Chiral Buckybowl Molecules

Buckybowls are polynuclear aromatic hydrocarbons that have a curved aromatic surface and are considered fragments of buckminsterfullerenes. The curved aromatic surface led to the loss of planar symmetry of the normal aromatic plane and may cause unique inherent chirality, so-called bowl chirality, which it is possible to thermally racemize through a bowl-to-bowl inversion process. In this short review, we summarize the studies concerning the special field of bowl chirality, focusing on recent practical aspects of attaining diastereo/enantioenriched chiral buckybowls through asymmetric synthesis, chiral optical resolution, selective chiral metal complexation, and chiral assembly formation.


Introduction
Studies on the physical and chemical characteristics of buckybowls have attracted significant interest owing to their direct structural correlation with fullerenes and potential applications in the realm of material science (Figure 1) [1][2][3].Corannulene 1 and sumanene 2, two prototypical buckybowls, are both highly curved π-conjugated aromatic molecules representing the substructures of fullerene.The smallest C 3v -symmetric fragment 2 comprises a central six-membered ring surrounded by three five-and three six-membered rings fused in an alternating fashion (Figure 1), whereas the smallest C 5 -symmetric fragment 1 contains a central five-membered ring surrounded by five six-membered rings (Figure 1).Regioselective allocation of the five-or six-membered ring subunits around the centroid pentagon or hexagon, respectively, gave the curved bowl-shaped structures.Certain buckybowls are inherently chiral and difficult to separate from the racemates due to their rapid bowl inversion [4].Controlling the bowl chirality of these π-conjugates and obtaining enantioenriched buckybowls are challenging tasks for chemists, while buckybowls application in chiral materials is highly promising [5][6][7][8][9][10][11][12][13][14].Further extension of the π-conjugation of these chiral enantioenriched buckybowls leads to homochiral carbon nanotubes, and these chiral carbon materials create new perspectives in chiral catalysis, chiral sensing, chiral separation sciences, etc. [15][16][17][18][19][20][21][22][23][24][25][26][27].A better understanding of bowl chirality will help researchers find a good way to control the chiral self-assembly of carbon nanotubes (CNTs) or fullerenes, which have already exhibited exciting potential as next-generation functional materials [28][29][30][31][32][33][34][35][36][37][38].While several buckybowl-related reviews have been published in recent years [6,[39][40][41][42][43][44], most of them focused on the synthetic strategy and derivation method.Only a few publications involve the stereochemistry or chirality of the buckybowls.In this short review, we summarize recent progress on the stereochemistry of buckybowls and their derivatives.It covers recent developments in the asymmetric synthesis and optical resolution of buckybowls from their racemates, and application of the enantioenriched buckybowls in the chiral aggregate formation and homochiral CNT syntheses [45,46].These structurally well-defined single-chiral CNTs exploit truly exciting applications ranging from space elevator to armchair quantum wire and bio-related applications [47].Chiral buckybowls that possess a stable concave or convex shape are suitable for the construction of chiral molecular recognition sites [48].They may also coordinate metals by virtue of their electron-rich/-deficient properties and thus have the potential for enantioselective organocatalysis [49,50].Chiral buckybowls, acting as chiral building blocks, can also be used for constructing helical assemblies and preparing homochiral crystalline organic materials.

Classification of Bowl Chirality
Bowl-shaped carbon-based π-conjugated aromatic compounds, so-called "buckybowls", are the partial structure of fullerenes or CNTs (Figure 1).These three-dimensional (3D) curved π-electron systems may possess inherent chirality and present many similarities to chiral fullerenes and CNTs, which is often defined as "bowl chirality" for convenience.The bowl-shaped polyaromatic hydrocarbon chiral buckybowl molecules can be broadly classified into three categories (Figure 2) based on the origin of their chirality.The conjugated bowl structure itself possesses the chirality, such as π-extended buckybowls, e.g., hemifullerene 3; bowl chirality caused by introduction of one or more substituents on the rim of the buckybowl, e.g., trimethylsumanene 4; and bowl chirality originated from the introduction of heteroatom into the π-bowl carbon skeleton, e.g., triazasumanene 5.While several buckybowl-related reviews have been published in recent years [6,[39][40][41][42][43][44], most of them focused on the synthetic strategy and derivation method.Only a few publications involve the stereochemistry or chirality of the buckybowls.In this short review, we summarize recent progress on the stereochemistry of buckybowls and their derivatives.It covers recent developments in the asymmetric synthesis and optical resolution of buckybowls from their racemates, and application of the enantioenriched buckybowls in the chiral aggregate formation and homochiral CNT syntheses [45,46].These structurally well-defined single-chiral CNTs exploit truly exciting applications ranging from space elevator to armchair quantum wire and bio-related applications [47].Chiral buckybowls that possess a stable concave or convex shape are suitable for the construction of chiral molecular recognition sites [48].They may also coordinate metals by virtue of their electron-rich/-deficient properties and thus have the potential for enantioselective organocatalysis [49,50].Chiral buckybowls, acting as chiral building blocks, can also be used for constructing helical assemblies and preparing homochiral crystalline organic materials.

Stereodescriptor System of Buckybowls
Until now, the absolute configuration of bowl chirality of chiral buckybowl molecules was followed by two independent stereodescriptor systems: The stereodescriptor C or A based on fullerene nomenclature [17,54] and the another stereodescriptor P or M based on the Cahn-Ingold-Prelog (CIP) sequence rule [32].The numbering system of fullerenes follows in general a helical numbering path, starting from the lowest set of numbers for substituents.To assign a descriptor in the chiral fullerene system, the viewer looks from the outside of the fullerene cage at the polygon to start the numbering and trace the path of numbering C(1)-C(6) (Figure 3a) [54].The chirality of buckybowls is assigned in the same way, and a viewer looks from the convex face and traces the path of numbering from atom C(1) to C(6) (Figure 3b) [17].If the numbering path describes a clockwise direction/travel, the configuration of the chiral bowl is designated as C. In contrast, if the path describes a counterclockwise direction/travel, the configuration of the chiral bowl is designated as A. Unlike common aromatic compounds, which often have a symmetry plane along the aromatic ring, buckybowls loss the cross-ring symmetry plane due to the curved shape.Unsubstituted buckybowls 1 and 2 are not chiral due to the presence of the reflection symmetry with respect to the mirror planes containing the rotational axis to show C 5v and C 3v symmetry, respectively.Introducing the addends may break the reflection symmetry to cause chirality, and alter their properties, such as bowl-to-bowl inversion [12][13][14][15][16][17][18][19][20][21][22], chirality [12,17,19,23], bowl depth [14,19,20], crystal structure [19,24,25], molecular recognition [2, 26,27,51] and supramolecular assembly [3,[28][29][30]52,53] behavior, metal complexation [23,[31][32][33][34][35][36], electronic conductivity [19,37,38], and so on.Although the chirality is an important element in three-dimensional curved π-electron systems, thus far there have been no reports of the enantioselective synthetic control of the bowl chirality.

Stereodescriptor System of Buckybowls
Until now, the absolute configuration of bowl chirality of chiral buckybowl molecules was followed by two independent stereodescriptor systems: The stereodescriptor C or A based on fullerene nomenclature [17,54] and the another stereodescriptor P or M based on the Cahn-Ingold-Prelog (CIP) sequence rule [32].The numbering system of fullerenes follows in general a helical numbering path, starting from the lowest set of numbers for substituents.To assign a descriptor in the chiral fullerene system, the viewer looks from the outside of the fullerene cage at the polygon to start the numbering and trace the path of numbering C(1)-C(6) (Figure 3a) [54].The chirality of buckybowls is assigned in the same way, and a viewer looks from the convex face and traces the path of numbering from atom C(1) to C(6) (Figure 3b) [17].If the numbering path describes a clockwise direction/travel, the configuration of the chiral bowl is designated as C. In contrast, if the path describes a counterclockwise direction/travel, the configuration of the chiral bowl is designated as A. Another stereodescriptor system P or M for chiral buckybowls uses the CIP sequence rule [32].Based on the CIP priority rule [55], the "non-fusion peripheral atoms" of buckybowl molecules are compared, and one atom with the highest CIP priority is chosen as the first priority (point of origin, ①, Figure 4) [17,18].Furthermore, for the next highest CIP priority atom ② versus ①, compare the two neighboring rim atoms attached to this original point, and subsequent atoms attached thereto.For a viewer looking from the concave face of the buckybowl molecule, i.e., looking "into" the cavity of the bowl, the path of CIP priority numbering is from the original point atom to the neighboring atom with higher priority (①→②).If the path of CIP priority numbering direction describes clockwise travel, the configuration of the chiral bowl is designated as P. Otherwise, if the path of CIP priority numbering travels in a counterclockwise direction, the descriptor is M. The numbering to specify the positions of substituents follows the nomenclature of fused ring systems.Another stereodescriptor system P or M for chiral buckybowls uses the CIP sequence rule [32].Based on the CIP priority rule [55], the "non-fusion peripheral atoms" of buckybowl molecules are compared, and one atom with the highest CIP priority is chosen as the first priority (point of origin,

