A competition between carbo and hetero Diels–Alder reactions of E-2-phenyl-1-cyano-1-nitroethene to cyclopentadiene in the light of B3LYP/6-31G(d) computational study

Unexpected course of reaction between (E)-2-aryl-1-cyano-1-nitroethenes and diazafluorene: why is there no 1,3-dipolar cycloaddition?

ABSTRACT

Reactions between (E)-2-aryl-1-cyano-1-nitroethenes and diazafluorene lead to acyclic 2,3-diazabuta-1,3-diene derivatives, instead of the expected pyrazoline systems. DFT calculations suggest that this is a consequence of formation of zwitterionic structure in the first stage of the reaction. It must be noted that this is a specific property of the (E)-2-aryl-1-cyano-1-nitroethenes group, in contrast to most other conjugated nitroalkenes.

GRAPHICAL ABSTRACT

A DFT computational study of the molecular mechanism of [3 + 2] cycloaddition reactions between nitroethene and benzonitrile N-oxides

DFT calculations were performed to shed light on the molecular mechanism of [3 + 2] cycloadditions of simple conjugated nitroalkenes to benzonitrile N-oxides. In particular, it was found that these processes proceed by a one-step mechanism through asynchronous transition states. According to the latest terminology, they should be considered polar but not stepwise processes.

On the Brassard's rule of regioselectivity in Diels–Alder reactions between haloquinones and polar dienes

DFT/MC/AIM computational methods and experimental tests were used to further explain the Brassard’s rule for the [4 + 2] polar Diels–Alder reaction between nucleophilic dienes and haloquinones, which proceeds with remarkably high regioselectivity.

A polar nature of benzoic acids extrusion from nitroalkyl benzoates: DFT mechanistic study

Using DFT calculations at various theory levels, quantum-chemical simulations of decomposition paths were performed for a series of nitroalkyl benzoates. It was discovered, that these reactions proceed via polar, but one-step mechanism. It turned out that depending on the nature of the substituent in the ester molecule and on medium polarity, the studied reactions may take place via transition states with varied synchronicity—from E1-like structures, to E1cb-like structures. A purely ionic, two-stage mechanism was not identified in any of the cases.

Nitroacetylene as dipolarophile in [2 + 3] cycloaddition reactions with allenyl-type three-atom components: DFT computational study

Abstract

[2 + 3] Cycloaddition reactions of nitroacetylene with allenyl-type three-atom components take place according to the polar, but a one-step mechanism. Alternatively to cycloadducts, during the reaction between the aforementioned reagents, zwitterionic structures with “extended” conformation may be formally created. However, this route is supported by neither kinetic nor thermodynamic factors.

Graphical Abstract

Experimental and theoretical studies of Diels-Alder reaction between methyl (Z)-2-nitro-3-(4-nitrophenyl)-2-propenoate and cyclopentadiene

Abstract

The Diels–Alder reaction between methyl (Z)-2-nitro-3-(4-nitrophenyl)-2-propenoate and cyclopentadiene yields a mixture of carbodiene Diels–Alder adducts. B3LYP/6–31G(d) simulations indicate that the conversion of addends into methyl (1R*,2S*,3S*,4R*)-2-nitro-3-(4-nitrophenyl)-bicyclo[2.2.1]hept-5-ene-2-carboxylate occurs via two-stage heterodiene Diels–Alder reaction and (in a second step) skeleton rearrangement of the primary cycloadduct, whereas the reaction leading to methyl (1R*,2R*,3R*,4R*)-2-nitro-3-(4-nitrophenyl)-bicyclo[2.2.1]hept-5-ene-2-carboxylate is a single-step process.

Graphical abstract

Kinetics of the [4+2] cycloaddition of cyclopentadiene with (E)-2-aryl-1-cyano-1-nitroethenes

ABSTRACT

The electrophilicity of (E)-2-aryl-1-cyano-1-nitroethenes is not sufficient to induce a zwitterionic course of their [4+2] cycloaddition to cyclopentadiene. The one-step mechanism of these reactions is indicated by the activation parameters, and the substituent and solvent effects on the reaction.

GRAPHICAL ABSTRACT

A B3LYP/6-31G(d) study of Diels-Alder reactions between cyclopentadiene and (E)-2-arylnitroethenes

The B3LYP/6-31G(d) simulations of competing CDA and HDA reactions between cyclopentadiene and (E)-2-arylnitroethenes prove that regardless of the medium polarity, the processes leading to respective 5-nitro-6-aryl-bicyclo-[2,2,0]-hept-2-enes 3,4 (paths A and B) should be most favoured, and the more electrophilic (E)-2-(p-nitrophenyl)-nitroethene should be more reactive than the less electrophilic (E)-2-(p-methoxyphenyl)-nitroethene. Asymmetry of the transition complexes on the favoured pathways increases with increase of medium polarity, but not sufficiently to enforce the zwitterionic mechanism. Analysis of competing pathways leading to HDA adducts proves that not all these compounds can be formed directly from the adducts. In particular, on the path C, the initially formed 5-nitro-6-aryl-bicyclo-[2,2,0]-hept-2-enes 3 is converted to 2-phenyl-4-aza-5-oxy-bicyclo-[3,4,0]-nona-3,7-diene N-oxides 5 as a result of a [3.3]-sigmatropic shift. On the paths D–F leading to 2-phenyl-4-aza-5-oxy-bicyclo-[3,4,0]-nonadienes N-oxides 6–8, the reaction proceeds according to a one-step mechanism.