Palladium-Catalyzed Chemoselective Activation of sp3 vs sp2 C-H Bonds: Oxidative Coupling To Form Quaternary Centers

Construction of quaternary alkyl motifs through palladium-catalyzed oxidative coupling of 1,3-dicarbonyl compounds with alkenes followed by C-C bond cleavage

A palladium-catalyzed coupling reaction has been developed for the generation of tertiary alkylation products by reacting olefins with diversely functionalized 1,3-dicarbonyls. The reaction involves the tertiary C-H alkylation of 1,3-dicarbonyls with olefins to produce a tertiary alcohol, followed by C-C bond cleavage.

Pd(II)-catalyzed cyclization of 2-methyl aromatic ketones with maleimides through weak chelation assisted dual C-H activation

A palladium-catalyzed dual C-H functionalization of substituted aromatic ketones and ester with maleimides leading to tricyclic heterocyclic molecules with good to excellent yield is reported. In this protocol, weak chelation of the carbonyl groups has been successfully utilized for the selective activation of the ortho-methyl C(sp3)-H bond instead of the ortho-C(sp2)-H bond in the presence of an external bidentate ligand Ac-Ile-OH. The reaction proceeds through two-fold C-H activation to generate a five-membered cyclic ring. The first C-H activation takes place selectively at the benzylic position followed by a second C-H bond activation at the meta position. The protocol demonstrates compatibility among diverse substituted aromatic ketones and ester as well as various substituted maleimides. Further derivatization of the tricyclic ketone to an alcohol exhibits the synthetic applicability of the protocol. Also, a plausible reaction mechanism has been proposed.

CuBr-mediated synthesis of 1,4-naphthoquinones via ring expansion of 2-aryl-1,3-indandiones

A CuBr-mediated direct insertion of alkenes into unstrained ring 2-aryl-1,3-indandiones is reported, which provides a one-carbon ring expansion strategy for the synthesis of 1,4-naphthoquinones. Entirely differing from the existing reports, the alkenes herein behave as C1 units to participate in annulation reactions. This transformation provides a facile route to access a class of highly functionalized 1,4-naphthoquinones with good yields, good functional-group tolerance and high atom-economy. Further research on the application of this reaction is currently underway in our laboratory.

Palladium-Catalyzed [3 + 2] Annulation of Aromatic Amides with Maleimides through Dual C-H Activation

A palladium-catalyzed [3 + 2] annulation of substituted aromatic amides with maleimides providing tricyclic heterocyclic molecules in good to moderate yields through weak carbonyl chelation is reported. The reaction proceeds via a dual C-H bond activation where the first C-H activation takes place selectively at the benzylic position followed by a second C-H bond activation at the meta position to afford a five-membered cyclic ring. An external ligand Ac-Gly-OH has been used to succeed in this protocol. A plausible reaction mechanism has been proposed for the [3 + 2] annulation reaction.

Pd-catalyzed Aerobic Cross-Dehydrogenative Coupling of Catechols with 2-Oxindoles and Benzofuranones: Reactivity Difference Between Monomer and Dimer

Persistent radicals, which are generated from 2-oxindole or benzofuranone dimers, are useful tools for designing the radical-based cross-coupling reaction to provide molecules containing a quaternary carbon. The persistent radical is accessible from both the dimer and monomer; however, the reactivity difference between these substrates for the oxidative cross-coupling reaction is not fully understood, most likely because of the mechanistic complexity. Here, we present details of an aerobic cross-dehydrogenative coupling (CDC) reaction using various monomers and catechols. UV-Vis analysis and mechanistic control experiments showed that the monomer is less reactive than the dimer under aerobic conditions. Our Pd(II)-BINAP-μ-hydroxo complex significantly improved the reactivity of the monomers for the aerobic CDC reaction with catechols, yielding results comparable to those of the corresponding dimer. The procedure, which enables the generation of the persistent radical in situ, is particularly useful when employing the monomer that is not readily converted to the corresponding dimer.

