Expanding Coefficient: A Parameter To Assess the Stability of Induced-Fit Complexes.

Here we propose a new parameter, the Expanding Coefficient (EC), that can be correlated with the thermodynamic stability of supramolecular complexes governed by weak secondary interactions and obeying the induced-fit model. The EC values show a good linear relationship with the log Kapp of the respective pseudorotaxane complexes investigated. According to Cram's Principle of Preorganization, the EC can be considered an approximate mechanical measure of the host's reorganization energy cost upon adopting the final bound geometry.

Study on the Influence of Chirality in the Threading of Calix[6]arene Hosts with Dialkylammonium Axles.

: The influence of chirality in calixarene threading has been studied by exploiting the "superweak anion approach". In particular, the formation of chiral pseudo[2]rotaxanes bearing a classical stereogenic center in their axle and/or wheel components has been considered. Two kind of pseudo[2]rotaxane stereoadducts, the "endo-chiral" and "exo-chiral" ones, having the stereogenic center of a cationic axle inside or outside, respectively, the calix-cavity of a chiral calixarene were preferentially formed with specifically designed chiral axles by a fine exploitation of the so-called "endo-alkyl rule" and a newly defined "endo-α-methyl-benzyl rule" (threading of a hexaalkoxycalix[6]arene with a directional (α-methyl-benzyl)benzylammonium axle occurs with an endo-α-methyl-benzyl preference). The obtained pseudorotaxanes were studied in solution by 1D and 2D NMR, and in the gas-phase by means of the enantiomer-labeled (EL) mass spectrometry method, by combining enantiopure hosts with pseudoracemates of one deuterated and one unlabeled chiral axle enantiomer. In both instances, there was not a clear enantiodiscrimination in the threading process with the studied host/guest systems. Possible rationales are given to explain the scarce reciprocal influence between the guest and host chiral centers.

Absolute Configuration Assignment of Chiral Resorcin[4]arenes from ECD Spectra.

Racemates of five chiral resorcin[4]arenes, four tetra-O-substituted and one hepta-O-substituted, have been resolved by enantioselective HPLC, and their ECD spectra have been recorded online by stopped-flow measurements. The absolute configuration has been assigned by comparison of the experimental ECD spectra with DFT and semiempirical calculations. For the four tetra-O-substituted resorcin[4]arenes, the ECD exciton couplet at longer wavelength depends on the chirality induced in the arene scaffold by the substituents rather than on the precise nature of the substituents themselves. Accordingly, the exciton chirality model with excitons localized on the arene scaffold, here generalized to Cn symmetry, accurately describes the relationship between stereochemistry and chiroptical properties for this couplet, while its application at shorter wavelengths is unsafe. For the significantly larger hepta-O-substituted system the assignment particularly benefits from the use of the semiempirical ZINDO method.

Nucleophilic Functionalization of the Calix[6]arene Para- and Meta-Position via p-Bromodienone Route.

It is here demonstrated that the p-bromodienone route, previously reported for calix[4]arenes, is also effective for the functionalization of the calix[6]arene macrocycle. Thus, alcoholic O-nucleophiles can be introduced at the calix[6]arene exo rim. In addition, the reaction of a calix[6]arene p-bromodienone derivative with an actived aromatic substrate, such as resorcinol, led to the first example of a meta-functionalized, inherently chiral calix[6]arene derivative.