Three-Dimensional Hydrocarbon Cores Based on Multiply Fused Cyclopentane and Indane Units: Centropolyindanes

A Geometrically Flexible Three-Dimensional Nanocarbon

The development of architecturally unique molecular nanocarbons by bottom-up organic synthesis is essential for accessing functional organic materials awaiting technological developments in fields such as energy, electronics, and biomedicine. Herein, we describe the design and synthesis of a triptycene-based three-dimensional (3D) nanocarbon, GFN-1, with geometrical flexibility on account of its three peripheral π-panels being capable of interconverting between two curved conformations. An effective through-space electronic communication among the three π-panels of GFN-1 has been observed in its monocationic radical form, which exhibits an extensively delocalized spin density over the entire 3D π-system as revealed by electron paramagnetic resonance and UV-vis-NIR spectroscopies. The flexible 3D molecular architecture of GFN-1, along with its densely packed superstructures in the presence of fullerenes, is revealed by microcrystal electron diffraction and single-crystal X-ray diffraction, which establish the coexistence of both propeller and tweezer conformations in the solid state. GFN-1 exhibits strong binding affinities for fullerenes, leading to host-guest complexes that display rapid photoinduced electron transfer within a picosecond. The outcomes of this research could pave the way for the utilization of shape and electronically complementary nanocarbons in the construction of functional coassemblies.

One-step syntheses of diaza-dioxa-fenestranes via the sequential (3 + 2) cycloadditions of linear precursors and their structural analyses

Fenestranes, in which four rings share one carbon atom, have garnered much attention because of their flattened quaternary carbon centers. In addition, the rigid and nonplanar structures of heteroatom-containing fenestranes are attractive scaffolds for pharmaceutical applications. We report one-step syntheses of diaza-dioxa-fenestranes via the sequential (3 + 2) cycloadditions. Our synthesis employs readily synthesizable, nonbranched acyclic allenyl precursors that facilitate sequential cycloaddition reactions. We report the synthesis of 22 heteroatom-containing and differently substituted fenestranes with rings of varying sizes. The prepared diaza-dioxa-fenestranes are subjected to X-ray crystallography and DFT calculations, which suggest that replacing the carbon atoms at the non-bridgehead positions in the fenestrane skeleton with nitrogen and oxygen atoms results in a slight flattening of the quaternary carbon center. Moreover, one of our synthesized c,c-[5.5.5.5]fenestranes containing two isoxazoline rings possesses the flattest quaternary carbon center among previously synthesized heteroatom-containing fenestrane versions.

Size Determination of Organic Cages by Diffusion NMR Spectroscopy

Reliable structure elucidation of covalent organic cage compounds remains challenging as routine analysis might leave ambiguities. Diffusion-ordered NMR spectroscopy (DOSY) allows insight into the molecular size and mass of the species present in solution, but a systematic evaluation of the diffusion behavior for cage assemblies is rarely considered. Here we report the synthesis of four series of covalent organic cages based on tribenzotriquinacenes and diboronic acids with varying geometry and exohedral substituents. We provide a guideline for the consistent measurement of diffusion coefficients from 1 H-DOSY NMR spectroscopy, which was utilized to study the diffusion behavior for the whole set of cages and selected examples from the literature. For structurally similar cages, a linear correlation between the solvodynamic volume and the molecular mass allows precise size determination. For more complex systems, multiple parameters, such as window size or rigid exohedral functionalization. further modulate cage diffusion in solution.

Centrohexaindane, a Unique Polyaromatic Hydrocarbon Bearing the Rare Cq (Cq )4 Core: Inelastic Neutron Scattering, Infrared and Raman Spectroscopy

The structure and vibrational spectroscopy of centrohexaindane, 1, was investigated. This unusual molecule has a quaternary carbon atom that is coordinated to four further such quaternary carbon atoms as its core, each pair of which is bonded to an ortho-phenylene unit. Previous NMR studies have shown that the molecule has tetrahedral (Td ) symmetry in solution. The infrared and Raman spectra of chloroform and deuterochloroform solutions of 1 are completely in agreement with this conclusion, as the only modes that are visible are those allowed for Td symmetry. This is not the case in the solid state: X-ray powder diffraction indicates that the unit cell is triclinic or monoclinic with a volume in excess of 4000 Å3 . The vibrational spectroscopy is consistent with C1 site symmetry and the presence of at least two molecules in the primitive cell. It is likely that the space group is centrosymmetric.