Chemo- and regioselective benzylic C(sp3)–H oxidation bridging the gap between hetero- and homogeneous copper catalysis

Selective C‒H functionalization in a pool of proximal C‒H bonds, predictably altering their innate reactivity is a daunting challenge. We disclose here, an expedient synthesis of privileged seven-membered lactones, dibenzo[c,e]oxepin-5(7H)-one through a highly chemoselective benzylic C(sp³)‒H activation. Remarkably, the formation of widely explored six-membered lactone via C(sp²)‒H activation is suppressed under the present conditions. The reaction proceeds smoothly on use of inexpensive metallic copper catalyst and di-tert-butyl peroxide (DTBP). Owing to the hazards of stoichiometric DTBP, further, we have developed a sustainable metallic copper/rose bengal dual catalytic system coupled with molecular oxygen replacing DTBP. A 1,5-aryl migration through Smiles rearrangement was realized from the corresponding diaryl ether substrates instead of expected eight-membered lactones. The present methodology is scalable, applied to the total synthesis of cytotoxic and neuroprotective natural product alterlactone. The catalyst is recyclable and the reaction can be performed in a copper bottle without any added catalyst.

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Catalytic strategy for chemo- and regioselective benzylic C–H activation

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Bulk copper catalysis merging with photocatalysis

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Reusable copper catalyst

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Reaction demonstrated in commercial copper bottle without external catalyst

Remote ortho-C-H functionalization via medium-sized cyclopalladation

Compared to the tremendous progress made in directed ortho-C-H functionalization via five- or six-membered cyclopalladation, protocols with the ability to selectively activate more remote C-H bonds through the intermediacy of larger, less favorable, seven- or eight-membered metalacycles are particularly challenging and remain rare. However, such a strategy would provide new retrosynthetic opportunities for generating structural diversity and complexity. Intense recent research based on the use of either mono-anionic bidentate or monodentate directing groups is characterizing this approach as an increasingly viable tool for selective C-C and C-X bond-forming reactions. This short review provides an overview of these strategies with an emphasis on mechanistic details, synthetic applicability, limitations, and key challenges.

DTBP-mediated cross-dehydrogenative coupling of 3-aryl benzofuran-2(3H)-ones with toluenes/phenols for all-carbon quaternary centers

We have developed a transition-metal free protocol for efficient cross-dehydrogenative coupling of 3-aryl benzofuran-2(3H)-ones and toluenes/phenols using DTBP as an oxidant.

Alternative and Uncommon Acylating Agents - An Alive and Kicking Methodology

Functionalizing and derivatising organic molecules is a centerpiece in organic synthesis. Succinctly manipulating and installing acyl moieties in organic molecules spurred the interest of chemists owing to its occurrence in natural products, bioactive molecules, pharmaceuticals, and advanced materials. Traditionally, access to acylation reaction was achieved by Friedel-Crafts reaction, Schotten-Baumann, and Vilsmeier-Haack acylation, however, these protocols own pitfalls. Further to make the acylation process attractive and environmentally friendly, toluene, aldehydes, alcohols, α-keto acids, amines, amides, esters, ethers, nitriles, alkynes, alkenes, ketenes, N-acylbenzotriazoles, ketones, thioacids, oximes, thiazolium carbinols, PIDA, diacyl disulfides and acyl salts were used as an acyl surrogates/reagents. Amusingly, these acylating reagents are considered uncommon and alternative to carboxylic acids, acid chlorides and acetic anhydrides. This short review aims to encompass the usage of acylating agents in transition-metal, metal-free, light-driven and other demanding conditions, and thus reveals their practicality.

Recent Advances on Benzofuranones: Synthesis and Transformation via C–H Functionalization

The benzofuranone structure is important in many fields, such as natural products, pharmaceuticals, building blocks, antioxidants, and dyes. The efficient synthesis and transformation of benzofuranones have attracted great attention in organic synthesis. They can be synthesized by the Friedel–Crafts reaction and intramolecular dehydration ring-closing and transition-metal-catalyzed reactions, among others. Their direct utilization in the preparation of other functional molecules further enhance their application. Due to their low pK a value and easy enolization, the transformation of benzofuranones via C(3)–H bond functionalization has been a hot issue since 2010. Herein, we highlight advances in the synthesis of benzofuranones and their transformation via C–H functionalization. Other transformations related to benzofuranones are also discussed.

