Proton Grease: An Acid Accelerated Molecular Rotor

Enantioconvergent synthesis of axially chiral amides enabled by Pd-catalyzed dynamic kinetic asymmetric aminocarbonylation

Atropisomeric biaryls bearing carbonyl groups have attracted increasing attention due to their prevalence in diverse bioactive molecules and crucial role as efficient organo-catalysts or ligands in asymmetric transformations. However, their preparation often involves tedious multiple steps, and the direct synthesis via asymmetric carbonylation has scarcely been investigated. Herein, we report an efficient palladium-catalyzed enantioconvergent aminocarbonylation of racemic heterobiaryl triflates with amines via dynamic kinetic asymmetric transformation (DyKAT). This protocol features a broad substrate scope and excellent compatibility for rapid construction of axially chiral amides in good to high yields with excellent enantioselectivities. Detailed mechanistic investigations discover that the base can impede the intramolecular hydrogen bond-assisted axis rotation of the products, thus allowing for the success to achieve high enantioselectivity. Moreover, the achieved axially chiral heterobiaryl amides can be directly utilized as N,N,N-pincer ligands in copper-catalyzed enantioselective formation of C(sp3)-N and C(sp3)-P bonds.

Structural Chemistry of C-N Axially Chiral Compounds

In the last several years, atropisomers owing to the rotational restriction around a C-N single bond (C-N axially chiral compounds) have attracted significant attention in the field of synthetic organic chemistry. In particular, the highly enantioselective synthesis of various C-N axially chiral compounds and their application to asymmetric reactions have been reported by many groups. On the other hand, studies on the structural chemistry of C-N axially chiral compounds have attracted scant attention in comparison with synthetic studies. For over 25 years, our group has explored asymmetric synthesis of C-N axially chiral compounds and their synthetic application. In the course of these synthetic studies, we found several notable structural properties in relation to the C-N bond rotation and an association of enantiomers (the relationship between the rotational stability and the structure or electronic effect, the chirality-dependent halogen bond, and the self-disproportionation of enantiomers). Furthermore, on the basis of these structural properties, the development of acid-mediated molecular rotors and the synthesis of isotopic atropisomers possessing high stereochemical purity and rotational stability were achieved. Through this Perspective, I wish to make the chemistry community aware that C-N axially chiral compounds are attractive molecules from the viewpoints of both synthetic organic chemistry and structural chemistry.

Rotational Behavior about the N3-Pyridyl Bond in 3-(Pyridin-2-yl)quinazolin-4-one and 4-Thione Derivatives

The rotational barriers about the N3-(2-pyridyl) bond in 2-iso-propyl-3-(pyridin-2-yl)quinazolin-4-one and the thione analogue were evaluated though VT-NMR measurement of a diastereotopic iso-propyl group followed by a line-shape simulation. In 3-(pyridin-2-yl)quinazoline-4-thione bearing a chiral center as the C2 substituent, the formation of dynamic diastereomers was detected by NMR. The rotational pathway about the N3-(2-pyridyl) bond and the stereochemistries of dynamic diastereomers were revealed through a computational study.

Supramolecular Control of the Temperature Responsiveness of Fluorescent Macrocyclic Molecular Rotamers

To fully harness the potential of molecular machines, it is crucial to develop methods by which to exert control over their speed of motion through the application of external stimuli. A conformationally strained macrocyclic fluorescent rotamer, CarROT, displays a reproducible and linear fluorescence decrease towards temperature over the physiological temperature range. Through the external addition of anions, cations or through deprotonation, the compound can access four discreet rotational speeds via supramolecular interactions (very slow, slow, fast and very fast) which in turn stop, reduce or enhance the thermoluminescent properties due to increasing or decreasing non-radiative decay processes, thereby providing a means to externally control the temperature sensitivity of the system. Through comparison with analogues with a higher degree of conformational freedom, the high thermosensitivity of CarROT over the physiological temperature range was determined to be due to conformational strain, which causes a high energy barrier to rotation over this range. Analogues with a higher degree of conformational freedom display lower sensitivities towards temperature over the same temperature range. This study provides an example of an information rich small molecule, in which programable rotational speed states can be observed with facile read-out.

