Mechanistic Insights into the Ni-Catalyzed Reductive Carboxylation of C-O Bonds in Aromatic Esters with CO 2 : Understanding Remarkable Ligand and Traceless-Directing-Group Effects

• Published in: Chemistry, an Asian journal Vol. 13; no. 12; pp. 1570 - 1581
• Main Authors: Han, Yan-Li, Zhao, Bing-Yuan, Jiang, Kun-Yao, Yan, Hui-Min, Zhang, Zhu-Xia, Yang, Wen-Jing, Guo, Zhen, Li, Yan-Rong
• Format: Journal Article
• Language: English
• Published: Germany 18.06.2018
• Subjects: carboxylation; density functional calculations; nickel; activation; cleavage reactions
• ISSN: 1861-4728; 1861-471X
• DOI: 10.1002/asia.201800257

The mechanism of the Ni -catalyzed reductive carboxylation reaction of C(sp )-O and C(sp )-O bonds in aromatic esters with CO to access valuable carboxylic acids was comprehensively studied by using DFT calculations. Computational results revealed that this transformation was composed of several key steps: C-O bond cleavage, reductive elimination, and/or CO insertion. Of these steps, C-O bond cleavage was found to be rate-determining, and it occurred through either oxidative addition to form a Ni intermediate, or a radical pathway that involved a bimetallic species to generate two Ni species through homolytic dissociation of the C-O bond. DFT calculations revealed that the oxidative addition step was preferred in the reductive carboxylation reactions of C(sp )-O and C(sp )-O bonds in substrates with extended π systems. In contrast, oxidative addition was highly disfavored when traceless directing groups were involved in the reductive coupling of substrates without extended π systems. In such cases, the presence of traceless directing groups allowed for docking of a second Ni catalyst, and the reactions proceed through a bimetallic radical pathway, rather than through concerted oxidative addition, to afford two Ni species both kinetically and thermodynamically. These theoretical mechanistic insights into the reductive carboxylation reactions of C-O bonds were also employed to investigate several experimentally observed phenomena, including ligand-dependent reactivity and site-selectivity.