Thiiran 1-Oxides

Formation.—Thiiran 1-oxides are conveniently prepared by the oxidation of thiirans with peroxybenzoic acid or m-chloroperoxybenzoic acid in 

3-Dibenzoyl-2,3-diphenylthiiran 1-oxides (24) behaved very differently to the 2,3-dimethylthiiran 1-oxides on heating, giving mainly benzil and thiobenzil together with a little cis- and /ranj-dibenzoylstilbene. The stereochemistry at sulphur had no effect upon the mode of decomposition. Similar results were obtained on photolysis, and a common intermediate for thermolysis and photolysis was postulated, possibly a vibrationally 

Alkyl chloromethyl ethers react with thiiran 1-oxide to give sulphenic esters (26), probably by way of an intermediate sulphoxonium salt (27). Thiiran oxide in FSOsH-SbF at — 78 C was protonated at sulphur and not at oxygen, according to n.m.r. studies.  

F Nrmati Mi.— Methods for the synthesis of thiiran 1-oxides and their chemical reactions have been reviewed. An equimolar mixture (72% yield) 

Wagenaar, E. M. van Rens, and B. Zwanenburg, Tetrahedron Letters, 1973,3589 B. Zwanenburg, A. Wagenaar, and L. Thijs, V Symposium on Organic Sulfur Chemistry, Lund, Sweden, June 1972, Intemat. J. Sulfur Chem. (A), 1972, 2, 223. 

Infermediates in Reacti ms.— Treatment of dichlorosulphine (93) with diphenyldiazomethanes (and other diaryldiazomethanes) yields 2-chloro-benzo[h]thiophen 1-oxide (95), also obtained along with the 1,1-dioxide on oxidation of 2,2-dichloro-3,3-diphenylthiiran (96). A mechanism involving a 

Chonical and Physical Pn rties.—The vertical ionization energies (9.66, 9.78 eV) of thiiran 1-oxide and a number of other sulphoxides (e.g. DMSO, 9.01, 10.17 eV) have been determined.  

The acid-catalysed reaction of thiiran 1-oxides with methanol. yields thiolsulphinates, e.g. (99) with acetic acid or dry hydrogen chloride, a mixture of disulphide (100) and thiolsulphonate (101) is obtained, and with ethanethiol, a mercaptoethyl disulphide (102). The ring opening is generally 

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The configurations of chiral thiiranes and thiirane 1-oxides can be determined by H NMR experiments in chiral solvents such as (R)-(-)- -phenyl-2,2,2-trifluoroethanol. The... 

The H NMR spectrum of thiirane 1-oxide is complex (AA BB ) at 60 MHz 24 lines are cfbserved consisting of two sets of 12 centered about a midpoint. The H NMR chemical shift in thiirane 1,1-dioxide is fairly sensitive to solvent variations partly because of the high dipole moment (4.4 D) of the sulfone. The benzene-induced shift, A5 (CeDe-CCLt), is large (-1.04 p.p.m.), as expected from the presence of a sulfone group. Oxygen-17 chemical shifts for thiirane 1-oxide and thiirane 1,1-oxide are -71 and +111 p.p.m. respectively, relative to H2O. 

The UV spectra of thiirane 1-oxide and (15,25)-(+)-2-methylthiirane 1-oxide show a broad maximum at about 205 nm (e —23 000). The latter shows a positive Cotton effect at low energy followed by a negative effect at high energy. The lowest excited states of thiirane 1-oxide involve excitations from the two lone pairs of the oxygen atom (79G19). 2,3-Diphenylthiirene 1-oxide and 1,1-dioxide show absorption due to the 1,2-diphenyl-ethylene chromophore. 

Acids are poor catalysts for ring cleavage of thiirane 1,1-dioxides but are good catalysts for reactions of thiirane 1-oxides with nucleophiles. These reactions of episulfoxides are believed to proceed by protonation of the oxygen atom (but see the NMR evidence cited above for 5-protonation in fluorosulfonic acid) and will be treated in the section on nucleophilic reactions. 

