Pd(II)-Catalyzed Cross-Coupling of C(sp2 )-H Bonds and Alkyl-, Aryl- and Vinyl-Boron Reagents via Pd(II)/Pd(0) Catalysis

Palladium (II)-Catalyzed C-H Activation with Bifunctional Ligands: From Curiosity to Industrialization

In 2001, our curiosity to understand the stereochemistry of C-H metalation with Pd prompted our first studies in Pd(II)-catalyzed asymmetric C-H activation (RSC Research appointment: 020 7451 2545, Grant: RG 36873, Dec. 2002). We identified four central challenges: 1. poor reactivity of simple Pd salts with native substrates; 2. few strategies to control site selectivity for remote C-H bonds; 3. the lack of chiral catalysts to achieve enantioselectivity via asymmetric C-H metalation, and 4. low practicality due to limited coupling partner scope and the use of specialized oxidants. These challenges necessitated new strategies in catalyst and reaction development. For reactivity, we developed approaches to enhance substrate-catalyst affinity together with novel bifunctional ligands which participate in and accelerate the C-H cleavage step. For site-selectivity, we introduced the concept of systematically modulating the distance and geometry between a directing template, catalyst, and substrate to selectively access remote C-H bonds. For enantioselectivity, we devised predictable stereomodels for catalyst-controlled enantioselective C-H activation based on the participation of bifunctional ligands. Finally, for practicality, we have developed varied catalytic manifolds for Pd(II) to accommodate diverse coupling partners while employing practical oxidants such as simple peroxides. These advances have culminated in numerous C-H activation reactions, setting the stage for broad industrial applications.

Transition-Metal-Catalyzed C–H Arylation Using Organoboron Reagents

Aryl rings are ubiquitous in the core of numerous natural product and industrially important molecules and thus their facile synthesis is of major interest in the scientific community and industry. Although multiple strategies enable access to these skeletons, metal-catalyzed C–H activation is promising due to its remarkable efficiency. Commercially available organoboron reagents, a prominent arylating partner in the cross-coupling domain, have also been utilized for direct arylation. Organoborons are bench-stable, inexpensive, and readily available coupling partners that promise regioselectivity, chemodivergence, cost-efficiency, and atom-economy without requiring harsh and forcing conditions. This critical, short review presents a summary of all major studies of arylation using organoborons in transition-metal catalysis since 2005.

1 Introduction

2 Arylation without Directing Group Assistance

2.1 Palladium Catalysis

2.2 Iron Catalysis

2.3 Gold Catalysis

3 Arylation with Directing Group Assistance

3.1 Palladium Catalysis

3.2 Ruthenium Catalysis

3.3 Rhodium Catalysis

3.4 Nickel Catalysis

3.5 Cobalt Catalysis

3.6 Copper Catalysis

4 Conclusion

Multiple Catalytic C-H Bond Functionalization for Natural Product Synthesis

In the past decade, multiple catalytic C-H bond functionalization has been successfully applied in natural product synthesis as a strategy to reduce the number of steps, increase overall yield and employ more easily available starting materials. This minireview presents selected examples making use of multiple C-H bond functionalization in conceptually different ways. First, linear syntheses are discussed, wherein multiple C-H functionalization is employed either from simple (hetero)cyclic cores, at a late stage, or to build polycyclic systems. Second, the use of multiple C-H functionalization as a strategic tool in convergent synthesis to access and couple complex fragments is discussed. Information on the scalability of the employed methods is provided when available. The presented cases indicate that multiple C-H functionalization strategies should play a great role to shape the future synthesis of functional complex molecules with improved sustainability.

Cu-Catalyzed C-H Alkenylation of Benzoic Acid and Acrylic Acid Derivatives with Vinyl Boronates

An efficient Cu-catalyzed C-H alkenylation with acyclic and cyclic vinyl boronates was realized for the first time under mild conditions. The scope of the vinyl borons and the compatibility with functional groups including heterocycles are superior than Pd-catalyzed C-H coupling with vinyl borons, providing a reliable access to multisubstituted alkenes and dienes. Subsequent hydrogenation of the product from the internal vinyl borons will lead to installation of secondary alkyls.

