Structure/energy correlation of bowl depth and inversion barrier in corannulene derivatives: combined experimental and quantum mechanical analysis.

Synthesis of a series of corannulene derivatives with varying bowl depths has allowed for a study correlating the structure (bowl depth) and the energy of bowl inversion. Substituents placed in the peri positions are repulsive and flatten the bowl, thus causing a decrease in the bowl inversion barrier. Conversely, annelation across the peri positions causes a deepening of the bowl, thus an increase in the bowl inversion barrier. Barriers between 8.7 and 17.3 kcal/mol have been measured, and their structures have been calculated using a variety of ab initio methods. The energy profile of an individual corannulene derivative is assumed to fit a mixed quartic/quadratic function from which an empirical correlation of bowl depth and inversion barrier that follows a quartic function is derived. Structure/energy correlations of this type speak broadly of the nature of enzymatic and catalytic activation of substrates.     

Chiral Atropisomeric Indenocorannulene Bowls: Critique of the Cahn–Ingold–Prelog Conception of Molecular Chirality

Chiral corannulenes abound, but suffer generally from configurational lability associated with bowl‐to‐bowl inversion, 1 thus obviating questions of stereogenicity and stereoelement construction. 2 In contrast, peri‐annulated corannulenes show greatly increased barriers for bowl‐to‐bowl inversion; specifically indenocorannulenes invert on a time scale too slow to observe by normal NMR methods and raise the possibility of creating chiral atropisomeric bowl‐shaped aromatics. 3 Two methods for preparing indenocorannulene from simple 2‐haloarylcorannulenes—silyl cation C–F activation, 4 and Pd‐mediated C–Cl activation^[5]—enable the synthesis of an array of such chiral atropisomeric indenocorannulenes. 6 Resolution of the enantiomers by high‐performance liquid chromatography over chiral support phases motivates the study of chiroptical properties, the assignment of absolute “Cartesian” configuration, and the assessment of configurational stability. 7 These studies bring into question any systematic assignment of nontrivial stereoelements (i.e. not the molecule in its entirety) and refute any assertion of congruence between “Cahn–Ingold–Prelog elements” and the physical or “Cartesian” basis of chirality.     

Redox‐Dependent Transformation of a Hydrazinobuckybowl between Curved and Planar Geometries

A red‐fluorescent heterobuckybowl with an embedded hydrazine structure was synthesized from a cyclobiphenothiazine derivative via a strained cyclobicarbazole. The hydrazinobuckybowl was found to possess bowl and twist structures in the neutral state, a shallow bowl structure in the monocation state, and a planar structure in the dication state by means of X‐ray crystallographic analysis, DFT calculations, and a comparison of experimental and calculated ¹³C NMR chemical shifts. The hydrazinobuckybowl is the first buckybowl that changes its geometry between curved bowl/twist structures and a planar structure depending on the oxidation state. The drastic geometrical change was possible as a result of the presence of two heteroatoms in the bowl skeleton and the multiple reversible redox reactions of the compound. Owing to the two kinds of bowl and twist conformations, the bowl‐inversion dynamics of the hydrazinobuckybowl were found to follow a triple‐well potential model.     

Shaping Antiaromatic π‐Systems by Metalation: Synthesis of a Bowl‐Shaped Antiaromatic Palladium Norcorrole

The synthesis of a bowl‐shaped antiaromatic molecule was achieved through the deformation of a planar antiaromatic porphyrinic π‐conjugation system by insertion of palladium into the small cavity of a metal‐free norcorrole. The bowl‐to‐bowl inversion dynamics of the antiaromatic Pd‐coordinated norcorrole was determined by variable‐temperature ¹H NMR spectroscopy. The metal‐free norcorrole was prepared from acid‐induced demetalation of a copper norcorrole, which was obtained from the intramolecular coupling of a bis(diiododipyrrin) copper complex with copper thiophenecarboxylate.     

