APEX Strategy Represented by Diels-Alder Cycloadditions-New Opportunities for the Syntheses of Functionalised PAHs

Intramolecular Diels-Alder Reaction of a Biphenyl Group in a Strained meta-Quaterphenylene Acetylene

At elevated temperatures, a strained, cyclic meta-quaterphenylene acetylene undergoes an intramolecular cyclization reaction to form benz[e]indeno[1,2,3-hi]acephenanthrylene. This reaction represents an example of a Diels-Alder reaction at the 2-, 1-, 1'-, and 2'-positions of a biphenyl derivative, a region analogous to the bay regions of perylene and other periacenes. The reaction proceeds cleanly with high conversion. Kinetics studies of a methylated derivative reveal that the ΔG‡ for the reaction is ∼40-41 kcal/mol, and computational models predict a similar value of Grel for the transition state of a concerted [4 + 2]-cycloaddition.

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.

Heteroatom Cycloaddition at the (BN)2 Bay Region of Dibenzoperylene

Cycloaddition-dehydration involving a BNBN-butadiene analogue at the bay region of a dibenzoperylene and a non-enolizable aldehyde provides a novel strategy for incorporation of the oxadiazadiborinane (B₂ N₂ CO) ring into the scaffold of a polycyclic aromatic hydrocarbon resulting in highly emissive compounds.

Diels-Alder Cycloaddition to the Bay Region of Perylene and Its Derivatives as an Attractive Strategy for PAH Core Expansion: Theoretical and Practical Aspects

PAHs (polycyclic aromatics hydrocarbons), the compound group that contains perylene and its derivatives, including functionalized ones, have attracted a great deal of interest in many fields of science and modern technology. This review presents all of the research devoted to modifications of PAHs that are realized via the Diels–Alder (DA) cycloaddition of various dienophiles to the bay regions of PAHs, leading to the π-extension of the starting molecule. This type of annulative π-extension (APEX) strategy has emerged as a powerful and efficient synthetic method for the construction of polycyclic aromatic hydrocarbons and their functionalized derivatives, nanographenes, and π-extended fused heteroarenes. Then, [4 + 2] cycloadditions of ethylenic dienophiles, -N=N-, i.e., diazo-dienophiles and acetylenic dienophiles, are presented. This subject is discussed from the organic synthesis point of view but supported by theoretical calculations. The possible applications of DA cycloaddition to PAH bay regions in various science and technology areas, and the prospects for the development of this synthetic method, are also discussed.