Effect of substituents on the complexation of aromatic and quinoid substrates with molecular tweezers and clips.
Mol. tweezers and clips of type I-III substituted with OAc, OH, OCONHPh, OMe, OCH2COOR and OCH2CONHR groups in the central spacer units were synthesized by modification, by std. methods, either of the known diacetoxy-substituted derivs. I (R1 = R2 = OAc, R3= H), II (R1 = R2 = OAc, R3= H) and III (R1 = R2 = OAc, R3= H), or of the correspondingly substituted bis-dienophiles IV (R1 = R2 = OAc, R3= H; n = 0,1). The synthesis of the dimethoxy-diacetoxy-substituted tweezer I (R1 = R2 = OAc, R3= OMe) could be accomplished through pressure-induced repetitive Diels-Alder reactions between the bis-dienophile IV (R1 = R2 = OAc, R3= H; n = 0) and the newly prepd. diene V (R1 = R2 = OAc, R3= H) and subsequent DDQ oxidn. The thermodn. parameters (Ka and DG) of complex formation between the new receptors and arom. substrates such as DCNB, TCNB, TCNQ and Kosower salt and the max. complexation-induced 1H NMR shifts (Ddmax.) were detd. by 1H NMR titrn. expts. The presence of substituents OH, OAc and OCONHPh in the central spacer units of the tweezers and clips I-III favors complex formation, whereas that of the substituents OMe, OCH2COOR and OCH2CONHR disfavors it. This finding can be explained in terms of the size and different conformations of these groups in the tweezer and clip mols. as calcd. by force-field (MMFF) techniques rather than of their influence on the electrostatic potential surfaces (EPSs) of the adjacent arom. rings as calcd. by quantum mech. methods. The complementary natures of the neg. EPSs inside the tweezer and clip cavities and the pos. EPSs of the substrates forming complexes explained the high selectivity of these receptors toward electron-deficient substrates. The finding that the self-assembly of the OCH2COOCH2CH3 side chain is only obsd. for the benzene-spaced tweezers I (R1 = R2 = OCH2CO2Et, R3= H) and I (R1 = R2 = OCH2CO2H, R3= H) confirms earlier results obtained for the intermol. complexation of these receptors. Accordingly, the benzene-spaced tweezers of type I selectively bind aliph. substrates, whereas the naphthalene-spaced tweezers of type II and clips of type III preferentially complex arom. substrates.
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