Synthesis of precisely functionalizable curved nanographenes via graphitization-induced regioselective chlorination in a mechanochemical Scholl Reaction

While the synthesis of nanographenes has advanced greatly in the past few years, development of their atomically precise functionalization strategies remains rare. The ability to modify the carbon scaffold translates to controlling, adjusting, and adapting molecular properties. Towards this end, here, we show that mechanochemistry is capable of transforming graphitization precursors directly into chlorinated curved nanographenes through a Scholl reaction. The halogenation occurs in a regioselective, high-yielding, and general manner. Density Functional Theory (DFT) calculations suggest that graphitization activates specific edge-positions for chlorination. The chlorine atoms allow for precise chemical modification of the nanographenes through a Suzuki or a nucleophilic aromatic substitution reaction. The edge modification enables modulation of material properties. Among the molecules prepared, corannulene-coronene hybrids and laterally fully π-extended helicenes, heptabenzo[5]superhelicenes, are particularly noteworthy.

pH-Responsive fluorescent supramolecular nanoparticles based on tetraphenylethylene-labelled chitosan and a six-fold carboxylated tribenzotriquinacene

We synthesized a new tetraphenylethylene-modified chitosan bioconjugate, CS-TPE, that shows the aggregation-induced emission effect. It can self-assemble into fluorescent polymeric nanoparticles in an aqueous solution at pH 5.3 either alone or with the water-soluble bowl-shaped six-fold carboxylated tribenzotriquinacene derivative TBTQ-C₆ via host-guest binding. The spherical nanoparticles formed by CS-TPE amphiphiles or TBTQ-C₆/CS-TPE supra-amphiphiles disintegrated under alkaline stimulation at pH 10.4 and the dispersion of the aggregates after the collapse in the presence of TBTQ-C₆ was greatly improved. In addition, the fluorescence of CS-TPE was significantly enhanced by introducing TBTQ-C₆, and remained relatively stable with variations in pH for both CS-TPE and TBTQ-C₆/CS-TPE. Such pH-responsive supramolecular spherical nanoparticles with stable fluorescence emission based on CS-TPE or TBTQ-C₆/CS-TPE may find applications in various fields, including the development of visual oral drug delivery systems.

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

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

Synthesis of a new water-soluble hexacarboxylated tribenzotriquinacene derivative and its competitive host–guest interaction for drug delivery

A new water-soluble hexacarboxylated tribenzotriquinacene derivative (TBTQ-CB6) was synthesized and used as a supramolecular drug carrier to load the model anticancer drugs dimethyl viologen (MV) and doxorubicin (DOX) via host–guest interactions. The drugs could be effectively released by spermine (SM), a molecule overexpressed in cancer cells, through host–guest competitive substitution since TBTQ-CB6 has a stronger binding affinity toward SM than MV and DOX. The host–guest interactions of the complexes of TBTQ-CB6 with MV, DOX and SM were investigated by NMR spectroscopy and fluorescence spectroscopy. The association stoichiometry of the complexes of TBTQ-CB6 with MV, DOX, and SM was found to be 1:1 with association constants of K_a = (7.67 ± 0.34) × 10⁴ M⁻¹, K_a = (6.81 ± 0.33) × 10⁴ M⁻¹, and K_a = (5.09 ± 0.98) × 10⁵ M⁻¹, respectively. The competitive substitution process was visualized by NMR titration. This novel TBTQ-based host–guest drug delivery system may have potential use in supramolecular chemotherapy.

Reactions of linear conjugated dienone structures with arenes under superelectrophilic activation conditions. An experimental and theoretical study of intermediate multicentered electrophilic species

Reactions of conjugated dienones with arenes under superelectrophilic activation conditions result in the formation of conjugated enones, indanes, and carbocyclic structures.