Bowl Inversion in an Exo‐type Supramolecule in the Solid State

Bowl inversion is a unique property of buckybowls. The polarity and assembly configuration of buckybowls are reversed after bowl inversion. So far, this unique phenomenon has been studied in solution and on surface, but not in solid state due to spatial constraint. Now a series of exo‐type supramolecular assemblies of trithiasumanene and nanographene are investigated. Tuning the electron density of the nanogaphene component was found to directly affect the binding constant of the complex. Reversible bowl inversion in the solid state was then successfully achieved by subjecting the trithiasumanene–nanographene assembly with the weakest binding strength to repeated heating–cooling cycles, which was unambiguously observed by single crystal X‐ray diffraction.     

Fluorous Corannulenes: Ab initio Predictions and the Synthesis of sym-Pentafluorocorannulene

Ten sym-penta and deca-X substituted corannulenes (1–10; X=H, F, CH₃, or CF₃) define a library of fluorous compounds comprising high symmetry non-planar aromatic compunds. They provide a group of structurally similar, yet physically distinct structures manifesting special chemical behavior related to their degree of fluorination. Owing to their bowl forms, corannulene derivatives are distinct from planar polynuclear aromatic compounds; they have relatively high dipole moments, accept 1–4 electrons, and display room temperature fluorescence as well as low temp phosphorescence. Electronic structure theory predicts the bowl inversion barrier and physical properties. The syntheses of sym-pentafluorocorannulene by an efficient late stage fluorination affords a key derivative to calibrate predictions.     

Fluorous Corannulenes: Ab initio Predictions and the Synthesis of sym‐Pentafluorocorannulene

Ten sym‐penta and deca‐X substituted corannulenes (1–10; X=H, F, CH₃, or CF₃) define a library of fluorous compounds comprising high symmetry non‐planar aromatic compunds. They provide a group of structurally similar, yet physically distinct structures manifesting special chemical behavior related to their degree of fluorination. Owing to their bowl forms, corannulene derivatives are distinct from planar polynuclear aromatic compounds; they have relatively high dipole moments, accept 1–4 electrons, and display room temperature fluorescence as well as low temp phosphorescence. Electronic structure theory predicts the bowl inversion barrier and physical properties. The syntheses of sym‐pentafluorocorannulene by an efficient late stage fluorination affords a key derivative to calibrate predictions.     

A Quintuple [6]Helicene with a Corannulene Core as a C5‐Symmetric Propeller‐Shaped π‐System

The synthesis and structural analysis of a quintuple [6]helicene with a corannulene core is reported. The compound was synthesized from corannulene in three steps including a five‐fold intramolecular direct arylation. X‐ray crystallographic analysis revealed a C₅‐symmetric propeller‐shaped structure and one‐dimensional alignment in the solid state. The enantiomers of the quintuple [6]helicene were successfully separated by HPLC, and the chirality of the two fractions was identified by CD spectroscopy. A kinetic study yielded a racemization barrier of 34.2 kcal mol^?1, which is slightly lower than that of pristine [6]helicene. DFT calculations indicate a rapid bowl‐to‐bowl inversion of the corannulene moiety and a step‐by‐step chiral inversion pathway for the five [6]helicene moieties.     

Is peri hydrogen repulsion responsible for flattening buckybowls? The effect of ring annelation to the rim of corannulene

B3LYP/6-31G* calculations indicate that annelation of three-, four-, or five-membered rings to the rim of corannulene results in lowering the bowl-to-bowl inversion barrier and flattens the bowl structure. The role of the peri hydrogen repulsion as a causative factor for such a behavior is disputed, the structure-energy relationships in this class of compounds were shown to fit with a mixed quartic-quadratic potential.     

