Two-armed neutral anion receptors (4, 5), were prepared and examined for their anion-binding ability using UV-vis, fluorescence and ¹H NMR spectra in DMSO. The results of non-linear curve fitting indicate that 4 or 5 form 1 : 1 stoichiometric complexes with dicarboxylate anions by multiple hydrogen bonding interactions and the sensitivity for recognition of dicarboxylate depends on the chain length of these dicarboxylate anions. Receptors 4 and 5 have no binding ability with acetate, dihydrogen phosphate and the halogen (Cl⁻, Br⁻, I⁻) anions. This demonstrates that receptors 4 or 5 could be used as chemical sensors for some special dicarboxylate anions.

Rh(II)-catalyzed multicomponent reaction achieved to give α-amino-β-indole ketones by trapping α-amino enols with vinylimine ions.