Active and Unidirectional Acceleration of Biaryl Rotation by a Molecular Motor

Light‐driven molecular motors possess immense potential as central driving units for future nanotechnology. Integration into larger molecular setups and transduction of their mechanical motions represents the current frontier of research. Herein we report on an integrated molecular machine setup allowing the transmission of potential energy from a motor unit onto a remote receiving entity. The setup consists of a motor unit connected covalently to a distant and sterically encumbered biaryl receiver. By action of the motor unit, single‐bond rotation of the receiver is strongly accelerated and forced to proceed unidirectionally. The transmitted potential energy is directly measured as the extent to which energy degeneration is lifted in the thermal atropisomerization of this biaryl. Energy degeneracy is reduced by more than 1.5 kcal mol^?1, and rate accelerations of several orders of magnitude in terms of the rate constants are achieved.     

Transmission of Unidirectional Molecular Motor Rotation to a Remote Biaryl Axis

Molecular motors undergo repetitive directional motions upon external energy input. A profound challenge is the defined transfer of directional motor motions to remote entities at the molecular scale. Herein, we present a molecular setup that allows for the transfer of the directional rotation of a light‐powered motor unit onto a remote biaryl axis via an ethylene glycol chain link. Based on a combination of X‐ray crystallographic analysis, ECD, and NMR experiments as well as a comprehensive theoretical assessment, we provide evidence for the coupled stepwise directional motions of both molecular units. With the presented setup, facile integration of molecular motor units into larger functional frameworks and complex molecular machines can be explored consciously in the future.     

Net directed 180° aryl-aryl bond rotation in a prototypical achiral biaryl lactone synthetic molecular motor

Net directed 180° bond rotation was achieved through diastereoselective ring-opening reactions in an achiral biaryl lactone using a chiral nucleophile followed by re-lactonization. The efficiency of the directed bond rotation has been determined by HPLC-MS to be 50% and 20% with two different chiral nucleophiles. These results demonstrate the potential for a prototype of a chemically driven synthetic molecular motor which has the advantages of both simplicity and flexibility in operation and is the first example of the use of a chiral auxiliary to induce transient axial chirality resulting in net directed bond rotation.     

Synthesis of Annelated Phospholes through Intramolecular C?H Activation by Monovalent Phosphorus

Electrophilic terminal phosphinidene complexes [Ar‐Ar‐P‐W(CO)₅] (Ar‐Ar: biaryl or an analogue thereof) undergo a spontaneous insertion of the phosphorus atom into the vicinal C H bonds to give annelated phospholes. Twelve examples are described, including biphenyl, thienyl, pyrrolyl, and benzofuryl groups as biaryl moieties. The activation energy of the insertion reaction is quite low (about 2 kcal mol⁻¹).     

Synthesis of Annelated Phospholes through Intramolecular CH Activation by Monovalent Phosphorus

Electrophilic terminal phosphinidene complexes [Ar‐Ar‐P‐W(CO)₅] (Ar‐Ar: biaryl or an analogue thereof) undergo a spontaneous insertion of the phosphorus atom into the vicinal C? H bonds to give annelated phospholes. Twelve examples are described, including biphenyl, thienyl, pyrrolyl, and benzofuryl groups as biaryl moieties. The activation energy of the insertion reaction is quite low (about 2 kcal mol^?1).     

Fluctuating Carbonaceous Networks with a Persistent Molecular Shape: A Saddle‐Shaped Geodesic Framework of 1,3,5‐Trisubstituted Benzene (Phenine)

A saddle‐shaped macromolecule has been synthesized. The molecule was designed as a geodesic saddle with 1,3,5‐trisubstituted benzene (named phenine) as the fundamental unit. The phenines were woven into a polygonal framework that was composed of 168 sp²‐hybridized carbon atoms. The saddle‐shaped structure with unique symmetry showed atypical conformational changes. The biaryl linkages in this molecule had a small energy barrier for rotation, and these structural fluctuations resulted in seven ¹H NMR resonances representing 84 aromatic hydrogen atoms. Nevertheless, the overall saddle shape of the molecule was persistent, and the “up” and “down” orientations of phenine moieties circulated to give average ¹H resonances. The structural characteristics of this molecule, including the anomalous entropy‐driven dimerization, may deepen our understanding of defect‐rich graphitic sheets.     

Donor‐Induced Helical Inversion of 1,1′‐Binaphthyl Connecting with Two Molybdenum Complexes

An atropisomeric biaryl molecule with a given absolute configuration could present two opposite helical conformations through the rotation around C? C single bond. To the best of our knowledge, the biaryl system is the simplest helical inversion model apart from stereomutation between two enantiomers. Herein, we first report such true helical inversion phenomena of biaryl compounds. Two [Mo^VIO₂(L)]‐type complexes, in which L is a tridentate dioxoanionic pyridine O,N,O‐ligand, are coalesced on the 2,2′,3,3′‐positions of an (R)‐1,1′‐binaphthyl unit and an intramolecular dioxo bridge is formed by two Mo?O???Mo interactions. Exterior strong donors can coordinate to molybdenum to interrupt this dioxo bridge and inversions from negative to positive chirality are explicitly observed by circular dichroism spectroscopy, consistent with single‐crystal X‐ray diffraction analyses.     