Interaction between anions and substituted molecular bowls

Complexes formed by anions and substituted molecular bowls were studied by means of computational methods. The bowls consisted of corannulene molecules substituted with five or ten F, Cl, or CN groups, whereas Cl(-), Br(-) and BF(4)(-) were the anions considered. Substitution with F, Cl and CN hardly affects the geometry of the bowl, but produces an inversion of the molecular electrostatic potential of the bowls, which becomes positive over the two faces of the bowl, therefore interacting favorably with anions. In all cases considered, the most stable complex presents the anion interacting with the concave side of the bowl. The strength of the interaction roughly follows the values of molecular electrostatic potential, being more stable as more positive is the potential. The preference of anions to interact with the concave side of the bowls has its origin in stronger electrostatic and dispersion interactions. Though the solvent produces an important decrease in the stability of the complexes, the results suggest the possibility of employing these substituted buckybowls as anion receptors with a preferential concave complexation, especially for large anions.     

反散射理论导出He-Ne相互作用势

以Ｈｅ原子束与Ｎｅ原子束交叉碰撞实验数据为基础。以反散射方法为手段，理论上导出Ｈｅ－Ｎｅ原子相互作用势，所得的势呈显出ｖａｎ ｄｅｒ Ｗａａｌｓ势的特征，即短程为强排斥，中程为碗形吸收，远程为零，这样的势能正确的，符合实验的弹发型用射微分截面。     

Asymmetric synthesis of a chiral buckybowl, trimethylsumanene

The first asymmetric synthesis of a chiral buckybowl, a C3 symmetric (C)-8,13,18-trimethylsumanene (1), was achieved by employing a synthetic strategy that translates chirality at sp3 centers into bowl chirality. The synthesis features a syn selective cyclotrimerization of an enantiopure halonorbornene derivative, tandem ring-opening/closing olefin metathesis reactions, and DDQ oxidation at low temperature. The bowl-to-bowl inversion energy of 1 was determined as 21.6 kcal/mol by circular dichroism spectra measurement.     

Heterobuckybowls: a theoretical study on the structure, bowl-to-bowl inversion barrier, bond length alternation, structure-inversion barrier relationship, stability, and synthetic feasibility

Hybrid density functional theory (DFT) calculations at the B3LYP/cc-pVDZ level have been performed on a series of heterobuckybowls, 3X, C(18)X(3)H(6) (X = O, NH, CH(2), BH, S, PH, PH(3), Si, SiH(2), and AlH). The minimum energy conformations and the transition states for bowl-to-bowl inversion, where the geometry is bowl shaped, are computed and characterized by frequency calculations. The geometries of heterotrindenes, 2X, C(12)X(3)H(6) (X = O, NH, CH(2), BH, S, PH, PH(3), Si, SiH(2), and AlH), were obtained, and the bond length alternation (Delta) in the central benzenoid ring shows remarkable sensitivity as a function of substituent with a wide range of fluctuations (-0.014 to +0.092 A). The Delta computed in 2BH was found to be comparable with the highest bond alternation reported to date in benzenoid frameworks. The inversion dynamics of these heterobowls and their bowl depths were fit to a mixed quartic/quadratic function. The size of the heteroatom seems to exclusively control the bowl depth and rigidity as well as the synthetic feasibility. In contrast, the bond length alternation seems to be controlled by electronic factors and not by the size of the substituted atom either in trindenes or in heterosumanenes. The thermodynamic stability of this class of compounds is very much comparable with trithiasumanene (3S), which has been synthesized recently. The chemical hardness (eta) was measured to assess the stability of the heterosumanenes. The strain energy buildup in a sequential ring closure strategy along two synthetic routes, namely a triphenylene route and a trindene route, were explored, and the trindene route was found to be highly favorable for making such compounds compared to the triphenylene route. However, in both routes the ease of the synthetic feasibility increases as the size of the heteroatom increases.     

