Intermolecular Dehydrogenative Heck Reactions

Bimolecular oxidative C–H alkynylation of α-substituted isochromans

The first bimolecular oxidative C–H functionalization of secondary benzylic ethers for tertiary ether synthesis has been established in high efficiency.

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.

Cooperative Palladium/Lewis Acid-Catalyzed Transfer Hydrocyanation of Alkenes and Alkynes Using 1-Methylcyclohexa-2,5-diene-1-carbonitrile

Catalytic transfer hydrocyanation represents a clean and safe alternative to hydrocyanation processes using toxic HCN gas. Such reactions provide access to pharmaceutically important nitrile derivatives starting with alkenes and alkynes. Herein, an efficient and practical cooperative palladium/Lewis acid-catalyzed transfer hydrocyanation of alkenes and alkynes is presented using 1-methylcyclohexa-2,5-diene-1-carbonitrile as a benign and readily available HCN source. A large set of nitrile derivatives (>50 examples) are prepared from both aliphatic and aromatic alkenes with good to excellent anti-Markovnikov selectivity. A range of aliphatic alkenes engage in selective hydrocyanation to provide the corresponding nitriles. The introduced method is useful for chain walking hydrocyanation of internal alkenes to afford terminal nitriles in good regioselectivities. This protocol is also applicable to late-stage modification of bioactive molecules.

One-Pot Magnetic Iron Oxide-Carbon Nanodot Composite-Catalyzed Cyclooxidative Aqueous Tandem Synthesis of Quinazolinones in the Presence of tert-Butyl Hydroperoxide

The development of synthetic protocols for biologically important molecules using biocompatible catalysts in aqueous medium holds the key in green and sustainable chemistry. Herein, a magnetically recoverable iron oxide-carbon dot nanocomposite has been demonstrated as an effective catalyst for cyclooxidative tandem synthesis of quinazolinones in aqueous medium using alcohols as starting materials. Fluorescent carbon dots, the newest entrant in the nanocarbon family, were used as the stabilizing agent for the iron oxide nanoparticles, and a continuous layer of carbon dots decorates the iron oxide nanoparticle surface as observed by transmission electron microscopy. The fluorescence studies demonstrated the effective electron transfer from carbon dots to the iron oxide nanoparticles resulting in complete quenching of emission owing to carbon dots, once it binds with iron oxide nanoparticles. The nanocatalyst showed high activity with significant reusability for the syntheses of quinazolinones in the presence of tert-butyl hydroperoxide (TBHP) in an aqueous medium. Controlled experiments revealed the synergistic effect of carbon dots in enhancing the catalytic activity of iron oxide, as they might influence the decomposition of TBHP into radicals owing to their peroxidase activity. These radicals stabilized over the nanoparticle surface are known to have increased lifetime compared to solution-based radicals. These surface-stabilized radicals then could catalyze the tandem reaction resulting in the formation of the quinazolinone derivatives in high yields.

Iridium-Catalyzed α-Selective Arylation of Styrenes by Dual C-H Functionalization

An IrI -system modified with a ferrocene derived bisphosphine ligand promotes α-selective arylation of styrenes by dual C-H functionalization. These studies offer a regioisomeric alternative to the Pd-catalyzed Fujiwara-Moritani reaction.

Oxidative R1-H/R2-H Cross-Coupling with Hydrogen Evolution

Oxidative R¹-H/R²-H cross-coupling with hydrogen evolution serves as one of the most atom-economical methods for constructing new chemical bonds. This reaction strategy avoids substrate prefunctionalization steps in traditional cross-coupling reactions. Besides, hydrogen gas, which is recognized as a source of green energy, is the only byproduct during the reaction process. The major challenge in this reaction strategy is to achieve selective bond formation and hydrogen evolution at the same time. Over the past few years, novel synthetic techniques especially photochemistry and electrochemistry have provided possibilities for oxidative cross-coupling with H₂ liberation. Both C-C and C-X bonds can be constructed without the use of any sacrificial reagents. In this perspective, we will discuss the concept of this reaction strategy and give an overview of its recent development.

Rhodium(III)-Catalyzed C-H Activation of α-Iminonitriles or α-Imino Esters and Cyclization with Acrylates to 2 H-Isoindoles

Rh(III) catalysts have played increasingly important roles in the activation of C-H bonds to build heterocyclic scaffolds. However, there are few reports on the more challenging synthesis of pharmaceutically important 2 H-isoindoles and fused 2 H-isoindoles. The process reported herein is an effective strategy to produce 2 H-isoindole or fused 2 H-isoindole derivatives via a Rh(III)-catalyzed transformation of α-iminonitriles or α-imino esters with acrylates.

