Abstract
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.
Highlights
Magnesium-BOX-catalyzed Diels−Alder reactions constitute one of the most important milestones of organometallic chemistry
An interesting case is represented by the classical Diels− Alder reaction between 3-acryloyl-1,3-oxazolidin-2-one 2 and cyclopentadiene 3 catalyzed by the BOX ligand (R)-1 and Mg(II) salts, which showed an enantioselectivity dependent on both the counterion and additives that behave as auxiliary ligands (Scheme 1).[5,7]
To explain the selectivity obtained in the reactions, simple tetrahedral or octahedral coordinated models were elaborated on the basis of the data retrieved from 1H NMR spectroscopic experiments,[8] but they only refer to the statical situation of the starting adduct between the 1,3-dicarbonyl substrate and the Mg-BOX complex and not to what occurs during the proceeding of the reaction in the presence of cyclopentadiene reactive partner, in particular at the transition state
Summary
Magnesium-BOX-catalyzed Diels−Alder reactions constitute one of the most important milestones of organometallic chemistry. We tried to give theoretical support to the proposed models by performing a computational study of the tetrahedral or octahedral models as well as the transitions state of the reaction reported in Scheme 1 with the aim to rationalize the observed enantio- and diastereoselectivities.[4−6] calculations were not able to reproduce all of the experimental results For such reason, we extended the computational study to starting adducts between 2 and Mg-BOX coordinatively different from those originally proposed and evaluated all of the possible roles of the counterions when they are not directly linked to the magnesium cation
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