Synthesis of 1-Trifluorometylindanes and Close Structures: A Mini Review

This review describes methods for the synthesis of 1-trifluomethylindanes and close structures, which are still quite rare and scarcely available compounds. There are two main approaches to obtain 1-CF3-indanes. The first one is the construction of an indane system from CF3 precursors; the main methods are acid-mediated Friedel–Crafts cyclization, transition metal-catalyzed [3+2] annulation, and free-radical transformations. The second approach is the trifluoromethylation of a ready-made indane core by various CF3 sources, such as Ruppert–Prakash or Togni reagents. Many of these synthetic procedures possess high regio- and stereo-selectivity, allowing the preparation of unique 1-CF3-indane structures. In recent years, great attention has been paid to the synthesis of 1-CF3-indanes, due to the discovery of important biologically active properties for these compounds.

Isoreticular Crystallization of Highly Porous Cubic Covalent Organic Cage Compounds*

Modular frameworks featuring well-defined pore structures in microscale domains establish tailor-made porous materials. For open molecular solids however, maintaining long-range order after desolvation is inherently challenging, since packing is usually governed by only a few supramolecular interactions. Here we report on two series of nanocubes obtained by co-condensation of two different hexahydroxy tribenzotriquinacenes (TBTQs) and benzene-1,4-diboronic acids (BDBAs) with varying linear alkyl chains in 2,5-position. n-Butyl groups at the apical position of the TBTQ vertices yielded soluble model compounds, which were analyzed by mass spectrometry and NMR spectroscopy. In contrast, methyl-substituted cages spontaneously crystallized as isostructural and highly porous solids with BET surface areas and pore volumes of up to 3426 m2  g-1 and 1.84 cm3  g-1 . Single crystal X-ray diffraction and sorption measurements revealed an intricate cubic arrangement of alternating micro- and mesopores in the range of 0.97-2.2 nm that are fine-tuned by the alkyl substituents at the BDBA linker.

Regioconvergent Synthesis of a π-Extended Tribenzotriquinacene-Based Wizard Hat-Shaped Nanographene

The successful enlargement of the curved π-electron periphery of the wizard hat-shaped polycyclic aromatic compound 1 is described. The target structure 2 features an m,m,p,m,m,p,m,m,p-nonaphenylene belt fused to a central tribenzotriquinacene unit. The synthesis involves a multiple regioconvergent Scholl-type dehydrocyclization as the key step. Spectroscopic, structural, and electronic properties of the title compound 2 are reported.

Endohedral Hydrogen Bonding Templates the Formation of a Highly Strained Covalent Organic Cage Compound*

A highly strained covalent organic cage compound was synthesized from hexahydroxy tribenzotriquinacene (TBTQ) and a meta-terphenyl-based diboronic acid with an additional benzoic acid substituent in 2'-position. Usually, a 120° bite angle in the unsubstituted ditopic linker favors the formation of a [4+6] cage assembly. Here, the introduction of the benzoic acid group is shown to lead to a perfectly preorganized circular hydrogen-bonding array in the cavity of a trigonal-bipyramidal [2+3] cage, which energetically overcompensates the additional strain energy caused by the larger mismatch in bite angles for the smaller assembly. The strained cage compound was analyzed by mass spectrometry and 1 H, 13 C and DOSY NMR spectroscopy. DFT calculations revealed the energetic contribution of the hydrogen-bonding template to the cage stability. Furthermore, molecular dynamics simulations on early intermediates indicate an additional kinetic effect, as hydrogen bonding also preorganizes and rigidifies small oligomers to facilitate the exclusive formation of smaller and more strained macrocycles and cages.

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

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

Synthesis and Optoelectronic Properties of a Quinoxalino-Phenanthrophenazine (QPP) Extended Tribenzotriquinacene (TBTQ)

A six‐step synthesis towards a tribenzotriquinacene (TBTQ) bearing three quinoxalinophenanthrophenazine (QPP) units is presented. The optoelectronic properties are investigated and the effect of the three‐dimensional arrangement of the individual QPP planes is examined using optical spectroscopy, electrochemical analysis and quantum‐chemical calculations.

