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

摘要

(Schiff base)vanadium(v) complexes 5 with tridentate inline 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-l-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-substi-tuted tetrahydrofuran 2-epi-magnosalicin.Treatment of co-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 tetrahy-dropyrans,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 bro-minating reagent.The molecular bromine generated thus is released into the solution at a steady rate and serves as a reagent for the synthesis of beta-brominated cyclic ethers from bis(homoallylic) alcohols in a second,vanadium-independent step.