Bowl Inversion/Racemization of Chiral Buckybowls
A unique property of buckybowls lies in that these molecules can thermally flip their curvature in solution via bowl-to-bowl inversion or concave-convex transition.During the bowl inversion process, the inherent bowl chirality inverts; this process is also called racemization (Figure 5a) [12,[17][18][19].The nonpolar polyaromatic small bowl molecules, like 1 and 2 can thermally flip through a planar transition state and the required energy to cross this transition barrier is defined as bowl inversion energy (ΔE) (Figure 5b) [20].In the solid state, buckybowl molecules exist in a stack structure and show excellent physical properties, such as electron conductivity [24].
The thermally driven bowl-to-bowl inversion is of interest to basic science and can find various applications in functional materials for sensors, chemical machines, or ferroelectric memories.The lifetime of enantiopure chiral buckybowls is controlled by the bowl inversion energy (ΔE).The enantiopure chiral buckybowls are expected to contribute to a variety of applications such as asymmetric molecular recognition, homochiral crystal organic materials, chiral self-assembly, and chiral organometallic catalysis [6,7,9].
The introduction of substituents or heteroatoms in the carbon skeleton of buckybowls changes the bowl inversion energy within the range of 20-40 kcal/mol, and therefore could change the lifetimes of chiral buckybowls considerably.Furthermore, the nature, position, number of substituents, and stereoelectronic effects also significantly influence the bowl shape, depth (x), lifetime of keeping chirality, electron-deficient/-rich nature, and other physicochemical properties [56].The bowl-inversion energy of corannulene derivatives was first determined by Scott and co-workers [13].The interesting dynamic behavior of various buckybowl molecules has been extensively studied by Scott [57], Siegel [14,16], Hirao [15,35], Sakurai [12,[17][18][19][20], and others.In particular, Sakurai and co-workers studied the effect of the substituent R at the benzylic position of sumanene 11, which possesses two different conformers, endo-R and exo-R, differing in the direction of the aromatic bowl shape, concave or convex (Figures 6 and 7

Bowl Inversion/Racemization of Chiral Buckybowls
A unique property of buckybowls lies in that these molecules can thermally flip their curvature in solution via bowl-to-bowl inversion or concave-convex transition.During the bowl inversion process, the inherent bowl chirality inverts; this process is also called racemization (Figure 5a) [12,[17][18][19].The nonpolar polyaromatic small bowl molecules, like 1 and 2 can thermally flip through a planar transition state and the required energy to cross this transition barrier is defined as bowl inversion energy (∆E) (Figure 5b) [20].In the solid state, buckybowl molecules exist in a stack structure and show excellent physical properties, such as electron conductivity [24].
The thermally driven bowl-to-bowl inversion is of interest to basic science and can find various applications in functional materials for sensors, chemical machines, or ferroelectric memories.The lifetime of enantiopure chiral buckybowls is controlled by the bowl inversion energy (∆E).The enantiopure chiral buckybowls are expected to contribute to a variety of applications such as asymmetric molecular recognition, homochiral crystal organic materials, chiral self-assembly, and chiral organometallic catalysis [6,7,9].
The introduction of substituents or heteroatoms in the carbon skeleton of buckybowls changes the bowl inversion energy within the range of 20-40 kcal/mol, and therefore could change the lifetimes of chiral buckybowls considerably.Furthermore, the nature, position, number of substituents, and stereoelectronic effects also significantly influence the bowl shape, depth (x), lifetime of keeping chirality, electron-deficient/-rich nature, and other physicochemical properties [56].The bowl-inversion energy of corannulene derivatives was first determined by Scott and co-workers [13].The interesting dynamic behavior of various buckybowl molecules has been extensively studied by Scott [57], Siegel [14,16], Hirao [15,35], Sakurai [12,[17][18][19][20], and others.In particular, Sakurai and co-workers studied the effect of the substituent R at the benzylic position of sumanene 11, which possesses two different conformers, endo-R and exo-R, differing in the direction of the aromatic bowl shape, concave or convex (Figures 6 and 7  Introduction of substituents will influence the geometries/conformations of buckybowls and bowl inversion energies as well.The effect of substituents in corannulene has been studied by Siegel and co-workers [14,16].They demonstrated that introducing the acyclic substituents in corannulene usually decreases the bowl depth and increases the bowl inversion energy.Based on the results of the experimental and theoretical study, they proposed a quantitative equation for bowl inversion energy, which relates the bowl depth to the bowl inversion energy (Figure 7) [14,16].Steric and electronic factors of substituents also affect the bowl structure and bowl inversion energy of sumanene.Recently, Sakurai Introduction of substituents will influence the geometries/conformations of buckybowls and bowl inversion energies as well.The effect of substituents in corannulene has been studied by Siegel and co-workers [14,16].They demonstrated that introducing the acyclic substituents in corannulene usually decreases the bowl depth and increases the bowl inversion energy.Based on the results of the experimental and theoretical study, they proposed a quantitative equation for bowl inversion energy, which relates the bowl depth to the bowl inversion energy (Figure 7) [14,16].Steric and electronic factors of substituents also affect the bowl structure and bowl inversion energy of sumanene.Recently, Sakurai Introduction of substituents will influence the geometries/conformations of buckybowls and bowl inversion energies as well.The effect of substituents in corannulene has been studied by Siegel and co-workers [14,16].They demonstrated that introducing the acyclic substituents in corannulene usually decreases the bowl depth and increases the bowl inversion energy.Based on the results of the experimental and theoretical study, they proposed a quantitative equation for bowl inversion energy, which relates the bowl depth to the bowl inversion energy (Figure 7) [14,16].Steric and electronic factors of substituents also affect the bowl structure and bowl inversion energy of sumanene.Recently, Sakurai Introduction of substituents will influence the geometries/conformations of buckybowls and bowl inversion energies as well.The effect of substituents in corannulene has been studied by Siegel and co-workers [14,16].They demonstrated that introducing the acyclic substituents in corannulene usually decreases the bowl depth and increases the bowl inversion energy.Based on the results of the experimental and theoretical study, they proposed a quantitative equation for bowl inversion energy, which relates the bowl depth to the bowl inversion energy (Figure 7) [14,16].Steric and electronic factors of substituents also affect the bowl structure and bowl inversion energy of sumanene.Recently, Sakurai and co-workers found that sumanene 2 attached with a mono-iodo substituent induces a slightly deeper bowl depth and a higher bowl inversion energy compared with other sumanene derivatives because of its steric effect, whereas the electronic effects caused by formyl and nitro substituents induced a shallower bowl depth and a lower bowl inversion energy [20].
Siegel and co-workers studied the bowl-to-bowl inversion of corannulene 1 and ethylcorannulene derivatives mediated by extended tetracationic cyclophane (Figure 8) [22,30].This synthetic host forms a supramolecular complex with 1 and ethylcorannulene via induced fit, and the bowl-to-bowl inversion process was accelerated by a factor of 10 at room temperature.Upon host-guest complexation, the transition state of the guest is stabilized through the stereoelectronic reorganization of the host, which switched from a strained conformation to an energetically favored conformation.Further, the experimental and DFT calculations for bowl-to-bowl inversion process of the host-guest complexion of 1 demonstrates the decrease of bowl-to-bowl inversion energy barrier of corannulene (Figure 8).and co-workers found that sumanene 2 attached with a mono-iodo substituent induces a slightly deeper bowl depth and a higher bowl inversion energy compared with other sumanene derivatives because of its steric effect, whereas the electronic effects caused by formyl and nitro substituents induced a shallower bowl depth and a lower bowl inversion energy [20].
Siegel and co-workers studied the bowl-to-bowl inversion of corannulene 1 and ethylcorannulene derivatives mediated by extended tetracationic cyclophane (Figure 8) [22,30].This synthetic host forms a supramolecular complex with 1 and ethylcorannulene via induced fit, and the bowl-to-bowl inversion process was accelerated by a factor of 10 at room temperature.Upon host-guest complexation, the transition state of the guest is stabilized through the stereoelectronic reorganization of the host, which switched from a strained conformation to an energetically favored conformation.Further, the experimental and DFT calculations for bowl-to-bowl inversion process of the host-guest complexion of 1 demonstrates the decrease of bowl-to-bowl inversion energy barrier of corannulene (Figure 8).

