High Configurational Stability Research Articles

π‐Radical Cascade to a Chiral Saddle‐Shaped Peropyrene

AbstractReactions of open‐shell molecular graphene fragments are typically thought of as undesired decomposition processes because they lead to the loss of desired features like π‐magnetism. Oxidative dimerization of phenalenyl to peropyrene shows, however, that these transformations hold promise as a synthetic tool for making complex structures via formation of multiple bonds and rings in a single step. Here, we explore the feasibility of using this “undesired” reaction of phenalenyl to build up strain and provide access to non‐planar polycyclic aromatic hydrocarbons. To this end, we designed and synthesized a biradical system with two phenalenyl units linked via a biphenylene backbone. The design facilitates an intramolecular cascade reaction to a helically twisted saddle‐shaped product, where the key transformations—ring‐closure and ring‐fusion—occur within one reaction. The negative curvature of the final peropyrene product, induced by the formed eight‐membered ring, was confirmed by single‐crystal X‐ray diffraction analysis and the helical twist was validated via resolution of the product's enantiomers that display circularly polarized luminescence and high configurational stability.

π-Radical Cascade to a Chiral Saddle-Shaped Peropyrene.

Reactions of open-shell molecular graphene fragments are typically thought of as undesired decomposition processes because they lead to the loss of desired features like π-magnetism. Oxidative dimerization of phenalenyl to peropyrene shows, however, that these transformations hold promise as a synthetic tool for making complex structures via formation of multiple bonds and rings in a single step. Here, we explore the feasibility of using this "undesired" reaction of phenalenyl to build up strain and provide access to non-planar polycyclic aromatic hydrocarbons. To this end, we designed and synthesized a biradical system with two phenalenyl units linked via a biphenylene backbone. The design facilitates an intramolecular cascade reaction to a helically twisted saddle-shaped product, where the key transformations-ring-closure and ring-fusion-occur within one reaction. The negative curvature of the final peropyrene product, induced by the formed eight-membered ring, was confirmed by single-crystal X-ray diffraction analysis and the helical twist was validated via resolution of the product's enantiomers that display circularly polarized luminescence and high configurational stability.

Studies on the Configurational Stability of Tropolone-Ketone-, Ester-, and Aldehyde-Based Chiral Axes.

Recent studies have revealed that tropolone-amide aryl C-C(O) rotational barriers are dramatically higher than those of analogous benzamide-based systems, and as a result, they have an increased likelihood of displaying high configurational stability. Studies on other tropolone-based chiral axes are important to assess the generality of this phenomenon. Herein, we describe a series of studies on the rotational barriers of tropolone-ketone, tropolone-ester, and tropolone-aldehyde chiral axes. These studies are complemented with computational modeling of the dynamics of these and analogous benzenoid variants to illuminate the impact that tropolone may have on aryl-C(O) configurational stability.

Stereospecific synthesis of silicon-stereogenic optically active silylboranes and general synthesis of chiral silyl Anions

Silicon-stereogenic optically active silylboranes could potentially allow the formation of chiral silyl nucleophiles as well as the synthesis of various chiral silicon compounds. However, the synthesis of such silicon-stereogenic silylboranes has not been achieved so far. Here, we report the synthesis of silicon-stereogenic optically active silylboranes via a stereospecific Pt(PPh3)4-catalyzed Si–H borylation of chiral hydrosilanes, which are synthesized by stoichiometric and catalytic asymmetric synthesis, in high yield and very high or perfect enantiospecificity (99% es in one case, and >99% es in the others) with retention of the configuration. Furthermore, we report a practical approach to generate silicon-stereogenic silyl nucleophiles with high enantiopurity and configurational stability using MeLi activation. This protocol is suitable for the stereospecific and general synthesis of silicon-stereogenic trialkyl-, dialkylbenzyl-, dialkylaryl-, diarylalkyl-, and alkylary benzyloxy-substituted silylboranes and their corresponding silyl nucleophiles with excellent enantiospecificity (>99% es except one case of 99% es). Transition-metal-catalyzed C–Si bond-forming cross-coupling reactions and conjugate-addition reactions are also demonstrated. The mechanisms underlying the stability and reactivity of such chiral silyl anion were investigated by combining NMR spectroscopy and DFT calculations.

