Titanium‐catalyzed esterification reactions: beyond Lewis acidity

• Published in: ChemCatChem Vol. 12; no. 20; pp. 5229 - 5235
• Main Authors: Wolzak, Lukas A., Vlugt, Jarl Ivar, Berg, Keimpe J., Reek, Joost N. H., Tromp, Moniek, Korstanje, Ties J.
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 20.10.2020
• Subjects: Basicity; Catalysts; Catalytic activity; Chemical reactions; Density functional theory; Esterification; Homogeneous Catalysis; Hydrogen bonding; Hydrogen bonding interactions; Lewis acid; Substrates; Titanium; Titanium, Amphoteric
• ISSN: 1867-3880; 1867-3899
• DOI: 10.1002/cctc.202000931

Esterification is a key reaction and is used in many synthetic and industrial processes, yet the detailed mechanism of operation of often‐used (Lewis acid) catalysts is unknown and subject of little research. Here, we report on mechanistic studies of a titanium aminotriphenolate catalyst, using stoichiometric and catalytic reactions combined with kinetic data and density functional theory (DFT) calculations. While often only the Lewis acidity of the Ti‐center is taken into account, we found that the amphoteric nature of this catalyst, combining this Lewis acidity with Brønsted basicity of a Ti‐bound and in situ formed carboxylate group, is crucial for catalytic activity. Furthermore, hydrogen bonding interactions are essential to pre‐organize substrates and to stabilize various intermediates and transition states and thus enhancing the overall catalytic reaction. These findings are not only applicable to this class of catalysts, but could be important for many other esterification catalysts. Beyond Lewis Acidity: Despite the omnipresence of esterification reactions in organic synthesis and industrial processes, mechanisms of operation of often applied (Lewis acid) catalyst are still unknown. While often Lewis acidity of the catalyst is considered to be of key importance we found that the amphoteric nature in combination with hydrogen bonding interactions are essential to pre‐organize substrates and to stabilize various intermediates and transition states.