Weakly Coordinating, Ketone-Directed Cp*Co(III)-Catalyzed C–H Allylation on Arenes and Indoles

Rh(III)-Catalyzed C-H Allylation of Aromatic Ketoximes with Vinylaziridines

The Rh(III)-catalyzed reaction of aromatic ketoximes with 2-vinylaziridines affords ortho-allylation products of the phenyl rings of aromatic ketoximes in moderate to excellent yields. The reaction requires 0.5 equiv of NaOAc as a base and occurs under mild conditions. The protocol exhibits ortho-monoallylation selectivity, wide scope of substrates, and good compatibility of functional groups.

Iridium(III)-catalyzed β-trifluoromethyl enone carbonyl-directed regioselective ortho-C(sp2)-H olefination

Due to the lower LUMO energy level at the β-position of α,β-unsaturated-β-trifluoromethyl enone than that of its non-fluorinated counterpart, there is a challenge to activate the sp2 C-H bond of aromatic rings. Herein, we have reported iridium(III)-catalyzed β-trifluoromethyl enone carbonyl-directed regioselective aromatic C(sp2)-H olefination with acrylates under oxidative conditions. Furthermore, coupling with natural product-derived acrylates, scale-up and product diversification have also been performed.

Construction of C2-indolyl-quaternary centers by branch-selective allylation: enabling concise total synthesis of the (±)-mersicarpine alkaloid

Herein we report a branch-selective allylation strategy for accessing C2-indolyl-all-carbon quaternary centers using allylboronic acids. This approach boasts broad functional group tolerance, scalability, and relies on easily accessible allyl alcohol precursors. Importantly, the C3-position of the indole remains free, offering a handle for further synthetic refinement. Mechanistic pathways, corroborated by density functional theory (DFT), suggest the involvement of an indolenine intermediate and a Zimmerman-Traxler-like transition state during allylboration. Demonstrating its efficacy, the method was applied to the total synthesis of the (±)-mersicarpine alkaloid and enabled formal synthesis of additional alkaloids, such as (±)-scholarisine G, (±)-melodinine E, and (±)-leuconoxine.

Ruthenium(II)-Catalyzed S(II)-Directed Aromatic C-H Allylation with Vinylaziridines

The ruthenium-catalyzed reaction of aryl methyl thioethers with vinylaziridines affords ortho-position mono- or bis-allylation products depending on substituents on the phenyl rings of sulfide substrates or the ratio of reactants. The reaction also features mild reaction conditions, good product yields, wide scope of substrates, good compatibility of functional groups, and the selective formation of E-configurated C-C double bonds.

Annulative π-Extension by Cp*Co(III)-Catalyzed Ketone-Directed peri-Annulation: An Approach to Access Fused Arenes

A masked-bay-region selective first-row transition-metal Cp*Co(III)-catalyzed annulative π-extension of arene-derived ketones is achieved to afford K-region-functionalized benzo[e]pyrenes, benzotetraphenes, and pyrenes. Comprehensive density functional theory studies buttress the mechanistic pathway comprising key steps like peri-C-H activation, alkyne 1,2-migratory insertion, and nucleophilic attack toward ketone, this attack being the rate-determining step. In addition, π-conjugated 1,1'-bipyrenes, potential photocatalyst pyrene-quinones, and putative n-type semiconductor cyano group-containing dibenzo[de,qr]tetracenes are also accessed.

Rh(III)-Catalyzed Enone Carbonyl/Ketone-Directed Aerobic C-H Olefination of Aromatics with Unactivated Olefins

A Rh(III)-catalyzed weak enone carbonyl/ketone-assisted aerobic oxidative C-H olefination of aromatics with unactivated alkenes has been developed. This protocol involves cross-dehydrogenative Heck-type olefination reaction of various substituted biologically relevant chalcones and aromatic ketones such as acetophenones and chromones with various functionalized unactivated olefins in moderate to good yields. Further, ortho-alkylation of chalcones with norbornene is also demonstrated. A possible reaction mechanism involving weak chelation-assisted C-H activation/insertion/β-hydride elimination was proposed and supported by the deuterium labeling studies.

