Molecular modelling calix arene conformers

Rather little attention has been given to the measurement and calculation of dipole moments of calixarenes. The most comprehensive experimental study includes data on nine calix[4]arenes with examples of each of the four up-down conformations, as shown in Table 3.2. As anticipated from simple inspection of molecular models, the sequence of decreasing dipole moment is cone > partial cone > 1,2-alternate > 1,3-alternate. A computational study affirms the sequence of decreasing dipole moment as cone > partial cone > 1,2-alternate > 1,3-alternate but yields a value of only 0.46 D for 4 and values of 1.46,... 

Considering a molecular model of the cyclic tetramers, Gutsche introduced the term calix[4]arenes [1,2] ( calix — vase, bowl, cup), a term that emphasizes the cup-like structure of these macrocycles, which have all four aromatic rings oriented in one direction (a so-called cone conformation). The word arene indicates the presence of aromatic rings in the macrocyclic ensembles. Later the term was extended to include large macrocyclic compounds ([l ]metacyclophanes) and macrocycles, in which the methylene bridge is replaced by a heteroatom. The size of the cycle is indicated by a number placed in square brackets between the roots caUx and arene nature and position... 

Confusion about the stereochemistry of the calix[4]- and [8]arenes already played a role in the work of Comforth et al. mentioned above. They attributed the different physical properties of the high and low melting fractions (HBC and LBC, respectively) to different conformers of the cyclic tetramer (a calix[4]arene), because on the basis of their space filling molecular models they believed that the phenolic OH groups could not rotate through the annulus. And this could give rise to four different rotational diastereoisomers (Fig. 1.5). 

Molecular modeling studies suggest that calix[4]arene derivatives 16-18 bearing methylene units between the calix[4]arene aromatic rings and the melamine moieties, prefer a pinched cone conformation with the melamine bearing aromatic rings twisted outward. This leads to an interatomic separation of more than 10 A between the two rosettes. The short diethyleneoxy spacer at the lower rim in 17 and 18 was introduced to increase the... 

As a part of our work on the synthesis of double calixcrown ethers [5], we reacted calix[4]arene with tetraethylene glycol ditosylate in the presence of cesium carbonate instead of potassium carbonate. Surprisingly, we isolated the l,2-calix[4]arene-fcw-crown-5 (3). Calixaiene (3) was deduced to be in the cone conformation from spectroscopic data. This conformation was ascertained by X-ray diffractometry. Preliminary metal binding properties of ligand (3) were anticipated by modeling with molecular mechanics. 

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