The Versatile and Strategic O-Carbamate Directed Metalation Group in the Synthesis of Aromatic Molecules: An Update

The aryl O-carbamate (ArOAm) group is among the strongest of the directed metalation groups (DMGs) in directed ortho metalation (DoM) chemistry, especially in the form Ar-OCONEt2. Since the last comprehensive review of metalation chemistry involving ArOAms (published more than 30 years ago), the field has expanded significantly. For example, it now encompasses new substrates, solvent systems, and metalating agents, while conditions have been developed enabling metalation of ArOAm to be conducted in a green and sustainable manner. The ArOAm group has also proven to be effective in the anionic ortho-Fries (AoF) rearrangement, Directed remote metalation (DreM), iterative DoM sequences, and DoM-halogen dance (HalD) synthetic strategies and has been transformed into a diverse range of functionalities and coupled with various groups through a range of cross-coupling (CC) strategies. Of ultimate value, the ArOAm group has demonstrated utility in the synthesis of a diverse range of bioactive and polycyclic aromatic compounds for various applications.

Pd-Catalyzed Cross-Couplings: On the Importance of the Catalyst Quantity Descriptors, mol % and ppm

This Review examines parts per million (ppm) palladium concentrations in catalytic cross-coupling reactions and their relationship with mole percentage (mol %). Most studies in catalytic cross-coupling chemistry have historically focused on the concentration ratio between (pre)catalyst and the limiting reagent (substrate), expressed as mol %. Several recent papers have outlined the use of “ppm level” palladium as an alternative means of describing catalytic cross-coupling reaction systems. This led us to delve deeper into the literature to assess whether “ppm level” palladium is a practically useful descriptor of catalyst quantities in palladium-catalyzed cross-coupling reactions. Indeed, we conjectured that many reactions could, unknowingly, have employed low “ppm levels” of palladium (pre)catalyst, and generally, what would the spread of ppm palladium look like across a selection of studies reported across the vast array of the cross-coupling chemistry literature. In a few selected examples, we have examined other metal catalyst systems for comparison with palladium.

Regiodivergent C-H Arylation of Triphenylene Derivatives Controlled by Electronic Effects of Diaryliodonium Salts

A regiodivergent C-H arylation of triphenylene derivatives with diaryliodonium salts was developed. The regiodivergence was controlled by electronic effects of diaryliodonium salts. When the aryl(mesityl)iodonium salts bearing strong electron-donating groups at the para-position of aryl groups were used, the arylation reactions occurred ortho to amide groups. However, if the aryl(mesityl)iodonium salts bearing electron-withdrawing groups or weak electron-donating groups at the para-position of aryl groups were utilized, the arylation reactions occurred meta to amide groups.

O2-Mediated transformation of 9-phenanthrenol: an approach to the synthesis of phenanthrenyl ketal and 9-fluorenones

We report the first example for O₂-mediated oxidation of 9-phenanthrenol to phenanthrenyl ketal under basic conditions, followed by transformation to 9-fluorenones.

C–H Functionalization Strategies in the Naphthalene Series: Site Selections and Functional Diversity

Naphthalene is certainly not a common arene. In contrast to benzene, the bicyclic feature of naphthalene offers multiple differentiable positions and thus a broad diversity of substitution patterns. Naphthalene is a central building block for the construction of elaborated polycyclic architectures with applications in broad domains such as life and materials sciences. As a result, C–H functionalization strategies specially designed for naphthalene substrates have become essential to install valuable substituents on one or both rings towards polysubstituted naphthalenes. This short review provides a focus on uncommon substitution patterns; however, classical ortho C–H activation is not covered.