Heterobuckybowls
Doping of heteroatoms to the carbon frameworks of bowl-shaped aromatic compounds drastically modulates their geometrical structure and physical and chemical properties.After the successful synthesis of corannulene and sumanene derivatives, researchers began to try to synthesize hetero atom-doped buckybowl molecules, so-called "heterobuckybowls".The introduction of heteroatoms into the carbon skeleton is expected to yield altered electronic properties of basic carbon skeleton buckybowls, especially electron-deficient or -rich in nature.Notably, the substitution of a hetero atom in its carbon skeleton also causes the geometrical change, especially the depth of the bowl.As a result of the change in depth of the bowl, the activation energy for the bowl-to-bowl inversion energy is also altered.The higher the bowl inversion energy is, the more stable the chiral conformer will be.Several heterobuckybowls have been reported in the literature, some of them chiral (Figure 9) [12,[58][59][60][61][62][63][64][65][66].Comparison of energy profile of bowl-to-bowl inversion of corannulene 1 (black) and tetracationic cyclophane: 1, complex (green) in Me 2 CO; absolute contributions of ground-state (GS) destabilization (0.5 kcal mol −1 ) and transition-state (TS) stabilization (2.0 kcal mol −1 ) to the overall energy-barrier decrease (∆∆G ‡ catalysis ) of the bowl-to-bowl inversion process of 1 inside the tetracationic cyclophane calculated by DFT (B97D/Def2-TZVPP).Reprinted from [22].© 2014 Nature Publishing Group.

Heterobuckybowls
Doping of heteroatoms to the carbon frameworks of bowl-shaped aromatic compounds drastically modulates their geometrical structure and physical and chemical properties.After the successful synthesis of corannulene and sumanene derivatives, researchers began to try to synthesize hetero atom-doped buckybowl molecules, so-called "heterobuckybowls".The introduction of heteroatoms into the carbon skeleton is expected to yield altered electronic properties of basic carbon skeleton buckybowls, especially electron-deficient or -rich in nature.Notably, the substitution of a hetero atom in its carbon skeleton also causes the geometrical change, especially the depth of the bowl.As a result of the change in depth of the bowl, the activation energy for the bowl-to-bowl inversion energy is also altered.The higher the bowl inversion energy is, the more stable the chiral conformer will be.Several heterobuckybowls have been reported in the literature, some of them chiral (Figure 9) [12,[58][59][60][61][62][63][64][65][66].The heteroatoms embedded in the reported heterobuckybowls are mainly the group 14, 15, and 16 elements, such as nitrogen (second row), silicon, phosphorus and sulfur (third row), germanium and selenium (fourth row), and tin and tellurium (fifth row) (Figure 9).Introduction of heteroatoms in the periphery of buckybowls usually resulted in a decrease of the bowl depth of heterobuckybowls, because the bond lengths of carbon-heteroatom increased with the atomic size (Figure 9).The bowl depths of trithiasumane 16 (0.79Å) and triselenasumane 18 (0.47 Å) [64] have a shallower bowl depth than that of normal sumanene 2 (1.11 Å) [24].Trisilicasumanene 12 [59,63] and tritellurasumanene 19 have a plane structure, whereas triazasumanene 10 has a deeper bowl depth of 1.30 Å because the C-N bond length (1.47 Å) is shorter than the C-C bond length (1.54 Å) [12].The high reactivity of nitrogen facilitates the formation of the highly strained triazasumanene derivative with higher bowl-to-bowl inversion energy.More recently, Saito and co-workers prepared a triphosphasumanene trisulfide 13 derivative with a large dipole moment (12.0 D) [67].The bowl depth of syn-isomer 13 is 0.46 Å, which is identical to triselenasumane 18 (0.47 Å) [64] and shallower than sumanene [24], whereas the anti-isomer of triphosphasumanene trisulfide 13 exists in an almost plane structure [67].

Asymmetric Synthesis of Buckybowl and Azabuckybowl
Synthesis of buckybowls was a highly challenging task for chemists.The first synthesis of the pristine corannulene was accomplished in 1966 by applying a long synthetic route [68].The synthetic investigation was postponed until Scott et al. [69] and Siegel et al. [70] reinitiated the synthesis of corannulene with a succinct synthetic route by using the flash vacuum pyrolysis (FVP) method in the early 1990s.In the FVP method, the planar π-conjugated precursors are directly converted into buckybowls under high temperature to yield a racemic mixture [71].Many buckybowls were thus synthesized using this FVP method, whereas FVP method was unsuccessful for the synthesis of 2 [72].In 1996, Siegel and co-workers developed the first solution phase synthesis The heteroatoms embedded in the reported heterobuckybowls are mainly the group 14, 15, and 16 elements, such as nitrogen (second row), silicon, phosphorus and sulfur (third row), germanium and selenium (fourth row), and tin and tellurium (fifth row) (Figure 9).Introduction of heteroatoms in the periphery of buckybowls usually resulted in a decrease of the bowl depth of heterobuckybowls, because the bond lengths of carbon-heteroatom increased with the atomic size (Figure 9).The bowl depths of trithiasumane 16 (0.79Å) and triselenasumane 18 (0.47 Å) [64] have a shallower bowl depth than that of normal sumanene 2 (1.11 Å) [24].Trisilicasumanene 12 [59,63] and tritellurasumanene 19 have a plane structure, whereas triazasumanene 10 has a deeper bowl depth of 1.30 Å because the C-N bond length (1.47 Å) is shorter than the C-C bond length (1.54 Å) [12].The high reactivity of nitrogen facilitates the formation of the highly strained triazasumanene derivative with higher bowl-to-bowl inversion energy.More recently, Saito and co-workers prepared a triphosphasumanene trisulfide 13 derivative with a large dipole moment (12.0 D) [67].The bowl depth of syn-isomer 13 is 0.46 Å, which is identical to triselenasumane 18 (0.47 Å) [64] and shallower than sumanene [24], whereas the anti-isomer of triphosphasumanene trisulfide 13 exists in an almost plane structure [67].

Asymmetric Synthesis of Buckybowl and Azabuckybowl
Synthesis of buckybowls was a highly challenging task for chemists.The first synthesis of the pristine corannulene was accomplished in 1966 by applying a long synthetic route [68].The synthetic investigation was postponed until Scott et al. [69] and Siegel et al. [70] reinitiated the synthesis of corannulene with a succinct synthetic route by using the flash vacuum pyrolysis (FVP) method in the early 1990s.In the FVP method, the planar π-conjugated precursors are directly converted into buckybowls under high temperature to yield a racemic mixture [71].Many buckybowls were thus synthesized using this FVP method, whereas FVP method was unsuccessful for the synthesis of 2 [72].In 1996, Siegel and co-workers Symmetry 2017, 9, 174 9 of 25 developed the first solution phase synthesis of 1 [73].Adopting this simple and short solution phase strategy, another pristine buckybowl 2 was obtained by Sakurai and co-workers [74].
In 2008, Sakurai and co-workers reported the first asymmetric synthesis of trimethylsumanene 4 using stepwise conversion strategy under milder reaction conditions [17].The synthetic strategy is described in Scheme 1.
In 2008, Sakurai and co-workers reported the first asymmetric synthesis of trimethylsumanene 4 using stepwise conversion strategy under milder reaction conditions [17].The synthetic strategy is described in Scheme 1.In this strategy, chiral norbornene possessing stereogenic carbon center was used as the precursor, which was converted into planar nonconjugated bowl by Pd catalyzed syn-selective cyclotrimerization [75,76], and the sp 3 chirality was thus transmitted to the bowl chirality of 4. The key concern of this strategy is the rate of racemization, which is directly correlated to the inversion barrier of buckybowls.Many buckybowls have a low bowl-to-bowl inversion barrier at room temperature and are unable to separate their enantiomers.For example, most corannulene derivatives are not able to enantiomerically isolate because of their bowl inversion energy of ca.11.5 kcal/mol (bowl-bowl-inversion is >20,000 times per second at room temperature) [14].In contrast, the bowl inversion energy of 2 and 4 is around 20.3 kcal/mol (one time per 143 s) [15,19] and 21.6 kcal/mol [19], respectively.Since the half-life of the bowl inversion of 4 are 2 h at 0 °C and 23 min at 20 °C [17], the isolation of an enantiomer would be possible if the last aromatization step is carried out at a low temperature.

