Two-dimensionally extended, polycyclic heteroaromatic molecules (heterocyclic nanographenes) are a highly versatile class of organic materials, applicable as functional chromophores and organic semiconductors. In this Review, we discuss the rich chemistry of large heteroaromatics, focusing on their synthesis, electronic properties, and applications in materials science. This Review summarizes the historical development and current state of the art in this rapidly expanding field of research, which has become one of the key exploration areas of modern heterocyclic chemistry.

Heterocyclic Nanographenes and Other Polycyclic Heteroaromatic Compounds: Synthetic Routes, Properties, and Applications

A C60H30 polycyclic aromatic hydrocarbon (PAH) that incorporates all 60 carbon atoms and 75 of the 90 carbon-carbon bonds required to form the fullerene C60has been synthesized in nine steps by conventional laboratory methods. Laser irradiation of this C60H30 PAH at 337 nanometers induces hydrogen loss and the formation of C60, as detected by mass spectrometry. A specifically labeled [13C3]C60H30 retains all three 13C atoms during the cage formation process. A structurally related C48H24 PAH that lacks the three peripheral benzene rings cannot be transformed into C60, whereas the next higher homolog, a C80H40 PAH, degrades to the C60H30 PAH, which then loses hydrogen to give [60]fullerene. These control experiments verify that the C60 is formed by a molecular transformation directly from the C60H30 PAH and not by fragmentation and recombination in the gas phase.

https://science.sciencemag.org/content/sci/294/5543/828.full.pdf

Groundwork for a Rational Synthesis of C60: Cyclodehydrogenation of a C60H30 Polyarene

The combination of neutral non-aromatic and zwitterionic aromatic contributing valence bond structures confers a distinctive chemical reactivity to quinone methides, which has attracted the interest of a tremendous number of chemist and biochemists. This chapter reviews reactions that generate quinone methides, and the results of mechanistic studies of the breakdown of quinone methides in nucleophilic substitution reactions. The following pathways for the formation of quinone methides are discussed; (a) photochemical reactions; (b) thermal heterolytic bond cleavage; (c) reactions that unmask a quinone oxygen; (d) nucleophilic aromatic substitution reactions of water at 4-methoxybenzyl carbocations; (e) oxidation reactions of phenols; (f) reductive-elimination reactions of quinones; (g) miscellaneous reactions. Results, and their interpretation, of studies from the laboratories of A. J. Kresge and J. P. Richard are reviewed. The highlights include: (a) Experiments to characterize the effect of O-protonation and O-methylation of the p-quinone methide oxygen to form the p-hydroxybenzyl and pmethoxybenzyl carbocations, respectively, on electrophile stability and reactivity. (b) A comparison of p-quinone methide, o-quinone methide and o-thioquinone methide. (c) Evidence that structure-reactivity relationships for addition of nucleophiles to quinone methides closely resemble those determined for nucleophile addition to strongly resonance stabilized carbocations. (d) Evidence that the aromatic zwitterionic valence bond structure makes only a relatively small contribution to the structure of p-quinone methide. (e) A comparison of the Marcus intrinsic barriers for addition of water to p-quinone methide and to formaldehyde. (f) A discussion of the effect of O-methylation of p-quinone methide and formaldehyde on the intrinsic barrier for water addition.

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The Generation and Reactions of Quinone Methides

Irradiation of 3-hydroxy-2-naphthalenemethanol (3a) and 2-hydroxy-1-naphthalenemethanol (4a) results in efficient (Φ254 = 0.17 and 0.20) dehydration and the formation of isomeric naphthoquinone methides, 2,3-naphthoquinone-3-methide (1) and 1,2-naphthoquinone-1-methide (2), respectively. In aqueous solution, naphthoquinone methides 1 and 2 undergo rapid hydration to regenerate starting materials (τH2O (1) = 7.4 ms and τH2O (2) = 4.5 ms at 25 °C). The hydration reaction is strongly catalyzed by the hydroxide ion but shows acid catalysis only at pH < 1. Reactive intermediates 1 and 2 can be intercepted by other nucleophiles, such as the azide ion (kN3(1) = 2.0 × 104 M−1 s−1 and kN3(2) = 3.0 × 104 M−1 s−1) or thiol (kSH(1) = 2.2 × 105 M−1 s−1 and kSH(2) = 3.3 × 105 M−1 s−1). Ethyl vinyl ether readily reacts with 1 and 2 (kDA(1) = 4.1 × 104 M−1 s−1 and kDA(2) = 6.0 × 104 M−1 s−1) to produce Diels−Alder adducts in excellent yield. o-Naphthoquinone methides 1 and 2 were also generated by photolysis of 3-ethoxym...

