Titanium Salalen Catalyzed Enantioselective Benzylic Hydroxylation

The titanium complex of the cis-1,2-diaminocyclohexane (cis-DACH) derived Berkessel-salalen ligand is a highly efficient and enantioselective catalyst for the asymmetric epoxidation of terminal olefins with hydrogen peroxide ("Berkessel-Katsuki catalyst"). We herein report that this epoxidation catalyst also effects the highly enantioselective hydroxylation of benzylic C-H bonds with hydrogen peroxide. Mechanism-based ligand optimization identified a novel nitro-salalen Ti-catalyst of the highest efficiency ever reported for asymmetric catalytic benzylic hydroxylation, with enantioselectivities of up to 98 % ee, while overoxidation to ketone is marginal. The novel nitro-salalen Ti-catalyst also shows enhanced epoxidation efficiency, as evidenced by e.g. the conversion of 1-decene to its epoxide in 90 % yield with 94 % ee, at a catalyst loading of 0.1 mol-% only.

Organic co-crystal semiconductors: a crystal engineering perspective

Organic semiconductors are being pursued with vigor for the development of efficient and smart electronics. As a brief tutorial account, we traverse the fundamentals and advancements in the area and provide a crystal engineering perspective.

Fluorination-Induced Evolution of Columnar Packing in Fluorous Triphenylenes and Benzotriphenylenes

Use of D_3h -symmetrical triphenylene (TRPH) as a substrate for high-temperature radical reactions with C₄ F₈ I₂ under varying conditions resulted in the introduction of four types of fluorinated substituents: ω-C₄ F₈ H, c-C₄ F₈ , c-C₄ F₄ , and c-C₄ HF₃ . In contrast to the previous work on direct (poly)substitutions with R_F groups in polycyclic aromatic hydrocarbons (PAHs), in this work regiospecificity, selectivity, and high yield were achieved for TRPH(C₄ F₈ ) and TRPH(C₄ F₈ )₃ . New single-crystal structural data for seven compounds combined with literature crystallographic data allowed for the first detailed and precise analysis of the effects of fluorous substituent types, their number, and their position(s) on the TRPH core on the solid-state packing, and more specifically, the degree of π-π overlap between neighboring molecules, which is linked to charge transport properties. Comparison of isostructural partially fluorinated benzotriphenylenes, 2,3-TRPH(C₄ F₄ ) and 2,3-TRPH(C₄ HF₃ ), revealed an unexpectedly large (30 %) drop of π-π overlap, when only one fluorine atom was replaced with the hydrogen atom in a C₄ F₄ moiety. Theoretical and potentially practical implications of this work may include further testing and elaboration of computational methods describing solid-state interactions and predictions of transport properties of organic semiconductors, and further advances in the molecular design of high-performing TRPH-based organic materials and supramolecular architectures for organic optoelectronics.

Stacked homodimers of substituted contorted hexabenzocoronenes and their complexes with C60 fullerene

Computations show that the tendency of contorted hexabenzocoronene (c-HBC) to form either homodimers or complexes with C₆₀ can be tuned by changing the curvature of the c-HBC via the addition of substituents.

Synergy between experimental and computational studies of aromatic stacking interactions

Aromatic stacking interactions are one of the most common types of non-covalent interactions. However, their fundamental origins and the ability to accurately predict their stability trends are still an active area of research. The study of aromatic stacking interactions has been particularly challenging. The interaction involves a delicate balance of multiple forces, and the aromatic surfaces can readily adopt different interaction geometries. Thus, the collaborative efforts of theoretical and experimental researchers have been essential to understand and build more accurate predictive models of aromatic stacking interactions.

Solution-phase dimerization of an oblong shape-persistent macrocycle

A new shape-persistent macrocycle comprising two 2,3-triphenylene moieties bridged by m-phenylene ethynylenes has been synthesized. UV-vis and fluorescence spectroscopies indicate limited interaction between the two triphenylene units. The compound, which has a pronounced oblong shape (the core measures approximately 2.2 × 1.0 nm), aggregates in CDCl3 and toluene-d8 to give stacked dimers, as indicated by the (1)H NMR signals corresponding to protons on or near the core, which shift upfield with increasing concentration. These changes in NMR shielding were modeled using DFT calculations on candidate dimer geometries. The best match to the experimental data was obtained for a dimer consisting of arene-arene stacking by 3.6 Å (on average) with a displacement along the molecular long axis of 3.5-4.0 Å. This displacement is larger than can be explained by the electronic effects of aromatic stacking interactions. Instead, the minimization of steric interactions between the side chains appears to control the dimer geometry, with the alkoxy groups of one molecule sliding into registry with the gaps along the periphery of the other. Such lateral displacement (as opposed to rotation) is a direct consequence of the extended oval shape of the compound.

Toward a more complete understanding of noncovalent interactions involving aromatic rings

Noncovalent interactions involving aromatic rings, which include π-stacking interactions, anion-π interactions, and XH-π interactions, among others, are ubiquitous in chemical and biochemical systems. Despite dramatic advances in our understanding of these interactions over the past decade, many aspects of these noncovalent interactions have only recently been uncovered, with many questions remaining. We summarize our computational studies aimed at understanding the impact of substituents and heteroatoms on these noncovalent interactions. In particular, we discuss our local, direct interaction model of substituent effects in π-stacking interactions. In this model, substituent effects are dominated by electrostatic interactions of the local dipoles associated with the substituents and the electric field of the other ring. The implications of the local nature of substituent effects on π-stacking interactions in larger systems are discussed, with examples given for complexes with carbon nanotubes and a small graphene model, as well as model stacked discotic systems. We also discuss related issues involving the interpretation of electrostatic potential (ESP) maps. Although ESP maps are widely used in discussions of noncovalent interactions, they are often misinterpreted. Next, we provide an alternative explanation for the origin of anion-π interactions involving substituted benzenes and N-heterocycles, and show that these interactions are well-described by simple models based solely on charge-dipole interactions. Finally, we summarize our recent work on the physical nature of substituent effects in XH-π interactions. Together, these results paint a more complete picture of noncovalent interactions involving aromatic rings and provide a firm conceptual foundation for the rational exploitation of these interactions in a myriad of chemical contexts.

Adjusting the Local Arrangement of π-Stacked Oligothiophenes through Hydrogen Bonds: A Viable Route to Promote Charge Transfer

We show that substituting quaterthiophene cores with strong H-bond aggregators, such as urea groups, provides an efficient way to adjust the mutual in-plane displacements of the semiconducting units and promote charge transfer. Our 2-D structure-property mapping reveals that the insertion of substituents induces up to 2.0 Å longitudinal and transversal displacements between the π-conjugated moieties. Some of these relative displacements lead to improved cofacial orbital overlaps that are otherwise inaccessible due to Pauli repulsion. Our results also emphasize that the fine-tuning of in-plane displacements is more effective than achieving "tighter" packing to promote charge-transfer properties.

Improving Hg-triggered gelation via structural modifications

The relationship between chemical structure and gelation ability was examined for a series of nine Hg-containing compounds. Both solid-state properties (dissolution enthalpies/entropies and packing structure) and gel properties (strength, morphology, cation selectivity, and anion tolerance) were examined. Overall, the results reveal a complex relationship between chemical structure and properties. The remediation potential of these Hg-triggered gelations was also investigated, revealing that >98% of the Hg(2+) in water can be removed through gel formation.