Calixarene backbone

Recently, a new bidentate hemispherical chelating bisphosphite ligand based on a calixarene backbone has been designed for linear selective hydroformylation of alkenes (Scheme 9) [54], Excellent levels of regioselectivity have been observed, and even the intrinsic branched-selective hydroformylation of styrene could be overruled by this system. However, the system suffers from low catalytic activity. 

The NHC class of nucleophilic carbenes are also bases. The proton-deuteron exchange of NHCs attached to macromolecules has been studied and the influence of counterion has been explored.99 Substitution, both directly on the imidazolium unit and on the pre-orientating calixarene backbone, was also studied. The results showed that substitution of the imidazolium salts has a large influence on the H-D exchange rates in wet methanol. These results were presented as having implications for Suzuki coupling. 

The optically responsive ligands described in the literature contain either a chromophore or fluorophore which is either joined to or part of the calixarene backbone. On complexation, the environment of the chromophore or fluorophore is... 

The sensor covalently joined a bithiophene unit with a crown ether macrocycle as the monomeric unit for polymerization (Scheme 1). The spatial distribution of oxygen coordination sites around a metal ion causes planarization of the backbone in the bithiophene, eliciting a red-shift upon metal coordination. They expanded upon this bithiophene structure by replacing the crown ether macrocycle with a calixarene-based ion receptor, and worked with both a monomeric model and a polymeric version to compare ion-binding specificity and behavior [13]. The monomer exhibited less specificity for Na+ than the polymer. However, with the gradual addition of Na+, the monomer underwent a steady blue shift in fluorescence emission whereas the polymer appeared to reach a critical concentration where the spectra rapidly transitioned to a shorter wavelength. Scheme 2 illustrates the proposed explanation for blue shift with increasing ion concentration. 

Scheme 2 A calixarene-based ion receptor with bithiophene backbone. The top scheme shows excitation of the receptor in the absence of coordinated ion with normal planarization and relaxation. The bottom scheme shows how ion binding restricts planarization and relaxation resulting in higher energy emission [13]...

Bis-macrocycles two macrocycles joined together Calixarenes basket-shaped macrocycles with phenyl backbones... 

Particular interest has been directed towards the use of the MNK reaction in supramolecular chemistry. The possibility of modifying a substrate backbone, by substituting an oxygen atom with sulfur, can be of extreme importance for changing a molecule s recognition properties. Indeed, this transformation has been exploited in typical hosts such as crown ethers and calixarenes via the MNK rearrangement. 

In addition to this enhancement, several other effects can be induced by the new conformation at the microscopic scale, such as the appearance of olf-diagonal molecular hyperpolarizability tensor components. The study of these microscopically aligned molecules has also provided new fundamental insights and was only made possible through creative synthetic chemistry. The systems that have been studied so far include dimers and tetramers such as the binaphthyl ethers and the calixarenes, and polymers with rigid backbones that force the pendant D ti-A chromophores into a non-centrosymmetric conformation. 

This section is dedicated to first-sphere coordination calixarenes connected to a metal for which another ligand might temporarily penetrate in the host cavity. For that purpose, the idea of rational catalyst design could be taken a step further by using sterically hindered chelated bisphosphines or bisphosphites with a calix[4]arene backbone. 

The selective detection of metal ions has also been achieved by the covalent attachment of molecular recognition moieties to the ICP backbone. The usual approach has been to synthesize a monomer or dimer containing the appropriate recognition group and this is subsequently polymerized (21). Swager (22) have prepared polythiophenes containing crown ethers and calixarenes covalently boimd to the bithiophene repeat imits, which exhibit controllable selectivity toward Li+, Na+, and K+ ions. 

Fig. 38.32 t-Butyl-calix[16]arene (a) inclusion complex with two acetonitrile molecules and two dichloromethane molecules (b) the backbone of the calixarene showing the two superimposed Celtic tores [63]... 

The ion binding properties of 11a and 11b were studied by solvent extraction experiments of selected alkali and transition metals such as Li, Na, K, Cs, Cu, Co, Cd and Hg from aqueous into dichloromethane organic phase. The polymers 11a and 11b have exhibited good extraction ability for mercury (II) cations. The low extraction ratio of 11b than 11a results from the number of calixarene moieties in the polymeric backbone. 

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