CHIRAL RECOGNITION OF DICARBOXYLATE ANIONS BY SAPPHYRIN-BASED RECEPTORS

The synthesis and characterization of the open-chain and cyclic sapphy rin dimers 2-4 and 7, bearing various bisamide spacers is reported. Th is family of receptors was shown to display excellent recognition prop erties for various dicarboxylate anions, as judged from mass spectrome tric analyses, U-tube aqueous L/CH2Cl2/aqueous II through-model-membra ne transport experiments, and equilibrium binding studies. These latte r were carried out in either methanol or dichloromethane using H-1 or H-2 NMR and visible spectroscopic titrations. The flexible, first-gene ration system 2, featuring a 1,3-bisamidopropane spacer was found to d isplay a high affinity for dicarboxylate anions even in polar solvents , such as methanol. Within a range of substrates, this receptor showed a strong preference toward linear over bent, and aromatic over alipha tic dicarboxylate anions, a fact that is readily rationalized in terms of extra, stabilizing pi-pi, C-H ...pi, or C-H ... N interactions. Th is latter C-H ... N hydrogen-binding motif was observed in the single crystal structure of the 1:1 complex formed between benzoate anion and the monoprotonated form of sapphyrin la. The second-generation, open- chain chiral sapphyrin dimers 3 and 4 (containing (1S,2S)-1,2-bisamido cyclohexane and (S)-2,2'-bisamido-1,1'-binaphthalene chiral auxiliarie s, respectively) were found to form strong complexes with N-carbobenzy loxy-protected aspartate and glutamate anions (K-a values are on the o rder of 10(4)-10(5) M-1 in 19:1 (v/v) dichloromethane-methanol), and d isplayed a preference for glutamate over aspartate, with receptor 4 sh owing a modest level of enantiomeric selectivity. The cyclic dimer 7 b inds these anions less effectively, but displays excellent chiral disc rimination between the D-and L-antipodal forms of N-carbobenzyloxy-pro tected glutamate anion.