Me
Thus, the aromatization step, converting the nonconjugated bowl into a conjugated bowl, was carried out using excess DDQ in a very short reaction time (1 min) at 0 °C, and the resulting (C)-4 quickly purified at <−20 °C.The isolated (C)-4 was stored at −80 °C.The chirality of (C)-4 was analyzed by the circular dichroism (CD) spectra at −40 °C, which show two positive Cotton effect curves at 247 nm and 284 nm, respectively (Figure 10a).In contrast, the intensity of CD signals gradually decreased at 10 °C (Figure 10b), indicating the racemization of (C)-4 through bowl inversion.The experimentally observed bowl inversion energy is 21.6 kcal/mol, which is close to the calculated value (21 kcal/mol).The chiral (C)-4 was converted into (C)-7 (Scheme 2) using (S)-MTPA as a derivatization reagent [17].The bowl inversion energy barrier of (C)-7 is further increased and the de values can be determined using NMR or HPLC analysis.In this strategy, chiral norbornene possessing stereogenic carbon center was used as the precursor, which was converted into planar nonconjugated bowl by Pd catalyzed syn-selective cyclotrimerization [75,76], and the sp 3 chirality was thus transmitted to the bowl chirality of 4. The key concern of this strategy is the rate of racemization, which is directly correlated to the inversion barrier of buckybowls.Many buckybowls have a low bowl-to-bowl inversion barrier at room temperature and are unable to separate their enantiomers.For example, most corannulene derivatives are not able to enantiomerically isolate because of their bowl inversion energy of ca.11.5 kcal/mol (bowl-bowl-inversion is >20,000 times per second at room temperature) [14].In contrast, the bowl inversion energy of 2 and 4 is around 20.3 kcal/mol (one time per 143 s) [15,19] and 21.6 kcal/mol [19], respectively.Since the half-life of the bowl inversion of 4 are 2 h at 0 • C and 23 min at 20 • C [17], the isolation of an enantiomer would be possible if the last aromatization step is carried out at a low temperature.
Thus, the aromatization step, converting the nonconjugated bowl into a conjugated bowl, was carried out using excess DDQ in a very short reaction time (1 min) at 0 • C, and the resulting (C)-4 quickly purified at <−20 • C. The isolated (C)-4 was stored at −80 • C. The chirality of (C)-4 was analyzed by the circular dichroism (CD) spectra at −40 • C, which show two positive Cotton effect curves at 247 nm and 284 nm, respectively (Figure 10a).In contrast, the intensity of CD signals gradually decreased at 10 • C (Figure 10b), indicating the racemization of (C)-4 through bowl inversion.The experimentally observed bowl inversion energy is 21.6 kcal/mol, which is close to the calculated value (21 kcal/mol).The chiral (C)-4 was converted into (C)-7 (Scheme 2) using (S)-MTPA as a derivatization reagent [17].The bowl inversion energy barrier of (C)-7 is further increased and the de values can be determined using NMR or HPLC analysis.It has been proven that the bowl inversion energy of buckybowls increased with bowl depth [14].Theoretical studies suggested that doping of nitrogen will lead to deeper and curved bowl depth compared to the all-carbon counterparts [77].Sakurai and co-workers reported the first chiral nitrogen-doped buckybowl (Scheme 3) [12].Pure chiral (C)-(−)-and (A)-(+)tris(methylthio)triazasumanene 9 was synthesized from corresponding enantiopure nonconjugated simple precursors, which was further converted into corresponding chiral sulfone derivative 10 by oxidation using meta-chloroperoxybenzoic acid (m-CPBA).From the single crystal analysis, the bowl depth of compound 10 is 1.30 Å, which is deeper than that of 2 (1.11 Å), indicating a higher bowl inversion barrier than the pristine 2. The bowl inversion energy of triazasumanene is calculated to be 39.9 kcal/mol by DFT method at the B3LYP/6-311 + G(d,p) level.In contrast, the calculated values for 2 and 4 are 18.3 and 19.2 kcal/mol, respectively.This means that chiral triazasumanene 10 takes 1.1 billion years to racemize at 20 °C.
Both chiral derivatives of 9 and 10 exhibit the opposite Cotton effect curves (Figure 11), which are a mirror image of the corresponding enantiomers.The CD spectra of 9 and 10 demonstrated the existence of bowl chirality and remained intensive even after a week, indicative of a chiral memory effect.To measure the rate constant and bowl inversion energy of 9, a racemization experiment was carried out at 488 K and analyzed by chiral HPLC.The racemization rate and energy of 9 were determined to be 1.24 × 10 −6 s −1 (488 K) and 42.2 kcal/mol, respectively [12].It has been proven that the bowl inversion energy of buckybowls increased with bowl depth [14].Theoretical studies suggested that doping of nitrogen will lead to deeper and curved bowl depth compared to the all-carbon counterparts [77].Sakurai and co-workers reported the first chiral nitrogen-doped buckybowl (Scheme 3) [12].Pure chiral (C)-(−)-and (A)-(+)tris(methylthio)triazasumanene 9 was synthesized from corresponding enantiopure nonconjugated simple precursors, which was further converted into corresponding chiral sulfone derivative 10 by oxidation using meta-chloroperoxybenzoic acid (m-CPBA).From the single crystal analysis, the bowl depth of compound 10 is 1.30 Å, which is deeper than that of 2 (1.11 Å), indicating a higher bowl inversion barrier than the pristine 2. The bowl inversion energy of triazasumanene is calculated to be 39.9 kcal/mol by DFT method at the B3LYP/6-311 + G(d,p) level.In contrast, the calculated values for 2 and 4 are 18.3 and 19.2 kcal/mol, respectively.This means that chiral triazasumanene 10 takes 1.1 billion years to racemize at 20 °C.
Both chiral derivatives of 9 and 10 exhibit the opposite Cotton effect curves (Figure 11), which are a mirror image of the corresponding enantiomers.The CD spectra of 9 and 10 demonstrated the existence of bowl chirality and remained intensive even after a week, indicative of a chiral memory effect.To measure the rate constant and bowl inversion energy of 9, a racemization experiment was carried out at 488 K and analyzed by chiral HPLC.The racemization rate and energy of 9 were determined to be 1.24 × 10 −6 s −1 (488 K) and 42.2 kcal/mol, respectively [12].
Scheme 2. Derivatization of (C)-4 using chiral derivatizing reagent for the determination of ee.
It has been proven that the bowl inversion energy of buckybowls increased with bowl depth [14].Theoretical studies suggested that doping of nitrogen will lead to deeper and curved bowl depth compared to the all-carbon counterparts [77].Sakurai and co-workers reported the first chiral nitrogen-doped buckybowl (Scheme 3) [12].
Pure chiral (C)-(−)and (A)-(+)-tris(methylthio)triazasumanene 9 was synthesized from corresponding enantiopure nonconjugated simple precursors, which was further converted into corresponding chiral sulfone derivative 10 by oxidation using meta-chloroperoxybenzoic acid (m-CPBA).From the single crystal analysis, the bowl depth of compound 10 is 1.30 Å, which is deeper than that of 2 (1.11 Å), indicating a higher bowl inversion barrier than the pristine 2. The bowl inversion energy of triazasumanene is calculated to be 39.9 kcal/mol by DFT method at the B3LYP/6-311 + G(d,p) level.In contrast, the calculated values for 2 and 4 are 18.3 and 19.2 kcal/mol, respectively.This means that chiral triazasumanene 10 takes 1.1 billion years to racemize at 20 • C.
Both chiral derivatives of 9 and 10 exhibit the opposite Cotton effect curves (Figure 11), which are a mirror image of the corresponding enantiomers.The CD spectra of 9 and 10 demonstrated the existence of bowl chirality and remained intensive even after a week, indicative of a chiral memory effect.To measure the rate constant and bowl inversion energy of 9, a racemization experiment was carried out at 488 K and analyzed by chiral HPLC.The racemization rate and energy of 9 were determined to be 1.24 × 10 −6 s −1 (488 K) and 42.2 kcal/mol, respectively [12].The above-discussed solution phase enantioselective synthesis of trimethylsumanene 4 and triazasumanene 9 and 10 represents a useful and versatile strategy for the construction of other homochiral curved π-electron systems from non-conjugated precursors.The chiral transmission from sp 3 stereogenic chirality to bowl chirality creates a new dimension for chiral-controlled 3D carbon materials like fullerene and CNTs.

