Mannich reaction derived novel boron complexes with amine-bis(phenolate) ligands: Synthesis, spectroscopy and in vitro/in silico biological studies

Abstract

The amine-bis(phenolate) ligands was prepared through a Mannich reaction utilizing two equivalents of 2,4-di-tert-butylphenol or 4-tert-butylphenol, two equivalents of formaldehyde and a single equivalent of 2-aminoethyl diphenylborinate as primary amine. This work deals with the synthesis and evaluation of new (B←N) and (B-O) units containing amine-bis(phenolate) boron complexes designed by combination of amine-bis(phenolate) ligands and various boronic acids in toluene reflux using a Dean-Stark apparatus to remove water formed as a by-product. The newly synthesized amine-bis(phenolate) ligands and their boron complexes were characterized using elemental analysis, and their probable structures were proposed based on 1H and 13C NMR, FT-IR, UV-Vis spectroscopy and LC-MS/MS spectrometry. The inhibition effects of the synthesized compounds on acetylcholinesterase (AChE) activity in vitro and their radical scavenging activities were evaluated. The AChE was effectively inhibited by compounds, with KI values in the range from 5.48 ± 0.65 to 40.56 ± 4.42 µM. The (L1B4) has more inhibition effect on AChE with KI value (6.37 ± 1.50 µM) in (L2) series, while (L2B4) has an inhibition effect with KI (5.48 ± 0.65 µM) in (L2) series. Also, the compounds showed about 18-45% DPPH and 26-85% ABTS radical scavenging activity. On the other hand, butylated hydroxy anisole (BHA), butylated hydroxytoluene (BHT) and Trolox showed about 72-96% DPPH and 94-96% ABTS radical scavenging activity, respectively in the same concentration (25-30 µg/mL). Besides, the (L1B4), (L1B5), and (L2B4) determined as the most active inhibitors were docked into the binding site of AChE to find the binding interactions of the boron complexes with the protein.