A systematic examination of the electronic and steric effects in the peracid oxidation of thianthrene 5-oxide (SSO) was undertaken and has revealed valuable information on the oxidative reactivity of the SSO mechanistic probe toward peroxidic oxidants. Thus, no significant steric effect was found on the bisulfoxide (SOSO)/sulfone (SSO(2)) product ratio for aliphatic peracids with varying size of alkyl groups [R = Me, Et, i-Pr, t-Bu, (n-Bu)(Et)CHCH(2)]. However, a significant electronic effect was observed for CH(3)CO(3)H and CF(3)CO(3)H, for which the X(SO) values were 0.16 and 0.01, i.e., predominant oxidation of the sulfide functionality to the SOSO product by these oxidants. In addition, substituted perbenzoic acids displayed this trend. In contrast, the nucleophilic peracid anion led exclusively (X(SO) = 1.00) to oxidation of the sulfoxide site to the SSO(2) product. The oxygen transfer appears to be orbital-controlled since the more electrophilic oxidant is clearly the more reactive as well as more chemoselective, i.e., preferred attack at the more nucleophilic sulfide site in SSO.

Thianthrene 5-oxide ( S O ) is established as a useful and convenient chemical probe to assess the electronic character (Xso) of oxygen transfer agents. Thus, HzOz under basic conditions (HOOH/nBu4NOH) gives an X,, value of 1.00, while HzOZ under acidic conditions (HOOH/HC104) affords an Xso value near zero. On this Xso scale, dimethyldioxirane (Xso = 0.13 at O T ) and methyl(trifluoromethy1)dioxirane (Xso = 0.10 at OOC) are, as expected, definitely strong electrophilic oxidants.During the last decade, the oxidation of thianthrene 5-oxide (SSO) has been used as a mechanistic probe to determine the electronic character of an oxidant (Scheme To reconcile this discrepancy, numerous mechanistic rationales have been suggested during the last few years. Thus, nucleophilic attack of the sulfoxide oxygen on the peroxide bond of the dioxirane[l31 was proposed, which would generate an intermediary persulfoxide, and subsequent rearrangement of the latter was supposed to give SS02. This hypothesis was recently convincingly disproved[l41. Alternatively, the possibility of an electron transfer p r o c e~s [~~'~I to afford the radical cation of SSO and the radical anion of DMD has been invoked, since the latter would undoubtedly act as a nucleophilic oxygen transfer agent. Nonetheless, quantum mechanical calculations[l61 confirmed our experiments that the SS02 should be the major product.In a recent application of the SSO probe for a biological oxidation, specifically the hemoprotein oxidizing species [7], in addition to assess its electrophilic nature, the cis-trans ratio of SOSO was determined to elucidate the preferred stereochemical approach of the biological oxidant towards thianthrene 5-oxide. In view of its substantially puckered conformation (dihedral angle ca. 133° [17]), the two sulfide lone pairs are stereotopically differentiated, axial attack by the oxidant would generate trans-SOSO and the equatorial

4-Dimethylaminopyridine-N-oxide 2 and 2',3',5'-triacetyladenosine-N1-oxide 4 are partially deoxygenated by dimethyldioxirane (DMD) to the corresponding amines 1 and 3; the formation of singlet oxygen suggests a polar rather than a radical mechanism, in which we propose SN2 attack of the N-oxide on the dioxirane peroxide bond.

Thianthrene 5-oxide (SSO) constitutes a useful mechanistic tool for the assessment of the electronic character of oxygen transfer agents by means of their X,, values, which reflect the extent of nucleophilic oxidation at the SO site in SSO. Treatment of dioxiranes 1 a-d with the SSO probe confirms that these are electrophilic oxidants (Xso < 0.15). Dioxirane sulfoxidation is sensitive to protic solvents and acids, which implies a polar mechanism (S,2) with nucleophilic attack of the sulfide electron pair on the dioxirane peroxide bond. In contrast, the carbonyl oxides 2a and b, diethyl persulfoxide (3), and adamantylideneadamantane perepoxide (4) are nucleophilic oxidants (Xso > 0.85). However, the cyclopentadienone carbonyl oxides 2c and d show low Xso values typical for electrophilic oxygen transfer agents. For these car-

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