A Ferrocenyl Kaleidoscope: Slow Interconversion of Six Diastereo-atropisomers of 2,6-Di-tert-butyl-9,10-diferrocenyltriptycene

Exploring the Acid-Catalyzed Reactions of 10,11-Epoxy-Dibenzo[a,d]cycloheptan-5-ol as the Synthetic Modules toward Polycyclic Aromatic Scaffolds

The structural diversity of polycyclic aromatic hydrocarbons (PAHs) offers exciting opportunities for their applications. Yet, selective synthesis of such conjugated networks poses a formidable challenge. Compared to the prominence of transition-metal-catalyzed cross-coupling and oxidative Scholl reactions, cationic rearrangement in the synthesis of polycyclic aromatic hydrocarbon is an underexplored subject. In this study, we reveal that cationic intermediate generated from epoxy dibenzocycloheptanol can be transformed into acenes, azulene-embedded PAHs, and dibenzocycloheptanone derivatives. Reactive patterns, including Meinwald rearrangement, Nazarov cyclization, transannular aryl migration, and transannular Friedel-Crafts cyclization were identified. Both substrate structures and reaction temperature affect the reaction pathways in predictable and manageable manners. A mechanistic scheme was postulated as the working model to guide the reactivity for further application. Substrates containing heterocyclic and ferrocenyl groups exhibit similar reactivity profiles. The inquiry culminates in the selective synthesis of 5, 7, 12, 14-tetrasubstituted C _2h and C _2v pentacene derivatives. Our results demonstrate that polycyclic aromatic hydrocarbons can be selectively prepared with this cation-initiated strategy by methodically tuning the reactivity.

Dynamics of a Molecular Rotor Exhibiting Local Directional Rotational Preference within Each Enantiomer

Directional internal rotation in molecular systems, generally controlled by chirality, is known to occur in natural and artificial systems driven by light or fueled chemically, but spontaneous directional molecular rotation is believed to be forbidden. We have designed a molecular rotor, whereby ferrocene and triptycene linked by a methylene bridge provide two rotational degrees of freedom. On the basis of experimental observations, in conjunction with computational data, we show that the two different modes of rotation are strongly coupled and the spatial orientation of the bistable ferrocene moiety controls the barrier to its own rotation about the triptycene axis. It is proposed that the barrier to clockwise 120° rotation across each individual triptycene blade is lower in the M-enantiomer and for counterclockwise 120° rotation, it is lower in its P-counterpart. These findings demonstrate the possibility of locally preferred thermal directional intramolecular rotation for each dynamically interconverting enantiomer.

Correlated motion and mechanical gearing in amphidynamic crystalline molecular machines

In this review we highlight the recent efforts towards the development of molecular gears with an emphasis on building molecular gears in the solid state and the role that molecular gearing and correlated motions may play in the function of crystalline molecular machines. We discuss current molecular and crystal engineering strategies, challenges associated with engineering correlated motion in crystals, and outline experimental and theoretical tools to explore gearing dynamics while highlighting key advances made to date.

Palladium-Catalysed Coupling Reactions En Route to Molecular Machines: Sterically Hindered Indenyl and Ferrocenyl Anthracenes and Triptycenes, and Biindenyls

Pd-catalysed Stille and Suzuki cross-couplings were used to prepare 9-(3-indenyl)-, 6, and 9-(2-indenyl)-anthracene, 7; addition of benzyne led to the 9-Indenyl-triptycenes, 8 and 9. In 6, [4 + 2] addition also occurred to the indenyl substituent. Reaction of 6 through 9 with Cr(CO)₆ or Re₂(CO)₁₀ gave their M(CO)₃ derivatives, where the Cr or Re was complexed to a six- or five-membered ring, respectively. In the 9-(2-indenyl)triptycene complexes, slowed rotation of the paddlewheel on the NMR time-scale was apparent in the η⁵-Re(CO)₃ case and, when the η⁶-Cr(CO)₃ was deprotonated, the resulting haptotropic shift of the metal tripod onto the five-membered ring also blocked paddlewheel rotation, thus functioning as an organometallic molecular brake. Suzuki coupling of ferrocenylboronic acid to mono- or dibromoanthracene yielded the ferrocenyl anthracenes en route to the corresponding triptycenes in which stepwise hindered rotations of the ferrocenyl groups behaved like molecular dials. CuCl₂-mediated coupling of methyl- and phenyl-indenes yielded their rac and meso 2,2′-biindenyls; surprisingly, however, the apparently sterically crowded rac 2,2′-Bis(9-triptycyl)biindenyl functioned as a freely rotating set of molecular gears. The predicted high rotation barrier in 9-phenylanthracene was experimentally validated via the Pd-catalysed syntheses of di(3-fluorophenyl)anthracene and 9-(1-naphthyl)-10-phenylanthracene.

