George E. Salnikov

The dynamic behavior of trifluoromethyl (CF₃) and tetrafluorophenyl (Ph) groups is studied in 1,2,4,5,7,8-hexafluoro-9-(trifluoromethyl)-9-(2,3,5,6- and 2,3,4,5-tetrafluorophenyl)fluorenes 1 and 2 by ¹⁹F NMR spectroscopy and quantum chemical calculations. At room temperature, the spectrum of fluorene 1 exhibits broadened signals of the CF₃ and Ph groups, which could be indicative of their synchronous rotation as meshed gears. However, the quantum chemical calculations do not confirm rigid synchrony, while accurate experimental exchange rate constants show that rotation occurs at similar but non-coincident rates. The calculation of the 2D energy surface along the rotation coordinates of both groups demonstrates that in the initial stage of rotation, the CF₃ and Ph fragments act as meshed gears, but later the engagement is lost. In fluorene 2, whose Ph group mimics a gear with a worn tooth, the CF₃ group rotation is substantially facilitated. It is established that the rotation rate is governed by steric interactions: unfavorable contacts appear for CF₃ in the transition state (raising the barrier), whereas for Ph they appear in the initial state (lowering the barrier).

The reaction of 1,2,4,5-tetrafluoro-3-(perfluoropropyl)benzene with 1,2,3,4-tetrafluorobenzene in an SbF₅ medium afforded a mixture of 1,2,3,4,5,7,8,9-octafluoro-9-(pentafluoroethyl)fluorene and 1,2,4,5,6,7,9-heptafluoro-9-(pentafluoroethyl)fluorene with a considerable predominance of the former. In a similar reaction with 1,2,4,5-tetrafluorobenzene at room temperature, 1,2,4,5,7,8,9-heptafluoro-9-(pentafluoroethyl)fluorene was formed in low yield, whereas conducting the reaction with an excess of the tetrafluorobenzene upon heating afforded 1,2,4,5,7,8-hexafluoro-9-(pentafluoroethyl)-9-(2,3,5,6-tetrafluorophenyl)fluorene in 52% yield. The hindered rotation of the tetrafluorophenyl group in this ethylphenylfluorene was studied.

N-Hydroxyphthalimide (NHPI) is widely used as a versatile N-oxyl catalyst. Herein, we disclose that this compound rearranges smoothly in superacids such as CF₃SO₃H (triflic acid, TfOH) or FSO₃H-SbF₅ with ring expansion to give 1H-benzo[d][1,2]oxazine-1,4(3H)-dione. However, when condensed with di-p-tolyl ether in TfOH or underwent ionic hydrogenation with cyclohexane in the presence of aluminum chloride, NHPI retains its skeleton, yielding 2-hydroxy-2',7'-dimethylspiro[isoindoline-1,9'-xanthen]-3-one and N-hydroxyisoindolinone, respectively. The catalytic properties of the obtained compounds were tested in the oxidation of fluorene to fluorenone with sodium chlorite, where the superior catalytic activity of N-hydroxyisoindolinone was demonstrated compared to that of both the starting NHPI and the resulting spiro derivative.

Методами спектроскопии ЯМР 19F и квантово-химических расчетов исследовано динамическое поведение трифторметильной (CF3) и тетрафторфенильной (Ph) групп в 1,2,4,5,7,8‐гексафтор‐9‐(трифторметил)-9‐(2,3,5,6‐ и 2,3,4,5-тетрафторфенил) флуоренах 1 и 2. При комнатной температуре в спектре флуорена 1 наблюдаются уширенные сигналы групп CF3 и Ph, которые могли бы свидетельствовать об их синхронном вращении как сцепленных шестерен. Однако, квантово-химические расчеты не подтверждают жесткой синхронности, а точные экспериментальные значения констант скоростей обмена атомов фтора показывают, что обмен происходит с близкими, но не совпадающими скоростями. Расчет двумерной энергетической поверхности по координатам поворота обеих групп демонстрирует, что на начальном этапе вращения фрагменты CF3 и Ph действуют как сцепленные шестерни, но в дальнейшем зацепление срывается. В флуорене 2, группа Ph которого имитирует шестерню со сточенным зубом, вращение CF3 существенно облегчается. Установлено, что скорость вращения определяется стерическими взаимодействиями: для CF3 неблагоприятные контакты проявляются в переходном состоянии (повышая барьер), а для Ph – в исходном состоянии (понижая барьер).