Chiral Resolution of Buckybowls
The optical resolution of racemic chiral buckybowls has also been accomplished using derivatization method [13,71,[78][79][80][81]. Derivatization at the outer rim of the buckybowl results in a deeper bowl with high bowl inversion energy and makes it resolvable from its racemates using The above-discussed solution phase enantioselective synthesis of trimethylsumanene 4 and triazasumanene 9 and 10 represents a useful and versatile strategy for the construction of other homochiral curved π-electron systems from non-conjugated precursors.The chiral transmission from sp 3 stereogenic chirality to bowl chirality creates a new dimension for chiral-controlled 3D carbon materials like fullerene and CNTs.

Chiral Resolution of Buckybowls
The optical resolution of racemic chiral buckybowls has also been accomplished using derivatization method [13,71,[78][79][80][81]. Derivatization at the outer rim of the buckybowl results in a deeper bowl with high bowl inversion energy and makes it resolvable from its racemates using The above-discussed solution phase enantioselective synthesis of trimethylsumanene 4 and triazasumanene 9 and 10 represents a useful and versatile strategy for the construction of other homochiral curved π-electron systems from non-conjugated precursors.The chiral transmission from sp 3 stereogenic chirality to bowl chirality creates a new dimension for chiral-controlled 3D carbon materials like fullerene and CNTs.
As shown above, 2 has less bowl inversion energy than 4, and it undergoes much slower racemization at 0 • C (t 1/2 = 2 h) [17].Based on this, Sakurai et al. reported the first asymmetric synthesis of chiral 4 [17], and the enantiomeric excess (ee) of 4 is determined by measuring the 1 H-NMR spectrum of 7, which was derivatized from 4 using Mosher's acid chloride (Scheme 2) [17,18].
Upon introducing stereogenic centers at the sp 3 carbons at the benzylic positions, it became more stable and could be stored for a long time.The enantioselectivity of 6 was determined to be 89% ee, which matches the 1 H-NMR analysis of chiral derivatized compound 4 (90% ee).The chiral (A)-6 buckybowl was eluted first with a retention time (t R ) of 38 min and another chiral buckybowl (C)-6 was eluted later with a retention time of 42 min (Figure 12a).The separated enantiomers of buckybowl 6 were analyzed using CD spectroscopy, which showed the mirror image Cotton effect curves generated from its inherent bowl chirality (Figure 12b).chiral HPLC [12,17,18].The purity of enantio-enriched chiral buckybowls and its bowl chirality is mainly analyzed using CD spectroscopy, NMR, and chiral HPLC methods [82][83][84][85][86][87].
As shown above, 2 has less bowl inversion energy than 4, and it undergoes much slower racemization at 0 °C (t1/2 = 2 h) [17].Based on this, Sakurai et al. reported the first asymmetric synthesis of chiral 4 [17], and the enantiomeric excess (ee) of 4 is determined by measuring the 1 H-NMR spectrum of 7, which was derivatized from 4 using Mosher's acid chloride (Scheme 2) [17,18].Upon introducing stereogenic centers at the sp 3 carbons at the benzylic positions, it became more stable and could be stored for a long time.The enantioselectivity of 6 was determined to be 89% ee, which matches the 1 H-NMR analysis of chiral derivatized compound 4 (90% ee).The chiral (A)-6 buckybowl was eluted first with a retention time (tR) of 38 min and another chiral buckybowl (C)-6 was eluted later with a retention time of 42 min (Figure 12a).The separated enantiomers of buckybowl 6 were analyzed using CD spectroscopy, which showed the mirror image Cotton effect curves generated from its inherent bowl chirality (Figure 12b).Reprinted from [18].© 2010 The Chemical Society of Japan.
Racemic (±)-21 was prepared by aerobic oxidation of racemic (±)-4 (Figure 12) [18].The estimated bowl inversion energy of 21 was 23.5 kcal/mol, which corresponds to ca. 44 h half-life at 10 °C.Racemic (±)-21 was optically resolved using a chiral HPLC (Figure 12c).The absolute configuration of each enantiomer was assigned based on the CD spectrum of the enantioenriched (C)-21 prepared from (C)-4.The bowl chirality of the optically resolved enantioenriched buckybowls (A)-and (C)-21 was analyzed using CD spectroscopy, which is shown in Figure 12d.In addition, the bowl inversion barrier of enantio-riched 21 was estimated to be 23.4 and 23.3 kcal/mol in CH3CN and CH2Cl2, respectively, by measuring the time-dependent decay of the intensity of CD spectra at 255 nm at 30 °C [18].These results led to the elucidation of the substituent effect and the correlation between bowl structure and bowl inversion energy [19].
Azasumanene 10 had a deeper bowl depth (1.30Å) than 2 (1.10 Å) and showed extremely stable bowl chirality (t1/2 = 54 billion years at 20 °C) because of its high bowl inversion energy (ΔE = 42.2 kcal/mol) [12].Similarly, C3 symmetric chiral 8 was prepared by a Pd-catalyzed cross-coupling reaction between chiral 9 and (p-trifluoromethyl)phenyl boronic acid (Figure 13) [88].The chirality of the obtained 8 was confirmed by chiral HPLC at 25 °C using a Daicel Chiralpak ® IA column (Figure 13a,b).The CD and UV spectra were measured in a CH2Cl2 solution of enantioenriched 8 derivatives Reprinted from [18].© 2010 The Chemical Society of Japan.Racemic (±)-21 was prepared by aerobic oxidation of racemic (±)-4 (Figure 12) [18].The estimated bowl inversion energy of 21 was 23.5 kcal/mol, which corresponds to ca. 44 h half-life at 10 • C. Racemic (±)-21 was optically resolved using a chiral HPLC (Figure 12c).The absolute configuration of each enantiomer was assigned based on the CD spectrum of the enantioenriched (C)-21 prepared from (C)-4.The bowl chirality of the optically resolved enantioenriched buckybowls (A)-and (C)-21 was analyzed using CD spectroscopy, which is shown in Figure 12d.In addition, the bowl inversion barrier of enantio-riched 21 was estimated to be 23.4 and 23.3 kcal/mol in CH 3 CN and CH 2 Cl 2 , respectively, by measuring the time-dependent decay of the intensity of CD spectra at 255 nm at 30 • C [18].These results led to the elucidation of the substituent effect and the correlation between bowl structure and bowl inversion energy [19].
Helix inversion renders P and M isomers, while corannulene bowl inversion results in concave and convex isomers, so it is predicted to have four isomers (convex-P, concave-P, convex-M, and concave-M) in equilibrium.Among the different conformers of corannulene-[n]helicenes, the conformer with a terminal helicene ring facing the convex surface of the bowl is more stable than the terminal ring facing the concave bowl surface, as the two forms display different degrees of steric congestion.For 22 and 23, the energy barriers for bowl-to-bowl inversions were calculated to be 10.6 kcal/mol and 10.1 kcal/mol, respectively, and the helix inversions were 20.9 kcal/mol and 34.6 kcal/mol, respectively.
The enantiomers of corannulene-[n]helicenes hybrid were successfully resolved using a chiral-stationary-phase HPLC.The kinetics of thermal racemization of obtained conformers with the ee of >99% was studied under different temperature, and the helix inversion barrier value of ΔG ⧧ = 33.5 kcal/mol obtained experimentally is in consensus with the calculated one (34.6 kcal/mol) (Figure 14b).This hybrid system indicates that the chirality mainly arises from the rigid [6]helicene unit, and the non-rigid corannulene unit undergoes a rapid bowl-to-bowl inversion under ambient conditions.
The authors also synthesized 24 and 25 to study the different magnetic shielding effects of the Recently, Yang and co-workers prepared azonia [6]helicene tethered with supramolecular host β-cyclodextrin, and used a chirality sensing probe for underivatized amino acids in water [89][90][91][92][93][94].Indeed, tethered azonia [6]helicene acts as a conformationally robust chiral auxiliary to improve the chiral recognition ability of native cyclodextrins.Scott and co-workers successfully synthesized the first corannulene-[n]helicenes hybrids 22-25 by combining two classical nonplanar conjugated systems: the chiral bowl-shaped π-system, corannulene, and the helically chiral helicene (Figure 14).These compounds show unique molecular dynamics in their enantiomerization processes, including inversion motions of both the bowl and the helix [95].Helix inversion renders P and M isomers, while corannulene bowl inversion results in concave and convex isomers, so it is predicted to have four isomers (convex-P, concave-P, convex-M, and concave-M) in equilibrium.Among the different conformers of corannulene-[n]helicenes, the conformer with a terminal helicene ring facing the convex surface of the bowl is more stable than the terminal ring facing the concave bowl surface, as the two forms display different degrees of steric congestion.For 22 and 23, the energy barriers for bowl-to-bowl inversions were calculated to be 10.6 kcal/mol and 10.1 kcal/mol, respectively, and the helix inversions were 20.9 kcal/mol and 34.6 kcal/mol, respectively.