Catalytic Atroposelective Synthesis of Axially Chiral Heterobiaryl Oxime Ethers via the One-Step Dynamic Kinetic Condensation Reaction

The catalytic atroposelective synthesis of axially chiral heterobiaryls was first developed through the direct one-step dynamic kinetic condensation reaction with the simple transformation of the C═O bond to the C═N bond, delivering a series of novel axially chiral heterobiaryl oxime ethers.

The Effect of Intramolecular Hydrogen Bonds on the Rotational Barriers of the Biaryl C-C Axis

Axially chiral compounds are attracting more attention recently. Although hydrogen bonds are reported as a vital weak force that influences the properties of compounds, the effect of intramolecular hydrogen bonds on the atropisomerization of the Caryl -Caryl single bonds has not yet been well quantitatively investigated. Here, a series of axially chiral biaryl compounds were synthesized to study the effect of hydrogen bonds on the rotational barriers of the biaryl C-C axis. Experimental studies demonstrated that the rotational barrier of hydrogen bonding biaryl 9 was significantly lower (46.7 kJ mol-1 ) than biaryl 10 without hydrogen bonds. Furthermore, theoretical studies revealed that the intramolecular hydrogen bond stabilized the transition state (TS) of tri-ortho-substituted biaryl 9, relieving the steric repulsion in the TS. We believe that this study will provide chemists with a deeper understanding of the atropisomerization process of axially chiral biaryl compounds.

Chiral Transient Ligand Enabled Enantioselective Synthesis of Atropisomers Decorated with Unactivated Olefins via a Palladium-Catalyzed C-H Olefination

Herein, atroposelective synthesis of axially chiral biaryls with unactivated olefins by a palladium-catalyzed C-H olefination using a chiral transient directing group strategy has been disclosed. This protocol is well compatible with a variety of biaryl-2-aldehydes as well as various olefins such as allyl sulfonamides and allyl sulfones to provide the atroposelective olefinated products in synthetically useful yields with excellent enantioselectivities up to >99% ee. In addition, a wide number of axially chiral biaryl alcohols were synthesized by the simple diversification of the products in excellent enantioselectivity.

Stereodynamic Strategies to Induce and Enrich Chirality of Atropisomers at a Late Stage

Enantiomers, where chirality arises from restricted rotation around a single bond, are atropisomers. Due to the unique nature of the origins of their chirality, synthetic strategies to access these compounds in an enantioselective manner differ from those used to prepare enantioenriched compounds containing point chirality arising from an unsymmetrically substituted carbon center. In particular stereodynamic transformations, such as dynamic kinetic resolutions, thermodynamic dynamic resolutions, and deracemizations, which rely on the ability to racemize or interconvert enantiomers, are a promising set of transformations to prepare optically pure compounds in the late stage of a synthetic sequence. Translation of these synthetic approaches from compounds with point chirality to atropisomers requires an expanded toolbox for epimerization/racemization and provides an opportunity to develop a new conceptual framework for the enantioselective synthesis of these compounds.

C-N Axially Chiral Heterobiaryl Isoquinolinone Skeletons Construction via Cobalt-Catalyzed Atroposelective C-H Activation/Annulation

Herein, the atroposelective construction of isoquinolinones bearing a C-N chiral axis has been successfully developed via a Co-catalyzed C-H bond activation and annulation process. This conversion can be effectively carried out in an environmentally friendly oxygen atmosphere to generate the target C-N axially chiral frameworks with excellent reactivities and enantioselectivities (up to >99% ee) in the absence of any additives. Additionally, the current protocol has proved to be an alternative approach for the C-N axial architectures fabrication under electrochemical conditions for cobalt/Salox catalysis, and this strategy allowed the efficient and atom-economical synthesis of various axially chiral isoquinolinones under mild reaction conditions.

On the Ability of the N-O Bond to Support a Stable Stereogenic Axis: Peptide-Catalyzed Atroposelective N-Oxidation

During studies of atroposelective, peptide-catalyzed N-oxidations of pyridines, we observed lower-than-expected barriers to atropisomerization for these stereodynamic processes under the reaction conditions. Mechanistic studies indicate a hydrogen bond-assisted racemization mechanism intrinsic to both the starting materials and products. We also identified a protonation-dependent barrier to rotation that operates for the starting materials alone. Nonetheless, several substrates were amenable to atroposelective N-oxidations via kinetic resolution, yielding krel values of up to 12.6 and the isolation of one N-oxide with >99:1 er after recrystallization.