基于全氟酞菁铜和碗烯双分子体系在银和石墨表面自组装行为的低温扫描隧道显微镜研究

Corannulene (COR) is considered a promising molecular building block for organic electronics owing to its intriguing geometrical and electronic properties. Intensive research efforts have been devoted to understanding the assembly behavior and electronic structure of COR and its derivatives on various metal surfaces via low-temperature scanning tunneling microscopy (LT-STM). Here we report the formation of binary molecular networks of copper hexadecafluorophthalocyanine (F₁₆CuPc)-COR self-assembled on the highly oriented pyrolytic graphite (HOPG) and Ag (111) substrates. Intermolecular hydrogen bonding between F₁₆CuPc and COR facilitates the formation of binary molecular networks on HOPG and further induces a preference for bowl-down configured COR molecules. This observed configuration preference disappears on Ag (111) substrate, where COR molecules lie on the substrate with their bowl openings pointing up and down randomly. We propose that strong interfacial interactions between the molecule and Ag (111) surface constrain the bowl inversion of the COR molecule, which thus retains its initial configuration upon adsorption.     

Synthesis of fluoranthenes and indenocorannulenes: elucidation of chiral stereoisomers on the basis of static molecular bowls

Cycloaromatization of a peri diyne with an external alkyne provides a general route to indeno-fused polynuclear aromatics. Fluoranthenes 9 (or 13) are easily accessible in good to excellent yields (75-99%; 18 examples) from the reaction of symmetric (or asymmetric) diynes 4 and alkynes 5 (or norbornadiene) in the presence of Wilkinson's catalyst. This formal [(2+2)+2] cycloaddition can also be applied to generate various indenocorannulenes 28 from 2,3-diethynylcorannulene derivatives 27 and alkynes 5. The indenocorannulenes 28 exist in a static bowl form at room temperature with bowl-to-bowl inversion barriers higher than 24 kcal/mol. This barrier renders the rate of inversion slow enough at room temperature to establish a class of chiral, bowl-shaped stereoiomers containing no tetrahedrally ligated atoms. The crystal structure of 28g provides insight into the bowl-shaped geometry of these compounds. This new synthetic method occurs under neutral conditions and tolerates various functional groups (e.g., alkyl, aryl, alcohols, and esters).     

Steric isotope effects gauged by the bowl-inversion barrier in selectively deuterated pentaarylcorannulenes

Motivated by a greater bowl depth and barrier to bowl inversion in sym-1,3,5,7,9-pentamanisylcorannulene compared to corannulene, an experimental plan is developed to measure the effective hydrogen/deuterium steric kinetic isotope effect (KIE). Symmetry arguments are used to design orthogonal isotope labeling patterns so that the barrier for the CD3 compound can be measured in the presence of the CH3 compound. This scheme eliminates the differential uncertainty in the temperature measurement by allowing both barriers to be measure in the same sample, which in turn reduces the error in determining the differential barrier. Ab initio computations corroborate the structure and isotope effect found experimentally. The predicted and determined steric KIE at 250 K is 1.08 (modified QUIVER at M06-2X/cc-pVDZ) and 1.22 +/- 0.06 (VT-NMR), respectively. The results stem from differences in zero-point energy of the CH and CD motions; however, the phenomenology makes the CD3 group appear effectively "stickier" than CH3. The more the C-H...X interaction steepens the well, the "stickier" C-D should appear to be relative to C-H--an important consideration for molecular recognition and one supported by stronger binding constants for deuterated substrates.     

A Nanoboat with Fused Concave N‐Heterotriangulene

The surface extension of all‐carbon based bowl‐shaped molecules, such as corannulene and sumanene, to synthesize even larger buckybowls has been widely studied, leaving other concave compounds with heteroatoms less considered. Herein we present a highly curved molecule synthesized via stepwise cyclization of fjords of a bisacridone derivative. Crystallographic analysis unambiguously confirmed a boat‐shaped structure with deformed bottom benzene ring. Theoretical calculation unravels an inversion process with an S‐shaped transition structure rather than a planar one. The enlarged boat demonstrates interesting properties, such as red shifts in absorption and emission spectra, enhanced emission intensity, and convergent frontier molecular orbital energy levels, in comparison to the related concave N‐heterotriangulene.     

