Halogenation Haloperoxidase Reactivity

If the substrate for oxygen transfer step in the back-reaction of Compound I to the resting state is a halogen atom, the overall process is denoted haloperoxidase 

The reaction has found little application so far however, one of the first examples of an asymmetric synthesis using peroxidases was the highly regio- and diastereo-selective halohydration of glycals to 2-deoxy-2-halo sugars in the presence of CPO [263]. Some examples of selective halogenation reactions of aromatic compounds using CPO optimized by directed evolution approaches are also known [264]. 

---

Heme-dependent haloperoxidases generate HOX as reactive species from H2O2 and X, which represents an X+ equivalent capable of undergoing electrophilic addition at electron-rich centers [270,271]. Aprototype biocatalyst of this group is the chloroperoxidase from Caldariomyces Jumago [272]. In many natural systems, such enzymes are responsible for the halogenation of electron-rich aromatic cores. 

Unfortunately, direct epoxidation of alkenes by metal-free haloperoxidases led to racemic epoxides [1331, 1332]. Since the reaction only takes place in the presence of a short-chain carboxylic acid (e.g., acetate or propionate), it is believed to proceed via an enzymatically generated peroxycarboxylic acid, which subsequently oxidizes the alkene without the aid of the enzyme. This mechanism has a close analogy to the lipase-catalyzed epoxidation of alkenes (Sect 3.1.5) and halogenation reactions catalyzed by haloperoxidases (Sect. 2.7.1), where enzyme catalysis is only involved in the formation of a reactive intermediate, which in turn converts the substrate in a spontaneous (nonenzymatic) foUowup reaction. 

---