1 Introduction

2 Synthesis of Benzofuranones

3 C–H Functionalization of Benzofuranones

4 Other Types of Reactions of Benzofuranones

5 Conclusion and Outlook

Inherent Selectivity of Pd C-H Activation from Different Metal Oxidation States

In investigating potential control factors that would permit a palladium-catalyzed benzylic vs arene C-H activation as previously reported by our group, it was discovered that the oxidation state of the homogenous palladium species influences the selectivity of C-H activation. DFT calculations show that Pd⁰ and Pd^I preferentially activate the sp³ C-H bond in toluene, whereas Pd^II and Pd^III preferentially activate the sp² C-H bond. This selectivity appears to originate from the steric environment created by the ligand framework on the palladium. As the palladium oxidation state increases, the number of ligand sites increases, which decreases the energetic favorability for activation of the weaker, yet more hindered sp³ C-H bond.

I2-Mediated Cross-Dehydrogenative Coupling and Amidation of 3-Aryl Benzofuranones with Aryl Amines for the Synthesis of 3,3-Diaryl Indolin-2-ones

We have developed a protocol for efficient synthesis of indolin-2-ones from benzofuranones and aryl amines using iodine as a mediator. A diverse range of benzofuranones and aryl amines undergo cross-dehydrogenative coupling and amidation of 3-aryl benzofuranones for the cascade reaction to generate products in 24-93% yields. This reaction can be easily scaled-up to give an indolin-2-one in a gram scale. Further chemical manipulation of the products enabled useful transformations of the phenol ring including alkylation, arylation, etc.

Direct Oxidation of Aryl Malononitriles Enabling a Copper-Catalyzed Intermolecular Alkene Carbochlorination

A copper-catalyzed carbochlorination of alkenes with aryl malononitriles and chloride is disclosed. This net oxidative transformation proceeds with activated and unactivated alkenes with moderate to excellent yields. Mechanism experiments suggest addition of the malononitrile radical to form a secondary carbon radical which is intercepted by a chloride source. The resultant products can be transformed into biologically important γ-lactones in one further step.

Mechanistic understanding enables chemoselective sp3 over sp2 C–H activation in Pd-catalyzed carbonylative cyclization of amino acids

An understanding on the factors controlling C(sp²)–H vs, C(sp³)–H selectivity in Pd-catalyzed carbonylative cyclization of γ-arylated valine derivatives has allowed to reverse the remarkable selectivity of Pd for aryl C(sp²)–H over C(sp³)–H cleavage.

Copper Catalyzed Oxidative Arylation of Tertiary Carbon Centers

We describe herein a Cu(OTf)₂ catalyzed oxidative arylation of a tertiary carbon-containing substrate including aryl malononitriles, 3-aryl benzofuran-2-ones, and 3-aryl oxindoles. In some cases, the nitrile groups of the aryl malononitriles undergo further reactions leading to lactones or imines. These reaction conditions are applicable for a range of arenes, including phenols, anilines, anisoles, and heteroarenes. Mechanistic studies support the formation of a cationic intermediate via a two-electron oxidation.

Formal Aerobic Oxidative Cross-Coupling of Benzofuranones with Azo Compounds Using Pd-μ-hydroxo Complex

We have developed a catalytic aerobic oxidative dimerization reaction of benzofuranones using a Pd(II)-µ-hydroxo complex. Radical-radical cross-coupling of the resulting dimers with azo compounds enabled the one-pot synthesis of structurally congested benzofuranones having two distinct vicinal all-carbon quaternary centers.

Catalytic Aerobic Cross-Dehydrogenative Coupling of Azlactones en Route to α,α-Disubstituted α-Amino Acids

We developed a catalytic aerobic method to synthesize α,α-disubstituted α-amino acids through cross-dehydrogenative coupling of azlactones. Combining an iron catalyst with a bisoxazolidine ligand resulted in high catalytic performance, and cross-coupling with an indole proceeded smoothly under aerobic conditions. A wide variety of α-aryl and aliphatic amino acid derived azlactones were applied to the present catalysis. In addition, a quaternary carbon could be constructed using oxindole and benzofuranone under aerobic conditions.

Cyanomethylation of Substituted Fluorenes and Oxindoles with Alkyl Nitriles

The first example of metal-free cyanomethylenation from alkyl nitriles of sp³ C-H bonds to afford quaternary carbon centers is described. This oxidative protocol is operationally simple and features good functional group compatibility. This method provides a novel approach to highly functionalized fluorene and oxindole derivatives, which are commonly used in material and pharmaceutical areas. Control experiments provide evidence of a radical reaction process.