Unravelling the reaction mechanism for the Claisen-Tishchenko condensation catalysed by Mn(I)-PNN complexes: a DFT study

Abstract

In this work, we study the potential catalytic role of previously identified Mn(I)-PNN complexes in the Claisen-Tishchenko reaction. An in-depth investigation of the reaction mechanism suggests that, after activation of the 16e pre-catalyst, a hydrogenated 18e active species is generated. Based on calculations, rate-limiting barriers in a range of ca. 15-20kcalmol(-1) are seen for a model process consisting in the esterification of acetaldehyde into ethyl acetate at 100 degrees C and 1atm reaction conditions (in toluene solution). Our hypothesis is centred on the role of the Mn centre as the only active site involved in both elementary steps, namely hydride borrowing and C-O bond formation. During this C-O bond formation step, diastereoisomers (R-N,R) and (R-N,S) [or their enantiomeric pairs (S-N,S) and (S-N,R)] can be generated, with calculations showing a preference towards the (R-N,R) pathway.