Water-soluble host-guest complexes between fullerenes and a sugar-functionalized tribenzotriquinacene assembling to microspheres

A sugar-functionalized water-soluble tribenzotriquinacene derivative bearing six glucose residues, TBTQ-(OG) 6 , was synthesized and its interaction with C60 and C70-fullerene in co-organic solvents and aqueous solution was investigated by fluorescence spectroscopy and ultraviolet-visible spectroscopy. The association stoichiometry of the complexes TBTQ-(OG) 6 with C60 and TBTQ-(OG) 6 with C70 was found to be 1:1 with binding constants of K a = (1.50 ± 0.10) × 105 M-1 and K a = (2.20 ± 0.16) × 105 M-1, respectively. The binding affinity between TBTQ-(OG) 6 and C60 was further verified by Raman spectroscopy. The geometry of the complex of TBTQ-(OG) 6 with C60 deduced from DFT calculations indicates that the driving force of the complexation is mainly due to the hydrophobic effect and to host-guest π-π interactions. Hydrophobic surface simulations showed that TBTQ-(OG) 6 and C60 forms an amphiphilic supramolecular host-guest complex, which further assembles to microspheres with diameters of 0.3-3.5 μm, as determined by scanning electron microscopy.

Regiocontrolled dimerization of asymmetric diazaheptacene derivatives toward X-shaped porous semiconductors

Conformationally rigid X-shaped PAHs are attracting interest due to their self-assembly into unique networks and as models to study through-space exciton and charge delocalization in one single molecule. We report here the synthesis of X-shaped PAHs by dimerization of diazaheptacene diimides. The diimide groups are employed to effectively direct the self-assembly into antiparallel dimer aggregates, which assist the compounds to undergo a regiocontrolled [4 + 4] dimerization, leading to an X-shaped conformation bearing electron-poor and -rich subunits. The resulting PAHs are found to pack in 2D layers with large open channels and infinite π⋯π arrays. Furthermore, these highly crystalline porous materials serve as electron-transporting materials in OFETs due to the long-range π-stacked arrays in the layers. This work presents a potentially generalizable strategy, which may provide a unique class of porous semiconductors for organic devices, taking advantage of their open channels.

The synthesis of conformationally rigid X-shaped PAHs by regiocontrolled cyclodimerization of diazaheptacene diimides is presented. The resulting porous materials exhibit enhanced semiconducting behaviors with large open channels.

A covalent organic cage compound acting as a supramolecular shadow mask for the regioselective functionalization of C60

A trigonal-bipyramidal covalent organic cage compound serves as an efficient host to form stable 1 : 1-complexes with C₆₀ and C₇₀. Fullerene encapsulation has been comprehensively studied by NMR and UV/Vis spectroscopy, mass spectrometry as well as single-crystal X-ray diffraction. Exohedral functionalization of encapsulated C₆₀via threefold Prato reaction revealed high selectivity for the symmetry-matched all-trans-3 addition pattern.

The taming of the Prato reaction: a covalent organic cage compound serves as a supramolecular template for the regioselective functionalization of C₆₀.

Pentadiamond: A Hard Carbon Allotrope of a Pentagonal Network of sp^{2} and sp^{3} C Atoms

A pentagonal covalent network consisting of sp^{2} and sp^{3} C atoms has been investigated based on the density functional theory. Our theoretical investigations clarified that the pentagonal covalent network is a metastable three-dimensional carbon allotrope with the Fm3[over ¯]m space group possessing remarkable mechanical properties: relatively high bulk modulus of 381 GPa together with a negative Poisson's ratio of -0.241. Accordingly, the pentagonal covalent network possesses extremely high Young's and shear moduli of 1691 and 1113 GPa, respectively, surpassing those of the diamond. The electronic structure of the pentagonal network is a semiconductor with an indirect band gap of 2.52 eV between L and X points for valence and conduction band edges, respectively, with the relatively small carrier masses.

Construction of Bisindolines via Oxidative Coupling Cyclization

Herein, we report a general approach for the efficient construction of three-dimensional bisindolines via oxidative coupling cyclization in an intermolecular manner. This reaction is featured by its operational simplicity, metal-free conditions, lack of protecting group, and high selectivity. Notably, a wide range of anilines are suitable in this intermolecular cyclization, furnishing corresponding bisindolines in up to 98% yield.

Iron(III)-Mediated Bicyclization of 1,2-Allenyl Aryl Ketones: Assembly of Indanone-Fused Polycyclic Scaffolds and Dibenzo[a,e]pentalene Derivatives

The rapid construction of three-dimensional fused carbocycles is a key challenge in synthetic chemistry. Herein, an unprecedented and practical tandem Nazarov/oxidative umpolung 4π-ring closure of readily available 1,2-allenyl aryl ketones mediated by iron(III) chloride has been developed, furnishing a new family of indanone-fused molecular architectures in moderate to excellent yields. The indanone-containing blocks can be efficiently converted to unsymmetrical dibenzo[a,e]pentalenes. Significantly, divergent synthetic applications have been achieved to provide densely functionalized polycyclic arrays.