Construction and Phosphorescence Behavior of S/Se-Heteroaromatics/Phenanthrene-Fused Hetero[9]helicenes.

Three S/Se-hetero[9]helicenes were synthesized by an assembly strategy of two building blocks, phenanthrene and ternary fused S/Se-heteroaromatic ring, and characterized by single-crystal X-ray diffraction analysis. Their phosphorescence emission gave a long-lived lifetime at the millisecond level. In addition, the high configurational stability of enantiomer (+)-[9]BH-1a was observed without change under both the conditions of 300 °C in the solid state for 18 h and 200 °C in solution of 1,2-dichlorobenzene for 10 h.

Thermochemistry of the Smallest Hyperbolic Paraboloid Hydrocarbon: A High-Level Quantum Chemical Perspective

[5.5.5.5]hexaene is a [12]annulene ring with a symmetrically bound carbon atom in its center. This is the smallest hydrocarbon with a hyperbolic paraboloid shape. [5.5.5.5]hexaene and related hydrocarbons are important building blocks in organic and materials chemistry. For example, penta-graphene—a puckered 2D allotrope of carbon—is comprised of similar repeating subunits. Here, we investigate the thermochemical and kinetic properties of [5.5.5.5]hexaene at the CCSD(T) level by means of the G4 thermochemical protocol. We find that this system is energetically stable relative to its isomeric forms. For example, isomers containing a phenyl ring with one or more acetylenic side chains are higher in energy by ∆H298 = 17.5–51.4 kJ mol−1. [5.5.5.5]hexaene can undergo skeletal inversion via a completely planar transition structure; however, the activation energy for this process is ∆H‡298 = 249.2 kJ mol−1 at the G4 level. This demonstrates the high configurational stability of [5.5.5.5]hexaene towards skeletal inversion. [5.5.5.5]hexaene can also undergo a π-bond shift reaction which proceeds via a relatively low-lying transition structure with an activation energy of ∆H‡298 = 67.6 kJ mol−1. Therefore, this process is expected to proceed rapidly at room temperature.

Helically Chiral Donor–Acceptor Double Hetero[4]helicenes with Circularly Polarized Thermally Activated Delayed Fluorescence

AbstractAlthough helically chiral heterohelicenes are widely adopted in chiroptical studies, those with circularly polarized thermally activated delayed fluorescence (CP‐TADF), which are highly valuable for circularly polarized organic light‐emitting diodes (CP‐OLEDs), are rarely realized. Moreover, the core skeletons of known CP‐TADF heterohelicenes are limited to the p‐ and n‐doped structures with multiresonance effect. In this study, a new type of helically chiral CP‐TADF heterohelicenes featuring a fused donor–acceptor core skeleton is successfully designed and constructed. Enantiomers possessing high configurational stability are achieved and separated, and they efficiently emit CP light with dissymmetry factors reaching 3.1 × 10−3 in doped films. Owing to the excellent photophysical properties, a high‐performance CP‐OLED with the maximum external quantum efficiency of up to 20.0% and an electroluminescence dissymmetry factor reaching 2.9 × 10−3 is obtained. This new type of CP‐TADF heterohelicenes provides a large space for the development of high‐performance CP‐TADF materials for various applications.

Synthesis and Optical Resolution of Configurationally Stable Zwitterionic Pentacoordinate Silicon Derivatives

AbstractStereogenic silicon centres in functionalised tetracoordinated organosilanes generally exhibit very high configurational stability under neutral conditions. This stability drops completely when higher coordination states of the silicon centre are reached due to rapid substituent exchange. Herein we describe the synthesis of chiral and neutral pentacoordinate silicon derivatives with high configurational stability. The zwitterionic nature of these air‐ and water‐tolerant species allows for the first time their direct and efficient optical resolution using chiral HPLC techniques. By means of this method, pentacoordinate silicon compounds exhibiting high Si‐inversion have been obtained as single enantiomers. A rationalisation of the enantiomerisation pathways has been also carried out using DFT calculations.

