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

Aromatic-Carbonyl Interactions as an Emerging Type of Non-Covalent Interactions

Aromatic-carbonyl (Ar···C═O) interactions, attractive interactions between the arene plane and the carbon atom of carbonyl, are in the infancy as one type of new supramolecular bonding forces. Here the study and functionalization of aromatic-carbonyl interactions in solution is reported. A combination of aromatic-carbonyl interactions and dynamic covalent chemistry provided a versatile avenue. The stabilizing role and mechanism of arene-aldehyde/imine interactions are elucidated through crystal structures, NMR studies, and computational evidence. The movement of imine exchange equilibria further allowed the quantification of the interplay between arene-aldehyde/imine interactions and dynamic imine chemistry, with solvent effects offering another handle and matching the electrostatic feature of the interactions. Moreover, arene-aldehyde/imine interactions enabled the reversal of kinetic and thermodynamic selectivity and sorting of dynamic covalent libraries. To show the functional utility diverse modulation of fluorescence signals is realized with arene-aldehyde/imine interactions. The results should find applications in many aspects, including molecular recognition, assemblies, catalysis, and intelligent materials.

Dual reactivity based dynamic covalent chemistry: mechanisms and applications

Dynamic covalent chemistry (DCC) focuses on the reversible formation, breakage, and exchange of covalent bonds and assemblies, setting a bridge between irreversible organic synthesis and supramolecular chemistry and finding wide utility. In order to enhance structural and functional diversity and complexity, different types of dynamic covalent reactions (DCRs) are placed in one vessel, encompassing orthogonal DCC without crosstalk and communicating DCC with a shared reactive functional group. As a means of adding tautomers, widespread in chemistry, to interconnected DCRs and combining the features of orthogonal and communicating DCRs, a concept of dual reactivity based DCC and underlying structural and mechanistic insights are summarized. The manipulation of the distinct reactivity of structurally diverse ring-chain tautomers allows selective activation and switching of reaction pathways and corresponding DCRs (C-N, C-O, and C-S) and assemblies. The coupling with photoswitches further enables light-mediated formation and scission of multiple types of reversible covalent bonds. To showcase the capability of dual reactivity based DCC, the versatile applications in dynamic polymers and luminescent materials are presented, paving the way for future functionalization studies.

Molecular orbital and topological electron density study of n → π* interactions: amides and thioamides cases

The n → π* interactions were studied in amides and thioamides systems models, through the analysis of the electron density topology along with the Natural Bonding Orbital (NBO) approach. The effect of the dispersion terms was assessed using different DFT functionals. The NBO, independent gradient model (IGM), and the analysis of the reduced density gradient outcomes show that dispersion forces play a significant role in the strength of n → π* interactions. The IGM results indicate that δg height values for n → π* interactions do not extend beyond 0.025. All the methods used in this work predict that n → π* interaction between pairs of thioamides is stronger than those between amides. However, the electron density topology-based methods were not able to replicate the trends in the relative force of this interaction found in the experimental and NBO results.

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.

Intermolecular n→π* Interactions in Supramolecular Chemistry and Catalysis

The n→π* interactions describing attractive force between lone pairs (lps) of nucleophile and carbonyl or polarized unsaturated bonds have recently attracted growing attentions in various disciplines. So far, such non-covalent driving force are mainly concentrated to intramolecular systems. Intermolecular n→π* interactions in principle could produce fascinated supramolecular systems or facilitate organic reactions, however, they remain largely underexplored due to the very weak energy of individual interaction. This review attempts to give an overview of the challenging intermolecular n→π* interactions, much efforts emphasize the supramolecular systems, catalytic processes and spectroscopic measurements.

Double n→π* Interactions with One Electron Donor: Structural and Mechanistic Insights

Double n→π* interactions between one common electron donor of the carbonyl oxygen and two individual acceptor aldehyde/imine units are presented. The structural and mechanistic insights were revealed through a collection of experimental and computational evidence. The orientation and further energetic dependence of orbital interactions were facilely regulated by the size of cyclic urea scaffolds, the bulkiness of aldehydes/imines, and the flexibility of imine macrocycles.

Intermolecular n→π* Interactions Based on a Tailored Multicarbonyl-Containing Macrocycle

Towards unexplored intermolecular n→π* interactions, presented herein are the synthesis, structure, self-assembly and function of a multicarbonyl-containing macrocycle calix[2]arene[2]barbiturate 1. X-ray single crystal diffraction reveals the presence of Cl⋅⋅⋅C=O interactions in CH₂ Cl₂ ⊂1 host-guest complex and multiple intermolecular C=O⋅⋅⋅C=O interactions between molecules 1 in crystalline state. The intermolecular C=O⋅⋅⋅C=O interactions as attractive driving force led to unprecedented self-assembly of nanotube with diameter around 1.4 nm and inner surface engineered by aromatic rings. SEM and TEM images of the self-assembly of 1 demonstrated temperature-dependent morphologies which allows the observation of spheres at 25 °C and rods at 0 °C, respectively. XRD analysis indicated consistent hexagonal patterns in the self-assembly and single crystal lattice, indicating the nanotubes driven by C=O⋅⋅⋅C=O interactions constitute the basic structural architectures of both aggregates. The nanoscopic tubes (pores) formed in the rodlike single crystal engendering the separation of moving dyes were preliminarily investigated by a single-crystal chromatography and crystal-packed column chromatography.

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.

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...

Analysis of the Orbital and Electrostatic Contributions to the Lone Pair-Aromatic Interaction Using Molecular Rotors

The attractive interaction between carbonyl oxygens and the π-face of aromatic surfaces was studied using N-phenylimide molecular rotors. The C═O···Ar interactions could stabilize the transition states but were half the strength of comparable C═O···C═O interactions. The C═O···Ar interaction had a significant electrostatic component but only a small orbital delocalization component.