1 Introduction

2 C–H Functionalization Using a Directing Group Located at Position 1

2.1 Functionalization on the Ring Bearing the DG: 1,3-Substitution Pattern

2.2 Functionalization on the Ring Bearing the DG: 1,4-Substitution Pattern

2.3 Functionalization on the Neighboring Ring: 1,6-, 1,7- and 1,8-Substitution Patterns

3 C–H Functionalization Using a Directing Group Located at Position 2

3.1 Functionalization on the Ring Bearing the DG: 2,4- and 2,1-Substitution Patterns

3.2 Miscellaneous Substitution Patterns

4 Bis C–H Functionalization

4.1 Symmetrical Bisfunctionalization: 1,2,8-Substitution Pattern

4.2 Symmetrical Bisfunctionalization: 2,3,1-Substitution Pattern

4.2 Unsymmetrical Bisfunctionalization: 2,3,1-Substitution Pattern

4.3 Symmetrical Bisfunctionalization: 2,4,8-Substitution Pattern

5 Conclusion and Outlook

Dual palladium-photoredox catalyzed chemoselective C-H arylation of phenylureas

A highly chemoselective C-H arylation of phenylureas has been accomplished using dual palladium-photoredox catalysis at room temperature without any additives, base or external oxidants. Regioselective C-H arylation of N,N'-diaryl substituted unsymmetrical phenylureas has also been accomplished by a careful choice of aryl groups.

Pd(ii)/Ag(i)-Cocatalyzed ortho direct arylation of O-phenylcarbamates with pinacol aryl boronates

Ortho direct arylation of O-phenyl carbamates using aryl boronates as coupling partners has been realized for the first time using Pd(OAc)₂ and AgOAc as co-catalyst.

Highly meta-selective halogenation of 2-phenylpyridine with a ruthenium(i) catalyst

A practical method for PhI(OCOCF₃)₂ mediated direct meta-C–H monohalogenation of 2-phenylpyridine and its derivatives via an Ru(i) catalyzed reaction is reported.

A meta-selective-C–H alkenylation of phenol-derivatives employing a traceless organosilicon template

A traceless organosilicon template-directed meta-selective-C–H alkenylation of phenols was realized with good yields and high selectivities.

Pd(II)-catalysed meta-C-H functionalizations of benzoic acid derivatives

Benzoic acids are highly important structural motifs in drug molecules and natural products. Selective C–H bond functionalization of benzoic acids will provide synthetically useful tools for step-economical organic synthesis. Although direct ortho-C–H functionalizations of benzoic acids or their derivatives have been intensely studied, the ability to activate meta-C–H bond of benzoic acids or their derivatives in a general manner via transition-metal catalysis has been largely unsuccessful. Although chelation-assisted meta-C–H functionalization of electron-rich arenes was reported, chelation-assisted meta-C–H activation of electron-poor arenes such as benzoic acid derivatives remains a formidable challenge. Herein, we report a general protocol for meta-C–H olefination of benzoic acid derivatives using a nitrile-based sulfonamide template. A broad range of benzoic acid derivatives are meta-selectively olefinated using molecular oxygen as the terminal oxidant. The meta-C–H acetoxylation, product of which is further transformed at the meta-position, is also reported.

While directed ortho-metalation is a well-established protocol, meta-functionalisation of aromatic groups is more challenging. Here, the authors report a directed meta-C-H functionalisation of benzoic acid derivatives, using a nitrile-based sulfonamide directing group.

Highly regioselective meta arylation of oxalyl amide-protected β-arylethylamine via the Catellani reaction

A first bidentate directing group assisted highly selective meta arylation of β-arylethylamine derivatives with various iodides is reported.

Ruthenium-Catalyzed meta-Selective C-H Bromination

The first example of a transition-metal-catalyzed, meta-selective C-H bromination procedure is reported. In the presence of catalytic [{Ru(p-cymene)Cl2 }2 ], tetrabutylammonium tribromide can be used to functionalize the meta C-H bond of 2-phenylpyridine derivatives, thus affording difficult to access products which are highly predisposed to further derivatization. We demonstrate this utility with one-pot bromination/arylation and bromination/alkenylation procedures to deliver meta-arylated and meta-alkenylated products, respectively, in a single step.