Redox chemistry of organoselenium compounds: Ab initio and density functional theory calculations on model systems for transition states and intermediates of the redox cycle of selenoenzymes

Ab initio [Hartree–Fock (HF)] and density functional theory (B3LYP) ECP calculations on intermediates and transition states of the reduction of phenylseleninic acid with hydrogen sulfide and of seleninic acid with benzenethiol as models for the species involved in the redox cycle of selenoenzymes are presented. Selenenyl sulfide is found to be the final product of the reduction reaction with sulfides. Further reduction to the selenol requires surmounting a substantial barrier (∼50 kcal mol−1). The final step of the redox cycle—oxidation of the selenol by hydrogen peroxide—is strongly exothermic. In case of the unsubstituted derivative H2Se three mechanisms for this oxidation are found [concerted and stepwise 1,2‐hydrogen and oxygen shift (TS6A, TS6B, resp.) and a four‐membered transition state TS6C]. For the oxidation of phenylselenol only TS6A and TS6B are obtained. For nearly all calculated structures the HF wave functions show a triplet instability. In contrast, at the B3LYP level only TS6C has a significant triplet instability. On the basis of QCISD(T)//QCISD results a mechanism involving TS6C seems highly unlikely.