Triperyleno[3,3,3]propellane triimides: achieving a new generation of quasi-D 3h symmetric nanostructures in organic electronics

Rigid three-dimensional (3D) polycyclic aromatic hydrocarbons (PAHs), in particular 3D nanographenes, have garnered interest due to their potential use in semiconductor applications and as models to study through-bond and through-space electronic interactions. Herein we report the development of a novel 3D-symmetric rylene imide building block, triperyleno[3,3,3]propellane triimides (6), that possesses three perylene monoimide subunits fused on a propellane. This building block shows several promising characteristics, including high solubility, large π-surfaces, electron-accepting capabilities, and a variety of reactive sites. Further, the building block is compatible with different reactions to readily yield quasi-D 3h symmetric nanostructures (9, 11, and 13) of varied chemistries. For the 3D nanostructures we observed red-shift absorption maxima and amplification of the absorption coefficients when compared to the individual subunits, indicating intramolecular electronic coupling among the subunits. In addition, the microplates of 9 exhibit comparable mobilities in different directions in the range of 10-3 cm2 V-1 s-1, despite the rather limited intermolecular overlap of the π-conjugated moieties. These findings demonstrate that these quasi-D 3h symmetric rylene imides have potential as 3D nanostructures for a range of materials applications, including in organic electronic devices.

Rhenium-Catalyzed Regioselective ortho-Alkenylation and [3 + 2 + 1] Cycloaddition of Phenols with Internal Alkynes

An operationally simple and direct rhenium-catalyzed ortho-alkenylation ( C-alkenylation) of unprotected phenols with alkynes was developed. The protocol provided ortho-alkenylphenols exclusively, and formation of para- or multiply alkenylated phenols and hydrophenoxylation ( O-alkenylation) products were not observed. The [3 + 2 + 1] cycloaddition of phenols and two alkynes via ortho-alkenylation was also demonstrated, in which the alkynes functioned as both two- and one-carbon units. These reactions proceeded with readily available starting materials under neutral conditions without additional ligands.

Spectroscopic characterisation of centropolyindanes

A highly promising class of three-dimensional polyaromatic hydrocarbons comprises the centropolyindanes. The characteristic feature of these compounds is the mutual fusion of several molecules of indane along the saturated C-C bonds of their cyclopentane rings. Among the polycyclic aromatic hydrocarbons, the centropolyindanes are special because of the saturated core of sp3-hybridised carbon atoms embedded in a three-dimensional environment of aromatic building blocks. While the centropolyindanes and their numerous derivatives have been studied in detail by NMR spectroscopy, mass spectrometry and X-ray diffraction, investigation of their vibrational features, and especially those of the neopentane core present in most cases, have not been performed so far. In the present paper, we report the first systematic study of a set of centropolyindanes by vibrational spectroscopy, using inelastic neutron scattering (INS), infrared and Raman spectroscopies.

Enantiopure Aromatic Saddles Bearing the Fenestrindane Core

The synthesis of enantiomerically pure, configurationally stable fenestrindane-based polyaromatic compounds with saddle-like structures is reported. Seven racemic fenestrane synthetic precursors were first screened by chiral HPLC for resolvability into enantiomers. Among the three resolvable precursors, a tribenzofenestrene derivative was resolved on a semipreparative scale, and the absolute configuration of the more slowly eluting enantiomer was established by X-ray crystallography. The enantiopure tribenzofenestrenes were then separately converted, in six steps, to the saddle-shaped fenestrindane derivatives in optically pure form. The two enantiomeric pairs of saddles were characterized by ¹H and ¹³C NMR spectroscopy, mass spectrometry, and circular dichroism spectroscopy. All new compounds reported herein represent the first enantiopure non-natural carbocyclic fenestranes isolated to date.

Rigidification of a macrocyclic tris-catecholate scaffold leads to electronic localisation of its mixed valent redox product

One-electron oxidation of the compound shown shows no evidence for intervalence charge transfer in the macrocylic ligand radical product. In contrast, related [{Pt(L)}₃(μ₃-ctc˙)]⁺ (H₆ctc = cyclotricatechylene), exhibits class II mixed valency.