In-situ and ex-situ co-pyrolysis studies of waste biomass with spent motor oil: Elucidating the role of physical inhibition and mixing ratio to enhance fuel quality

Simultaneous renewable energy generation is an imperative part of sustainable hazardous waste management. Therefore, the present work explicates the co-pyrolysis of rice stubble (RS) waste biomass and spent motor oil (SMO) to upgrade the obtained bio-oil. Moreover, two different modes, namely, in-situ and ex-situ, were implemented to analyze the effect of physical inhibition. Monothetic analysis approach was followed to determine optimum process conditions. A substantial increment of ∼ 85% was observed in bio-oil yield for RS: SMO (1:1) in-situ operation whilst the only RS biomass pyrolysis. Moreover, the HHV increased by ∼ 2.15 times after co-pyrolysis with a considerable reduction (62.70%) in water content. Consequently, the paraffin content increased to 79.14 vol% with an iso-paraffin index of 0.285. Subsequently, a possible reaction mechanism is also proposed to evaluate results comprehensively. Altogether, the co-pyrolysis of these feedstocks resulted in improved aliphatic content and reduced oxygenates, encouraging its adequacy as an alternate fuel.

Palladium-Catalyzed Three-Component Cross-Coupling of Conjugated Dienes with Indoles Using Ethynylbenziodazolones as Electrophilic Alkynylating Reagents

A palladium-catalyzed regioselective 1,2-alkynyl-carbonalization of conjugated dienes with ethynylbenziodazolone (EBZ) and indoles has been developed for the first time. Various molecules containing alkenyl, alkynyl, and indole groups were readily obtained. Moreover, the resulting products can be applied to various derivatizations. This protocol uses EBZ as an electrophilic alkynylating reagent, avoiding the byproduct of dimerization of alkynes.

Cobalt(III)-Catalyzed Chemo- and Regioselective [4 + 2]-Annulation of Aromatic Sulfoxonium Ylides with 1,3-Diynes

Air-stable, highly abundant, and cost-effective Co(III)-catalyzed redox-neutral [4 + 2]-annulation of aromatic sulfoxonium ylides with 1,3-diynes providing useful substituted 1-naphthol derivatives in a regioselective manner is described. Further, the prepared 1-naphthols having internal alkyne were converted into useful polycarbocyclic molecules and spiro-dienone derivatives in good-to-excellent yields. A possible reaction mechanism involving ortho C-H activation as a key step was proposed and supported by deuterium labeling and kinetic isotope labeling studies.

Looking deep into C-H functionalization: the synthesis and application of cyclopentadienyl and related metal catalysts

Metal catalyzed C-H functionalization offers a versatile platform for methodology development and a wide variety of reactions now exist for the chemo- and site-selective functionalization of organic molecules. Cyclopentadienyl-metal (CpM) complexes of transition metals and their correlative analogues have found widespread application in this area, and herein we highlight several key applications of commonly used transition-metal Cp-type catalysts. In addition, an understanding of transition metal Cp-type catalyst synthesis is important, particularly where modifications to the catalyst structure are required for different applications, and a summary of this aspect is given.

Cp*Co(III)-catalyzed thiocarbamate directed C−H aminocarbonyl-ation, amination, and cascade annulation of pyrroles

Cobalt(III)-catalyzed thiocarbamate directed aminocarbonylation and amination of C−H bond are described to access diverse amides. Biologically relevant pyrrolo[1,2-c]imidazoles were readily accessed via one-pot intramolecular cyclization at thiocarbamoyl directing group. Notably,...

Cobalt-catalysed acyl silane directed ortho C–H functionalisation of benzoyl silanes

Acyl silanes can be engaged as weakly coordinating directing groups in cobalt catalysed C–H functionalisation reactions to prepare benzoyl silanes that are highly amenable to subsequent synthetic manipulations yet inaccessible via existing methods.

Iridium(iii)-catalyzed two-fold C–H alkylation of BINOLs with allyl alcohols

Ir(iii)-Catalyzed C–H alkylation of BINOL units has been well examined by using allyl alcohols as coupling partners.

Recent Advances in the High-Valent Cobalt-Catalyzed C-H Functionalization of N-Heterocycles

Direct functionalization of heterocycles using C-H activation widely relies on the precious metal complexes. In past decade, the use of earth abundant and inexpensive transition metal to functionalize heterocycles has become an attractive alternate strategy. This concept is also interesting due to the unique reactivity pattern of these inexpensive metals. In this context we and other research groups have utilized the high-valent cobalt complexes as an inexpensive and readily available catalyst for the functionalization of heterocycles. In this review, we intend to brief recent progress made in the area of high-valent cobalt complexes catalyzed C-H functionalization of N-containing heterocycles.

