ing that the 'JC.H coupling constants for the inner atoms (C3, C4) are larger than those for the terminal atoms (C2, C5, and C6)."] In analogy to the known CpCo(q4-1,3diene) compounds, we assume that each diene unit is pla-narlX1 but that the two outer diene units are slightly bent with respect to the inner diene."] The electron delocalization in 4 is essentially suppressed, which means that biphenyl in 4 is best described by a Kekule structure. Although there are numerous examples of the reduction or suppression of the aromaticity of benzene, naphthalene, or anthracene by q2-, q3-, or q4complexation to a suitable metal complex compound 4 is, to the best of our knowledge, the first example for the transformation of an aromatic system with 12 x-electrons into a system of three q4-bonded dienes. Experimental ProcedureA solution of 5 (4.9 g, 10.7 mmol) and 6 (1.8 g, 21.4 mmol) in 80 mL of ether was prepared at -80°C and then allowed to warm over several hours to 20°C. After 24 h, the ether was removed under vacuum and the residue was extracted with pentane. After filtration to remove the LiCI, the solvent was removed from the filtrate under vacuum. The remaining LiCl was removed by extraction with pentane, filtration, and removal of the pentane from the filtrate. This procedure was carried out three times. Finally, the residue was dried at 20"C/IO-4 torr to remove the biphenyl. The residue was dissolved in 2 0 0 m L of pentane and filtered, and the filtrate was concentrated to 40 mL. Upon standing at -S O T , the concentrated solution slowly afforded 4 (3.6 g, 4.9 mmol, 69%) as black crystals. Correct elemental analysis and mass spectrum. Comparison of the "C-NMR data of 4 with those of the s-cis/trans isomers of [Cp2Zr(q4-butadiene)] also indicates that the inner diene unit has the s-cis configuration; see also G. Erker,
Die seit langem bekannte Kondensation von Resorcin mit Acetaldehyd ergibt das Tetraphenolat H (R=CH3. Diese napfförmige Wirtverbindung zeichnet sich durch cyclische Wasserstoffbrücken und delokalisierte negative Ladung, aus, was zu einer hohen Symmetrie und Stabilität und zu den höchsten bisher bekannten Bindungskonstanten für Methylammonium‐Ionen vom Cholintyp und ähnlichen Derivaten führt.
Der auf Silicagelschichten immobilisierte Makrocyclus 1 (siehe vorstehende Zuschrift) ermöglicht eine neue Art des Studiums von Wirt‐Gast‐Wechselwirkungen. Der selektive Einschluß eines organischen Neutralmoleküls im Hohlraum des Makrocyclus beruht zum einen auf der Wechselwirkung zwischen lipophilen Teilen von Wirt‐ und Gastmolekül und zum anderen auf der elektrostatischen Anziehung zwischen polaren Substituenten im Substrat und den positiven Ladungen im Wirt.
131 Fluorescence spectroscopic measurements at higher concentrations, and methods which are based on changes in solubility or partition coefficients by the addition of complexing host compounds, do not enable any differentiation between surface association and inclusion [2dJ and, in certain cases give errors of more than a 100% in the complexation energies (H. [']. We have found that easily handleable chromatographic techniques are accessible by immobilization of the azacyclophane 1 on silica gel layers. As a rule, the azacyclophane derivative does not hinder detection by fluorescence under the UV lamp or by customary developing reagents [']. On application of the dissolved substances the starting spots are usually less spread out than on untreated layers. We hoped, by means of this technique, also to obtain information about the nature of host-guest interactions.-Experiments with aromatic guest molecules, inter alia with azo dyes, show that only compounds with acidic groups, particularly groups such as R=S03H, are selectively bound. Hereby, we define the selectivity S by the percent decrease in Rf obtained with 1. Besides the acidity of the substituents, the geometry of the hydrocarbon framework also has a decisive influence on the selectivity (cf. 5, 6 ) ; this is consistent with the order of lipophilicity and complexation observed, inter alia, in solution by NMR spectro~copy~~~. For the selective binding of different substrates, a most effective inclusion as possible in the cavity of immobilized 1 is essential. This follows from the generally far higher selectivities of naphthyl derivatives compared to the corresponding phenyl derivatives as well as from the considerably lower selectivities which are found with the immobilized open-chain analogue 2 : Here the S values obtained with sulfonic acids are only 20-40% instead of an average of 88%.Carboxylic acids are still complexed selectively if the lipophilic residue fits in the cavity of 1 (Scheme 2, compare 10 with 11 and 13) or if the electrostatic attraction can be realized by several substituents (cf. 12, 15). Phenolic groups are not acidic enough to lead to selectivity in methanol. On the other hand, the addition of water in this case leads, as also in solution"], to a strengthening of the binding (cf. 10, Scheme 2). A simple strategy for increasing the selectivity in the case of substrates with weakly dissociating groups consists in the use of an alkaline solvent, which enables novel separations to be realized, also in the case of natural products (Scheme 3). In the case of amino acids and peptides, protecting groups on the nitrogen which reduce the betaine participation and enhance the lipophilicity usually effect an increase in the selectivity.
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