The enantiomers of corannulene-[n]helicenes hybrid were successfully resolved using a chiral-stationary-phase HPLC.The kinetics of thermal racemization of obtained conformers with the ee of >99% was studied under different temperature, and the helix inversion barrier value of ∆G ‡ = 33.5 kcal/mol obtained experimentally is in consensus with the calculated one (34.6 kcal/mol) (Figure 14b).This hybrid system indicates that the chirality mainly arises from the rigid [6]helicene unit, and the non-rigid corannulene unit undergoes a rapid bowl-to-bowl inversion under ambient conditions.
The authors also synthesized 24 and 25 to study the different magnetic shielding effects of the convex and concave faces.The results show that the ring-current effect from the concave face of corannulene led to 1 H-NMR signal upfield shifts of 2 ppm to 5 ppm.However, the t-Bu group of 24 is shifted upfield by only 0.65 ppm compared to the t-Bu group in 25, which is away from the convex face.This comparison demonstrates that the magnetic shielding of the concave face is much greater than that of the convex face.Similarly, various stereodynamic (non-rigid/fluctuating) systems 26-29 based on a corannulene bowl that possesses more than one stereogenic unit were synthesized and their selected diastereomeric conformers were studied via bowl-to-bowl inversion (Figure 14c) [57,[96][97][98].
Symmetry 2017, 9, 174 14 of 25 corannulene led to 1 H-NMR signal upfield shifts of 2 ppm to 5 ppm.However, the t-Bu group of 24 is shifted upfield by only 0.65 ppm compared to the t-Bu group in 25, which is away from the convex face.This comparison demonstrates that the magnetic shielding of the concave face is much greater than that of the convex face.Similarly, various stereodynamic (non-rigid/fluctuating) systems 26-29 based on a corannulene bowl that possesses more than one stereogenic unit were synthesized and their selected diastereomeric conformers were studied via bowl-to-bowl inversion (Figure 14c) [57,[96][97][98].The enantiopure or enriched chiral buckybowls could be obtained by asymmetric synthesis or optical resolution of racemates using chiral HPLC.The enantiopure buckybowls with inherent chirality are expected to be applicable not only for asymmetric molecular recognition, novel chiral organic materials creation [15,24,39,99], and chiral ligands for transition metals, but also for precursors of chiral fullerenes and CNTs in chemical synthesis.Methods and approaches that control the bowl chirality can potentially be applied to the related chiral fullerenes and CNTs as well.
The enantiopure or enriched chiral buckybowls could be obtained by asymmetric synthesis or optical resolution of racemates using chiral HPLC.The enantiopure buckybowls with inherent chirality are expected to be applicable not only for asymmetric molecular recognition, novel chiral organic materials creation [15,24,39,99], and chiral ligands for transition metals, but also for precursors of chiral fullerenes and CNTs in chemical synthesis.Methods and approaches that control the bowl chirality can potentially be applied to the related chiral fullerenes and CNTs as well.
In 2007, Hirao and co-workers reported the first selective concave-binding of CpFe + (Cp = C 5 H 5 ) to 2 (Figure 15) [33].The metalation of 2 was carried out in solvent-free conditions using ferrocene at 120 • C.Under the ligand exchange method with excesses of CpFe + , the monometalated concave complex 30 was selectively afforded with a η 6 coordination (Figure 15a).In order to study the substituent effect, a methyl group was introduced into the cyclopentadiene ring, resulting in the concave selective complex 31 (Figure 15b).The bowl depth of 2 was not affected upon selective concave complexation of CpFe + (1.07-1.13Å), whereas in MeCpFe + complexation the coordinatized side is flatted to 0.98 Å and the noncoordinated side was not affected (1.13 Å) (Figure 15a,b).Introduction of a chiral substituent sec-Bu to the Cp ring causes the target compound 32 as chiral (Figure 15c).The peaks of H a split in 1 H-NMR due to the presence of a chiral sec-butyl group.Meanwhile, the concave-selective complexation was confirmed by the split of H b endo .Furthermore, the chirality of complex 32 was proved by CD spectral analysis, which shows positive CD peaks at 272, 303, and 517 nm, respectively [34].
side is flatted to 0.98 Å and the noncoordinated side was not affected (1.13 Å) (Figure 15a,b).Introduction of a chiral substituent sec-Bu to the Cp ring causes the target compound 32 as chiral (Figure 15c).The peaks of Ha split in 1 H-NMR due to the presence of a chiral sec-butyl group.Meanwhile, the concave-selective complexation was confirmed by the split of Hb endo.Furthermore, the chirality of complex 32 was proved by CD spectral analysis, which shows positive CD peaks at 272, 303, and 517 nm, respectively [34].Four years later, the first chiral selective convex-face complex based on pentasubstituted C5-symmetric bowl corannulene was reported by Siegel and co-workers [36].Four kinds of compounds were synthesized to investigate the effect of different substituents in bicyclo[2.2.1]hepta-2,5-diene (nbd) and corannulene derivatives (R = H, Me and t-Bu).The crystal structure of 34 is given in Figure 16a.From the stereochemical analysis, 35 and 36 were predicted to have diastereomers 35a/35b pair and 36a/36b pair, respectively, arising from the rotation of chiral nbd ligand (Figure 16b).The complex is more likely to yield a static form with the steric repulsion between nbd ligand and corannulene.The calculated energy gaps between 35a/35b pair and 36a/36b pair are 1.88 kcal/mol and 3.54 kcal/mol, respectively.However, 35a/35b was observed with a ratio of 2.5:1, while only 36a was detected in the latter pair.These phenomena confirmed the hypothesis that the complex tends to exist in the static form with the steric hindrance.
The Cotton effects of 33 and 36 were observed in the CD spectra and confirmed the steric repulsion hypothesis.The distinct bathochromic shift of 33 and 36 compared to the free chiral nbd ligand was ascribed to the electron transfer from nbd ligand to the corannulene.This kind of curved π-bowl chiral complexation has the potential to function as a catalyst in asymmetrical organometallic chemistry and promote the understanding of selective π-bowl coordination.Four years later, the first chiral selective convex-face complex based on pentasubstituted C 5 -symmetric bowl corannulene was reported by Siegel and co-workers [36].Four kinds of compounds were synthesized to investigate the effect of different substituents in bicyclo[2.2.1]hepta-2,5-diene (nbd) and corannulene derivatives (R = H, Me and t-Bu).The crystal structure of 34 is given in Figure 16a.From the stereochemical analysis, 35 and 36 were predicted to have diastereomers 35a/35b pair and 36a/36b pair, respectively, arising from the rotation of chiral nbd ligand (Figure 16b).The complex is more likely to yield a static form with the steric repulsion between nbd ligand and corannulene.The calculated energy gaps between 35a/35b pair and 36a/36b pair are 1.88 kcal/mol and 3.54 kcal/mol, respectively.However, 35a/35b was observed with a ratio of 2.5:1, while only 36a was detected in the latter pair.These phenomena confirmed the hypothesis that the complex tends to exist in the static form with the steric hindrance.The Cotton effects of 33 and 36 were observed in the CD spectra and confirmed the steric repulsion hypothesis.The distinct bathochromic shift of 33 and 36 compared to the free chiral nbd ligand was ascribed to the electron transfer from nbd ligand to the corannulene.This kind of curved π-bowl chiral complexation has the potential to function as a catalyst in asymmetrical organometallic chemistry and promote the understanding of selective π-bowl coordination.

