Partial Cone Conformation Research Articles

Calixarenes 21: The conformations and structures of the products of aroylation of the, calix[4]arenes

The aroylatlon of eallx[4]arene (1), p-tert -butylealix[4]arene (2) and p -allylcalix[4]arene (3) with benzoyl chloride and a variety of p -substituted benzoyl chlorides has been carried out via AlCl 3-catalyzed reaction and NaH-induced reaction. The products are tetra-aroylates that exist in the cone, partial cone, and/or 1,3-alternate conformation. It is shown that the distribution anong these conformers is dependent on a variety of factors including temperature, aroylating agent, calixarene, and solvent. When 1 is eaterified by the AlCl 3 procedure lower temperature tends to slightly favor the partial cone conformation, while higher temperature favors the 1,3-alternate conformation. When 3 is esteritied by the NaH method aroyl chlorides carrying electron-releasing p -substituents favor the 1,3-alternate conformers; those carrying electron-withdrawing substituents favor the cone conformers, the distribution between the conformers correlating quite well with the Hammett σ constants for the p -substituents of the aroylating agents. When 2 is esterified by the NaH method it shows a greater tendency to produce the cone conformer than does 3, the former undergoing slower conformational inversion than the latter. On the basis of these observations it is postulated that the conformer distribution is dependent on the relative rates of the conformational inversion process and the aroylation process, viz. faster aroylation leading to cone conformers or partial cone conformers and slower aroylation leading to 1,3-alternate conformers.

Calixarenes 9 : Conformational isomers of the ethers and esters of calix[4]arenes

Calix[4]arene ( 1A), p-t-butylcalix[4]arene ( 1B), and p-allylcalix[4]arene ( 1C) have been converted to various derivatives, including the methyl, ethyl, allyl, benzyl and trimethylsilyl ethers and the acetates. Although the parent calixarenes exist preferentially in the cone conformation, they are conformationally flexible and at room temperature interconvert at a rate of ca 100 sec −1. All but the methyl ethers, on the other hand, are conformationally rigid at room temperature. The preferred conformations in most cases are the cone and partial cone, depending on the derivative formed (i.e. methyl and ethyl ethers favor the partial cone; benzyl and trimethylsilyl ethers favor the cone). In the cases of the allyl ethers and the acetates the p-substituents appear to influence the conformational outcome (i.e. 1A and 1B form the allyl ethers in the partial cone and cone conformation, respectively; 1A and 1B form the acetates in the 1,3-alternate and partial cone conformation, respectively). The conformationally rigid calixarene derivatives in the cone and partial cone conformations belong to the small group of synthetic compounds that contain a permanent cavity (a “changeless calix”) whose dimensions are large enough to encapsulate other molecules.