C-N Axially Chiral Heterobiaryl Skeletons Construction via Cobalt-Catalyzed Atroposelective Annulation

Herein, the atroposelective construction of five-six heterobiaryl skeleton-based C-N chiral axis has been successfully accomplished via a Co-catalyzed C-H bond activation and annulation process, in which the isonitrile was employed as the C1 source and the 8-aminoquinoline moiety served as both directing group and integral part of C-N atropisomers, respectively. This conversion can be effectively carried out in an environmentally friendly oxygen atmosphere, generating the target axial heterobiaryls with excellent reactivities and enantioselectivities (up to >99% ee) in the absence of any additives, and the obtained 3-iminoisoindolinone products with a five membered N-heterocycle exhibit high atropostability. Additionally, the C-N axially chiral monophosphine backbones derived from this protocol possess the potential to become an alternative ligand platform.

Main Group Molecular Switches with Swivel Bifurcated to Trifurcated Hydrogen Bond Mode of Action

Artificial molecular machines have captured the full attention of the scientific community since Jean-Pierre Sauvage, Fraser Stoddart, and Ben Feringa were awarded the 2016 Nobel Prize in Chemistry. The past and current developments in molecular machinery (rotaxanes, rotors, and switches) primarily rely on organic-based compounds as molecular building blocks for their assembly and future development. In contrast, the main group chemical space has not been traditionally part of the molecular machine domain. The oxidation states and valency ranges within the p-block provide a tremendous wealth of structures with various chemical properties. Such chemical diversity─when implemented in molecular machines─could become a transformative force in the field. Within this context, we have rationally designed a series of NH-bridged acyclic dimeric cyclodiphosphazane species, [(μ-NH){PE(μ-N^tBu)₂PE(NH^tBu)}₂] (E = O and S), bis-P^V₂N₂, displaying bimodal bifurcated R²₁(8) and trifurcated R³₁(8,8) hydrogen bonding motifs. The reported species reversibly switch their topological arrangement in the presence and absence of anions. Our results underscore these species as versatile building blocks for molecular machines and switches, as well as supramolecular chemistry and crystal engineering based on cyclophosphazane frameworks.

Cation controlled rotation in anionic pillar[5]arenes and its application for fluorescence switch

Controlling molecular motion is one of hot topics in the field of chemistry. Molecular rotors have wide applications in building nanomachines and functional materials, due to their controllable rotations. Hence, the development of novel rotor systems, controlled by external stimuli, is desirable. Pillar[n]arenes, a class of macrocycles, have a unique planar chirality, in which two stable conformational isomers pR and pS would interconvert by oxygen-through-the-annulus rotations of their hydroquinone rings. We observe the differential kinetic traits of planar chirality transformation in sodium carboxylate pillar[5]arene (WP5-Na) and ammonium carboxylate pillar[5]arene (WP5-NH₄), which inspire us to construct a promising rotary platform in anionic pillar[5]arenes (WP5) skeletons. Herein, we demonstrate the non-negligible effect of counter cations on rotational barriers of hydroquinone rings in WP5, which enables a cation grease/brake rotor system. Applications of this tunable rotor system as fluorescence switch and anti-counterfeiting ink are further explored.

Chemically Driven Rotatory Molecular Machines

Molecular machines are at the frontier of biology and chemistry. The ability to control molecular motion and emulating the movement of biological systems are major steps towards the development of responsive and adaptive materials. Amazing progress has been seen for the design of molecular machines including light-induced unidirectional rotation of overcrowded alkenes. However, the feasibility of inducing unidirectional rotation about a single bond as a result of chemical conversion has been a challenging task. In this Review, an overview of approaches towards the design, synthesis, and dynamic properties of different classes of atropisomers which can undergo controlled switching or rotation under the influence of a chemical stimulus is presented. They are categorized as molecular switches, rotors, motors, and autonomous motors according to their type of response. Furthermore, we provide a future perspective and challenges focusing on building sophisticated molecular machines.