Nanotechnology, from quantum mechanical calculations up to drug delivery

There are several reasons why nanotechnology is currently considered as the leader among the most intensively developing research trends. Nanomatter often exhibits new properties, other than those of the morphology of a continuous solid. Also, new phenomena appear at the nanoscale, which are unknown in the case of microcrystalline objects. For this reason, nanomaterials have already found numerous applications that are described in this review. However, among intensively developed various branches of nanotechnology, nanomedicine and pharmacology stand out particularly, which opens new possibilities for the development of these disciplines, gives great hope for the creation of new drugs in which toxicological properties are reduced to a minimum, reduces the doses of medicines, offers targeted treatment and increases diagnostic possibilities. Nanotechnology is the source of a great revolution in medicine. It gives great hope for better and faster treatment of many diseases and gives hope for a better tomorrow. However, the creation of new "nanodrugs" requires a special understanding of the properties of nanoparticles. This article is a review work which determines and describes the way of creating new nanodrugs from ab initio calculations by docking and molecular dynamic applications up to a new medicinal product, as a proposal for the personalized medicine, in the early future.

Chiral Cyclohexyl-Fused Spirobiindanes: Practical Synthesis, Ligand Development, and Asymmetric Catalysis

1,1'-Spirobiindane has been one type of privileged skeleton for chiral ligand design, and 1,1'-spirobiindane-based chiral ligands have demonstrated outstanding performance in various asymmetric catalysis. However, the access to enantiopure spirobiindane is quite tedious, which obstructs its practical application. In the present article, a facile enantioselective synthesis of cyclohexyl-fused chiral spirobiindanes has been accomplished, in high yields and excellent stereoselectivities (up to >99% ee), via a sequence of Ir-catalyzed asymmetric hydrogenation of α,α'-bis(arylidene)ketones and TiCl₄ promoted asymmetric spiroannulation of the hydrogenated chiral ketones. The protocol can be performed in one pot and is readily scalable, and has been utilized in a 25 g scale asymmetric synthesis of cyclohexyl-fused spirobiindanediol (1 S,2 S,2' S)-5, in >99% ee and 67% overall yield for four steps without chromatographic purification. Facile derivations of (1 S,2 S,2' S)-5 provided straightforward access to chiral monodentate phosphoramidites 6a-c and a tridentate phosphorus-amidopyridine 11, which were evaluated as chiral ligands in several benchmark enantioselective reactions (hydrogenation, hydroacylation, and [2 + 2] reaction) catalyzed by transition metal (Rh, Au, or Ir). Preliminary results from comparative studies showcased the excellent catalytic performances of these ligands, with a competency essentially equal to the corresponding well-established privileged ligands bearing a regular spirobiindane backbone. X-ray crystallography revealed a close resemblance between the structures of the precatalysts 20 and 21 and their analogues, which ultimately help to rationalize the almost identical stereochemical outcomes of reactions catalyzed by metal complexes of spirobiindane-derived ligands with or without a fused cyclohexyl ring on the backbone. This work is expected to stimulate further applications of this type of readily accessible skeletons in development of chiral ligands and functional molecules.

Organic Synthesis: Wherefrom and Whither? (Some Very Personal Reflections)

This perspective represents a (highly personal) examination of the past, present and future of synthetic organic chemistry. The central thesis posits that the confluence of factors that led to the "Golden Age of Natural Product Synthesis" in the second half of the twentieth century can be traced back to the identification of the therapeutic potential of steroid hormones culminating in the introduction of oral contraceptives. The tremendous benefits of those activities to the development of organic synthesis as a vibrant discipline led to the exponential increase in strategies and methods and the ability to tackle, larger and larger molecules of greater and greater complexity. The existential challenge to the health of organic synthesis is whether a similarly dynamic future can be anticipated and if so, to what end and how. Musings on potential answers to those questions are presented.

Stereoselective synthesis of enantiomerically pure bowl-shaped hydroxytribenzotriquinacenes

A bowl-shaped (M)-2-hydroxytribenzotriquinacene was obtained in enantiomerically pure form by stereoselective enzyme-catalysed ester hydrolysis of the racemic TBTQ-based aryl acetate.

