Design and Synthesis of Kekulè and Non-Kekulè Diradicaloids via the Radical Periannulation Strategy: The Power of Seven Clar's Sextets.

This work introduces an approach to uncoupling electrons via maximum utilization of localized aromatic units, i.e., the Clar's π-sextets. To illustrate the utility of this concept to the design of Kekulé diradicaloids, we have synthesized a tridecacyclic polyaromatic system where a gain of five Clar's sextets in the open-shell form overcomes electron pairing and leads to the emergence of a high degree of diradical character. According to unrestricted symmetry-broken UCAM-B3LYP calculations, the singlet diradical character in this core system is characterized by the y0 value of 0.98 (y0 = 0 for a closed-shell molecule, y0 = 1 for pure diradical). The efficiency of the new design strategy was evaluated by comparing the Kekulé system with an isomeric non-Kekulé diradical of identical size, i.e., a system where the radical centers cannot couple via resonance. The calculated singlet-triplet gap, i.e., the ΔEST values, in both of these systems approaches zero: -0.3 kcal/mol for the Kekulé and +0.2 kcal/mol for the non-Kekulé diradicaloids. The target isomeric Kekulé and non-Kekulé systems were assembled using a sequence of radical periannulations, cross-coupling, and C-H activation. The diradicals are kinetically stabilized by six tert-butyl substituents and (triisopropylsilyl)acetylene groups. Both molecules are NMR-inactive but electron paramagnetic resonance (EPR)-active at room temperature. Cyclic voltammetry revealed quasi-reversible oxidation and reduction processes, consistent with the presence of two nearly degenerate partially occupied molecular orbitals. The experimentally measured ΔEST value of -0.14 kcal/mol confirms that K is, indeed, a nearly perfect singlet diradical.

Oxidative Photocyclization of Aromatic Schiff Bases in Synthesis of Phenanthridines and Other Aza-PAHs.

The oxidative photocyclization of aromatic Schiff bases was investigated as a potential method for synthesis of phenanthridine derivatives, biologically active compounds with medical applications. Although it is possible to prepare the desired phenanthridines using such an approach, the reaction has to be performed in the presence of acid and TEMPO to increase reaction rate and yield. The reaction kinetics was studied on a series of substituted imines covering the range from electron-withdrawing to electron-donating substituents. It was found that imines with electron-withdrawing substituents react one order of magnitude faster than imines bearing electron-donating groups. The 1H NMR monitoring of the reaction course showed that a significant part of the Z isomer in the reaction is transformed into E isomer which is more prone to photocyclization. The portion of the Z isomer transformed showed a linear correlation to the Hammett substituent constants. The reaction scope was expanded towards synthesis of larger aromatic systems, namely to the synthesis of strained aromatic systems, e.g., helicenes. In this respect, it was found that the scope of oxidative photocyclization of aromatic imines is limited to the formation of no more than five ortho-fused aromatic rings.

Negative Charge as a Lens for Concentrating Antiaromaticity: Using a Pentagonal "Defect" and Helicene Strain for Cyclizations.

Incorporation of a five-membered ring into a helicene framework disrupts aromatic conjugation and provides a site for selective deprotonation. The deprotonation creates an anionic cyclopentadienyl unit, switches on conjugation, leads to a >200 nm red-shift in the absorbance spectrum and injects a charge into a helical conjugated π-system without injecting a spin. Structural consequences of deprotonation were revealed via analysis of a monoanionic helicene co-crystallized with {K+ (18-crown-6)(THF)} and {Cs+ 2 (18-crown-6)3 }. UV/Vis-monitoring of these systems shows a time-dependent formation of mono- and dianionic species, and the latter was isolated and crystallographically characterized. The ability of the twisted helicene frame to delocalize the negative charge was probed as a perturbation of aromaticity using NICS scans. Relief of strain, avoidance of antiaromaticity, and increase in charge delocalization assist in the additional dehydrogenative ring closures that yield a new planarized decacyclic dianion.

Upconversion of Reductants.

The many applications of photon upconversion-conversion of low-energy photons into high-energy photons-raises the question of the possibility of "electron upconversion". In this Review, we illustrate how the reduction potential can be increased by using the free energy of exergonic chemical reactions. Electron (reductant) upconversion can produce up to 20-25 kcal mol-1 of additional redox potential, thus creating powerful reductants under mild conditions. We will present the two common types of electron-upconverting systems-dissociative (based on unimolecular fragmentations) and associative (based on the bimolecular formation of three-electron bonds). The possible utility of reductant upconversion encompasses redox chain reactions in electrocatalytic processes, photoredox cascades, design of peroxide-based medicines, firefly luminescence, and reductive repair of DNA photodamage.

Radical Alkyne peri-Annulation Reactions for the Synthesis of Functionalized Phenalenes, Benzanthrenes, and Olympicene.

Radical cyclization reactions at a peri position were used for the synthesis of polyaromatic compounds. Depending on the choice of reaction conditions and substrate, this flexible approach led to Bu3 Sn-substituted phenalene, benzanthrene, and olympicene derivatives. Subsequent reactions with electrophiles provided synthetic access to previously inaccessible functionalized polyaromatic compounds.