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.