Novel Model Sulfur Compounds as Mechanistic Probes for Enzymatic and Biomimetic Oxidations

Abstract

AbstractTo test for the intermediacy of sulfide radical cations in biomimetic and enzymatic oxidations, the sulfides PhSCH3 (1a), PhSCH2Ph (1b), PhSCHPh2 (1c), PhSCPh3 (1d), CH3SCHPh2 (2), PhSCH2CH=CH2 (3), PhSCH2CH=CHPh (4) and CH3SCH2CH=CHPh (5) were studied, and their results were compared to those obtained for the corresponding chemical electron transfer (CET) and photoinduced electron transfer (PET) oxidations. The radical cations generated from 3−5 by CET in the presence of cerium(IV) ammonium nitrate (CAN) yielded only fragmentation products from the alkyl cations and the thiyl radicals (RS·), whereas 2·+ afforded both fragmentation and mainly α‐deprotonation products. Photochemical treatment of the sulfides 1a and 1b with C(NO2)4 gave only the corresponding sulfoxides, while fragmentation was the main pathway for the photoreactions of 1c, 2 and 5, and for 1d only this latter process was observed. These results support our selection of the sulfides RSCHPh2, RSCH2CH=CHPh (R = Me, Ph) and PhSCPh3 as models for the biomimetic and enzymatic studies. As evidenced by the sulfoxides and sulfones detected as unique products both in protic and in aprotic solvents, it is proposed that the mechanism of the biomimetic sulfoxidations of sulfides 1c and 2−5 by TPPFeIIICl is direct oxygen transfer. Three enzymes − Coprinus cinereus peroxidase (CiP), horseradish peroxidase (HRP) and chloroperoxidase (CPO) − were studied in the oxidation of sulfides 1a, 2, 4 and 5. The use of a racemic alkyl hydroperoxide in the CiP enzymatic oxidation of sulfides 5 and 2 yielded the corresponding sulfoxides (23 and 29%) and the aldehyde or benzophenone (5%), respectively. These results suggest the involvement of an ET process for the CiP‐catalysed oxidation. Fragmentation products were observed in the enzymatic oxidation of sulfide 4 with HRP, which confirms the previously proposed ET mechanism. On the other hand, the CPO‐enzymatic oxidation of sulfide 5 yielded only the corresponding sulfoxide, as would be expected for a direct oxygen‐transfer or oxene mechanism. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)