Understanding the uniqueness of the stepwise [4 + 1] cycloaddition reaction between conjugated nitroalkenes and electrophilic carbene systems with a molecular electron density theory perspective

Abstract

The [4 + 1] cycloaddition (41CA) and [2 + 1] cycloaddition (21CA) reactions of the α-halogeno-nitroalkenes with carbene have been studied using the molecular electron density theory through density functional theory calculations at the MPWB1K(PCM)/ 6-311G(d,p) computational level. The 41CA reaction take place through a polar twostep mechanism involving the formation of a zwitterionic intermediate. The competitive 21CA reaction path proceeds according to nonconcerted two-stage one-step. Bonding evolution theory analysis of 41CA reaction allows the distinguishing of four groups along the reaction path. The reaction begins with rupture of the C C double bond of the alkene moiety and formation of the pseudoradical center. Formation of the O C and C C new single bonds begins in subsequent phases. In the last stage of the 41CA reaction path, we observed the formation of an N C new double bond. In the case of 21CA reaction path, we observed the successive rupture of the C C double bond of the alkene moiety, formation of C C new single bond and pseudoradical center, and creation of a second C C single bond.

A DFT Study on the Molecular Mechanism of Additions of Electrophilic and Nucleophilic Carbenes to Non-Enolizable Cycloaliphatic Thioketones

“…In the first case (route A), the formation of the thiirane ring occurs via stepwise mechanism, with the intervention of thiocarbonyl ylide as a reactive intermediate [40]. Alternatively (route B), the thiirane ring can be formed according to the one-step [2 + 1] cycloaddition scheme [26,41].…”

“…Within this TS, a single new ϭ bond is formed between C(1) and S(2) occurs via stepwise mechanism, with the intervention of thiocarbonyl ylide as a reactive intermediate [40]. Alternatively (route B), the thiirane ring can be formed according to the one-step [2 + 1] cycloaddition scheme [26,41]. The computational wb97xd/6-311g(d,p)(PCM) study was performed based on the model reaction of dichlorocarbene (1a) with 2,2,4,4-tetramethylcyclobutane-1,3-dithione (2a).…”

Understanding the molecular mechanism of γ‐elimination of nitrous acid in the framework of the molecular electron density theory

“…The ELF localization domains were represented by using the Paraview software at an isovalue of 0.75 a.u 40,41 . A similar approach has been successfully used to explain the mechanism of different types of reactions 23–25,42–45 …”

“…In the older literature, for thermal elimination reactions, the “pericyclic,” one‐step mechanism was proposed. The latest discoveries in the field of organic mechanisms show, however, that stepwise mechanisms with an ionic intermediate were detected regarding the reactions which in academic literature were treated as “pericyclic”, such as [3 + 2] cycloadditons, 18–22 Diels–Alder reactions, 23,24 [4 + 1] cycloadditions 25 . Next, Domingo generally undermines the “pericyclic” approach regarding the most of one‐step reactions 26,27 .…”

On the Question of Stepwise [4+2] Cycloaddition Reactions and Their Stereochemical Aspects

“…Currently, in addition to conjugated 1,3-dienes, conjugated nitroalkenes [5][6][7] are tested quite frequently as hetero-analogues of the dienes. The role of hetero-analogues of ethene as components of the cycloaddition reaction includes molecular segments containing nitrogen [8,9], oxygen [10,11], sulphur [12,13], selenium [14], and others.…”