Cation-ir interaction

Neda, I, Sakhaii, P, Wassmann, A, Niemeyer, U, Gunther, E, Engel, J, A practical synthesis of benzyl a- and allyl p-D-glucopyranosides regioselectively substituted with (CH2)30H groups stereocon-troUed p-galactosidation by cation ir-interaction. Synthesis, 1625-1632, 1999. 

Comparison of calculated desolvation free energies with IQ, values suggested the importance of ligand hydrophobicity (low desolvation free energy) for effective cation-ir interaction of the homodimer (245) with peripheral site(s). 

B. Maigret, D. A. Pearfman, and P. A. Kollman, Molecular dynamics potential of mean force calculations a study of the toluene-ammonium ir-cation interactions, J. Am. Chem. Soc. 118, 2998 3005 (1996). (b) J. W. Caldwell, and P. A. Kollman, Cation-ir interactions nonadditive effects are critical in their accurate representation, J. Am. Chem. Soc. 117, 4177-4178 (1995). 

Given our discussion of the cation-water interactions in an earlier section, it is of interest to compare their characteristics with the cation-ir interactions. It can be seen from Figure 3, that the total interaction energies of the cation-water complexes are comparable in magnitude to the corresponding cation-pyrrole complexes. 

In our recent work, we have also investigated the interactions of anions with electron-deficient TT-systems. Except for the greater relevance of dispersion energies, the anion-ir interactions are in many respects similar to the cation-ir interactions [110]. 

Figure 2.9 (a) The alkene BiBLE ligand 2.34 and (b) its Na+ complex. The sodium ion is bound via cation-ir interactions supported by the crown ether donor set. 

Dunbar RC. Complexation of Na and K to aromatic amino acids a density functional computational study of cation-ir interactions. J Phys Chem A. 2000 104 8067-74. 

Gapeev A, Dunbar RC. Cation-ir interactions and the gas-phase thermochemistry of the Na+/phenylalanine complex. JAm Chem Soc. 2001 123 8360-5. 

There appear to be two reasons for the retained strength of the cation-ir interaction in water. First, remember that one component of the cation-ir interaction, the benzene, is hydrophobic. So, to cover one face of it with an ion might be favorable in water (see the discussion of the hydrophobic effect given below). 

The electrostatic potential surfaces of simple aromatics also nicely rationalize the substituent effects on the cation-ir interaction (Figure 3.8 C). These effects are not what might be immediately expected. Usually we think of phenol as electron rich, and so it is a bit surprising that it is not a better cation-ir binder than benzene. However, the electrostatic potential surfaces fully support this result and the other results of Figure 3.8. To a considerable extent, the cation-ir interaction is more affected by the inductive influence of a substituent than by ir donation. 

Synthetic receptors such as cyclophanes can substantially exploit the cation-ir interaction in binding (see Section 4.2.5). Also, in crystal packing and many catalytic systems, cation-iT interactions can be important players. 

The perfluoroarene effect has also been probed using cyclophane systems. In particular the interaction of a cation with an arene and a fluorinated arene has been probed in several systems. An interesting comparison between the it effects found for standard arenes and perfluoroarenes can be made simply by examining the electrostatic surface maps shown in Appendix 2 for benzene and hexafluorobenzene. The charges derived from the quadrupole moments for benzene and hexafluorobenzene are opposite on the faces of the arenes, negative for benzene (red in Appendix 2) and positive for hexafluorobenzene (blue in Appendix 2). As expected, fluorination turns an attractive cation-ir interaction into a repulsive cation-fluoroarene interaction, and the difference can be on the order of several kcal / mol. 

The host behavior of oxacalixarene 49 with the electrochemically active guests ferrocene (F) and cobaltocene (C) has been studied by Shimizu. A host-guest complex results from cation-ir interactions between the hydroxyl-substituted aromatic rings of 49 with the positively charged ferrocenium (F" ) in CH2CI2. Binding... 

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