Rotational Behavior of N-(5-Substituted-pyrimidin-2-yl)anilines: Relayed Electronic Effect in Two N-Ar Bond Rotations

N-Methyl-2-methoxymethylanilines 1 bearing various 5-substituted-pyrimidin-2-yl groups were prepared, and their rotational behaviors were explored in detail. It was revealed that the rotational barriers around two N-Ar bonds increase in proportion to the electron-withdrawing ability of substituents X at the 5-position.

A Rotation-Inversion Dual-Motion Molecular Switch: Race for NMR Signaling

The interplay between the thermal helical inversion (THI) of the stiff-stilbene moiety and the rotation of the dimethylamino (DMA) group in 1 results in a dependence of the DMA NMR signals on the THI kinetics in (E)-1 but the rotation kinetics in (Z)-1, because the faster motion mode is responsible. Consequently, the photochemical switching from (E)-1 to (Z)-1 illustrates the phenomenon of "switchable motion detection" by the same set of NMR signals in a dual-motion molecular system.

Atropselective synthesis of N-aryl pyridones via dynamic kinetic resolution enabled by non-covalent interactions

The dynamic kinetic resolution of C-N atropisomeric pyridones was achieved via asymmetric phase-transfer catalysis, exploiting a rotational barrier-lowering hydrogen bond in the starting materials. X-ray and NMR experiments revealed the presence of a barrier-raising ground state CH⋯π interaction in the product, supported by DFT calculations. A co-crystal of the quinidine-derived phase-transfer catalyst and substrate reveals key substrate-catalyst non-covalent interactions.

Dynamics of the alkyne → copper(i) interaction and its use in a heteroleptic four-component catalytic rotor

The dynamics of alkyne → copper(i) interactions has been determined and used to self-assemble a fast nanorotor, which underwent a self-catalyzed click transformation to a triazole rotor, an interesting process for the production of biohybrid devices.

Electrostatically-gated molecular rotors

The ability to control molecular-scale motion using electrostatic interactions was demonstrated using an N-phenylsuccinimide molecular rotor with an electrostatic pyridyl-gate. Protonation of the pyridal-gate forms stabilizing electrostatic interactions in the...

The crystal structure of 4-chloro-2-(quinolin-8-yl)isoindoline-1,3-dione, C17H9ClN2O2

C17H9ClN2O2, monoclinic, P21/c (no. 14), a = 13.6888(4) Å, b = 8.3775(2) Å, c = 12.9180(3) Å, β = 110.515(3)°, V = 1387.46(7) Å3, Z = 4, R gt (F) = 0.0420, wR ref (F 2) = 0.1224, T = 293(2) K.

Complex molecular logic gates from simple molecules

Molecular logic gates (MLGs) are compounds that can solve Boolean logic operations to give an answer (OUTPUT) upon receiving a stimulus (INPUT). These derivatives can be used as biological sensors and are promising substitutes for the present logic gates. Although MLGs with complex molecular structures have been reported, they often show stability problems. To address this problem, we describe herein six stable pseudo-hemiindigo-derived MLGs capable of solving complex logic operations. MLGs 7, 8, 9, and 10 can solve a complex logic operation connecting 4 logic gates using 2 different wavelengths (445 nm and 400 nm) and the presence of p-TsOH and triethylamine (TEA) as inputs; MLG 11 solves a complex logic operation connecting 3 logic gates and uses 3 inputs, one wavelength of 445 nm and the presence of p-TsOH and TEA; and MLG 12 can only solve one logic operation (INH) and uses only the presence of p-TsOH and TEA as an input. Each operating method of the MLGs was evaluated with several techniques; proton interactions with MLGs were screened with NMR by titrating with p-TsOH, the photochemical properties were examined with absorption ultraviolet-visible (UV-Vis) spectroscopy, and the isomerization dynamics were examined with NMR using the two wavelengths for isomerization (photostationary isomer). The results indicate that the pseudo-hemiindigo-derived MLGs described herein can be applied as multiplexers or data selectors that are necessary for the transient flow of information for biological and computer systems. Finally, to design different MLGs and a system that can treat more information as complex logic gates (demultiplexers), two and three MLGs were mixed in different experiments. In both cases, four inputs were employed (445 nm, 400 nm, p-TsOH and TEA), yielding more outputs. Detailed information about the system dynamics was obtained from NMR experiments.

