DFT Study on the Molecular Mechanism of the [4 + 2] Cycloaddition between Thiobenzophenone and Arylalkenes via Radical Cations

Inducing high exo selectivity in Diels-Alder reaction by dimethylborane substituent: a DFT study

In this work, the role of Lewis acid-base (LAB) interaction on the stereoselectivity of the Diels-Alder (DA) reaction has been studied by DFT in gas and solution (dichloromethane) phases. The calculations were performed at the B3LYP/6-311G++ (d, p) level. Two different series of DA reactions were investigated: (1)-three mono-substituted cyclopentadienes + dimethyl(vinyl)borane; (2)-five α,β-unsaturated carbonyl compounds + cyclopenta-2,4-dien-1-yldimethylborane. The reacting diene and dienophile pairs were chosen to restrict LAB interaction to the exo reaction pathway. It was found that in some of the examined cases, the favorable LAB interaction is so strong that it can lead to a completely exo-selective DA reaction. Furthermore, a novel multistep synthetic method was hypothesized for preparing exo cycloadduct with near 100% stereoselectivity. Our results can open up new avenues toward the rational design of exo-selective DA reactions for synthesizing novel bioorganic compounds.

Hetero-cycloreversions mediated by photoinduced electron transfer

Discovered more than eight decades ago, the Diels-Alder (DA) cycloaddition (CA) remains one of the most versatile tools in synthetic organic chemistry. Hetero-DA processes are powerful methods for the synthesis of densely functionalized six-membered heterocycles, ubiquitous substructures found in natural products and bioactive compounds. These reactions frequently employ azadienes and oxadienes, but only a few groups have reported DA processes with thiadienes. The electron transfer (ET) version of the DA reaction, though less investigated, has emerged as a subject of increasing interest. In the last two decades, researchers have paid closer attention to radical ionic hetero-cycloreversions, mainly in connection with their possible involvement in the repair of pyrimidine(6-4)pyrimidone photolesions in DNA by photolyases. In biological systems, these reactions likely occur through a reductive photosensitization mechanism. In addition, photooxidation can lead to cycloreversion (CR) reactions, and researchers can exploit this strategy for DNA repair therapies. In this Account, we discuss electron-transfer (ET) mediated hetero-CR reactions. We focus on the oxidative and reductive ET splitting of oxetanes, azetidines, and thietanes. Photoinduced electron transfer facilitates the splitting of a variety of four-membered heterocycles. In this context, researchers have commonly examined oxetanes, both experimentally and theoretically. Although a few studies have reported the cycloreversion of azetidines and thietanes carried out under electron transfer conditions, the number of examples remains limited. In general, the cleavage of the ionized four-membered rings appears to occur via a nonconcerted two-step mechanism. The trapping of the intermediate 1,4-radical ions and transient absorption spectroscopy data support this hypothesis, and it explains the observed loss of stereochemistry in the products. In the initial step, either C-C or C-X bond breaking may occur, and the preferred route depends on the substitution pattern of the ring, the type of heteroatom, and various experimental conditions. To better accommodate spin and charge, C-X cleavage happens more frequently, especially in the radical anionic version of the reaction. The addition or withdrawal of a single electron provides a new complementary synthetic strategy to activate hetero-cycloreversions. Despite its potential, this strategy remains largely unexplored. However, it offers a useful method to achieve C═X/olefin metathesis or, upon ring expansion, to construct six-membered heterocyclic rings.

Theoretical investigations toward the [4 + 2] cycloaddition of ketenes with N-benzoyldiazenes catalyzed by N-heterocyclic carbenes: mechanism and enantioselectivity

Density functional theory (DFT) calculations have been performed to provide the first detailed computational study on the mechanism and enantioselectivity for the [4 + 2] cycloaddition reaction of ketenes with N-benzoyldiazenes catalyzed by N-heterocyclic carbenes (NHCs). Two possible mechanisms have been studied: first is the "ketene-first" mechanism (mechanism A), and second is the novel "diazene-first" mechanism (mechanism B). The calculated results reveal that mechanism B is more favorable than mechanism A because it is not only of lower energy barrier but also more consistent with the provided general experimental procedure (Huang, X.-L.; He, L.; Shao, P.-L.; Ye, S. Angew. Chem., Int. Ed.2009, 48, 192-195). The enantioselectivity-determining step is demonstrated to present during the first process of cycloaddition, and the main product configuration is verified to agree with the experimental ee values very well. This study should be of some worth on forecasting how different substituent groups of catalysts and/or reactants affect the enantioselectivity of products. The obtained novel mechanistic insights should be valuable for not only rational design of more efficient NHC catalysts but also understanding the general reaction mechanism of [4 + 2] cycloaddition of ketenes.

Zwitterion formation in titan ice analogs: reaction between HC3N and NH3

A zwitterion is formed in the laboratory at low temperatures in the solid phase from the thermal reaction of HC(3)N and NH(3). We report for the first time its infrared spectrum. We study its reaction using Fourier transform infrared spectroscopy. Its reaction rate is estimated to be k(T) = 2.9 × 10(5) exp(-2.3 ± 0.1 (kJ mol(-1))/RT). Calculations using density functional theory (B3LYP/6-31g**) are used to characterize all the species (complexes, zwitterions, and transition states) and are in good agreement with the infrared spectra. The structure of the zwitterion is determined planar and it is characterized by a N-C bond around 1.5 Å.