Enantioselective Electrophilic Aromatic Nitration: A Chiral Auxiliary Approach

Kinetically controlled synthesis of rotaxane geometric isomers

Geometric isomerism in mechanically interlocked systems-which arises when the axle of a mechanically interlocked molecule is oriented, and the macrocyclic component is facially dissymmetric-can provide enhanced functionality for directional transport and polymerization catalysis. We now introduce a kinetically controlled strategy to control geometric isomerism in [2]rotaxanes. Our synthesis provides the major geometric isomer with high selectivity, broadening synthetic access to such interlocked structures. Starting from a readily accessible [2]rotaxane with a symmetrical axle, one of the two stoppers is activated selectively for stopper exchange by the substituents on the ring component. High selectivities are achieved in these reactions, based on coupling the selective formation reactions leading to the major products with inversely selective depletion reactions for the minor products. Specifically, in our reaction system, the desired (major) product forms faster in the first step, while the undesired (minor) product subsequently reacts away faster in the second step. Quantitative 1H NMR data, fit to a detailed kinetic model, demonstrates that this effect (which is conceptually closely related to minor enantiomer recycling and related processes) can significantly improve the intrinsic selectivity of the reactions. Our results serve as proof of principle for how multiple selective reaction steps can work together to enhance the stereoselectivity of synthetic processes forming complex mechanically interlocked molecules.

Through-Space, Lone-Pair Promoted Aromatic Substitution: A Relay Mechanism Can Beat Out Direct Activation

We report a detailed experimental and theoretical analysis of through-space arene activation with halogens, tetrazoles and achiral esters and amides. Contrary to previously assumed direct activation through σ-complex stabilization, our results suggest that these reactions proceed by a relay mechanism wherein the lone pair-containing activators form exothermic π-complexes with electrophilic nitronium ion before transferring it to the probe ring through low barrier transition states. Noncovalent interactions (NCI) plots and Quantum Theory of Atoms in Molecules (QTAIM) analyses depict favorable interactions between the Lewis base (LB) and the nitronium ion in the precomplexes and the transition states, suggesting directing group participation throughout the mechanism. The regioselectivity of substitution also comports with a relay mechanism. In all, these data pave the way for an alternate platform of electrophilic aromatic substitution (EAS) reactions.

Donor-Acceptor Tribenzotriquinacene-Based Molecular Wizard Hats Bearing Three ortho-Benzoquinone Units

Two π-extended bay-bridged tribenzotriquinacenes ("TBTQ wizard hats") 12 and 16 bearing three mutually conjugated, alternating veratrole-type and ortho-benzoquinone units were synthesized. The electronic properties of these complementarily arranged, nonplanar push-pull systems are affected by the fusion with the rigid, C₃ -symmetric TBTQ core to a different extent, as revealed by X-ray structural analysis, UV-vis spectroscopy and cyclovoltammetry. The combination of three quinone units within the original TBTQ core and three veratrole-type bay bridging units in 12 gives rise to a more efficiently π-conjugated chromophore, as reflected by the shallower shape of wizard hat and its absorption in the visible up to 750 nm in comparison to 16. Congener 12 contains an aromatic 18-π electron system in contrast to the cross-conjugated analog 16. X-ray structure analysis of the precursor dodecaether 15 revealed the formation of a cage-like supramolecular dimer, in which the peripheral dioxane-type ether groups interlace by twelve noncovalent C-H⋅⋅⋅⋅⋅O bonds.

Aromatic Electrophilic Directing for Fluorescence and Room-Temperature Phosphorescence Modulation

The ability to modulate luminescence is crucial for organic light-emitting molecules. However, the correlation between molecular structure and emission is not always obvious and systematic. Here, using a well-established empirical rule on electrophilic substitution involving directing groups in organic chemistry, we present a model system, where two luminophores are covalently linked to benzene ortho, meta, and para to each other, to demonstrate that the rule can also be useful in predicting the fluorescence and phosphorescence behaviors of these disubstituted benzene molecules. The benzene ring works as a "molecular wire" that transduces electron density when the two luminophores form ortho- and para-isomers, while little to no transduction can be noted for the meta-isomer, based on well-established organic chemistry. We anticipate that many more "textbook examples" of electronic directing in organic chemistry can be used for systematic modulation of molecular fluorescence and room-temperature phosphorescence.

Synthesis of Enantiopure Hydrocarbon Cages Based on an Optically Resolved C3-Symmetric Triaminotribenzotriquinacene

The synthesis of the enantiomerically pure, D₃-symmetric covalent hydrocarbon cages (+)-(M,M)-4 and (-)-(P,P)-4 bearing two C₃-symmetrically functionalized tribenzobenzotriquinacene (TBTQ) vertices is reported. The enantiomerically pure TBTQ building blocks (+)-(M)-5 and (-)-(P)-5 were prepared via the diastereomeric TBTQ triamides obtained by use of both Boc-d- and Boc-l-phenylglycine as chiral auxiliaries.

Chiral Derivatives of 2-Aminotribenzotriquinacene: Synthesis and Optical Resolution

Starting from a hitherto unknown 2-aminotribenzotriquinacene, several 2-amino-3-X-substituted TBTQ derivatives, all bearing a single ortho-difunctionalized indane wing, were synthesized as rigid and chiral building blocks for the potential construction of complex supramolecular architectures. Efficient access to two pairs of enantiomeric TBTQ derivatives, namely, the peripheral ortho-nitroaniline (X = NO₂) and the related anthranilic acid (X = CO₂H), was developed using chiral auxiliaries as the resolving reagents. The structure of the intermediate diastereomers was confirmed by ¹H and ¹³C NMR spectroscopy, high-resolution mass spectroscopy (HRMS), and polarimetry. The absolute configuration of the optically active derivatives was confirmed by quantum chemical time-dependent density functional theory (TD-DFT) calculations of the theoretical electronic circular dichroism (ECD) spectra and by single-crystal X-ray structure analysis of a synthesis intermediate.

Solvent-Controlled Racemic Resolution of C3-Symmetric Trihydroxytribenzotriquinacenes

A racemic C₃-symmetric trihydroxytribenzotriquinacene was resolved on a large scale by fractional crystallization of the corresponding (1S)-camphanic esters, achieving both enantiopure enantiomers (>99% ee) in 35% and 32% yields. The method relies on a distinct solvent-controlled discrimination process between the diastereomers. The enantiopure trihydroxytribenzotriquinacenes were converted into four other enantiopure building blocks, which are valuable precursors for supramolecular and materials chemistry to illustrate the utility of the synthesized compounds.

The Chirality Induction and Modulation of Polymers by Circularly Polarized Light

Chirality is a natural attribute nature of living matter and plays an important role in maintaining the metabolism, evolution and functional activities of living organisms. Asymmetric conformation represents the chiral structure of biomacromolecules in living organisms on earth, such as the L-amino acids of proteins and enzymes, and the D-sugars of DNA or RNA, which exist preferentially as one enantiomer. Circularly polarized light (CPL), observed in the formation regions of the Orion constellation, has long been proposed as one of the origins of single chirality. Herein, the CPL triggered asymmetric polymerization, photo-modulation of chirality based on polymers are described. The mechanisms between CPL and polymers (including polydiacetylene, azobenzene polymers, chiral coordination polymers, and polyfluorene) are described in detail. This minireview provides a promising flexible asymmetric synthesis method for the fabrication of chiral polymer via CPL irradiation, with the hope of obtaining a better understanding of the origin of homochirality on earth.