Phthalimide and N-phenylphthalimide smoothly condense with di-p-tolyl ether in triflic acid (CF₃SO₃H, TfOH) to obtain the corresponding spiro[isoindoline-1,9'-xanthen]-3-ones. Structural analogs of phthalimide, such as phthalic anhydride and 1,3-indandione (but not saccharin), show similar reactivity. In contrast, N-(tetrafluoropyridin-4-yl)phthalimide reacts with DTE by an alternative pathway, yielding isobenzofuran dispiro derivative. The mechanistic aspects of these reactions are discussed on the basis of in situ NMR and theoretical (DFT) studies, providing insights on the key intermediacy of O,O-diprotonated forms of the starting compounds.

The [M₄−Hal]⁻ (M=the title compound; Hal=Cl, Br, and I) complexes were isolated in the form of salts of [Et₄N]⁺ cation and characterized by XRD, NMR, UV‐Vis, DFT, QTAIM, EDD, and EDA. Their stoichiometry is caused by a cooperative interplay of σ‐hole‐driven chalcogen (ChB) and hydrogen (HB) bondings. In the crystal, [M₄−Hal]⁻ are connected by the π‐hole‐driven ChB; overall, each [Hal]⁻ is six‐coordinated. In the ChB, the electrostatic interaction dominates over orbital and dispersion interactions. In UV‐Vis spectra of the M+[Hal]⁻ solutions, ChB‐typical and [Hal]⁻‐dependent charge‐transfer bands are present; they reflect orbital interactions and allow identification of the individual [Hal]⁻. However, the structural situation in the solutions is not entirely clear. Particularly, the UV‐Vis spectra of the solutions are different from the solid‐state spectra of the [Et₄N]⁺[M₄−Hal]⁻; very tentatively, species in the solutions are assigned [M−Hal]⁻. It is supposed that the formation of the [M₄−Hal]⁻ proceeds during the crystallization of the [Et₄N]⁺[M₄−Hal]⁻. Overall, M can be considered as a chromogenic receptor and prototype sensor of [Hal]⁻. The findings are also useful for crystal engineering and supramolecular chemistry.

Noncatalytic halogenation of adamantane with Br ₂ or ICl proceeds according to the cluster mechanism featuring high kinetic order in halogen and a sharp decrease in DFT energy barrier when additional halogen molecules are involved in the system.

Density functional theory studies have confirmed that N,C-diprotonation of α,β-unsaturated nitriles, and likewise O,C-diprotonation of isoelectronic ketones, provide the best rationale for their electrophilic reactions with weak nucleophiles, such as benzene.

The influence of fluorination on the acid‐base properties and the capacity of structurally related 6–5 bicyclic compounds – 1,3‐benzodiazole 1, 1,2,3‐benzotriazole 2 and 2,1,3‐benzoselenadiazole 3 to σ‐hole interactions, i. e. hydrogen (1 and 2) and chalcogen (3) bondings, is studied experimentally and computationally. The tetrafluorination increases the Brønsted acidity of the diazole and triazole scaffolds and the Lewis acidity of selenadiazole scaffold decreases the basicity. Increased Brønsted acidity facilitates anion binding via the formation of hydrogen bonds; particularly, tetrafluorinated derivative of 1 (compound 4) binds Cl⁻. Increased Lewis acidity of tetrafluorinated derivative of 3 (compound 10), however, is not enough for binding with Cl⁻ and F⁻ via chalcogen bonds in contrast to previously studied Te analog of 10. It is suggested that the maximum positive values of molecular electrostatic potential at the σ‐holes, V_S,max, can be a reasonable metric for design and synthesis of new anion receptors with selenadiazole‐diazole/triazole hybrids as a special target. Related chlorinated compounds are also discussed.

The mechanism of the noncatalytic bromination of carboranes was studied experimentally and theoretically. We found that the reactions of o- and m-carboranes 1 and 2 with elemental bromine are first order in the substrate but unusually high (approximately fifth) order in bromine. The calculated energy barriers of these reactions decrease sharply as more bromine molecules are added to the quantum-chemical system. A considerable primary deuterium kinetic isotope effect for the bromination of 2 indicates that the rate-limiting stage is B–H bond breakage. According to quantum-chemical reaction path calculations, the bond breakage proceeds after the intrusion of a bromine atom into the B–H σ-bond. The 9-Br and 9-OH substituents in carborane 1 strongly retard the bromination of the corresponding derivatives. The bromination mechanism of 9-OH-1 is complex and includes neutral, deprotonated, and protonated forms of the carborane. The high experimental kinetic reaction order in bromine, together with quantum chemical modeling, points to a specific mechanism of bromination facilitated by anionic bromine clusters which significantly stabilize the transition state.