Decarboxylative direct arylation processes

In 2009, Larrosa and co-workers developed the first decarboxylative direct C-H arylation methodology that allowed the intermolecular coupling of a variety of electron-poor benzoic acids with N-pivaloylindoles (090L5506). Remarkably, this process occurs with high chemo- and regioselectivity in both coupling partners and led to the formation of C3-arylated products exclusively, with good yields (Scheme 45). The authors proposed... 

In a process developed by Myers et al., aromatic carboxylic acids were directly employed as substrates for Heck olefinations, albeit in the presence of a large excess of silver carbonate [38]. This base both facilitates the decarboxylation step and acts as an oxidant, generating arylpalladium(II) intermediates. In related processes, arylphosphonic [39] and arylboronic acids [40] were used as aryl sources in the presence of an oxidant. 

Decarboxylation of excited singlets of aromatic esters (Eq. 13.6) is a concerted process, and it can account for a significant fraction of the reaction by appropriately substituted aryl esters, especially in bulk polymers and other media that are capable of imposing conformational constraints on guest molecules.Because photoinduced decarboxylation occurs before lysis of the aryl esters, it does not influence the rates at which the A B singlet pair react. For that reason, the relative yields of decarboxylation products need not be considered in analyses of the radical pairs unless their formation precludes sufficient radical pair production for their easy direct or indirect detection. 

Heck carbonylation involving the oxidative addition of aryl halides is not applicable to aliphatic halides, since alkyl halides react directly with nucleophiles. Tsuji developed a process of carbonylating allyl carbonates to form carboxylic esters by palladium-catalyzed carbonylation that is applicable to aliphatic substrates [60]. The process probably involves (a) the oxidative addition of allyl carbonates to Pd(0) species to form r/ -allyl palladium species, (b) CO insertion into the allyl-Pd bond to give acylpalladium species, (c) decarboxylation of the carbonate ligand to give alkoxide, and (d) liberation of butenoate esters by combination with the alkoxides as shown in Scheme 1.21. 

---