From Fenestrindane towards Saddle-Shaped Nanographenes Bearing a Tetracoordinate Carbon Atom

Two saddle-shaped polycyclic aromatic compounds (8 a and 8 b) bearing an all-cis-[5.5.5.5]fenestrane core surrounded by an o,p,o,p,o,p,o,p-cyclooctaphenylene belt were synthesized and characterized by NMR spectroscopy and mass spectrometry. The key step of this synthesis involves the formation of four cycloheptatriene rings from the corresponding electron-rich 1,4,9,12-tetraarylfenestrindane derivatives 7 a and 7 b in Scholl-type cyclizations. The structural details of the D_2d -symmetric saddle compound 8 a were determined by X-ray crystallography, and the properties of 8 a and 8 b were studied by UV/Vis and fluorescence spectroscopy and cyclic voltammetry.

Propellanes-From a Chemical Curiosity to "Explosive" Materials and Natural Products

Propellanes are a unique class of compounds currently consisting of well over 10 000 representatives, all featuring two more or less inverted tetrahedral carbon atoms that are common to three bridging rings. The central single bond between the two bridgeheads is significantly weakened in the smaller entities, which leads to unusual reactivities of these structurally interesting propeller-like molecules. This Review highlights the synthesis of such propellanes and their occurrence in material sciences, natural products, and medicinal chemistry. The conversion of [1.1.1]propellane into bridgehead derivatives of bicyclo[1.1.1]pentane, including oligomers and polymers with bicyclo[1.1.1]penta-1,3-diyl repeat units, is also featured. A selection of natural products with larger propellane subunits are discussed in detail. Heteropropellanes and inorganic propellanes are also addressed. The historical background is touched in brief to show the pioneering work of David Ginsburg, Günther Snatzke, Kenneth B. Wiberg, Günter Szeimies, and others.

A cyclopenta[hi]acephenanthrylene bearing two benzoannelated [3.3.3]propellane units: extension of triptindane chemistry

The McMurry reaction of triptindan-9-one (2), a three-fold benzoannelated Cs-symmetrical [3.3.3]propellane ketone, gave trans-9,9′-bitriptindanylidene (5), a sterically crowded stilbene hydrocarbon bearing two E-oriented triptindane moieties, in good yield. Photoisomerization of 5 generated the corresponding cis-stilbene 8 in a photostationary E:Z mixture (55:45), which adopts a similarly crowded C2-symmetrical conformation that is apparently static on the NMR timescale. Photocyclodehydrogenation of 5 via 8 in benzene solution afforded the title hydrocarbon 6, a 1,2,9,10-tetrahydrocyclopenta[hi]acephenanthrylene merged with two triptindane units, in 85% yield. X-ray structure analysis of 6 revealed an essentially planar phenanthrene unit but significant steric repulsion between the pairs of opposite methylene groups of the [3.3.3]propellane cores, giving rise to a C2-symmetrical conformation. Reaction of 2 under modified McMurry conditions (DME instead of THF as a solvent) gave the saturated dimer, 9,9′-bitriptindanyl 7, as a mixture of diastereomers. Attempts to synthesize “columnene” (4), an elusive barrelene derivative fused with two triptindane caps, by three-fold McMurry reaction of triptindane-9,10,11-trione (3) failed.

Decasilahexahydrotriquinacene and Decasilaisotwistane: σ Conjugation on a Bowl Surface

The first bowl-shaped oligosilane, hexadecamethyldecasilahexahydrotriquinacene (1), and a related oligosilane, hexadecamethyldecasilaisotwistane (2), were synthesized, and their structures and properties were studied. The results revealed importance of σ conjugation on a bowl surface: the HOMOs of 1 are σ orbitals delocalized on the bowl surface, whereas the LUMO is a pseudo π* orbital on the convex and concave sides of the bowl surface.

Acid-promoted cyclization of 2,4-diaryl-1,1,1-trifluorobut-3-en-2-oles and their TMS-ethers into CF3-indenes

2,4-Diaryl-1,1,1-trifluorobut-3-en-2-oles and their TMS-ethers in H₂SO₄ at room temperature in just 2 min are quantitatively cyclized into 1-aryl-3-trifluoromethyl-1H-indenes.