Layered double hydroxides exchanged with tungstate as biomimetic catalysts for mild oxidative bromination

摘要

The manufacture of a range of bulk and fine chemicals, including flame retardants, disinfectants and antibacterial and antiviral drugs, involves bromination. Conventional bromination methods typically use elemental bromine, a pollutant and a safety and health hazard. Attempts to develop alternative and more benign strategies have been inspired by haloperoxidase enzymes, which achieve selective halogenation at room temperature and nearly neutral pH by oxidizing inorganic halides with hydrogen peroxide. The enzyme vanadium bromoperoxidase has attracted particular interest in this regard, and several homogeneous inorganic catalysts mimicking its activity are available, although they are limited by the requirement for strongly acidic reaction media. A heterogenous mimic operating at neutral pH has also been reported, but shows only modest catalytic activity. Here we describe a tungstate-exchanged layered double hydroxide that catalyses oxidative bromination and bromide-assisted epoxidation reactions in a selective manner. We find that the catalyst is over 100 times more active than its homogeneous analogue. The low cost and heterogeneous character of this system, together with its ability to operate efficiently under mild conditions using bromides rather than elemental bromine, raise the prospect of being able to develop a clean and efficient industrial route to brominated chemicals and drugs and epoxide intermediates.