Palladium(II)-Catalyzed ortho-Arylation of Aromatic Alcohols with a Readily Attachable and Cleavable Molecular Scaffold

A palladium(II)-catalyzed C-H arylation process of alcohols has been developed. The strategy utilizes a novel quinoline-based hemiacetal scaffold that can direct the selective C-H bond functionalization. This reaction provides a useful method to construct biaryl compounds of benzyl alcohols in good to excellent yields. The new molecular scaffold can be readily attached, removed, and recovered.

Orchestrated triple C-H activation reactions using two directing groups: rapid assembly of complex pyrazoles

A sequential triple C-H activation reaction directed by a pyrazole and an amide group leads to the well-controlled construction of sterically congested dihydrobenzo[e]indazole derivatives. This cascade reaction demonstrates that the often problematic competing C-H activation pathways in the presence of multiple directing groups can be harvested by design to improve step economy in synthesis. Pyrazole as a relatively weak coordinating group is shown to direct Csp3-H activation for the first time.

Palladium(ii)-catalysed ortho-arylation of N-benzylpiperidines

A simple and mild protocol for saturated N-heterocycle directed ortho-C–H arylation under Pd-catalysis is reported.

Iron-catalyzed C(sp(2))-H bond functionalization with organoboron compounds

We report here that an iron-catalyzed directed C-H functionalization reaction allows the coupling of a variety of aromatic, heteroaromatic, and olefinic substrates with alkenyl and aryl boron compounds under mild oxidative conditions. We rationalize these results by the involvement of an organoiron(III) reactive intermediate that is responsible for the C-H bond-activation process. A zinc salt is crucial to promote the transfer of the organic group from the boron atom to the iron(III) atom.

Ligand-accelerated ortho-C-H alkylation of arylcarboxylic acids using alkyl boron reagents

A protocol for the Pd(II)-catalyzed ortho-C-H alkylation of phenylacetic and benzoic acids using alkylboron reagents is disclosed. Monoprotected amino acid ligands (MPAA) were found to significantly promote reactivity. Both potassium alkyltrifluoroborates and alkylboronic acids were compatible coupling partners. The possibility of a radical alkyl transfer to Pd(II) was also investigated.

Palladium-Catalyzed C-H Arylation Using Aryltrifluoroborates in Conjunction with a MnIII Oxidant under Mild Conditions

This paper describes the development of a mild Pd-catalyzed C-H arylation reaction using potassium aryltrifluoroborates in conjunction with Mn(OAc)₃ as the oxidant. The scope of this transformation is explored with a variety of different aryltrifluoroborates and arylpyridine substrates. Preliminary mechanistic studies suggest that the reaction proceeds via a high-valent Pd mechanism with C-H activation occurring at or before the rate determining step.

Mild palladium-catalyzed C-H alkylation using potassium alkyltrifluoroborates in combination with MnF3

A Pd-catalyzed method for ligand-directed C-H alkylation with organoboron reagents is described. The combination of potassium organotrifluoroborates, MnF3, and a Pd(II) catalyst effects pyridine and amide-directed C-H alkylation. These reactions proceed under mild conditions (25-40 °C in weakly acidic media), are effective for installing methyl and 1° alkyl groups, and do not require promoters such as benzoquinone.

Pd(II)-catalyzed enantioselective C-H activation of cyclopropanes

Systematic ligand development has led to the identification of novel mono-N-protected amino acid ligands for Pd(II)-catalyzed enantioselective C-H activation of cyclopropanes. A diverse range of organoboron reagents can be used as coupling partners, and the reaction proceeds under mild conditions. These results provide a new retrosynthetic disconnection for the construction of enantioenriched cis-substituted cyclopropanecarboxylic acids.