Chloroethyldisulfides are obtained by electrophilic attack on the sulfur atom of thiiranes by sulfenyl halides (Scheme 39). Sulfur dichloride and disulfur dichloride react similarly to give more sulfur-rich derivatives di- and tri-sulfenyl halides, and tri- and tetra-sulfides (Scheme 42). A 1 1 ratio of sulfur halide to thiirane gives the di- or tri-sulfenyl halide a 2 1 ratio the tri- or tetra-sulfide. Thiirane 1-oxides are cleaved by sulfenyl halides to thiolsulfinates (Scheme 43) (74JAP7440461). 

One example of nucleophilic attack by a rr-electron system on a sulfur atom of a thiirane 1-oxide is shown in Scheme 51. S-Alkylthiirenium ions react with tetramethylethylene to transfer the S-alkyl group yielding the alkyne and an S-alkyl-2,2,3,3-tetramethylthiiranium ion (79MI50600). 

The reaction of thiirane 1-oxides with water or methanol is usually acid-catalyzed and gives /3-substituted sulfenic acids which dimerize to thiolsulfinates (54 Scheme 70) (72JA5786). If acetic acid is used a mixture of disulfide (55) and thiolsulfonate (56) is obtained. Treatment of thiirane 1,1-dioxides with hydroxide ion may involve attack on carbon as well as on sulfur as exemplified by 2-phenylthiirane 1,1-dioxide (Scheme 71). 

Thiirane 1-oxide undergoes acid-catalyzed ring opening by ethanethiol to give ethyl 2-ethylthioethyl disulfide. Treatment of thiirane 1,1-dioxide with thiolate anions, sodium sulfide or thiourea gives /3-mercaptosulfinic acid derivatives (75S55). Thiiranium ions are attacked at carbon by most sulfur nucleophiles (79ACR282), but see Section 5.06.3.4.3 for exceptions. 

Methylene thiirane is obtained by thermolysis of several spirothiirane derivatives which are formally Diels-Alder adducts of methylenethiirane and cyclopentadiene or anthracene <78JA7436). They were prepared via lithio-2-(methylthio)-l,3-oxazolines (c/. Scheme 121). A novel synthesis of the allene episulfide derivatives, 2-isopropylidene-3,3- dimethylthiirane (good yield) or its 5-oxide (poor yield), involves irradiation of 2,2,3,3-tetramethyl-cyclopropanethione or its 5-oxide (81AG293). Substituents on the thiirane ring may be modified to give new thiiranes (Section 5.06.3.9). The synthesis of thiirane 1-oxides and thiirane 1,1-dioxides by oxidation is discussed in Section 5.06.3.3.8 and the synthesis of 5-alkylthiiranium salts by alkylation of thiiranes is discussed in Section 5.06.3.3.4. Thiirene 1-oxides and 1,1-dioxides may be obtained by dehydrohalogenation of 2-halothiirane 1-oxides and 1,1-dioxides (Section 5.06.4.1.2). 

Thiirane is more bactericidal than oxirane, and derivatives of 2-mei captomethylthiirane inhibit tuberculosis. The following pharmacological uses have been reported for compounds derived from thiirane derivatives gold complexes of the adducts of diethylphosphine and thiirane (antiarthritic), adducts of thiiranes and malononitrile (antibacterial, blood vessel dilators, muscle relaxants, sedatives), thermolysis products of thiirane 1-oxides and adducts of thiirane 1-oxides with sulfenyl chlorides (antibacterial), adducts of 2,3-diarylthiirene 1,1-dioxides with ynamines (antibacterial, parasiticidal), adducts of 2,3-diarylthiirene 1,1-dioxides with enamines (antifertility), adducts of p-aminophenylacetic esters with thiirane (immunosuppressants), adducts of amines and thiiranes (radioprotective drugs). 

JIn the cyclic compound thiirane 1-oxide. See text for meaning of BDE in this case. 

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