Ruthenium-Catalyzed meta-Selective CAr-H Bond Formylation of Arenes

Ruthenium-Catalyzed Meta-Selective Trifluoroisopropylation of Arenes

This work reports the mild and efficient Ru-catalyzed trifluoroisopropylation of arenes using 2-bromo-1,1,1-trifluoropropane. Various bioactive molecules, such as purine and nucleoside derivatives, were well-suited for this transformation, affording the corresponding products in moderate-to-good yields. This method provides an efficient strategy for synthesizing trifluoroisopropyl molecules for drug discovery.

Mechanochemical Duff Reaction in Solid Phase for Easy Access to Mono- and Di-formyl Electron-Rich Arenes

A sustainable alternative to the century-old Duff reaction was developed by adopting a solid-phase mechanochemical route. A series of mono-formyl electron-rich arenes were prepared in high yields in silica as the solid reaction media using a combination of hexamethylenetetramine (HMTA) as the formyl source and a small amount of H2SO4 in a mixer mill. The use of toxic, costly, and low-boiling trifluoroacetic acid was avoided in the new mold of the mechanochemical Duff reaction. The mono-formyl phenols were obtained with exclusive ortho-selectivity, whereas unprecedented para-formylation was observed for other electron-rich aromatics. By controlling the stoichiometry of HMTA, the method offers easy access to di-formylated phenols as well. The scalability of the reaction was validated with selected substrates at the gram-scale level. In a case study, a mechanochemical tandem reaction was explored in the synthesis of a rhodol derivative. The solvent-free, metal-free mild method of formylation, with the absence of tedious work-up steps and shorter reaction times using an inexpensive mineral acid, is a sustainable alternative to the available methods for aromatic formylation.

meta-Allylation of Arenes via Ruthenium-Catalyzed Cross-Dehydrogenative Coupling

A successful example of oxidative meta-dehydrogenative allylation of arenes with alkenes has been developed using Ru(PPh₃)₃Cl₂ as a catalyst and DTBP as an oxidant. In the allylation process, pyrimidines, pyrazoles, and purines, found widely in nucleosides, were effective auxiliary groups. Gram-scale experiments took place smoothly under optimized conditions. Mechanistic studies indicated that ruthenium-catalyzed meta-dehydrogenative allylation was a free-radical process. The allylation process developed herein provides an efficient and practical strategy to prepare versatile meta-allylated arenes.

Remote C–H Functionalizations by Ruthenium Catalysis

Synthetic transformations of otherwise inert C–H bonds have emerged as a powerful tool for molecular modifications during the last decades, with broad applications towards pharmaceuticals, material sciences, and crop protection. Consistently, a key challenge in C–H activation chemistry is the full control of site-selectivity. In addition to substrate control through steric hindrance or kinetic acidity of C–H bonds, one important approach for the site-selective C–H transformation of arenes is the use of chelation-assistance through directing groups, therefore leading to proximity-induced ortho-C–H metalation. In contrast, more challenging remote C–H activations at the meta- or para-positions continue to be scarce. Within this review, we demonstrate the distinct character of ruthenium catalysis for remote C–H activations until March 2021, highlighting among others late-stage modifications of bio-relevant molecules. Moreover, we discuss important mechanistic insights by experiments and computation, illustrating the key importance of carboxylate-assisted C–H activation with ruthenium(II) complexes.

1 Introduction

2 Stoichiometric Remote C–H Functionalizations

3 meta-C–H Functionalizations

4 para-C–H Functionalizations

5 meta-/ortho-C–H Difunctionalizations

6 Conclusions

A simple removable aliphatic nitrile template 2-cyano-2,2-di-isobutyl acetic acid for remote meta-selective C–H functionalization

The remote meta-selective C–H functionalization of arenes using first aliphatic nitrile template 2-cyano-2,2-di-isobutyl acetic acid under mild conditions is presented.

Ruthenium-Catalyzed Meta-Selective C-H Difluoromethylation of Phenol Derivatives

With pyrimidine as the directing group, we achieved the meta-selective difluoromethylation of phenol derivatives using ruthenium as a catalyst. This synthetic scheme provided an efficient method for the syntheses of fluorine-containing phenol derivatives. A wide variety of phenol derivatives were well-suited, affording the corresponding products in moderate-to-good yields.