Titanium‐catalyzed esterification reactions: beyond Lewis acidity

Abstract

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.

Combined Computational and Experimental Investigation on the Mechanism of CO2 Hydrogenation to Methanol with Mn-PNP-Pincer Catalysts

“…They conclude that the elimination of water from the ortho ester intermediate is the TDTS for this reaction with a computed ES of 23 kcal mol −1 . 59 This lies well below the overall ES and the experimentally determined activation barrier for the overall hydrogenation cycle in which Ti(O i Pr) 4 is applied (vide infra). The coordination of the Lewis acid with the Mn complex I is unfavorable by roughly 25 kcal mol −1 and does therefore also not contribute to the operating cycle (XVI in the SI).…”

Parts-Per-Million (Salen)Fe(III) Homogeneous Catalysts for the Production of Biodiesel from Waste Cooking Oils

“…In any case, these structures demonstrate that (salen)Fe(III) complexes have two mutually cis coordination sites available during catalytic reactions. This might be relevant to (trans)esterification catalysis, since it has been argued that the role of the acidic catalyst in such reactions goes beyond that of simple Lewis acid and may involve more than a single carboxylate [50,51].…”

Mechanistic elucidation of monoalkyltin(iv)-catalyzed esterification

“…The importance of having an amphoteric catalyst in esterification reactions is not only restricted to tinbased catalysts as we have recently demonstrated for titaniumbased esterification catalysts. 34 Furthermore, the function of the n-butyl tail on the tin catalyst is to enforce a sevencoordinate tin center which remains during the whole catalytic cycle as established by DFT calculations.…”