Bowl-shaped molecules

If both ethylene subunits of a l,3-hexadien-5-yne derivative are members of benzene entities, the cycloaromatization, caused by flash vacuum thermolysis, may give rise to bowl-shaped molecules such as corannulene or semibuckminsterfullerene. However, in those cases, the initial step is not a Hopf cydization but an isomerization of the ethynyl to a vinylidenecarbene group [125]. 

In the approach of Puddephatt et al., the P-phenyl-phosphonitocavitand 2 was obtained by the reaction of phenylphosphonous chloride on re-sorc[4]arene lb (1, R=CH2CH2C6H5) in presence of pyridine as base. The reaction is stereoselective and yielded the bowl-shaped molecule 2 with the four P-phenyl groups directed outwards and the four lone pairs directed inwards ini configuration) [45-49] (Scheme 6). Molecular mechanics calculations performed on the six possible isomers of 2, showed that the iiii isomer is preferred and the orientation of one phenyl group toward the macrocyclic cavity is probable iiio isomer), but two or more phenyl groups oriented inwards are highly unlikely [48]. 

Bowl-shaped molecules like 245 and 246 inspired Cram to propose cage molecules like 247 [2] that could host incarcerated smaller guests, hence the name carcerands. The lattermolecule remained hypothetical butthe Cram group... 

Molecular cavities have been intensively studied over the last decades by supramolecular chemists, since they can be used for molecular recognition, catalysis, or transport processes across membranes. Bowl-shaped molecules such as calixarenes (28), res-orcinarenes (29), or cyclotriveratrylene (CTV, 30) derivatives constitute useful building blocks for cavities since their shapes and sizes can be controlled by covalent synthesis to a remarkable degree (Figure 3.15). The inherent flexibility of calixarenes and resorcinarenes can be limited either by substitution or by bridging, respectively. 

Some other capsules have been described in which both complimentary functions, carboxylic acids and nitrogen-containing aromatic rings, are respectively attached to the rims of two bowl-shaped molecules. Reinhoudt has prepared selfassembling, multi-hydrogen bonding molecular capsules in which a calix[4]arene substituted with four carboxylic functions at the upper rim interacts with a complimentary calix[4]arene with four pyridines attached to the lower rim. These capsules have been identified by H-NiMR, IR and VPO measurements but encapsulation properties have not been reported [106]. 

Bohmer has reported the ability of a bowl-shaped molecule known as a calixarene to self-assemble in the solid state to form a homodimer.35 A calix[4]arene with four urea groups attached to the upper rim self-assembled via 16 hydrogen bonds to give a cavity with an approximate volume of 200 A3 (Fig. 17). The cavity hosted a highly... 

In addition to calixarenes, Sherman has reported the ability of a resorcin[4]arene, a bowl-shaped molecule with eight hydroxyl groups at the upper rim, to form a homodimer in the solid state.39 Deprotonation of two hydroxyl groups of a re-sorcin[4]arene using l,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as a base produced a dianion that self-assembled to form an anionic capsule. The dimer hosted a molecule of pyrazine. Four FI+ DBU ions also interacted with the periphery of the dimer via four N+ H O hydrogen bonds (Fig. 19). 

We have recently shown that it is possible to extend the cavities of bowl-shaped molecules known as resorcin[4]arenes (e.g. C-methylcalix[4]resorcinarene 2), su-pramolecularly, using hydrogen bond acceptors as extender units. As a starting point, we chose readily available 2 as a platform for the assembly process. Indeed, solid-state studies had revealed the ability of 2 to adopt a bowl-like conformation... 

This new centre does not appear in the natural product 76. It is oxidised to the ketone and both alkenes reduced catalytically, both hydrogen molecules being added from the exo-face to set up both chiral centres in one step. The THF is finally oxidised to the lactone with Ru(IV). After the initial Michael addition, controlled by the sulfoxide, every chiral centre depends on the folded or bowl-shaped molecule. 

High-temperature pyrolysis techniques are now applied with surprising selectivity, leading to all sorts of bowl-shaped molecules, which are substructures of C60 and higher fullerenes. Combinations of saturated and unsaturated, acyclic and cyclic fragments with acetylenic subunits have led to two-dimensional and three-dimensional arrays and all-carbon networks. 

Good control is available when working with bowl shaped molecules such as cis-decalins (12), e.g. (9) and (10). as approach of reagents must be from outside the bowl (13), i.e. from thes rme side as the ring junction hydrogens. 

Compound 45 is nonplanar, based on molecular model calculations, due to the presence of five-membered rings, and therefore offers the possibility to create bowl-shaped molecules via flash vacuum pyrolysis (FVP) [86, 89]. The C150 disc 47 is the largest disc with threefold symmetry yet made [86], whereas the giant nanographene 48 with its 222 carbon atoms is the largest PAH to be synthesized and characterized to date [88]. 

Compared to planar polycyclic aromatic hydrocarbons (PAHs), the curved structure of buckybowls endows them with additional interesting physical properties. For example, a bowl-shaped molecule has a dipole moment and a self complimentary shape that could lead to the formation of polar crystals. Moreover, buckyballs and carbon nanotubes are well known for their (potential) applications as electro-optical organic materials. Studies of buckybowls can provide fundamental information on buckyballs and carbon nanotubes. 

They further demonstrated that benzo[k]fluoranthene 46 could be cyclized to a bowl-shaped molecule 47 by fourfold intramolecular C-H/C-X coupling. 

A related construct is the resorcinol analog of a calixarene. It is also a bowl-shaped molecule that can be used in many ways. In such structures there is a conformational constraint from the additional hydrogen bonding motif shown. 

Matching the convex shape of Ceo to a concave siuface has been used to gain control over the inherently weak association between a host moiety and a molecular guest. A built-in shape- and size-specific receptor site ensures efficient binding, but it also exerts a major challenge to find the right match. The following examples form remarkably stable composites with fullerenes nano-sized, bowl-shaped molecules or container molecules with curved, open-ended cavities, such as cyclodextrins, benzotri(benzonorbomadienes), calixarenes, calixnaphthalenes and cyclotriveratrylene. ... 

FIGURE 11.3 Peripheral steric protection by bowl-shaped molecules. 

FIGURE 11.10 Bowl-shaped molecule with all-carbon and acyclic framework. 

Figure 9. Beni and bowl-shaped molecules that are capable of exhibiting mesophases with reduced symmetries.

Figure 10. Parallel and antiparallel packing of columns of bowl-shaped molecules.

In addition to the nematic phase (termed Nq), disc or bowl-shaped molecules may stack in layers (discotic smectic), or much more often, to columns forming so-called columnar (Col) discotic or columnar pyramidic... 

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