Biomimetic Oxidative Bromination by cis-Dioxidotungsten(VI) Complexes of Salan Type N,N’-Capped Linear Tetradentate Amino Bisphenol

Abstract

Reaction of [WVIO2(acac)2] (Hacac=acetylacetone) with salan-type dibasic tetradentate ONNO donor Mannich bases derived from ethylenediamine, formaldehyde and 2,4-di-tert-butylphenol (H2L1), 2-tert-butyl-4-methylphenol (H2L2), 2,4-dimethylphenol (H2L3) and 2,4-dichlorophenol (H2L4) in a 1 : 1 ([WVIO2(acac)2] : H2L) molar ratio in refluxing MeOH gave the corresponding cis-dioxidotungsten(VI) complexes [WVIO2L1] (1), [WVIO2L2] (2), [WVIO2L3] (3) and [WVIO2L4] (4), respectively. Characterization by elemental analysis, various spectroscopic (FT-IR, UV-vis, 1H and 13C NMR) studies, DFT calculations and single-crystal X-ray analysis of 2 and 3 suggest six-coordinated octahedral α-cis (symmetric) isomeric form of the complexes where ligands coordinate through the two phenolate oxygen and two amine nitrogen atoms (in a cis-α type symmetric binding mode) with one of the N atoms of the ligand and one of the terminal O atoms of the cis-WO2 group in the axial position. These complexes are potential catalyst precursors for the oxidative bromination of thymol and styrene. Thymol upon bromination gave three products, namely, 2-bromothymol, 4-bromothymol, and 2,4-dibromothymol; later one being the major product. Oxidative bromination of styrene resulted in 2-bromo-1-phenylethanol and 1-phenylethane-1,2-diol; the later one is the result of nucleophilic attack of water on the α as well as β carbons both of the initially formed 1,2-dibromo-1-phenylethane. © 2021 Wiley-VCH GmbH