were oxidized with tert ‐butyl hydroperoxide to their corresponding aldehydes in situ and efficiently coupled with 2‐arylpyridines to form aryl ketones under solvent‐free conditions. Furthermore, catalyst C could be easily recovered by simple filtration and reused for five cycles without any significant decrease in its activity.     

Synthesis of ent‐Ketorfanol via a C–H Alkenylation/Torquoselective 6π Electrocyclization Cascade

The asymmetric synthesis of ent‐ketorfanol from simple and commercially available precursors is reported. A Rh^I‐catalyzed intramolecular C? H alkenylation/torquoselective 6π electrocyclization cascade provides a fused bicyclic 1,2‐dihydropyridine as a key intermediate. Computational studies were performed to understand the high torquoselectivity of the key 6π electrocyclization. The computational results demonstrate that a conformational effect is responsible for the observed selectivity. The ketone functionality and final ring are introduced in a single step by a redox‐neutral acid‐catalyzed rearrangement of a vicinal diol to give the requisite carbonyl, followed by intramolecular Friedel–Crafts alkylation.     

A concise synthesis of highly substituted imidazoles via copper-mediated oxidative C–H functionalization

We herein report a simple and concise route for the synthesis of highly substituted imidazole derivatives via copper-mediated oxidative C–H functionalization in good to high yields. The advantage of the reaction lies in its mild reaction conditions and readily available starting materials.     

Visible‐Light‐Enabled Ruthenium‐Catalyzed meta‐C−H Alkylation at Room Temperature

Visible‐light‐induced ruthenium catalysis has enabled remote C?H alkylations with excellent levels of position control under exceedingly mild conditions at room temperature. The metallaphotocatalysis occurred under exogenous‐photosensitizer‐free conditions and features an ample substrate scope. The robust nature of the photo‐induced mild meta‐C?H functionalization is reflected by the broad functional group tolerance, and the reaction can be carried out in an operationally simple manner, setting the stage for challenging secondary and tertiary meta‐C?H alkylations by ruthenaphotoredox catalysis.     

Palladium‐Catalyzed [2+2+1] Spiroannulation via Alkyne‐Directed Remote C−H Arylation and Subsequent Arene Dearomatization

Palladium‐catalyzed alkene‐directed cross‐coupling of aryl iodide with another aryl halide through C?H arylation opens a unique avenue for unsymmetrical biaryl‐derived molecules. However, homo‐coupling of aryl iodides often erodes the overall synthetic efficiency. Reported herein is a highly chemoselective Pd⁰‐catalyzed alkyne‐directed cross‐coupling of aryl iodides with bromophenols, which was subsequently followed by phenol dearomatization to furnish a very attractive [2+2+1] spiroannulation. Notably, possible homo‐coupling of aryl iodides was not observed at all. Mechanistic studies indicated that a five‐membered aryl/vinyl palladacycle most likely accounts for promoting the key step of biaryl cross‐coupling.     

A facile and efficient synthesis of 3‐cyanoindoles by a simple palladium(II)‐catalyzed C─H activation of indoles

An efficient trans ‐PdCl₂(NH₂CH₂COOH)₂‐catalyzed direct C3‐cyanation of indole C─H bonds is described. Notably, free (N─H)‐indoles reacted smoothly using the procedure, and the desired product 3‐cyanoindoles were obtained in good to excellent yields.     

Domino Pd0‐Catalyzed C(sp3)–H Arylation/Electrocyclic Reactions via Benzazetidine Intermediates

The Pd⁰‐catalyzed C(sp³)‐H arylation of 2‐bromo‐N‐methylanilides leads to unstable benzazetidine intermediates that rearrange to benzoxazines through 4π electrocyclic ring‐opening and 6π electrocyclization. The introduction of a bulky, non‐activatable amide group on the nitrogen atom was key to favor the challenging reductive elimination step and disfavor undesired reaction pathways.     

Palladium‐Catalyzed Construction of Heteroatom‐Containing π‐Conjugated Systems by Intramolecular Oxidative CH/CH Coupling Reaction

Synthesis of heteroatom‐containing ladder‐type π‐conjugated molecules was successfully achieved via a palladium‐catalyzed intramolecular oxidative C?H/C?H cross‐coupling reaction. This reaction provides a variety of π‐conjugated molecules bearing heteroatoms, such as nitrogen, oxygen, phosphorus, and sulfur atoms, and a carbonyl group. The π‐conjugated molecules were synthesized efficiently, even in gram scale, and larger π‐conjugated molecules were also obtained by a double C?H/C?H cross‐coupling reaction and successive oxidative cycloaromatization.     

Copper Catalyzed C−H Activation

Activation of C?H bonds and their application in cross coupling chemistry has received a wider interest in recent years. The conventional strategy in cross coupling reaction involves the pre‐functionalization step of coupling reactants such as organic halides, pseudo‐halides and organometallic reagents. The C?H activation facilitates a simple and straight forward approach devoid of pre‐functionalization step. This approach also addresses the environmental and economical issues involved in several chemical reactions. In this account, we have reported C?H bond activation of small organic molecules, for example, formamide C?H bond can be activated and coupled with β‐dicarbonyl or 2‐carbonyl substituted phenols under oxidative conditions to yield carbamates using inexpensive copper catalysts. Phenyl carbamates were successfully synthesized in moderate to good yields by cross dehydrogenative coupling (CDC) of phenols with formamides using copper catalysts in presence of a ligand. We have also prepared unsymmetrical urea derivatives by oxidative cross coupling of formamides with amines using copper catalysts. Synthesis of N,N‐dimethyl substituted amides, 5‐substituted‐γ‐lactams and α‐acyloxy ethers was carried out from carboxylic acids using recyclable CuO nanoparticles. Copper nanoparticles afforded N‐aryl‐γ‐amino‐γ‐lactams by oxidative coupling of aromatic amines with 2‐pyrrolidinone. Reusable transition metal HT‐derived oxide catalyst was used for the synthesis of N,N‐dimethyl substituted amides by the oxidative cross‐coupling of carboxylic acids and substituted benzaldehydes. Overview of our work in this area is summarized.