Chiral Assembly of Buckybowls
Generally, various cyclic systems have inherent chirality because of their conformational stability and rigidity, which make them useful as chiroptical sensing probes [108][109][110].Their preorganized binding sites can recognize optically active molecular species selectively via noncovalent interactions and gave a conformationally stable chiral organic structures.Furthermore, essential molecular chiral components amplify to form 3D supramolecularly assembled architectures and oriented nanoscale assemblies [111,112].The nanoscale chirality of these 3D chiral supramolecular assemblies has potential application in functional soft materials.
Curved π-conjugated buckybowl corannulenes mainly form noncolumnar structures through a CH-π interaction rather than a π-π stacking interaction [28][29][30].C5-Symmetric corannulene undergoes rapid bowl-to-bowl inversion/racemization at ambient or even low temperatures, and is unable to resolve into their enantiomers.Aida and co-workers reported a C5v-symmetric decasubstituted liquid crystalline (LC) corannulene derivative tethered with thioalkyl-amide-tribranched paraffinic side chains that forms self-assembled hexagonal columnar LC structures [3].The nonpolar corannulene derivative can align homeotropically to the electrode surface with an applied electric field, and the alignment of corannulene was memorized for a long time.Hydrogen bond formation among the amide groups plays a key role in the formation of LC.
In order to resolve their enantiomers, the substituents were appended at the outer rim of corannulene viz.thioalkyl chains and amide groups (Figure 17A) [21].C5-symmetric corannulene-based chiral initiators and monomers carrying amide-appended thioalkyl side chains were synthesized from 1,3,5,7,9-pentachlorocorannulene according to methods analogous to those reported by Scott and co-workers [113].The amide derivatives form a unimolecular closed cage through "intramolecular" H-bonding interactions, which represents the first unimolecular host that is responsive to the chiral hydrocarbon solvent and obeys the majority rule in a chiral environment.
Non-amide derivative 37 does not show chiroptical activity even in chiral limonene, and no CD signal could be seen for 38R despite a chiral center in the side chain, whereas amide derivative 40M showed chiroptical activity in the presence of chiral limonene.Compounds 41MR and 41MS, in which amide linkers at the periphery of corannulene are attached to a chiral side chain, show

Chiral Assembly of Buckybowls
Generally, various cyclic systems have inherent chirality because of their conformational stability and rigidity, which make them useful as chiroptical sensing probes [108][109][110].Their preorganized binding sites can recognize optically active molecular species selectively via noncovalent interactions and gave a conformationally stable chiral organic structures.Furthermore, essential molecular chiral components amplify to form 3D supramolecularly assembled architectures and oriented nanoscale assemblies [111,112].The nanoscale chirality of these 3D chiral supramolecular assemblies has potential application in functional soft materials.
Curved π-conjugated buckybowl corannulenes mainly form noncolumnar structures through a CH-π interaction rather than a π-π stacking interaction [28][29][30].C 5 -Symmetric corannulene undergoes rapid bowl-to-bowl inversion/racemization at ambient or even low temperatures, and is unable to resolve into their enantiomers.Aida and co-workers reported a C 5v -symmetric decasubstituted liquid crystalline (LC) corannulene derivative tethered with thioalkyl-amide-tribranched paraffinic side chains that forms self-assembled hexagonal columnar LC structures [3].The nonpolar corannulene derivative can align homeotropically to the electrode surface with an applied electric field, and the alignment of corannulene was memorized for a long time.Hydrogen bond formation among the amide groups plays a key role in the formation of LC.
In order to resolve their enantiomers, the substituents were appended at the outer rim of corannulene viz.thioalkyl chains and amide groups (Figure 17A) [21].C 5 -symmetric corannulene-based chiral initiators and monomers carrying amide-appended thioalkyl side chains were synthesized from 1,3,5,7,9-pentachlorocorannulene according to methods analogous to those reported by Scott and co-workers [113].The amide derivatives form a unimolecular closed cage through "intramolecular" H-bonding interactions, which represents the first unimolecular host that is responsive to the chiral hydrocarbon solvent and obeys the majority rule in a chiral environment.
Non-amide derivative 37 does not show chiroptical activity even in chiral limonene, and no CD signal could be seen for 38 R despite a chiral center in the side chain, whereas amide derivative 40M showed chiroptical activity in the presence of chiral limonene.Compounds 41M R and 41M S , in which amide linkers at the periphery of corannulene are attached to a chiral side chain, show chiroptical activity even in the achiral solvents.The computational simulation studies suggested that the formed stereoisomeric unimolecular closed cages exist in four equilibrated structures (Figure 17B).These amide derivatives form four kinds of "intramolecular" closed cages, and its cyclic amide H-bonded networks take clockwise and anticlockwise geometries.The corannulene bowl chirality axis and the direction of the intramolecular H-bonding (C=O .The compound 40M exhibited opposite Cotton effect curves in chiral solvents (S)-limonene and (R)-limonene, respectively.Likewise, chiral 41MR/41MS showed chiroptical activity even in an achiral solvent such as methylcyclohexane, displaying mutually mirror-imaged CD spectra (Figure 18a).However, upon addition of a protonic solvent like ethanol, which is known to deteriorate the H-bonding, the 41MR/41MS became CD-silent due to the breaking of the intramolecular H-bonding network.In methylcyclohexane, the CD spectral change of 40M was monitored in a wide temperature range from −40 to 40 °C.When the temperature is lowered to −40 °C, the %de of 40M is increased to 45%.At 20 °C, %de of 41MR is enhanced to 80% when (R)-limonene was used as the solvent, whereas 41MR becomes CD-silent when (S)-limonene is used as the solvent (Figure 18b).A similar trend is followed by 41MS (Figure 18b), which indicates that the chiral side-chain of 40M could cooperate with the chiral solvent, either positively or negatively, in desymmetrizing the bowl inversion equilibrium of the corannulene core.A sigmoidal signal was observed in the CD spectra which demonstrated that the majority rule works in the unimolecular system [21].The compound 40M exhibited opposite Cotton effect curves in chiral solvents (S)-limonene and (R)-limonene, respectively.Likewise, chiral 41M R /41M S showed chiroptical activity even in an achiral solvent such as methylcyclohexane, displaying mutually mirror-imaged CD spectra (Figure 18a).However, upon addition of a protonic solvent like ethanol, which is known to deteriorate the H-bonding, the 41M R /41M S became CD-silent due to the breaking of the intramolecular H-bonding network.In methylcyclohexane, the CD spectral change of 40M was monitored in a wide temperature range from −40 to 40 • C. When the temperature is lowered to −40 • C, the %de of 40M is increased to 45%.At 20 • C, %de of 41M R is enhanced to 80% when (R)-limonene was used as the solvent, whereas well (Figure 19).As the oligomer elongates, the first chain-growth supramolecular polymerization is realized.Surprisingly, the chiral chain-growth polymerization can also be achieved via chiral initiator 41IR/41IS.When chiral initiator 41IS was added with racemic monomers 41MR and 41MS, chiral helical assemblies were achieved from the analysis of size-exclusion chromatography (SEC), meaning that only 41MS can polymerize.As a result, chiral resolution of 41MR and 41MS was also realized successfully in the same way [8].Differing from the classical chiral assembly formation in a macroscopic organic system, chiral microscopic assemblies of buckybowl hemifullerene at the surface of inorganic crystal were achieved by Fasel and co-workers [114].They discovered how chiral bowl-shape hemifullerenes, derivatives of sumanene (Figure 20A), restructure on the copper surface atoms into chiral structures such as chiral nanowires and chiral islands.Unlike other researchers using polar molecules with carbonyl groups as chiral surface modifiers [115,116], Fasel et al. [114] turned their attention to metal-aromatic coordination bonds and found hemifullerenes suitable to induce chirality on a copper surface.Furthermore, the chiral island and nanowire (Figure 20B) were found to form with different alignments of hemifullerenes.The results indicate the emergence of a chiral shape on Cu surface is of great importance to help investigate the elementary mechanism of chiral recognition or induction occurring on an achiral crystal surface.Unfortunately, efforts to separate the enantiomers adsorbed on the Cu surface were not successful [114].
Tethering the various alkyl chains with different length, number, substitution position, and nature of chiral carbons to the outer rim π-conjugated buckybowls is an effective way to tune the chiral assembly formation, the function of the system and their soft materialization.Extension of π-conjugation through the outer rim of the corannulene provided a single chiral CNT [114].Furthermore, the chiral island and nanowire (Figure 20B) were found to form with different alignments of hemifullerenes.The results indicate the emergence of a chiral shape on Cu surface is of great importance to help investigate the elementary mechanism of chiral recognition or induction occurring on an achiral crystal surface.Unfortunately, efforts to separate the enantiomers adsorbed on the Cu surface were not successful [114].