Highly Curved Bowl‐Shaped Fragments of Fullerenes: Synthesis,Structural Analysis,and Physical Properties

Highly curved buckybowls 3 , 4 , and 5 were synthesized from planar precursors, fluoranthenes 8 , benzo[k]fluoranthenes 10 and naphtho[1,2‐k]‐cyclopenta[cd]fluoranthenes 12 , respectively, using straightforward palladium‐catalyzed cyclization reactions. These fluoranthene‐based starting materials were easily prepared from 1,8‐bis(arylethynyl)naphthalenes 6 . Both buckybowls 3 and 4 are fragments of C₆₀, whereas 5 is a unique subunit of C₇₀. The curved structures were identified by X‐ray crystallography, and they are deep bowls. The maximum π‐orbital axis vector (POAV) pyramidalization angle in both 3 and 4 is 12.8°. Such a high curvature is very rarely obtained. Buckybowls 5 are less curved than the others because they have a lower density of five‐membered rings, analogous to the tube portion of C₇₀. Cyclopentaannulation increases the bowl depths of 3 and 4 , but not the maximum POAV pyramidalization angle. Among the eight buckybowls studied herein, five form polar crystals. The bowl‐to‐bowl inversion dynamics of these buckybowls can be classified into two types; one has a planar transition structure, whereas the other has an S‐shaped transition structure. A larger longitudinal length of these buckybowls corresponds to a stronger preference for the latter. The photophysical properties of these buckybowls were examined and compared with those of C₆₀ and C₇₀. Buckybowls 5 have absorption bands at wavelengths greater than 450 nm, which are similar to those of C₇₀. The chiral resolution of the mono‐substituted buckybowl 4 ac was also studied by using HPLC with a chiral column.     

Synthesis and Properties of 2,3‐Dihydro‐1H‐corannuleno[2,3‐cd]pyridine (=2,3‐Dihydro‐1H‐dibenzo[1,10 : 6,7]fluorantheno[3,4‐cd]pyridine) Derivatives: Heterocyclic peri‐Anellated Corannulenes

The anellation of a 6‐membered ring to the 2,3‐position of corannulene (=dibenzo[ghi,mno]fluoranthene; 1 ) leads to curved aromatic compounds with a significantly higher bowl‐inversion barrier than corannulene (see Fig. 1). If the bridge is −CH₂−NR−CH₂−, a variety of linkers can be introduced at the N(2) atom, and the corresponding curved aromatics act as versatile building blocks for larger structures (see Scheme). The locked bowl, in combination with an amide bond (see 9 and 10 ), gives rise to corannulene derivatives with chiral ground‐state conformations, which possess the ability to adapt to their chiral environment by shifting their enantiomer equilibrium slightly in favor of one enantiomeric conformer. Rim annulation of corannulene seems to display a significantly lower electron‐withdrawing effect than facial anellation on [5,6]fullerene‐C₆₀‐I_h, as determined by an investigation of the basicity at the N‐atom of CH₂−NR−CH₂ (see 4 vs. 15 in Fig. 2).     

Enantioselective addition of diethylzinc to benzaldehyde catalysed by chiral, bridged resorcinarenes: a stereoselectivity model based on chirality transfer

The enantioselective addition of diethylzinc to benzaldehyde catalysed by a range of chiral bridged resorcinarenes has been studied, and the results used as a means of probing cooperative effects in the resorcinarene bowl. A structure-activity relationship has emerged in which bridged resorcinarenes with little available room in the bowl (e.g., 3b, 3c) favour R-enantioselectivity in the product, while those promoting cooperative effects in the bowl via coordination sites in the bridge (3e) or strong donor protecting groups (3j) favour S-enantioselectivity. A mechanistic hypothesis based on Noyori's model to account for these trends has been put forward in which stereoselectivity is dependent on two factors as the ratio of axially diastereomeric anti-zincoxazines as well as the exo or endo bias of active zinc. The model explains why ee's are never greater than around 50% and indicates asymmetric induction to be due to the axial chirality in the bowl as a result of chirality transfer from the bridge, rather than due to induction via the central chirality in the line of the bridge. As a result, the model stimulates some new fascinating possibilities in enzyme mimicry.