Synthesis and Optical Resolution of Configurationally Stable Zwitterionic Pentacoordinate Silicon Derivatives.

Stereogenic silicon centres in functionalised tetracoordinated organosilanes generally exhibit very high configurational stability under neutral conditions. This stability drops completely when higher coordination states of the silicon centre are reached due to rapid substituent exchange. Herein we describe the synthesis of chiral and neutral pentacoordinate silicon derivatives with high configurational stability. The zwitterionic nature of these air- and water-tolerant species allows for the first time their direct and efficient optical resolution using chiral HPLC techniques. By means of this method, pentacoordinate silicon compounds exhibiting high Si-inversion have been obtained as single enantiomers. A rationalisation of the enantiomerisation pathways has been also carried out using DFT calculations.

Catalyst-Controlled Stereoselective Barton-Kellogg Olefination.

Overcrowded alkenes are expeditiously prepared by the versatile Barton–Kellogg olefination and have remarkable applications as functional molecules owing to their unique stereochemical features. The induced stereodynamics thereby enable the controlled motion of molecular switches and motors, while the high configurational stability prevents undesired isomeric scrambling. Bistricyclic aromatic enes are prototypical overcrowded alkenes with outstanding stereochemical properties, but their stereocontrolled preparation was thus far only feasible in stereospecific reactions and with chiral auxiliaries. Herein we report that direct catalyst control is achieved by a stereoselective Barton–Kellogg olefination with enantio‐ and diastereocontrol for various bistricyclic aromatic enes. Using Rh2(S‐PTAD)4 as catalyst, several diazo compounds were selectively coupled with a thioketone to give one of the four anti‐folded overcrowded alkene stereoisomers upon reduction. Complete stereodivergence was reached by catalyst control in combination with distinct thiirane reductions to provide all four stereoisomers with e.r. values of up to 99:1. We envision that this strategy will enable the synthesis of topologically unique overcrowded alkenes for functional materials, catalysis, energy‐ and electron transfer, and bioactive compounds.

Thermoconformational Behavior of Cellulose Nanofiber Films as a Device Substrate and Their Superior Flexibility and Durability to Glass.

The design and high-throughput manufacturing of thin renewable energy devices with high structural and atomic configurational stability are crucial for the fabrication of green electronics. Yet, this concept is still in its infancy. In this work, we report the extraordinary durability of thin molecular interlayered organic flexible energy devices based on chemically tuned cellulose nanofiber transparent films that outperform glass by decreasing the substrate weight by 50%. The nanofabricated flexible thin film has an exceptionally low thermal coefficient of expansion of 1.8 ppm/K and a stable atomic configuration under a harsh fabrication condition (over 190 °C for an extended period of 5 h). A flexible optoelectronic device using the same renewable cellulose nanofiber film substrate was found to be functionally operational over a life span of 5 years under an intermittent operating condition. The success of this device's stability opens up an entirely new frontier of applications of flexible electronics.

Enantiopure planar chiral [2.2]paracyclophanes: Synthesis and applications in asymmetric organocatalysis

This short review focuses on enantiopure planar chiral [2.2]paracyclophanes (pCps), a fascinating class of molecules that possess an unusual three-dimensional core and intriguing physicochemical properties. In the first part of the review, different synthetic strategies for preparing optically active pCps are described. Although classical resolution methods based on the synthesis and separation of diastereoisomeric products still dominate the field, recent advances involving the kinetic resolution of racemic compounds and the desymmetrization of meso derivatives open up new possibilities to access enantiopure key intermediates on synthetically useful scales. Due to their advantageous properties including high configurational and chemical stability, [2.2]paracyclophanes are increasingly employed in various research fields, ranging from stereoselective synthesis to material sciences. The applications of [2.2]paracyclophanes in asymmetric organocatalysis are described in the second part of the review. While historically enantiopure pCps have been mainly employed by organic chemists as chiral ligands in transition-metal catalysis, these compounds can also be used as efficient catalysts in metal-free reactions and may inspire the development of new transformations in the near future.