Photochemical generation and the reactivity of o-naphthoquinone methides in aqueous solutions.

Novel Syntheses of Fluorenones via Nitrile-Directed Palladium-Catalyzed C–H and Dual C–H Bond Activation

On the mechanism of the directed ortho and remote metalation reactions of N,N-dialkylbiphenyl 2-carboxamides.

Enaminones have in the past decade received renewed interest in organic synthesis by acting as highly versatile building blocks. Particularly, the featured amino and carbonyl group in the enaminone structure endow them with the ability of tolerating water via hydrogen bonding effect. Thus, designing synthetic methods in aqueous medium with enaminones as the main building blocks has received extensive attention and notable success. Based on our longstanding efforts and interests in enaminone chemistry, we review herein the research advances in enaminone participated organic synthesis in aqueous medium. 

Recent Advances in Reactions using Enaminone in Water or Aqueous Medium

The photosolvolysis of 1- and 2-hydroxy-9-fluorenols 4-6 has been studied in aqueous solution. All of these 9-fluorenols photosolvolyze efficiently in 1:1 H 2O-CH3OH, to give the corresponding methyl ether products in high chemical and quantum yields. Whereas the photosolvolysis of the parent 9-fluorenol (2 ,R=H ) isknown to proceed via the very short-lived and formally ground-state antiaromatic 9-fluorenyl cation ( 1, R = H), the photosolvolysis of 1- hydroxy-9-fluorenol (4) proceeds via a much longer-lived (≈5-10 s) fluorenyl quinone methide 9, which is trappable by ethyl vinyl ether via a (4+2) cycloaddition reaction to give a chroman derivative. Interestingly, 2-hydroxy-9- fluorenol (5) photosolvolyzes via a very short-lived intermediate with similar lifetimes as observed for the 9-fluorenyl cation (1, R = H), although a corresponding fluorenyl quinone methide intermediate is accessible for this compound. This study demonstrates that the mechanism of photosolvolysis of these types of compounds can be dramatically altered when an aryl hydroxy group is present.

Contrasting behaviour in the photosolvolysis of 1- and 2-hydroxy-9-fluorenols in aqueous solution

Abbreviated Synthesis of Benzo[c]phenanthrene, Benz[a]anthracene and their Respective 5-Methyl Derivatives via Combined Metalation-Palladium-Catalyzed Cross Coupling Strategy

The Dess-Martin periodinane (DMP) reagent-mediated reactions of tertiary enaminones with potassium thiocyanate for the synthesis of thiazole-5-carbaldehydes are developed. The product formation involves cascade hydroxyl thiocyanation of the C═C double bond, intramolecular hydroamination of the C≡N bond, and thiazole annulation by condensation on the ketone carbonyl site, representing novel reaction pathways in the reactions between enaminones and thiocyanate salt. DMP plays dual roles in mediating the free radical thiocyanation and inducing the unconventional selective thiazole-5-carbaldehyde formation by masking the in situ generated formyl group during the reaction process.

Bao-ping Zhao

Papers

On the mechanism of the directed ortho and remote metalation reactions of N,N-dialkylbiphenyl 2-carboxamides.

Recent Advances in Reactions using Enaminone in Water or Aqueous Medium

Contrasting behaviour in the photosolvolysis of 1- and 2-hydroxy-9-fluorenols in aqueous solution

Abbreviated Synthesis of Benzo[c]phenanthrene, Benz[a]anthracene and their Respective 5-Methyl Derivatives via Combined Metalation-Palladium-Catalyzed Cross Coupling Strategy

Thiazole-5-carbaldehyde Synthesis by Cascade Annulation of Enaminones and KSCN with Dess-Martin Periodinane Reagent.