Mingsheng Tang

DFT Study on the Mechanisms and Diastereoselectivities of Lewis Acid-Promoted Ketene–Alkene [2 + 2] Cycloadditions: What is the Role of Lewis Acid in the Ketene and C = X (X = O, CH2, and NH) [2 + 2] Cycloaddition Reactions?

The detailed mechanisms and diastereoselectivities of Lewis acid-promoted ketene-alkene [2 + 2] cycloaddition reactions have been studied by density functional theory (DFT). Four possible reaction channels, including two noncatalyzed diastereomeric reaction channels (channels A and B) and two Lewis acid (LA) ethylaluminum dichloride (EtAlCl2) catalyzed diastereomeric reaction channels (channels C and D), have been investigated in this work. The calculated results indicate that channel A (associated with product R-configurational cycloputanone) is more energy favorable than channel B (associated with the other product S-configurational cyclobutanone) under noncatalyzed condition, but channel D leading to S-configurational cyclobutanone is more energy-favorable than channel C, leading to R-configurational cycloputanone under a LA-promoted condition, which is consistent with the experimental results. And Lewis acid can make the energy barrier of ketene-alkene [2 + 2] cycloaddition much lower. In order to explore the role of LA in ketene and C = X (X = O, CH2, and NH) [2 + 2] cycloadditions, we have tracked and compared the interaction modes of frontier molecular orbitals (FMOs) along the intrinsic reaction coordinate (IRC) under the two different conditions. Besides by reducing the energy gap between the FMOs of the reactants, our computational results demonstrate that Lewis acid lowers the energy barrier of the ketene and C = X [2 + 2] cycloadditions by changing the overlap modes of the FMOs, which is remarkably different from the traditional FMO theory. Furthermore, analysis of global reactivity indexes has also been performed to explain the role of LA catalyst in the ketene-alkene [2 + 2] cycloaddition reaction.

Theoretical Study on the Reaction Mechanism between 6-Benzyl-6-azabicyclo[2.2.1]hept-2-ene and Benzoyl Isocyanate to Urea and Isourea

Reaction mechanisms of the 6-benzyl-6-azabicyclo[2.2.1]hept-2-ene with benzoyl isocyanate have been investigated using density functional theory (DFT) at the B3LYP/6-31G(d,p) level of theory. The reaction proceeding along six competitive channels includes two categories. That is, two channels are formally [3,3]-sigmatropic rearrangements and four channels are [4+2] cycloadditions. For urea, the formally [3,3]-sigmatropic rearrangement channel and the [4+2] cycloaddition channels are competitive since they have similar barriers. However, the [4+2] cycloaddition channels are energetically favorable pathways to lead to isourea, with the highest barrier of 12.77 kcal/mol. These polar Diels-Alder (P-DA) reactions are controlled by the charge transfer (CT) at the transition states. Moreover, the main products of this reaction include urea and isourea. Furthermore, difference of two new bond lengths at transition states indicate that the [4+2] cycloadditions in this reaction are asynchronous processes, which is in good agreement with the experiment.

Theoretical Investigations toward the Asymmetric Insertion Reaction of Diazoester with Aldehyde Catalyzed by N-Protonated Chiral Oxazaborolidine: Mechanisms and Stereoselectivity

In recent years, the N-protonated chiral oxazaborolidine has been utilized as the Lewis acid catalyst for the asymmetric insertion reaction, which is one of the most challenging topics in current organic chemistry. Nevertheless, the reaction mechanism, stereoselectivity, and regioselectivity of this novel insertion reaction are still unsettled to date. In this present work, the density functional theory (DFT) investigation has been performed to interrogate the mechanisms and stereoselectivities of the formal C-C/H insertion reaction between benzaldehyde and methyl α-benzyl diazoester catalyzed by the N-protonated chiral oxazaborolidine. For the reaction channel to produce the R-configured C-C insertion product as the predominant isomer, the catalytic cycle can be characterized by four steps: (i) the complexation of the aldehyde with catalyst, (ii) addition of the other reactant methyl α-benzyl diazoester, (iii) the removal of nitrogen concerted with the migration of phenyl group or hydrogen, and (iv) the dissociation of catalyst from the products. Our computational results show that the carbon-carbon bond formation step is the stereoselectivity determining step, and the reaction pathways associated with [1, 2]-phenyl group migration occur preferentially to those pathways associated with [1, 2]-hydrogen migration. The pathway leading to the R-configured product is the most favorable pathway among the possible stereoselective pathways. All these calculated outcomes align well with the experimental observations. The novel mechanistic insights should be valuable for understanding this kind of reaction.