Ferrocenyl-Pyrenes, Ferrocenyl-9,10-Phenanthrenediones, and Ferrocenyl-9,10-Dimethoxyphenanthrenes: Charge-Transfer Studies and SWCNT Functionalization

The synthesis of 1-Fc- (3), 1-Br-6-Fc- (5 a), 2-Br-7-Fc- (7 a), 1,6-Fc₂ - (5 b), 2,7-Fc₂ -pyrene (7 b), 3,6-Fc₂ -9,10-phenanthrenedione (10), and 3,6-Fc₂ -9,10-dimethoxyphenanthrene (12; Fc=Fe(η⁵ -C₅ H₄ )(η⁵ -C₅ H₅ )) is discussed. Of these compounds, 10 and 12 form 1D or 2D coordination polymers in the solid state. (Spectro)Electrochemical studies confirmed reversible Fc/Fc⁺ redox events between -130 and 160 mV. 1,6- and 2,7-Substitution in 5 a (E°'=-130 mV) and 7 a (E°'=50 mV) influences the redox potentials, whereas the ones of 5 b and 7 b (E°'=20 mV) are independent. Compounds 5 b, 7 b, 10, and 12 show single Fc oxidation processes with redox splittings between 70 and 100 mV. UV/Vis/NIR spectroelectrochemistry confirmed a weak electron transfer between Fe^II /Fe^III in mixed-valent [5 b]⁺ and [12]⁺ . DFT calculations showed that 5 b non-covalently interacts with the single-walled carbon nanotube (SWCNT) sidewalls as proven by, for example, disentangling experiments. In addition, CV studies of the as-obtained dispersions confirmed exohedral attachment of 5 b at the SWCNTs.

Nonconjugated Hydrocarbons as Rigid-Linear Motifs: Isosteres for Material Sciences and Bioorganic and Medicinal Chemistry

Nonconjugated hydrocarbons, like bicyclo[1.1.1]pentane, bicyclo[2.2.2]octane, triptycene, and cubane are a unique class of rigid linkers. Due to their similarity in size and shape they are useful mimics of classic benzene moieties in drugs, so-called bioisosteres. Moreover, they also fulfill an important role in material sciences as linear linkers, in order to arrange various functionalities in a defined spatial manner. In this Review article, recent developments and usages of these special, rectilinear systems are discussed. Furthermore, we focus on covalently linked, nonconjugated linear arrangements and discuss the physical and chemical properties and differences of individual linkers, as well as their application in material and medicinal sciences.

Mechanisms and Beyond: Elucidation of Fluxional Dynamics by Exchange NMR Spectroscopy

Detailed mechanistic information is crucial to our understanding of reaction pathways and selectivity. Dynamic exchange NMR techniques, in particular 2D exchange spectroscopy (EXSY) and its modifications, provide indispensable intricate information on the mechanisms of organic and inorganic reactions and other phenomena, for example, the dynamics of interfacial processes. In this Review, key results from exchange NMR studies of small molecules over the last few decades are systemised and discussed. After a brief introduction to the theory, the key types of dynamic processes are identified and fundamental examples given of intra- and intermolecular reactions, which, in turn, could involve, or not, bond-making and bond-breaking events. Following that logic, internal molecular rotation, intramolecular stereomutation and molecular recognition will first be considered because they do not typically involve bond breaking. Then, rearrangements, substitution-type reactions, cyclisations, additions and other processes affecting chemical bonds will be discussed. Finally, interfacial molecular dynamics and unexpected combinations of different types of fluxional processes will also be highlighted. How exchange NMR spectroscopy helps to identify conformational changes, coordination and molecular recognition processes as well as quantify reaction energy barriers and extract detailed mechanistic information by using reaction rate theory in conjunction with computational techniques will be shown.

Ferrocene-containing non-interlocked molecular machines

Ferrocene is chemically robust and readily functionalized which enables its facile incorporation into more complex molecular systems. This coupled with ferrocene's reversible redox properties and ability to function as a “molecular ball bearing” has led to the use of ferrocene as a component in wide range of non-interlocked synthetic molecular machine systems.