Rapid, Microwave-Assisted Perdeuteration of Polycyclic Aromatic Hydrocarbons

Homolytic Versus Heterolytic Bond Breaking in Functionalized [R-C20 H10 ]+ Systems

The comprehensive theoretical investigation of stability of functionalized corannulene cations [R-C₂₀ H₁₀ ]⁺ with respect to two alternative bond-breaking mechanisms, namely, homolytic or radical ([R-C₂₀ H₁₀ ]⁺  → R^•  + C₂₀ H₁₀ ^+• ) and heterolytic or cationic ([R-C₂₀ H₁₀ ]⁺  → R⁺  + C₂₀ H₁₀ ), was accomplished. The special focus was on the influence of the nature of R-group on the energetics of the bond cleavage. Detailed study of energetics of both mechanisms has revealed that the systems with small alkyl groups such as methyl tend to undergo bond breaking in accordance with homolytic mechanism. Subsequent elongation of the chain of the R-group resulted in shifting the paradigm, making heterolytic path more energetically favorable. Subsequent analysis of different components of the bonding between R-group and corannulene polyaromatic core helped to shed light on trends observed. In both mechanisms, the covalent contribution was found to be dominating, whereas ionic part contributes ~25-27%. Two leading components of ΔE_orb , C₂₀ H₁₀ → R and R → C₂₀ H₁₀ , were identified with NOCV-EDA approach. While the homolytic pathway is best described as R → C₂₀ H₁₀ process, the heterolytic mechanism shows domination of the C₂₀ H₁₀ → R term. Surprisingly, the preparation energy (ΔE_prep ) was identified as a key player in stability tendencies found. In other words, the relative stability of corresponding molecular fragments (here R-groups as the corannulene fragment remains the same for all systems) in their cationic or radical forms determine the preference given to a specific bond breaking path and, as consequence, the total stability of target functionalized cations. These conclusions were further confirmed by extending a set of R-groups to conjugated (allyl, phenyl), bulky (iPr, tBu), β-silyl (CH₂ SiH₃ , CH₂ SiMe₃ ), and benzyl (CH₂ Ph) groups. © 2019 Wiley Periodicals, Inc.

Concyclic CH-π arrays for single-axis rotations of a bowl in a tube

The hydrogen bond is undoubtedly one of the most important non-covalent interactions. Among the several types of the hydrogen bonds, the CH-π interaction is a relatively new notion that is being recognised in chemistry and biology. Although the CH-π hydrogen bond and conventional hydrogen bonds share common features such as directionality, this weak interaction has played a secondary role in molecular recognition. In this study, we have devised a host-guest complex that is assembled solely by the CH-π hydrogen bonds. Multivalent interactions of a bowl-shaped hydrocarbon with its peripheral hydrogen atoms are made possible via CH-π hydrogen bonds by adopting a tubular hydrocarbon as a host for their enthalpy-driven complexation. Concyclic arrays of weak hydrogen bonds further allow dynamic rotational motions of the guest in the host. Solid-state analysis with crystallographic and spectroscopic methods reveal a single-axis rotation of the bowl in the tube.

Stability of functionalized corannulene cations [R-C20 H10 ](+) : An influence of the nature of R-Group

The first comprehensive theoretical study of stability of hub-functionalized corannulene cations [R-C20 H10 ](+) as the function of the nature of R-group was accomplished. The initial set of linear alkyl R-group of different length (R=(CH2 )n CH3 , n = 0-9) was augmented by groups which form stable organic cations, such as tert-butyl, isopropyl, allyl, and phenyl. Investigation of relative stability (with bonding energy as the measure) was accompanied by detailed study of changes in aromaticity using a large set of descriptors, as well as by the evaluation of energetics of possible migration of R-group from the hub-site to the spoke-position. Decrease in stability of functionalized corannulene cations with lengthening of R-group and/or replacing it with branched alkyl group was found to be the general trend. At the same time, π-conjugated groups such as allyl or phenyl ones, stabilize the system. All methods/approaches applied unambiguously indicated that the actual stability of the hub-functionalized corannulene cations is indeed a multi faceted phenomenon. Important contributions come from different interplay between attractive (ΔEorb vs. ΔEelstat ) and repulsive (ΔEPauli ) components of the bonding, from changes in aromatic behavior of rings in polyaromatic fragment, and from activation barrier for the process of migration of R-group. © 2016 Wiley Periodicals, Inc.

Aromatic stabilization of functionalized corannulene cations

The first comprehensive theoretical investigation of aromaticity in functionalized corannulene cations of general formula [CH3-C20H10](+) was accomplished. The experimentally known system [CH3-hub-C20H10](+) was augmented by two other possible isomers, namely, rim- and spoke-ones. Changes in aromaticity, when going from neutral corannulene to its functionalized cations, were monitored with the help of descriptors of different nature such as structure-based HOMA, topological PDI and FLU, and magnetic NICS. A highly efficient tool for analysis and visualization of delocalization and conjugation named ACID was also utilized. In the final step, a complete set of (1)H and (13)C chemical shifts was calculated and compared with the available experimental data. Conservation of aromaticity of 6-membered rings along with vanishing anti-aromatic character of central 5-membered rings was found to be the main reason for the exceptional stability of the hub-isomer. At the same time, functionalization of the corannulene moiety at the rim- or spoke-site resulted in dramatic elimination of aromaticity of 6-membered rings, whereas anti-aromatic character of the central ring remained. Altogether, it led to much lower stability of these isomers in comparison with that of the hub-one.

Functionalized corannulene carbocations: a structural overview

A detailed structural overview of a family of bowl-shaped polycyclic aromatic carbocations of the type [C20 H10 R](+) with different R functionalities tethered to the interior surface of corannulene (C20 H10 ) is provided. Changing the identity of the surface-bound groups through alkyl chains spanning from one to four carbon atoms and incorporating a different degree of halogenation has led to the fine tuning of the bowl structures and properties. The deformation of the corannulene core upon functionalization has been revealed based on X-ray crystallographic analysis and compared for the series of cations with R=CH3 , CH2 Cl, CHCl2 , CCl3 , CH2 CH3 , CH2 CH2 Cl, and CH2 CH2 Br. The resulting carbocations have been isolated with several metal-based counterions, varying in size and coordinating abilities ([AlCl4 ](-) , [AlBr4 ](-) , [(SnCl)(GaCl4 )2 ](-) , and [Al(OC(CF3 )3 )4 ](-) ). A variety of aggregation patterns in the solid state has been revealed based on different intermolecular interactions ranging from cation-anion to π-π stacking and to halogen⋅⋅⋅π interactions. For the [C20 H10 CH2 Cl](+) ion crystallized with several different counterions, the conformation of the R group attached to the central five-membered ring of corannulene moiety was found to depend on the solid-state environment defined by the identity of anions. Solution NMR and UV/Vis investigations have been used to complement the X-ray diffraction studies for this series of corannulene-based cations and to demonstrate their different association patterns with the solvent molecules.

Efficient synthesis of new asymmetric tripodal ligands using microwave irradiation, and their crystal structures

An efficient method for synthesis of asymmetric tripodal ligands was described by using microwave irradiation.

Arenium acid catalyzed deuteration of aromatic hydrocarbons

The arenium acid [mesitylene-H](+) has been shown to be an extraordinarily active H/D exchange catalyst for the perdeuteration of polycyclic aromatic hydrocarbons. The reactions take place under ambient conditions in C6D6 as an inexpensive deuterium source. High isolated yields and excellent degrees of deuterium incorporation were achieved using the substrates p-terphenyl, fluoranthene, pyrene, triphenylene, and corannulene.