Recent Advances in Catalytic Asymmetric Construction of Atropisomers

Atropisomerism is a stereochemical behavior portrayed by three-dimensional molecules that bear rotationally restricted σ bond. Akin to the well-represented point-chiral molecules, atropisomerically chiral compounds are finding increasing utilities in many disciplines where molecular asymmetry is influential. This provides steady demand on atroposelective synthesis, where numerous synthetic pursuits have been rewarded with conceptually novel and streamlined methods while expanding the structural diversity of atropisomers. This review summarizes key achievements in stereoselective preparation of biaryl, heterobiaryl, and nonbiaryl atropisomers documented between 2015 and 2020. Emphasis is placed on the synthetic strategies for each structural class, while examples are cited to illustrate the potential applications of the accessed atropochiral targets.

Ruthenium(iv) N-confused porphyrin μ-oxo-bridged dimers: acid-responsive molecular rotors

Ruthenium(iv) N-confused porphyrin μ-oxo-bridged complexes were synthesized via oxidative dimerization of a ruthenium(ii) N-confused porphyrin complex using 2,2,6,6-tetramethylpiperidine 1-oxyl. The deformed core planes in the dimers conferred a relatively high ring rotational barrier of ca. 16 kcal mol⁻¹. Rotation of the complexes was controlled by protonating the peripheral nitrogen.

Ring rotation of ruthenium(iv) N-confused porphyrin μ-oxo-dimer was controlled by protonation at the peripheral nitrogen moieties.

Effects of n → π* Orbital Interactions on Molecular Rotors: The Control and Switching of Rotational Pathway and Speed

The role of n → π* orbital interactions in the rotational pathway and barrier of biaryl-based molecular rotors was elucidated through a combined experimental and computational study. The n → π* interaction in the transition state can lead to the acceleration of rotors. The competition between the n → π* interaction and hydrogen bonding further enabled the reversal of the pathway and greasing/braking the rotor in response to acid/base stimuli, thereby creating a switchable molecular rotor.

Dynamics of Hydrogen Bonding in Three-Component Nanorotors

The dynamics of hydrogen bonding do not only play an important role in many biochemical processes but also in Nature's multicomponent machines. Here, a three-component nanorotor is presented where both the self-assembly and rotational dynamics are guided by hydrogen bonding. In the rate-limiting step of the rotational exchange, two phenolic O-H-N,N(phenanthroline) hydrogen bonds are cleaved, a process that was followed by variable-temperature 1 H NMR spectroscopy. Activation data (ΔG≠ 298 =46.7 kJ mol-1 at 298 K, ΔH≠ =55.3 kJ mol-1 , and ΔS≠ =28.8 J mol-1  K-1 ) were determined, furnishing a rotational exchange frequency of k298 =40.0 kHz. Fully reversible disassembly/assembly of the nanorotor was achieved by addition of 5.0 equivalents of trifluoroacetic acid (TFA)/1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) over three cycles.

Large transition state stabilization from a weak hydrogen bond

A series of molecular rotors was designed to study and measure the rate accelerating effects of an intramolecular hydrogen bond. The rotors form a weak neutral O–H⋯O Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C hydrogen bond in the planar transition state (TS) of the bond rotation process. The rotational barrier of the hydrogen bonding rotors was dramatically lower (9.9 kcal mol⁻¹) than control rotors which could not form hydrogen bonds. The magnitude of the stabilization was significantly larger than predicted based on the independently measured strength of a similar O–H⋯OC hydrogen bond (1.5 kcal mol⁻¹). The origins of the large transition state stabilization were studied via experimental substituent effect and computational perturbation analyses. Energy decomposition analysis of the hydrogen bonding interaction revealed a significant reduction in the repulsive component of the hydrogen bonding interaction. The rigid framework of the molecular rotors positions and preorganizes the interacting groups in the transition state. This study demonstrates that with proper design a single hydrogen bond can lead to a TS stabilization that is greater than the intrinsic interaction energy, which has applications in catalyst design and in the study of enzyme mechanisms.

