Use of WO42- on layered double hydroxides for mild oxidative bromination and bromide-assisted epoxidation with H2O2

Tungstate, exchanged on a (Ni,Al) layered double hydroxide, is applied as a heterogeneous catalyst in the oxidation of bromide with H2O2 and the ensui ng electrophilic bromination of olefins. The high halogenation activity of the catalyst in essentially neutral conditions mimicks the activity of V-br omoperoxidase enzymes. In water, aromatic and aliphatic olefins are selecti vely converted to bromohydrins; in methanol, methoxybromides are produced. In appropriate solvent conditions, the bromohydroxylation of geminally di-, tri-, and tetrasubstituted olefins proceeds via dehydrobromination to the epoxide. Evidence for this mechanism is provided by kinetic and labeling ex periments. This one-pot alternative for the two-step halohydrin epoxidation process is enabled by the mild pH conditions: bromide is effective in subs toichiometric, catalytic amounts. All new catalytic procedures are characte rized by a high oxidative stability of the catalyst, high productivity of t he catalyst on weight basis, high W turnover frequencies in ambient conditi ons (up to 50 mol of product per W per h), and high chemo-, regio-, and ste reoselectivities.