C2- and C1-Symmetric Configurationally Stable Pyrene-Fused [5]Helicenes Connected via Hexagonal and Heptagonal Rings.

In this paper, we describe the stereospecific synthesis and functional properties of C2- and C1-symmetric pyrene-fused [5]helicene molecules 1 and 2 connected via hexagonal and heptagonal rings, respectively. Both molecules showed high configurational stability and distinct functional properties, which were attributed to the fusing mode of [5]helicene with the pyrene and molecular symmetry. The estimated Gibbs activation energy for enantiomerization of 2 is one of the highest reported values for any π-conjugated molecules incorporating [5]helicene moiety.

Asymmetric construction of tetrahedral chiral zinc with high configurational stability and catalytic activity

Chiral metal complexes show promise as asymmetric catalysts and optical materials. Chiral-at-metal complexes composed of achiral ligands have expanded the versatility and applicability of chiral metal complexes, especially for octahedral and half-sandwich complexes. However, Werner-type tetrahedral complexes with a stereogenic metal centre are rarely used as chiral-at-metal complexes because they are too labile to ensure the absolute configuration of the metal centre. Here we report the asymmetric construction of a tetrahedral chiral-at-zinc complex with high configurational stability, using an unsymmetric tridentate ligand. Coordination/substitution of a chiral auxiliary ligand on zinc followed by crystallisation yields an enantiopure chiral-only-at-zinc complex (> 99% ee). The enantiomer excess remains very high at 99% ee even after heating at 70 °C in benzene for one week. With this configurationally stable zinc complex of the tridentate ligand, the remaining one labile site on the zinc can be used for a highly selective asymmetric oxa-Diels-Alder reaction (98% yield, 87% ee) without substantial racemisation.

Circularly polarized luminescence of enantiopure carboline-based europium cryptates under visible light excitation

The enantiomers of carboline-based cryptates were successfully resolved by chiral HPLC. These complexes show high configurational stability under harsh conditions and their absolute configuration was determined by comparing theoretical and experimental electronic circular dichroism spectra. The enantiopure cryptates exhibit strong circularly polarized luminescence with a maximum dissymmetry factor glum = 0.25 for the f-f transition 5D0→7F1 (λ = 594 nm) under visible light excitation at λex = 400 nm.

Umpolung Reactivity of in Situ Generated Phosphido-Boranes: An Entry to P-Stereogenic Aminophosphine-Boranes.

The synthesis of P-stereogenic aminophosphine-boranes has been developed on the basis of umpolung reactivity of in situ generated alkylarylphosphido-boranes, which are normally configurationally unstable intermediates. In our case, their high configurational stability was due to the slow release of the hydroxyalkyl protecting group, together with the fast formation of the iodophosphanylborane in the presence of N-iodosuccinimide. The subsequent substitution reaction was found to proceed in moderate to good yields and in a very high stereospecifity (es) using a variety of amines as nucleophiles.

Configurational Stability of [5]Helicenes.

A series of [5]helicenes difunctionalized in the fjord region with either fluoro, methoxy, or methyl groups was synthesized via photochemical and benzylic coupling route. Resolution of each compound into enantiomers and determination of the Gibbs activation energies of enantiomerization (ΔG⧧(T)) revealed high configurational stability in all three cases. The ΔG⧧(T) values of difunctionalized [5]helicenes were compared with those of their monofunctionalized analogues and the parent [5]helicene. Within this series, an exponential correlation between the torsional twist and ΔG⧧(T) was found. The dimethyl derivative exhibits one of the highest configurational stabilities among [n]helicenes reported to date, comparable to that of [9]helicene.