Metal-Free Dehydrogenative Diels-Alder Reactions of Prenyl Derivatives with Dienophiles via a Thermal Reversible Process

An efficient dehydrogenative Diels-Alder reaction of prenyl derivatives with dienophiles has been developed. The reaction exhibits broad substrate scope and provides efficient access to cyclohexene derivatives with good to excellent yields. A reasonable mechanism involving a metal-free thermal reversible process is proposed.

Efficient and stereodivergent synthesis of unsaturated acyclic fragments bearing contiguous stereogenic elements

Synthetic organic strategies that enable the catalytic and rapid assembly of a large array of organic compounds possessing multiple stereocenters in acyclic systems are somewhat rare, especially when it comes to reaching today’s high standards of efficiency and selectivity. In particular, the catalytic preparation of a three-dimensional molecular layout of a simple acyclic hydrocarbon skeleton possessing several stereocenters from simple and readily available reagents still represents a vastly uncharted domain. Here, we report a rapid, modular, stereodivergent and diversity-oriented unified strategy to construct acyclic molecular frameworks bearing up to four contiguous and congested stereogenic elements, with remarkably high levels of stereocontrol and in only three catalytic steps from commercially available alkynes. A regio- and diastereoselective catalytic Heck migratory insertion reaction of alkenylcyclopropyl carbinols merging selective C–C bond cleavage of a cyclopropane represents the key step.

Tandem Catalysis: Transforming Alcohols to Alkenes by Oxidative Dehydroxymethylation

We report a Rh-catalyst for accessing olefins from primary alcohols by a C-C bond cleavage that results in dehomologation. This functional group interconversion proceeds by an oxidation-dehydroformylation enabled by N, N-dimethylacrylamide as a sacrificial acceptor of hydrogen gas. Alcohols with diverse functionality and structure undergo oxidative dehydroxymethylation to access the corresponding olefins. Our catalyst protocol enables a two-step semisynthesis of (+)-yohimbenone and dehomologation of feedstock olefins.

Regioselective Copper-Catalyzed Oxidative Coupling of α-Alkylated Styrenes with Tertiary Alkyl Radicals

A radical-mediated oxidative cross-coupling of readily accessible α-alkylated styrenes with 1,3-dicarbonyl compounds utilizing a combination of Cu(OAc)₂ and air as a catalytic system is described. Rather than requiring α-halocarbonyl compounds, this efficient approach enables direct installation of tertiary functionalized alkyl motifs to olefins with simple carbonyl derivatives. The novel protocol is characterized with high allylic selectivities via a competing β-H elimination. Both radical-clock and -trapping experiments provided clear-cut evidence for the intermediacy of an α-keto carbon-centered radical.

Towards Sustainable C-H Functionalization Reactions: The Emerging Role of Bio-Based Reaction Media

In the last decade, transition-metal catalyzed C-H functionalization reactions have progressed enormously, becoming a useful tool in organic synthesis and a practical alternative to well-established methodologies. Very recently, research efforts have also been devoted to developing more sustainable C-H functionalization protocols, in order to increase their applicability. One of the most promising approaches in this sense is represented by the substitution of common reaction media with bio-based solvents. In the present contribution a general perspective on the benefits of this approach is given, followed by key literature examples.

HOTf-Catalyzed Alkyl-Heck-type Reaction

The Heck reaction, along with other cross-coupling reactions, led to a revolution in organic chemistry. In the last 50 years, metal-catalyzed, photo-induced, or base-mediated Heck and Heck-type reactions have been elegantly developed. Brønsted acid-catalyzed Heck (or Heck-type) reactions are still unknown, however. By introducing alkyl peroxides as the key intermediates, primary, secondary, and tertiary aliphatic carboxylic acids are therefore applied here in a one-pot Brønsted acid-catalyzed Heck-type reaction, to deliver E-alkenes exclusively in most cases. The use of HOTf is vital to the reaction, whose mechanism is supported by both experimental and computational results. This method can be expanded to the direct alkylation of complex natural products.

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First acid-catalyzed Heck-type reaction

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Aliphatic acids are utilized as the sources of alkyl functionalities

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E-alkenes exclusively in most cases

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Strong acid effect

Direct oxidative C-H alkynylation of N-carbamoyl tetrahydroisoquinolines and dihydroisoquinolines

An efficient oxidative C-H alkynylation of N-carbamoyl tetrahydroisoquinolines mediated by a TEMPO oxoammonium salt has been established. A variety of electronically varied N-carbamoyl tetrahydroisoquinolines reacted with a range of alkynyl potassium trifluoroborates smoothly under mild metal-free conditions. Dihydroisoquinolines were also suitable components for the reaction. The synthetic applicability of the method for facile access to structurally diverse bioactive molecules was further demonstrated.