The selective oxidation of thioanisole to sulfoxide using a highly efficient electroenzymatic cascade system

Abstract

Sulfoxides are remarkably useful in asymmetric synthesis and drug development. Herein, we propose a new electroenzymatic cascade route to selectively synthesize methyl phenyl sulfoxide rather than methyl phenyl sulfone by the oxidation of thioanisole. Chloroperoxidase (CPO) functionalized with 1-ethyl-3-methylimidazolium bromide (IL_EMB) is encapsulated into dendritic N-doped mesoporous carbon nanospheres (NMCNs) modified with polyethyleneimine (NMCNs-PEI) to form a biohybrid (CPO-IL_EMB@NMCNs-PEI). The in situ generation of hydrogen peroxide (H₂O₂) species by the two-electron oxygen reduction reaction (2e⁻ORR) on NMCNs-PEI initiates the subsequent selective oxidation of thioanisole by CPO-IL_EMB. NMCNs-PEI shows not only high electroactivity for the 2e⁻ORR but also good biocompatibility for CPO-IL_EMB immobilization. The mesopores in NMCNs-PEI can provide a protective space for the encapsulated enzyme, ensuring the operational stability of the enzyme and avoiding the escape of H₂O₂, which result in high catalytic activity. The experimental results suggest that the catalytic efficiency of the electroenzymatic cascade system with the CPO-IL_EMB@NMCNs-PEI biohybrid is up to 4.5 times higher than that of the free CPO-IL_EMB catalytic system with H₂O₂ generated by NMCNs-PEI for the oxidation of thioanisole to produce methyl phenyl sulfoxide.