Conclusions and Outlook
Tethering the various alkyl chains with different length, number, substitution position, and nature of chiral carbons to the outer rim π-conjugated buckybowls is an effective way to tune the chiral assembly formation, the function of the system and their soft materialization.Extension of π-conjugation through the outer rim of the corannulene provided a single chiral CNT [114].

Conclusions and Outlook
In conclusion, this review summarizes the recent progress of studies on the bowl chirality, focusing on the two kinds of buckybowl compounds, corannulenes and sumanenes.These buckybowls possess curved aromatic rings, and introducing substituents on their aromatic periphery results in the loss of planar symmetry to generate a unique kind of chirality, so-called bowl chirality.The curved aromatic surface is possibly flipped, and such a bowl-to-bowl inversion causes switching of the bowl chirality.The size and shape of the substituents, a doped heteroatom in the aromatic bowl, and introducing a metal complex on the surface of buckybowls will significantly influence the bowl-to-bowl inversion energy as well as the racemization rate.Enantio-/diastereo-pure/enriched chiral buckybowls could be obtained by chiral resolution or chiral synthesis.The introduction of a chiral chain to the rim of the buckybowl dominated the asymmetric synthesis of chiral buckybowls.Sumanene 2 usually has higher bowl-to-bowl inversion energy than corannulene 1, and asymmetric synthesis of chiral trimethylsumanene 4 and triazasumanene 9 and 10 has been realized using chiral non-conjugated precursors.The selective chiral assemblies of buckybowls with unique chemical and electronic properties may contribute to the development of the field of electronic devices and energy storage.In addition, the synthesis of homochiral CNTs continuously attracts scientists and chiral buckybowls may open up new opportunities in this area.The chiral assemblies of buckybowls are attractive, in relation to topics such as chiral amplification, chiral induction, and chiral memory.There is no doubt that the study of chiral buckybowls is still immature yet highly promising; further investigation will provide more insight into this particular area and lead to applications in various fields such as materials, electronics, and photonics.

Figure 1 .
Figure 1.Chemical structures and bowl-shaped fragments of 1 (left) and 2 (right) as rim hydrogenated subunits of spherical buckminsterfullerene C 60 .

Figure 3 .
Figure 3. (a) Three-dimensional diagram of C60 with the subunit of sumanene including enantiomeric travel numbering schemes and the related stereodescriptor systems C and A; (b) The stereodescriptor system of some C3 symmetric chiral buckybowls.

Figure 3 .
Figure 3. (a) Three-dimensional diagram of C 60 with the subunit of sumanene including enantiomeric travel numbering schemes and the related stereodescriptor systems C and A; (b) The stereodescriptor system of some C 3 symmetric chiral buckybowls.

Figure 4 .
Figure 4.The P and M stereodescriptor system for some C3 symmetric chiral buckybowl molecules based on the CIP priority rule. )[56].

Figure 4 .
Figure 4.The P and M stereodescriptor system for some C 3 symmetric chiral buckybowl molecules based on the CIP priority rule.

Scheme 3 .Figure 11 .
Scheme 3. Strategy for the asymmetric synthesis of chiral triazasumanene 9 and 10; Insert: X-ray crystal ORTEP drawing structure of 10; all hydrogen atoms omitted for clarity: (A) top view (with thermal ellipsoids set at 50% probability); (B) side view with bowl depth.Reprinted from[12].© 2012 Nature Publishing Group.

Figure 15 .
Figure 15.Synthetic strategy for the concave selective coordination complex formation of [RCpFe(sumanene)]PF6. (a) Crystal structure of the cation of 30 with thermal ellipsoids set at 50% probability; side view with bowl depth; (b) Crystal structure of the cation of 31 with thermal ellipsoids set at 40% probability (the PF6 ion and acetone are omitted for clarity), side view with bowl depth; (c) Schematic illustration of chiral concave-face selective coordination of sumanene.(a) is reprinted from [33], © 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim; (b) is reprinted from [34], © 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 15 .
Figure 15.Synthetic strategy for the concave selective coordination complex formation of [RCpFe(sumanene)]PF 6 .(a) Crystal structure of the cation of 30 with thermal ellipsoids set at 50% probability; side view with bowl depth; (b) Crystal structure of the cation of 31 with thermal ellipsoids set at 40% probability (the PF 6 ion and acetone are omitted for clarity), side view with bowl depth; (c) Schematic illustration of chiral concave-face selective coordination of sumanene.(a) is reprinted from [33], © 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim; (b) is reprinted from [34], © 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 17 .
Figure 17.(A) Chemical structures of C5-symmetric corannulene derivatives 37-41 bearing chiral or achiral side chains; (B) Schematic representations of the bowl inversion equilibrium using simplified molecular model 39 for C5-symmetric corannulenes carrying thioalkyl side chains featuring intramolecularly H-bonded amide units.Blue-and red-colored arc-shaped arrows represent the directions of H→SR and H-bonded N-H→O=C arrays, respectively, along the corannulene periphery.Subscripts C and A for 39 denote the clockwise and anticlockwise directions (tentatively defined), respectively.Four possible stereoisomers were energy minimized at the SCS-MP2/ def2-TZVPP//DFT-D3-TPSS/defTZVP level.(B) is reprinted from [21].© 2014 American Chemical Society.

Figure 17 .
Figure 17.(A) Chemical structures of C 5 -symmetric corannulene derivatives 37-41 bearing chiral or achiral side chains; (B) Schematic representations of the bowl inversion equilibrium using simplified molecular model 39 for C 5 -symmetric corannulenes carrying thioalkyl side chains featuring intramolecularly H-bonded amide units.Blue-and red-colored arc-shaped arrows represent the directions of H→SR and H-bonded N-H→O=C arrays, respectively, along the corannulene periphery.Subscripts C and A for 39 denote the clockwise and anticlockwise directions (tentatively defined), respectively.Four possible stereoisomers were energy minimized at the SCS-MP2/def2-TZVPP//DFT-D3-TPSS/defTZVP level.(B) is reprinted from [21].© 2014 American Chemical Society.

Figure 19 .
Figure 19.Schematic representations of the chiral chain-growth supramolecular polymerization of chiral pentasubstituted corannulene derivative.Reprinted from [8]. © 2015 Science American Association for the Advancement of Science (AAAS).

Figure 19 .
Figure 19.Schematic representations of the chiral chain-growth supramolecular polymerization of chiral pentasubstituted corannulene derivative.Reprinted from [8]. © 2015 Science American Association for the Advancement of Science (AAAS).

Figure 20 . 33 _ 4 ]
Figure 20.(A) Chemical structure of hemifullerene 3 enantiomers; (B) a. Structural model of the observed step edge with alternating [ _ _ 4] and [ _ 334] segments and the formation of chiral kink sites; b. structural model of R kinks decorated by M enantiomers and S kinks decorated by P enantiomers; c. structural model of a chiral island stabilized by hemifullerene enantiomers; d. structural model of a chiral Cu nanowire stabilized by M-hemifullerene along the [ _ 33 . .H-N) network rotate in the same clockwise or anticlockwise directions (denoted as AA and CC); 39 AA and 39 CC structures are more stable and energetically favored than different directions (denoted as AC and CA; 39 AC and 39 CA ).-bonded networks take clockwise and anticlockwise geometries.The corannulene bowl chirality axis and the direction of the intramolecular H-bonding (C=O … H-N) network rotate in the same clockwise or anticlockwise directions (denoted as AA and CC); 39AA and 39CC structures are more stable and energetically favored than different directions (denoted as AC and CA; 39AC and 39CA). H