Homo- and heterodivalent crown-ammonium pseudorotaxanes with different spacers connecting the two axle ammonium binding sites have been synthesized and characterized by NMR spectroscopy and ESI mass spectrometry. The homodivalent pseudorotaxanes are investigated with respect to the thermodynamics of divalent binding and to chelate cooperativity. The shortest spacer exhibits a chelate cooperativity much stronger than that of the longer spacers. On the basis of crystal structure, this can be explained by a noninnocent spacer, which contributes to the binding strength in addition to the two binding sites. Already very subtle changes in the spacer length, i.e., the introduction of an additional methylene group, cause substantial changes in the magnitude of cooperative binding as expressed in the large differences in effective molarity. With a similar series of heterodivalent pseudorotaxanes, the spacer effects on the barrier for the intramolecular threading step has been examined with the result that the shortest spacer causes a strained transition structure and thus the second binding event occurs slower than that of the longer spacers. The activation enthalpies and entropies show clear trends. While the longer spacers reduce the enthalpic strain that is present in the transition state for the shortest member of the series, the longer spacers become entropically slightly more unfavorable because of conformational fixation of the spacer chain during the second binding event. These results clearly show the noninnocent spacers to complicate the analysis of multivalent binding. An approximate description which considers the binding sites to be connected just by a flexible chain turns out to be more a rough approximation than a good model. The second conclusion from the results presented here is that multivalency is expressed in both the thermodynamics and the kinetics in different ways. A spacer optimized for strong binding is suboptimal for fast pseudorotaxane formation.
A quite simple, achiral benzo-21-crown-7-substituted bis(urea) low-molecular weight gelator hierarchically assembles into helical fibrils, which further develop into bundles and finally form a stable gel in acetonitrile. The gel-sol transition can be controlled by three different molecular recognition events: K + binding to the crown ethers, pseudorotaxane formation with secondary ammonium ions and Cl À binding to the urea units. Addition of a cryptand that scavenges the K + ions and Ag + addition to remove the chloride and bases/acids, which mediate pseudorotaxane formation, can reverse this process. With the gelator, and these chemical stimuli, a number of different systems can be designed that behave as logic gates. Depending on the choice of components, OR, AND, XOR, NOT, NOR, XNOR and INHIBIT gates have been realized. Thus, the gel-sol transition as a property of the system as a whole is influenced in a complex manner. For some cases, the type of logic gate is defined by input signal concentration so that an even more complex reaction of the gel towards the two input signals is achieved.
The ability of an E-configured azobenzene guest to undergo photoisomerisation is controlled by the presence of a complementary host. Addition of base/acid allowed for a weakening/strengthening of the interactions in the divalent pseudo[2]rotaxane complex and hence could switch on/off photochromic activity.
A detailed thermodynamic analysis of the axle-wheel binding in di- and trivalent secondary ammonium/[24]crown-8 pseudorotaxanes is presented. Isothermal titration calorimetry (ITC) data and double mutant cycle analyses reveal an interesting interplay of positive as well as negative allosteric and positive chelate cooperativity thus providing profound insight into the effects governing multivalent binding in these pseudorotaxanes.
N,N-Dimethylhydrazones of propenal-and 2-methylpropenal and their derivatives and homologues (vinylogous azaenamines) were allowed to react with N,N-dimethylformiminium chloride in moisture-free dimethylformamide to yield singly, doubly, and even triply aminomethylated products. They can be easily separated and characterized as crystalline hydrochlorides. The reaction takes place at the w-position of the p-system. This is a consequence of the conjugative interaction of the electron-donating aminohydrazone group with the double bond system in analogy to the enamines. The formation of dialkylhydrazones from unsaturated aldehydes thus causes the umpolung of the formerly electrophilic d 3 -building blocks into a nucleophile. Depending on the reaction conditions and confirmed by crystal structures and 2D NMR experiments, control can be exerted over the degree of substitution: Up to trisubstituted products were obtained for the 2-methylpropenal derivative. The hydrochlorides can be easily deprotonated to yield the free aminohydrazone bases. The back-conversion of the aminohydrazones into the corresponding amino aldehydes is possible under acidic conditions. The isoelectronic exchange of C1 in enamines I 3 by nitrogen formally leads to aldehyde hydrazones III (Scheme 1). Such hydrazones can be formally regarded as aza-enamines. Some time ago, we could demonstrate that besides this formal connection, also real relations exist in the reactivities of both structural classes. Both react with electrophiles E + , the enamines at C2 to yield II and the aldehyde hydrazones at the azomethine C-atom giving rise to IV. Consequently, the azomethine carbon corresponds to the enamine C2 carbon atom. Both owe this property to the conjugative interaction between the electron-donating dialkylated nitrogen atom and the carbon-carbon and carbon-nitrogen double bonds in the sense of mesomeric structures I¢ and III¢. The aldehyde hydrazones are N,Ndialkylated, for example, as in the pyrrolidino, piperidino, or dimethylamino derivatives, because they proved to be the most effective electron donors in enamine chemistry. 3 Examples for this behavior of aldehyde N,N-dialkylhydrazones are the acylation with the Vilsmeier reagent 4a-d,f and trifluoroacetic anhydride, 5a,b the carboxylation with sulfonyl isocyanates, 4b,e,6a-d as well as the alkylation with electrophilic olefins (Scheme 1). 7a-cThe reactions occur contrary to the normal polarity of the imino structural element as expressed in III¢¢; thus, such hydrazones proved to react as umpoled d 1 -nucleophilic aldehyde derivatives. Moreover, they are neutral acyl anion equivalents and thus differ from these strongly alkaline reagents. 8a-c This aza-enamine principle is also applicable to vinylogous aldehyde hydrazones. Consequently the dimethylhydrazones of the a,b-unsaturated aldehydes V are attacked by electrophiles at the vinylogous C3-position leading to VI (Scheme 2). This represents an umpolung of the Michael acceptor reactivity. Therefore, these neutral d 3 -nucleophilic building block...
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