The (Schiff base)vanadium(V) Complex Catalyzed Oxidation of Bromide − A New Method for the in situ Generation of Bromine and Its Application in the Synthesis of Functionalized Cyclic Ethers

Abstract

Abstract(Schiff base)vanadium(V) complexes 5 with tridentate imine auxiliaries served as catalysts for the oxidation of Br− with tert‐butyl hydroperoxide (TBHP) in nonaqueous solvents. This reaction has been applied for the conversion of substituted 4‐penten‐1‐ols into 5‐exo‐bromo‐cyclized products, including a diastereomerically pure heterocyclic precursor used in a synthesis of the all‐trans‐configured 2,3,4,5‐substituted tetrahydrofuran 2‐epi‐magnosalicin. Treatment of ψ‐substituted bis(homoallylic) alcohols with the reagent combination of pyHBr, TBHP, and a vanadium(V) catalyst 5 afforded 6‐endo‐cyclized products, i.e. brominated tetrahydropyrans, as major compounds. The results from 51V NMR, ESI‐MS, and supporting reactivity‐selectivity studies indicated that the mechanism of the new bromination reaction consists of vanadium‐dependent and vanadium‐independent steps. A (Schiff base)vanadium(V) compound 5 is required for activation of TBHP via in situ formation of the corresponding tert‐butylperoxy complex. This reagent oxidizes Br−, which under the reaction conditions provides Br2 as the active brominating reagent. The molecular bromine generated thus is released into the solution at a steady rate and serves as a reagent for the synthesis of β‐brominated cyclic ethers from bis(homoallylic) alcohols in a second, vanadium‐independent step. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)