Ruthenium-Catalyzed Regioselective Direct Ortho -Acyloxylation of Azoarenes with Carboxylic Acids via C-H Bond Activation

Nitrate promoted mild and versatile Pd-catalysed C(sp2)-H oxidation with carboxylic acids

A nitrate-promoted Pd-catalysed mild cross-dehydrogenative C(sp²)-H bond oxidation of oximes or azobenzenes with diverse carboxylic acids has been developed. In contrast to the previous catalytic systems, this protocol features mild conditions (close to room temperature for most cases) and a broad substrate scope (up to 64 examples), thus constituting a versatile method to directly prepare diverse O-aryl esters. Moreover, the superiority of the nitrate additive in this mild transformation was further determined by experimental and computational evidence.

Ruthenium(ii)-catalyzed acyloxylation of the ortho-C–H bond in 2-aroyl-imidazoles with carboxylic acids

The reaction of 2-aroyl-imidazoles with carboxylic acids using [RuCl₂(p-cymene)]₂ as the catalyst and Ag₂CO₃ as the oxidant results in ortho-C–H acyloxylation to afford acyloxylation products.

Ruthenium(ii)-catalysed selective C(sp2)-H bond benzoxylation of biologically appealing N-arylisoindolinones

Site- and regio-selective aromatic C-H bond benzoxylations were found to take place using biologically appealing N-arylisoindolinones under ruthenium(ii) catalysis in the presence of (hetero)aromatic carboxylic acid derivatives as coupling partners. Besides the presence of two potential C(sp²)-H sites available for functionalization in the substrates, exclusive ortho selectivity was achieved in the phenyl ring attached to the nitrogen atom. Notably, the reactions occurred in a selective manner as only mono-functionalized products were formed and they tolerated a large number of functional chemical groups. The ability of the cyclic tertiary amide within the isoindolinone skeleton to act as a weak directing group in order to accommodate six-membered ring ruthenacycle intermediates appears to be the key to reach such high levels of selectivity. In contrast, the more sterically demanding cyclic imides were unreactive under identical reaction conditions.

Cross-dehydrogenative coupling reactions between arenes (C-H) and carboxylic acids (O-H): a straightforward and environmentally benign access to O-aryl esters

Transition-metal catalyzed cross-dehydrogenative-coupling reactions encompass highly versatile and atom economical methods for the construction of various carbon-carbon and carbon-heteroatom bonds by combining two C(X)-H (X = heteroatom) bonds. Along this line, direct acyloxylation of C-H bonds with carboxylic acids has emerged as a powerful and green approach for the synthesis of structurally diverse esters. In this focus-review we will describe recent progress in direct esterification of aromatic C-H bonds with special emphasis on the mechanistic features of the reactions. Literature has been surveyed until the end of February 2019.