Role of Anion in Determining the Stereoselectivity of Mg-Ph-BOX-Catalyzed Diels-Alder Reactions: A Computational Study

In the realm of enantioselective Diels-Alder reactions, a role of primary importance is held by Mg-BOX catalysis. The main features of both catalysts and ligand in directing the stereoselective outcome have been extensively studied in several papers mainly through 1H NMR and X-ray diffraction (XRD) techniques. However, over the years, no computational studies have been reported to support the models proposed to rationalize the observed stereoselectivity for the reaction between 3-acryloyl-1,3-oxazolidin-2-one and cyclopentadiene catalyzed by the BOX ligand (R,R)-(+)-2,2′-isopropylidenebis(4-phenyl-2-oxazoline) and Mg(II) salts. To approach the problem, we performed a density functional theory (DFT) computational study, aiming to locate the preferred transitions states deriving from these proposed models, but we only found a correspondence in selectivities for the reaction catalyzed by Mg(OTf)2, where the model suggests an octahedral complex with the two triflate anions coordinating magnesium. For the other cases [i.e., Mg(ClO4)2, Mg(ClO4)2·2H2O, and MgI2·I2], the commonly accepted tetrahedral or octahedral models suggest no involvement of the perchlorate or iodide anions, but the corresponding calculations did not reproduce the experimental selectivities. Only when we considered also in these cases coordination complexes involving their presence, the observed selectivities were reproduced, thus opening new insights to better understand the role and the action of the counterion to determine the stereochemical outcome of these reactions.