Configurationally Stable Doubly Bridged Biphenyl Azocines through Copper-Catalyzed Double Carbene Insertions into the Corresponding Azepines

Doubly bridged biphenyl azocines can be prepared in a single step through copper-catalyzed reactions of a doubly bridged biphenyl azepine and diazodiester reagents. Double [1,2]-Stevens rearrangements occur at 100 °C to afford doubly tethered eight-membered rings (49 to 61%) as trans and cis regioisomers (1:1 ratio). These products present an axial chirality. ECD and VCD analyses of the separated enantiomers (CSP-HPLC) were used to assign the absolute configuration. High configurational stability is observed for both regioisomers as racemization does not occur after 1 week of heating at 208 °C in dodecane (ΔG ‡ > 41 kcal·mol–1). Interestingly, reactions performed at 40 °C retain a certain level of enantiospecificity (82–86%), avoiding, for the most part, thermal racemization of the starting material.

Heteroaggregation of graphene oxide with minerals in aqueous phase.

Upon release into waters, sediments, and soils, graphene oxide (GO) may interact with fine mineral particles. We investigated the heteroaggregation of GO with different minerals, including montmorillonite, kaolinite, and goethite, in aqueous phase. GO significantly enhanced the dispersion of positively charged goethite (>50%) via heteroaggregation, while there was no interaction between GO and negatively charged montmorillonite or kaolinite. Electrostatic attraction was the dominant force in the GO-goethite heteroaggregation (pH 4.0-8.5), and the dissolved Fe ions (<0.16 mg/L) from goethite were unable to destabilize GO suspension. The GO-goethite heteroaggregation was further quantitatively investigated through GO adsorption study. All adsorption isotherms of GO at different solution pH (4.0 and 6.5) followed the Linear model. The apparent intercept (1.0-6.9 mg/g) was observed for all the adsorption isotherms, indicating that this fraction of adsorbed GO was difficult to desorb from goethite (defined here as irreversible adsorption) under the tested conditions. Desorption hysteresis was observed, which could be explained by the formation of multilayered GO-goethite complex with high configurational stability. These findings are useful for understanding the interaction of GO with mineral surfaces, and potential fate and toxicity of GO under natural conditions in aquatic environments, as well as in soils and sediments.

Racemization Mechanisms and Electronic Circular Dichroism of [4]Heterohelicenium Dyes: A Theoretical Study

Triarylmethylium cations with the three rings linked by two bridging groups constitute a special class of [4]heterohelicenium dyes that combine high configurational stability with the optical properties of classic dye compounds. The racemization barriers and electronic circular dichroism of seven [4]heterohelicenium analogues are investigated with density functional theory to consider different design strategies. The racemization barriers are examined with the B3LYP functional utilizing the basis set 6-31+G* with respect to bridging heteroatoms in the helicenium motif and with different bulky substituents in the helix pitch. The racemization barriers of the [4]heterohelicenium are found to be highly dependent on the relative size of the bridging atom. Nucleophilic attack at the carbenium ion leads to formation of center adducts for which the racemization barriers are found to be lowered by 8-12 kJ/mol. This finding of a nucleophilic racemization catalysis may be rationalized by the loss of conjugation upon formation of an sp(3)-hybridized carbon in the center adducts. The effect of the heteroatom substitution and center adduct formation is reflected in the electronic properties of the compound calculated with the Coulomb-attenuated method CAM-B3LYP with the basis set 6-311++G**. The excitation energies are found to be highly dependent on the twisting of the helicenium framework and only weakly influenced by the electronic nature of the bridging substituent. The electronic circular dichroism is evaluated, as the rotational strength is found to be highly dependent on both the overall molecular structure and substitution pattern but with no simple correlation between structure and circular dichroism (CD) response. The calculations reveal that the magnitude of rotational strength in most cases is dominated by the angle between the electronic and magnetic transition dipole moments. Finally, it is found that computational screening of many different structures and substituents might be needed to find target structures with maximized rotational strength.