A chiroptical molecular switch with distinct chiral and photochromic entities and its application in optical switching of a cholesteric liquid crystal

Two new structurally related photoswitches are described, in which azobenzene photochromism is combined with the chirality of a 2,2'-dihydroxy-1,1'-binaphthyl unit. In system 1 the chiral binaphthyl moiety is bridged by a methylene tether, locking the biaryl chirality while in system 2 the biaryl core is unbridged and has considerable conformational flexibility. Both compound are capable of inducing cholesteric liquid crystalline phases and proved to be good photoswitches both in solution and in a liquid crystalline matrix. Compound 2 is capable of completely reversing the liquid crystalline chirality which is unique for a chiroptical molecular switch where the switching unit and the chiral moiety are separate entities.     

An improved neutron reflection setup for the determination of H and (O+C)/H in oil samples

An improved experimental setup based on the thermalization and reflection of neutrons has been used for the determination of total H content and (O+C)/H atomic ratio in oil samples. The count rate response function for this setup gives a relative error of about ±1.5% and a detection limit of 0.09 H w% in the case of different hydrogeneous samples of 300–500 cm³ volumes. Results obtained for motor and household oils as well as for crude oil samples from Nigeria and Hungary are presented.This work was supported by the Hungarian Research Foundation (Contract No. T 016713).     

Synthesis and characterization of a functionalized chiral biaryl capable of exhibiting unidirectional bond rotation

A class of chiral biaryl molecules that have been designed to undergo unidirectional rotation about the aryl–aryl bond may show promise for future application in the area of synthetic molecular motors. These asymmetric molecules should be capable of channeling chemical energy into repeated 360° rotation in one direction. Herein we report the synthesis and characterization of one such biaryl molecule (1).     

Ab initio and molecular mechanics study of n-phenyl phthalimide and its crystal structure

Ab initio molecular orbital calculations are reported on the energetics for torsional motion of N-phenyl phthalimide using 3-21G, 6-31G, and 6-31G^** basis sets and incorporating electron correlation effects for selected geometries. With the largest basis set, a minimum energy is found for a torsion angle of 59.2°. Atomic charges are assigned to the molecules on the basis of a least-squares fit to the molecular electrostatic potential. This information is then used in molecular mechanics calculations of the crystal structure, where the calculated unit cell parameters are in good agreement with those observed experimentally.     

Steric strain and electron-impact. The behaviour of some n,n'-dimethyl-1,1-binaphthyls,some n,n'-dimethylbiphenyls and model compounds

For the title compounds I(M), A[M ? H]⁺ and A[M ? Me]⁺ have been determined. In the biaryl compounds, unlike the xylene and the two dimethylnaphthalenes studied, A[M ? Me]⁺ is noticeably dependent on the position of the methyl group. Deuterium labelling of the methyl groups in the biaryl compounds reveals that more than one process is involved in the formation of the [M ? Me]⁺ ion. In contrast, only in the case of 2,2′-dimethylbiphenyl is the appearance potential of the [M ? H]⁺ ion position dependent. The labelling results suggest complete scrambling of the hydrogen atoms before formation of the [M ? H]⁺ ions. Some comments are made on the problem of trying to relate differences in strain energies and ionisation and appearance potentials in alkyl substituted aromatics.     

ChemInform Abstract: B+13: A Photodriven Molecular Wankel Engine.

Based on DFT and Born—Oppenheimer molecular dynamics calculations a model molecular Wankel motor, the dual‐ring structure B₁₃⁺, driven by circularly polarized IR radiation near 3 THz is proposed as a potential building block for nanomachines.     

Syntheses of all-methylated ellagitannin, isorugosin B and rugosin B

Ellagitannins possess a wide range of biological activities and remarkable structural diversity, which commonly include an axially chiral biaryl unit. This paper describes syntheses of all-methylated versions of isorugosin B and rugosin B, which are regioisomeric, ellagitannin-related compounds. The key features of these syntheses involve the construction of an axially chiral biaryl function on a sugar moiety through a Pd-catalyzed intramolecular biaryl coupling reaction, Bringmann’s atroposelective lactone opening reaction, and a two-step ester formation. This is the first synthetic approach for generating ellagitannins featuring a valoneoyl group.     

Synthesis of Axially Chiral Biaryl‐2‐amines by PdII‐Catalyzed Free‐Amine‐Directed Atroposelective C−H Olefination

A simple and ubiquitously present group, free amine, is used as a directing group to synthesize axially chiral biaryl compounds by Pd^II‐catalyzed atroposelective C?H olefination. A broad range of axially chiral biaryl‐2‐amines can be obtained in good yields with high enantioselectivities (up to 97 % ee). Chiral spiro phosphoric acid (SPA) proved to be an efficient ligand and the loading could be reduced to 1 mol % without erosion of enantiocontrol in gram‐scale synthesis. The resulting axially chiral biaryl‐2‐amines also provide a platform for the synthesis of a set of chiral ligands.     