C-H bond functionalization by high-valent cobalt catalysis: current progress, challenges and future perspectives

Over the last decade, high-valent cobalt catalysis has earned a place in the spotlight as a valuable tool for C-H activation and functionalization. Since the discovery of its unique reactivity, more and more attention has been directed towards the utilization of cobalt as an alternative to noble metal catalysts. In particular, Cp*Co(III) complexes, as well as simple Co(II) and Co(III) salts in combination with bidentate chelation assistance, have been extensively used for the development of novel transformations. In this review, we have demonstrated the existing trends in the C-H functionalization methodology using high-valent cobalt catalysis and highlighted the main challenges to overcome, as well as perspective directions, which need to be further developed in the future.

Weakly Coordinating, Hydroxyl Directed Ruthenium Catalyzed C-H Alkylation of Ubiquitous Benzyl Alcohols with Maleimides

Benzyl alcohols have been employed as effective coupling partners in Ru-catalyzed C-H functionalization reactions, and their annulation with maleimides then offers efficient synthesis of useful ortho substituted succinimide aromatic aldehydes and ketones. Detailed mechanistic studies have been demonstrated by performing preliminary reactions, deuterium studies, and competitive experiments.

Cp*CoIII-Catalyzed C(7)-H Bond Annulation of Indolines with Alkynes

An efficient protocol for the synthesis of biologically essential pyrroloquinolinones has been developed under Cp*Co^III catalysis, which involves a cascade reaction of C(7)-H alkenylation with alkynes followed by nucleophilic addition. A wide variety of internal alkynes including enyne, diyne, and ynamide and more challenging terminal alkynes were successfully employed for the annulation in good to excellent yield with high regioselectivity.

O-Directed C-H functionalization via cobaltacycles: a sustainable approach for C-C and C-heteroatom bond formations

This review focuses on providing comprehensive highlights of the recent advances in the field of cobalt-catalysed C-H functionalization and related synthetic concepts, relying on these through oxygen atom coordination. In recent years, 3d transition metal (Fe, Co, Cu & Ni) catalysed C-H functionalization reactions have received immense attention on account of its higher abundance and low cost, as compared to noble metals such as Ir, Rh, Ru and Pd. Among the first-row transition metals, cobalt is one of the extensively used metals for sustainable synthesis due to its unique reactivity towards the functionalization of inert C-H bonds. The functionalization of the inert C-H bond necessitates a proximal directing group. In this context, strongly coordinating nitrogen atom directed C-H functionalizations have been well explored. Nevertheless, strongly coordinating nitrogen-containing scaffolds, such as pyridine, pyrimidine, and 8-aminoquinoline, have to be installed and removed in a separate process. In contrast, C-H functionalizations through weakly coordinating atoms, such as oxygen, are largely underdeveloped. Since the oxygen atom is a part of many readily available functional groups, such as aldehydes, ketones, carboxylic acids, and esters, it could be used as directing groups for selective C-H functionalization reactions without any modification. Thus, the use of 3d transition metals, such as cobalt, along with weakly coordinating (oxygen) directing groups for C-H functionalization reactions are more sustainable, especially for the large-scale production of pharmaceuticals in industries. During the last decade, notable progress has been made using this concept. Nonetheless, almost all the reports are restricted to the formation of C-C and C-N bond. Therefore, there is a wide scope for developing this area for the formation of other bonds, such as C-X (halogens), C-B, C-S, and C-Se.

Thioamide-Directed Cp*Co(III)-Catalyzed C-H Allylation of Ferrocenes

Herein, the first Cp*Co(III)-catalyzed C-H allylation of ferrocene thioamides with allyl carbonates has been developed. This reaction is compatible with a wide range of functional groups, providing various allylated ferrocene derivatives in up to 90% yields. In addition, the C-H allylation protocol is also compatible with the use of vinylcyclopropanes as allylating reagents by merging C-H and C-C activation into one catalytic system. Mechanistic studies revealed that the thiocarbonyl-directing group plays a vital role in C-H activation.

Transition metal-catalyzed C–H functionalizations of indoles

This review summarises a wide range of transformations on the indole skeleton, including arylation, alkenylation, alkynylation, acylation, nitration, borylation, and amidation, using transition-metal catalyzed C–H functionalization as the key step.

Cp*Co(III)-Catalyzed Ketone-Directed ortho-C-H Activation for the Synthesis of Indene Derivatives

A weakly coordinating, carbonyl-assisted C-H activation of aromatic systems with α,β-unsaturated ketone and subsequent aldol condensation has been developed using a Cp*Co(CO)I₂ catalyst. The developed method is the first example of indene synthesis by cobalt-catalyzed C-H activation. In addition, the reaction requires mild reaction conditions and easily accessible starting materials, and it shows excellent functional group compatibility.

