Experimentally, a reversal of chemoselectivity has been observed in catalyzed Diels–Alder reactions of α,β-unsaturated aldehydes (e.g., (2E)-but-2-enal) and ketones (e.g., 2-hexen-4-one) with cyclopentadiene. Indeed, using the triflimidic Brønsted acid Tf2NH as catalyst, the reaction gave a Diels–Alder adduct derived from α,β-unsaturated ketone as a major product. On the other hand, the use of tris(pentafluorophenyl)borane B(C6F5)3 bulky Lewis acid as catalyst gave mainly the cycloadduct of α,β-unsaturated aldehyde as a major product. Our aim in the present work is to put in evidence the role of the catalyst in the reversal of the chemoselectivity of the catalyzed Diels–Alder reactions of (2E)-but-2-enal and 2-Hexen-4-one with cyclopentadiene. The calculations were performed at the ωB97XD/6-311G(d,p) level of theory and the solvent effects of dichloromethane were taken into account using the PCM solvation model. The obtained results are in good agreement with experimental outcomes.
Experimentally (G. Mlostoń et al., J. Fluor. Chem. 190 (2016) 56-60), it has been found that the type of the obtained cycloadduct of the [3+2] cycloaddition (32CA) reaction of thiocarbonyl S-methanides with α,β-unsaturated ketones depends strongly on the location of the trifluoromethyl group. In the case of enones containing the CF3CH=CH moiety, the 32CA reaction occurs chemo-and regioselectively onto the C=C double bond giving trifluoromethylated tetrahydrothiophene derivatives. On the other hand, enones containing the CF3-C=O fragment react as carbonyl heteroethylenes leading to trifluoromethylated
Experimentally (G. Mlostoń et al., J. Fluor. Chem. 2016, 190, 56–60), it has been found that the type of the obtained cycloadduct of the [3+2] cycloaddition (32CA) reaction of thiocarbonyl S‐methanides with α,β‐unsaturated ketones depends strongly on the location of the trifluoromethyl group. In the case of enones containing the CF3CH=CH moiety, the 32CA reaction occurs chemo‐ and regioselectively onto the C=C double bond giving trifluoromethylated tetrahydrothiophene derivatives. On the other hand, enones containing the CF3−C=O fragment react as carbonyl heteroethylenes leading to trifluoromethylated 1,3‐oxathiolanes also in a chemo‐ and regioselective manner. Our aim in the present work is to perform a theoretical study of the all chemo‐, regio‐, and stereo‐isomeric reaction paths of these 32CA reactions within the Molecular Electron Density Theory. Activation Gibbs free energies, calculated at the B3LYP/6‐311G(d,p) level in tetrahydrofurane at −40 °C, show that the ortho/endo reaction path giving the trifluoromethylated tetrahydrothiophene is more favoured, while the meta/endo reaction path leading to trifluoromethylated 1,3‐oxathiolanes is more preferred in total agreement with experimental findings. The low activation barriers in combination of the Electron Localization Function topological analysis of the most relevant points along the Intrinsic Reaction Coordinate reveals the pseudomonoradical character of the studied 32CA reactions.
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