Stereochemistry of isoplagiochin C, a macrocyclic bisbibenzyl from liverworts

Cyclic bisbibenzyls, like isoplagiochins C (1) and D (2), are stereochemically intriguing molecules: Although not equipped with any of the traditional stereogenic elements that render molecules conformationally stable per se, they are sometimes isolated in an optically active form and are thus chiral at room temperature. The paper describes quantum chemical calculations, in particular investigations of the conformational space and molecular dynamics simulations, showing that the helicity is a property of the entire molecule, whose ring strain makes the molecule configurationally stable overall, with (formally) three stereogenic elements (two biaryl axes and one helical stilbene unit). Only one of the biaryl axes (the 'upper' one, joining C-12' and C-14) has a stable configuration, leading to a population of four interconverting diastereomers, yet without racemization at room temperature. On the basis of these conformational and dynamic calculations, the circular dichroism spectrum of isoplagiochin C (1) was calculated, leading to the first assignment of the absolute configuration of a cyclic bisbibenzyl. Accordingly, 1 has the P-configuration at the stereochemically stable biaryl axis and constitutes a mixture of diastereomers with respect to the other biaryl axis and the helical stilbene unit. From the temperature dependence of the racemization rates, an enantiomerization barrier of 101.6 kJ/mol was determined. Likewise, for the first time for cyclic bisbibenzyls, the enantiomeric ratio of this natural product was determined, by chromatography on a chiral phase with CD-coupling. Accordingly, 1 from Plagiochila deflexa is not enantiomerically pure, but occurs in a 85:15 ratio in favor of the enantiomer that has the P-configuration at the stereochemically stable axis.     

Rate constants for reactions O(3P) + X2 → OX + X (X = Br,I) determined by an RRKM-type statistical algorithm employing Monte Carlo simulation

Thermal and crossed molecular beam experiments have independently given information on the reactions O(³P) + Br₂ → OBr + Br and O(³P) + I₂ → OI + I, which proceed through long-lived reaction complexes. Our previously described statistical RRKM-type simulation algorithm is now modified to calculate detailed and macroscopic rate constants for these reactions. Good agreement is found with the experimental thermal rate constant for O + I₂, while reasonable results are found for O + Br₂ when a potential energy barrier with a height of 4.2 kJ/mol is included in the entrance channel. A somewhat larger energy barrier may be needed to reach good agreement with experiments. Angular momentum restrictions are shown to considerably lower the rate constants for these reactions. It is demonstrated that the detailed reaction model largely influences the energy dependence of the detailed rate constants, and also that extrapolations to other temperatures for thermal rate constants depend strongly on the potential energy model used.     

Access to P‐ and Axially Chiral Biaryl Phosphine Oxides by Enantioselective CpxIrIII‐Catalyzed C−H Arylations

An enantioselective C?H arylation of phosphine oxides with o‐quinone diazides catalyzed by an iridium(III) complex bearing an atropchiral cyclopentadienyl (Cp^x) ligand and phthaloyl tert‐leucine as co‐catalyst is reported. The method allows access to a) P‐chiral biaryl phosphine oxides, b) atropo‐enantioselective construction of sterically demanding biaryl backbones, and also c) selective assembly of axial and P‐chiral compounds in excellent yields and diastereo‐ and enantioselectivities. Enantiospecific reductions provide monodentate chiral phosphorus(III) compounds having structures and biaryl backbones with proven importance as ligands in asymmetric catalysis.     

Exploring the Mechanism of a Chiral N-Alkyl Imine-Based Light-Driven Molecular Rotary Motor at MS-CASPT2//CASSCF and MS-CASPT2//(TD) DFT Levels

The working mechanism including the photoisomerization and thermal isomerization steps of a chiral N-alkyl imine-based motor synthesized by Lehn et al. are revealed by MS-CASPT2//CASSCF and MS-CASPT2//(TD-)DFT methods. For the photoisomerization process of the imine-based motor, it involves both the bright (π,π*) state and the dark (n,π*) state. In addition, the MECI has similar geometry and energy to the minimum of the S₁ state, which shows that the process is barrierless and keeps the unidirectionality of rotation well; the result confirms the imine-based motor is a good candidate for a light-driven molecular rotary motor. For the thermal isomerization process of the imine-based motor, there are two even isomerization paths: one with the mechanism of the in-plane N inversion, the energy barriers of which are 29.6 kcal mol⁻¹ at MS3-CASPT2//CAM-B3LYP level and 29.2 kcal mol⁻¹ at MS3-CASPT2//CASSCF level; the other with the mechanism of ring inversion of the cycloheptatriene moiety, with energy barriers of 28.1 kcal mol⁻¹ at MS3-CASPT2//CAM-B3LYP level and 18.1 kcal mol⁻¹ at MS3-CASPT2//CASSCF level. According to the structural feature of the stator moiety, the imine molecule can be used as a two-step or a four-step light-driven rotary motor.