A molecular electron density theory study to understand the interplay of theory and experiment in nitrone-enone cycloaddition

Synthesis of New Isoxazolidine Derivatives Utilizing the Functionality of N-Carbonylpyrazol-Linked Isoxazolidines

Using Ni(II) as the catalyst, electron-deficient 3,5-dimethylacryloylpyrazole olefin was reacted with C,N-diarylnitrones alone for 10 min to prepare novel five-member heterocyclic products, 4-3,5-dimethylacryloylpyrazole isoxazolidines with 100% regioselectivity and up to 99% yield. And then, taking these cycloadducts as substrates, six kinds of derivatization reactions, like ring-opening, nucleophilic substitution, addition-elimination and reduction, were studied. Experimental results showed that all kinds of transformations could obtain the target products at a high conversion rate under mild conditions, a finding which provided the basic methods for organic synthesis methodology research based on an isoxazolidine skeleton.

Theoretical Insight into the Reversal of Chemoselectivity in Diels-Alder Reactions of α,β-Unsaturated Aldehydes and Ketones Catalyzed by Brønsted and Lewis Acids

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.

A molecular electron density theory study for [3+2] cycloaddition reactions of N-benzylcyclohexylnitrone with methyl-3-butenoate

The [3+2] cycloaddition (32CA) reactions of N-benzylcyclohexylnitrone 1 with methyl-3-butenoate 2 have been studied within molecular electron density theory (MEDT) at the B3LYP/6-311++G(d,p) level of theory.

Unravelling the regio- and stereoselective synthesis of bicyclic N,O-nucleoside analogues within the molecular electron density theory perspective

The [3 + 2] cycloaddition (32CA) reactions of 1-pyrroline-1-oxide with N-vinyl nucleobases leading to bicyclic N,O nucleoside analogues have been studied within the molecular electron density theory (MEDT) at the MPWB1K/6-311G(d,p) computational level. These non-polar zwitterionic type 32CA reactions take place through a one-step mechanism with minimal global electron density transfer (GEDT) at the TSs and the exo/ortho approach mode as the energetically favoured reaction path. The 32CA reactions of N-vinyl nucleobases with thymine and cytosine substituents respectively show the activation enthalpies of 15.2 and 12.5 kcal mol⁻¹ in toluene. The reactions are irreversible due to strong exothermic character of − 35.4–− 26.4 kcal mol⁻¹ in toluene. The bonding evolution theory (BET) study suggests that these 32CA reactions take place through the coupling of pseudoradical centres with earlier C–C bond formation and the formation of new C–C and C–O covalent bonds has not been started in the TSs. Non-covalent interactions (NCI) are predicted at the TSs from the visualization of NCI gradient isosurfaces.