Iridium-Catalyzed Enantioselective Intermolecular Indole C2-Allylation

The enantioselective intermolecular C2-allylation of 3-substituted indoles is reported for the first time. This directing group-free approach relies on a chiral Ir-(P, olefin) complex and Mg(ClO4 )2 Lewis acid catalyst system to promote allylic substitution, providing the C2-allylated products in typically high yields (40-99 %) and enantioselectivities (83-99 % ee) with excellent regiocontrol. Experimental studies and DFT calculations suggest that the reaction proceeds via direct C2-allylation, rather than C3-allylation followed by in situ migration. Steric congestion at the indole-C3 position and improved π-π stacking interactions have been identified as major contributors to the C2-selectivity.

Carbamates: A Directing Group for Selective C-H Amidation and Alkylation under Cp*Co(III) Catalysis

The selective reactivity of carbamate and thiocarbamate toward alkylation and amidation is reported under stable, high-valent, cost-effective cobalt(III) catalysis. This method reveals the wide possibility of designing a different branch of synthetically challenging yet highly promising asymmetric catalysts based on BINOL and SPINOL scaffolds. Late-stage C-H functionalization of l-tyrosine and estrone was also achieved through this approach. The mechanistic study shows that a base-assisted internal electrophilic substitution mechanism is operative here.

Cobalt(iii)-catalyzed ketone-directed C–H vinylation using vinyl acetate

Weakly coordinating, ketone-directed C–H vinylation using vinyl acetate is reported here for a wide range of aromatic ketones such as acetophenones, diaryl ketones, chromones and chalcones under cost-effective and air-stable cobalt(iii)-catalysis.

Mild and Practical Indole C2 Allylation by Allylboration of in situ Generated 3-Chloroindolenines

C2 allylation of indole derivatives is a challenging but important transformation given the biological relevance of the products. Herein we report a selective C2 allylation strategy that proceeds via allylboration of in situ-generated 3-chloroindolenines. The reaction is mild, practical, and compatible with a wide range of C3-substituted indoles. As allylboronates are readily accessible from commercial precursors, various substituted allyl moieties can be introduced using the same protocol. To showcase the utility of this method we applied it to the synthesis of the natural product, tryprostatin B.

A Brønsted Acid Catalyzed Cascade Reaction for the Conversion of Indoles to α-(3-Indolyl) Ketones by Using 2-Benzyloxy Aldehydes

A Brønsted acid catalyzed, operationally simple, scalable route to several functionalized α-(3-indolyl) ketones has been developed and the long-standing regioisomeric issue has been eliminated by choosing appropriate carbonyls. A readily available and cheap bottle reagent was used as the catalyst. This protocol was also applicable to the synthesis of densely functionalized α-(3-pyrrolyl) ketones. A detailed mechanistic study confirmed the involvement of enolether as a reaction intermediate. Several postsynthetic modifications along with easy access to β-carboline, tryptamines, tryptophols, and spiro-indolenine proclaim the synthetic utility of this powerful building block. On the basis of this concept, functionalized carbazoles were constructed by a cascade annulation strategy.

Synthesis of Benzofulvenes via Cp*Co(III)-Catalyzed C-H Activation and Carbocyclization of Aromatic Ketones with Internal Alkynes

A highly efficient and practical synthesis of benzofulvenes was developed via ketone-directed Cp*Co(III)-catalyzed sequential C-H activation, addition, cyclization, and dehydration cascade processes between simple aromatic ketones and alkynes. The reaction tolerates a variety of functional groups, and various benzofulvenes were efficiently synthesized in 42-92% yields.

Cobalt(ii)-catalyzed regioselective C-H halogenation of anilides

A cobalt-catalyzed regioselective C-H halogenation methodology is reported herein. The highlight of this work is the highly selective C-H functionalization of anilides, which results in high-yielding, versatile, and practical halogenated products. Thereby, brominations, chlorinations and iodinations of many electron-rich and electron-deficient anilides were achieved in a highly selective fashion. Mechanistic studies with respect to the pathway of the reaction are also described.

Synthesis of 1-naphthols via Cp*Co(iii)-catalyzed C–H activation and cyclization of sulfoxonium ylides with alkynes

A highly practical synthesis of 1-naphthols was developed via Cp*Co(iii)-catalyzed C–H activation and cyclization between sulfoxonium ylides and alkynes.

3d Transition Metals for C-H Activation

C-H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C-H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C-H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C-H activation until summer 2018.