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

(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 51 V 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 Br 2 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)