Palladium(II)‐Catalyzed Tandem Synthesis of Acenes Using Carboxylic Acids as Traceless Directing Groups

Abstract

A straightforward synthetic strategy for generating useful anthracene derivatives was developed involving palladium(II)-catalyzed tandem transformation with carboxylic acids as traceless directing groups. Carboxyl-directed C-H alkenylation, carboxyl-directed secondary C-H activation and rollover, intramolecular C-C bond formation, and decarboxylative aromatization are proposed as the key steps in the tandem reaction pathway. This novel synthetic route utilizes a broad range of substrates and provides a convenient synthetic tool that allows access to acenes.

Efficient Synthesis of Anthraquinones from Diaryl Carboxylic Acids via Palladium(II)‐Catalyzed and Visible Light‐Mediated Transformations

“…After successfully achieving the visible light‐mediated transformation of 9‐ester‐substituted anthracene 2 a into anthraquinone 4 a , we next explored the prospects of the proposed one‐pot sequence of mechanistically different events using diphenyl carboxylic acid ( 1 a ), which can be easily obtained from the arylation of ethyl acetate, with ethyl acrylate as model substrates (Table ). Although KOAc was most effective in the photooxidation process (Table ), K 2 CO 3 proved to be the most efficient base for the overall reactions, especially at the stage of formation of anthracene . After much experimentation employing a Pd‐catalytic system under visible light, we were pleased to find that the desired anthraquinone 4 a was formed in 23% yield (entry 2), proving that the tandem process was indeed operating.…”

“…We envisioned that the 9‐ester‐substituted anthracenes 2 generated in situ, as a result of the tandem reactions, could be further utilized for spontaneously generating transannular endoperoxides 3 via a visible light‐induced photooxidation process . Subsequently, the generated endoperoxide intermediates 3 might further undergo a base‐assisted fragmentation pathway to form anthraquinones 4 with removal of the ethyl acetate leaving group .…”

A comprehensive overview of directing groups applied in metal-catalysed C–H functionalisation chemistry

“…One example of a decarboxylative annulation was the formation of polyarenes by C–H functionalisation of diphenyl acetic acids ( Scheme 13F ). 159 The transformation was catalysed by Pd(OAc) 2 with a MPAA as ligand. First the substrate was alkenylated via typical carboxyl-directed C–H activation.…”

The use of carboxylic acids as traceless directing groups for regioselective C–H bond functionalisation

“…Hong and co-workers approached the synthesis of (hetero)acenes from di(hetero)aryl acetic acids and ethyl acrylate by developing a palladium-catalysed cascade reaction involving a C-H bond functionalisation/decarboxylation sequence ( Scheme 19). 40 This reaction is believed to be initiated with the carboxylate-directed palladium-catalysed C-H vinylation reaction, followed by C-H bond cyclisation. Upon decarboxylation and isomerisation of the double bond, the final (hetero)acene scaffold was obtained.…”