Cation-induced complexation

Poly(U) cannot combine with s-SPG in nonsalt or neutral solutions, as seen in Table 4.5. Recently, however, we found that the addition of a small amount of cation can induce complexation. 

Since the complexation is sensitive to the cation concentration, it can be switched on and off by changing the KCl concentration. Cryptand [2,2,2] is a well-known strong ligand for potassium, the effective (or free) potassium cation concentration 

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The cationic aqua complexes prepared from traws-chelating tridentate ligand, R,R-DBFOX/Ph, and various transition metal(II) perchlorates induce absolute enantio-selectivity in the Diels-Alder reactions of cyclopentadiene with 3-alkenoyl-2-oxazoli-dinone dienophiles. Unlike other bisoxazoline type complex catalysts [38, 43-54], the J ,J -DBFOX/Ph complex of Ni(C104)2-6H20, which has an octahedral structure with three aqua ligands, is isolable and can be stored in air for months without loss of catalytic activity. Iron(II), cobalt(II), copper(II), and zinc(II) complexes are similarly active. 

Tandem reaction strategies can accomplish several synthetic objectives in a single step.6 The rapidity with which they can build up molecular complexity is a most useful and impressive virtue. For example, cation-induced, biomimetic polyolefinic cyclizations7 are among the most productive and atom-economical8 single-step transformations known in organic chemistry. In one of the most spectac-... 

Ligand 73 was prepared directly from a single enantiomer of the corresponding naphthol of QUINAP 60, an early intermediate in the original synthesis, and both enantiomers of BINOL. Application in hydroboration found that, in practice, only one of the cationic rhodium complexes of the diastereomeric pair proved effective, (aA, A)-73. While (aA, A)-73 gave 68% ee for the hydroboration of styrene (70% yield), the diastereomer (aA, R)-73 afforded the product alcohol after oxidation with an attenuated 2% ee (55% yield) and the same trend was apparent in the hydroboration of electron-poor vinylarenes. Indeed, even with (aA, A)-73, the asymmetries induced were very modest (31-51% ee). The hydroboration pre-catalyst was examined in the presence of catecholborane 1 at low temperatures and binuclear reactive intermediates were identified. However, when similar experiments were conducted with QUINAP 60, no intermediates of the same structural type were found.100... 

The bis-benzo-15-crown-5 ferrocene compound [7] containing two vinylic linkages was formed in a mixture of three isomeric components, the cis-cis, cis-trans and trans-trans isomers, which proved inseparable. However, the precedent of insignificant differences found between the magnitudes of the metal cation-induced anodic shifts in the ferrocenyl redox potentials of the respective separated cis and trans isomers [2a] and [2b] led us to use the same isomeric mixture of [7] throughout the subsequent FABMS and electrochemical group 1 and 2 metal cation complexation experiments,... 

The molybdenum complex 1, a typical high-valent Schrock-type carbene, efficiently catalyzes the self-metathesis of styrene. On the other hand, the cationic iron complex 3 does not induce metathesis but stoichiometrically cyclopropanates styrene. The tungsten complex 2, again a Fischer-type carbene complex, mediates... 

Several AFM studies examined the effect of buffer conditions on the formation of motifs with DNA molecules. For instance, it has been foimd that the multivalent cations induce the condensation of DNA molecules into higher ordered structures, including toroids and rods [122], More specifically, Zn and Mg ions induce the formation of DNA kinked and perfect circles, respectively [123] (Fig. 16). Also, higher concentrations of spermidine induce the formation of complex flower-shaped structures with single crossover points [122] and increased concentrations of ethanol lead to complex and looped structures [ 124] (Fig. 17). 

Gromov, S.P., Ushakov, E.N., Fedorova, O.A., Baskin, I.I., Buevich, A.V., Andryukhina, E.N., Alfimov, M.V., Johnels, D., Edlund, U.G., Whitsel, J.K., Fox, M.A (2003) Novel photoswitcable receptors synthesis and cation-induced assembly into dimeric complexes leading to stereospecific [2+2]-photocycloaddition of styryl dyes containing a 15-crown-5 ether unit, J. Org. Chem., 68, 6115-6125. 

Cation-induced supramolecular formation of crown ether substituted phthalocya-nines can also be regarded as due to host-guest interactions. Fig. 10 shows the proposed structure for the K+- and Ca2+-induced dimer of metallophthalocya-nines fused with four 15-crown-5 units, which serves as a typical example. The formation of these cofacial dimers has been found to go through a two-step three-stage process. The earlier works have been briefly reviewed [93, 94], Recently, Jiang et al. have prepared a series of copper(II) phthalocyanines fused with one to four 15-crown-5 unit(s) [CuPc(15-C-5) ] (n = 1 1) by heating a solution of the double-decker complexes Eu(Pc)[Pc(15-C-5) ] (n = 1-4) with Cu(OAc)2 in... 

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