Diels–Alder Cycloaddition with CO, CO2, SO2, or N2 Extrusion: A Powerful Tool for Material Chemistry

Phenyl, naphthyl, polyarylphenyl, coronene, and other aromatic and polyaromatic moieties primarily influence the final materials’ properties. One of the synthetic tools used to implement (hetero)aromatic moieties into final structures is Diels–Alder cycloaddition (DAC), typically combined with Scholl dehydrocondensation. Substituted 2-pyranones, 1,1-dioxothiophenes, and, especially, 1,3-cyclopentadienones are valuable substrates for [4 + 2] cycloaddition, leading to multisubstituted derivatives of benzene, naphthalene, and other aromatics. Cycloadditions of dienes can be carried out with extrusion of carbon dioxide, carbon oxide, or sulphur dioxide. When pyranones, dioxothiophenes, or cyclopentadienones and DA cycloaddition are aided with acetylenes including masked ones, conjugated or isolated diynes, or polyynes and arynes, aromatic systems are obtained. This review covers the development and the current state of knowledge regarding thermal DA cycloaddition of dienes mentioned above and dienophiles leading to (hetero)aromatics via CO, CO2, or SO2 extrusion. Particular attention was paid to the role that introduced aromatic moieties play in designing molecular structures with expected properties. Undoubtedly, the DAC variants described in this review, combined with other modern synthetic tools, constitute a convenient and efficient way of obtaining functionalized nanomaterials, continually showing the potential to impact materials sciences and new technologies in the nearest future.

4-Amino-2-butyl-7-methoxycarbonylthiazolo[4,5-c]quinoline

4-Amino-imidazo-, oxazolo-, and thiazoloquinolines are key structural scaffolds in the design of nucleoside base analogs for use as therapeutic agents. Current strategies for arriving at diverse substitutions at the C6–C9 positions of the thiazolo- and oxazoloquinolines, however, are limited due to difficulties in arriving at the thiazoloquinoline-5N-oxide intermediate using electron deficient aromatic systems. Here, we demonstrate a synthetic route to obtain substituted thiazoloquinolines with electron-withdrawing groups at the C7 position. The target compound, 4-amino-2-butyl-7-methoxycarbonylthiazolo[4,5-c]quinoline, is obtained in eight steps using a 7-bromo surrogate as a precursor to the successful generation of the N-oxide intermediate, and final transformation via Pd-mediated C7-acylation.

On the Application of EELS for Investigating Nanostructure of Soot From Different Fuels

Soot is characterized by a multiscale structural organization and Transmission Electron Microscope (TEM) is the only diagnostic tool giving access to it. However, being a diffraction-based technique, TEM images only aromatic systems and thus, it is particularly useful to combine it with electron energy-loss spectroscopy (EELS), able to provide quantitative information about the relative abundance of sp3 and sp2 hybridized carbon. In this paper a method for the EELS spectrum analysis of carbonaceous materials recently developed for electron-irradiated graphite and glassy carbon composition analysis has been applied for the first time on soot samples, in order to test its performance in soot nanostructure study in combination with TEM and High Resolution TEM (HRTEM). Soot samples here analysed were collected in the soot inception region of premixed flames of different hydrocarbon fuels. EELS, in agreement with TEM and HRTEM, showed a quite disordered and heterogeneous structure for young soot, without any significant distinction between soot formed from methane and ethylene fuels.

Nickel-catalyzed oxidative fluoroalkylation of aryl halides via paired electrolysis

The paired electrolysis triggers multiple oxidative and reductive processes to occur simultaneously, which ensures the steady transformation of the intermediates to the desired coupling products. However, fluoroalkyl radicals have not been harnessed for metal-catalyzed cross-coupling with aryl halides under electrochemical conditions. This work describes a general strategy for rapid access to various fluoromethyl and difluoromethyl aromatics by paired electrolysis. The contradiction between anodic oxidation of fluoroalkyl sulfinates and cathodic reduction of low-valent nickel catalysts can be well addressed under mild cell conditions, allowing for direct introduction of fluorinated functionalities into aromatic systems.

Applicability of Baird’s Rule for In-Plane Aromaticity

The concept of aromaticity is essential for chemists to understand the nature of many chemical substances. “In-plane” aromaticity is a unique type of aromaticity found in some π-conjugated systems consisting of radially oriented p-orbitals. Although in-plane aromaticity has been theoretically predicted about 40 years ago, its intrinsic properties are much less well understood than those of conventional Hückel aromatic systems with perpendicularly oriented p-orbitals because of lack of available examples of in-plane aromatic molecules. Here we demonstrate, from both theoretical and experimental aspects, that in-plane aromaticity of cycloparaphenylenes is reversed upon excitation to the lowest triplet state. In-plane non-aromatic neutral forms with a 4N-electron system and in-plane aromatic dications with a 4N+2-electron system turn out to be aromatic and anti-aromatic, respectively, indicating that Baird’s rule holds for the in-plane aromatic system. Armchair carbon nanotube fragments based on the cycloparaphenylene were also found to follow Baird’s rule from a magnetic viewpoint.<br>

Applicability of Baird’s Rule for In-Plane Aromaticity

The concept of aromaticity is essential for chemists to understand the nature of many chemical substances. “In-plane” aromaticity is a unique type of aromaticity found in some π-conjugated systems consisting of radially oriented p-orbitals. Although in-plane aromaticity has been theoretically predicted about 40 years ago, its intrinsic properties are much less well understood than those of conventional Hückel aromatic systems with perpendicularly oriented p-orbitals because of lack of available examples of in-plane aromatic molecules. Here we demonstrate, from both theoretical and experimental aspects, that in-plane aromaticity of cycloparaphenylenes is reversed upon excitation to the lowest triplet state. In-plane non-aromatic neutral forms with a 4N-electron system and in-plane aromatic dications with a 4N+2-electron system turn out to be aromatic and anti-aromatic, respectively, indicating that Baird’s rule holds for the in-plane aromatic system. Armchair carbon nanotube fragments based on the cycloparaphenylene were also found to follow Baird’s rule from a magnetic viewpoint.<br>

Nickel-Catalyzed Ipso-Borylation of Silyloxyarenes via C–O Bond Activation

A mild method to convert silyloxyarenes to pinacol boronic esters using nickel catalysis is described. In contrast to other borylation protocols of relatively inert C–O bonds, the method is competent in activating the carbon-oxygen bond of silyloxyarenes in isolated aromatic systems lacking a directing group. The catalytic functionalization of benzyl silyl ethers was also achieved under these conditions. Orthogonal reactivity of silyloxyarenes vs other electrophiles was demonstrated in the case of several cross-coupling reactions wherein the unaffected silyloxarene could be functionalized subsequently.

Nickel-Catalyzed Ipso-Borylation of Silyloxyarenes via C–O Bond Activation

A mild method to convert silyloxyarenes to pinacol boronic esters using nickel catalysis is described. In contrast to other borylation protocols of relatively inert C–O bonds, the method is competent in activating the carbon-oxygen bond of silyloxyarenes in isolated aromatic systems lacking a directing group. The catalytic functionalization of benzyl silyl ethers was also achieved under these conditions. Orthogonal reactivity of silyloxyarenes vs other electrophiles was demonstrated in the case of several cross-coupling reactions wherein the unaffected silyloxarene could be functionalized subsequently.