A series of molecular rotors was designed to study and measure the rate accelerating effects of an intramolecular hydrogen bond.

Enantioselective Synthesis of N-C Axially Chiral Compounds by Cu-Catalyzed Atroposelective Aryl Amination

N-C axially chiral compounds have emerged recently as appealing motifs for drug design. However, the enantioselective synthesis of such molecules is still poorly developed and surprisingly no metal-catalyzed atroposelective N-arylations have been described. Herein, we disclose an unprecedented Cu-catalyzed atroposelective N-C coupling that proceeds at room temperature. Such mild reaction conditions, which are a crucial parameter for atropostability of the newly generated products, are operative thanks to the use of hypervalent iodine reagents as a highly reactive coupling partners. A large panel of the N-C axially chiral compounds was afforded with very high enantioselectivity (up to >99 % ee) and good yields (up to 76 %). Post-modifications of thus accessed atropisomeric compounds allows further expansion of the diversity of these appealing compounds.

Relayed Proton Brake in N-Pyridyl-2-iso-propylaniline Derivative: Two Brakes with One Proton

The addition of methanesulfonic acid to N-(2,6-dimethylpyridin-4-yl)-N-methyl-2-iso-propylaniline led to the selective protonation of the pyridine nitrogen atom, resulting in a significant deceleration of the rotation rates around both N-pyridyl and N-(i-Pr)phenyl bonds through a relayed brake mechanism.

Hydrogen-Bond-Enabled Dynamic Kinetic Resolution of Axially Chiral Amides Mediated by a Chiral Counterion

Non-biaryl atropisomers are valuable in medicine, materials, and catalysis, but their enantioselective synthesis remains a challenge. Herein, a counterion-mediated O-alkylation method for the generation of atropisomeric amides with an er up to 99:1 is outlined. This dynamic kinetic resolution is enabled by the observation that the rate of racemization of atropisomeric naphthamides is significantly increased by the presence of an intramolecular O-H⋅⋅⋅NCO hydrogen bond. Upon O-alkylation of the H-bond donor, the barrier to rotation is significantly increased. Quantum calculations demonstrate that the intramolecular H-bond reduces the rotational barrier about the aryl-amide bond, stabilizing the planar transition state for racemization by approximately 40 kJ mol-1 , thereby facilitating the observed dynamic kinetic resolution.

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.

Cation exchange reversibly switches rotor speed and is monitored by a networked fluorescent reporter

The three-step transformation of a turnstile into a zinc rotor (8 kHz) and then into a copper rotor (30 kHz) was achieved with the last transformation being monitored by a fluorescence reporter.

A multistage rotational speed changing molecular rotor regulated by pH and metal cations

Despite having significant applications in building nanomachines, molecular rotors with the rotational speed modulations to multiple stages in a wide range of frequency have not yet been well established. Here, we report the discovery of a stimuli-responsive molecular rotor, the rotational speed of which in the slow-to-fast range could be modulated to at least four stages triggered by acid/base and metal cations. The rotor itself rotates rapidly at ambient or elevated temperature but displays a restricted rotation after deprotonation due to the produced intramolecular electrostatic repulsion. Subsequent addition of Li⁺ or Na⁺ cations introduces an electrostatic bridge to stabilize the transition state of the deprotonated rotor, thus giving a cation-radius-dependent acceleration of the rotation to render the rotor running at a mid-speed. All the stimuli are highly reversible. Our studies provide a conceptual approach for constructing multistage rotational-speed-changing molecular rotors, and further, the practical nanomachines.

Molecular rotors with rotational speed modulation have not yet been well established. Here, the authors report a pH and metal cation triggered molecular rotor, which allows for a four stage speed modulation in the slow-to-fast frequency range.

Guest control of a hydrogen bond-catalysed molecular rotor

Herein, the control of a molecular rotor using hydrogen bonding guests is demonstrated. With a properly positioned phenol substituent, the N-arylimide rotors can form an intramolecular hydrogen bond that catalyses the rotational isomerization process. The addition of the guests disrupts the hydrogen bond and raises the rotational barrier, slowing the rotation by two orders of magnitude.