Thiiranes from alkenes

Sulfur dichloride is difficult to handle it disproportionates readily, has an unpleasant odor, and tends to introduce extra, unwanted chlorine atoms. Several reagents have been developed, especially by Harpp and co-workers, in which these undesirable features have been modified or eliminated. Succinimide-N-sulfenyl chloride (28) and phthalimide-N-sulfenyl chloride (29) are both stable crystalline compounds which undergo many of the reactions of the sulfur dichloride itself. They can, for example, be used in a facile, high yield synthesis of thiiranes from alkenes.33... 

For the formation of thiiranes from alkenes, it seems best to use the arenethiosulfenyl chloride method since this appears to result in the highest yields. While the sulfur monochloride method can be used, the yields are ically poor to moderate over a wide range of substrates. For both methods there are some common problems which include competing group reactivity (other alkenes or electron-rich groups) and, of course, the stench associated with many sulfides. 

A new method for the preparation of thiirans from alkenes employs suc-cinimide- or phthalimide-N-sulphenyl chlorides to form addition compounds, e.g. (320 n = 1, 2, or 4), which, on treatment with LiAlH4 at -78 °C, give the corresponding cycloalkene sulphides (58—97%).The method has also been successfully applied to the preparation of thiirans from styrene, methylene-cyclohexane, and norbornene. 

Readily available 2-chloroalkyl-0,0-diethylthiophosphates 304, prepared from alkenes and diethoxyphosphoranesul-fenyl chloride, reacted, during 3 h at — 78 °C, with tetrabutylammonium fluoride trihydrate in THF to give thiiranes 305 in moderate or good yield <1997HAC429>. A one-pot version of this reaction is known (Scheme 90 Table 16). 

Thiiranes can be prepared directly from alkenes using specialized reagents. Thiourea with a tin catalyst gives the thiirane, for example. " Interestingly, internal alkynes were converted to 1,2-dichorothiiranes by reaction with S2CI2 (sulfur monochloride).It is noted that epoxides are converted to thiiranes with ammonium thiocyanate and a cerium complex. " A trans-thiiration reaction occurs with a molybdenum catalyst, in which an alkene reacts with styrene thiirane to give the new thiirane. 

Fluorothiiranes are also prepared by cyclization of open-chain precursors. Treatment of sulfenyl chloride 92 with the triethylamine-boron triiluoride complex gave thiirane 30 in 68% yield. Tris(diethylamino)phosphine transformed disulfide 94, prepared from alkene 93 and sulfur monochloride, into thiirane 95. An indirect route from a chlorodisulfide that has the advantage of utilizing both halves of the molecule is exemplified by the transformation of 96 into thiirane 97. ... 

Thiirans are available from alkenes via succinimide-iV-sulphenyl chloride or phthalimide-N-sulphenyl chloride adducts (Scheme 56). ... 

Thiirane 1,1-dioxides extrude sulfur dioxide readily (70S393) at temperatures usually in the range 50-100 °C, although some, such as c/s-2,3-diphenylthiirane 1,1-dioxide or 2-p-nitrophenylthiirane 1,1-dioxide, lose sulfur dioxide at room temperature. The extrusion is usually stereospeciflc (Scheme 10) and a concerted, non-linear chelotropic expulsion of sulfur dioxide or a singlet diradical mechanism in which loss of sulfur dioxide occurs faster than bond rotation may be involved. The latter mechanism is likely for episulfones with substituents which can stabilize the intermediate diradical. The Ramberg-Backlund reaction (B-77MI50600) in which a-halosulfones are converted to alkenes in the presence of base, involves formation of an episulfone from which sulfur dioxide is removed either thermally or by base (Scheme 11). A similar conversion of a,a -dihalosulfones to alkenes is effected by triphenylphosphine. Thermolysis of a-thiolactone (5) results in loss of carbon monoxide rather than sulfur (Scheme 12). 

Electrophilic attack on the sulfur atom of thiiranes by alkyl halides does not give thiiranium salts but rather products derived from attack of the halide ion on the intermediate cyclic salt (B-81MI50602). Treatment of a s-2,3-dimethylthiirane with methyl iodide yields cis-2-butene by two possible mechanisms (Scheme 31). A stereoselective isomerization of alkenes is accomplished by conversion to a thiirane of opposite stereochemistry followed by desulfurization by methyl iodide (75TL2709). Treatment of thiiranes with alkyl chlorides and bromides gives 2-chloro- or 2-bromo-ethyl sulfides (Scheme 32). Intramolecular alkylation of the sulfur atom of a thiirane may occur if the geometry is favorable the intermediate sulfonium ions are unstable to nucleophilic attack and rearrangement may occur (Scheme 33). 

The formation of alkenes from thiirane dioxides may not be stereospecifically controlled in the presence of a sufficiently strong base and sufficiently acidic protons in the three-membered ring. Under such conditions (essentially those typical for the Ramberg Backlund reaction), epimerization via a carbanion intermediate produces an equilibrium mixture of thiirane dioxides19,99 and consequently a mixture of cis- and trans-alkenes. 

The elimination of sulfur dioxide from thiirane dioxides leading to the corresponding alkenes is not the only result of base-induced reactions other products are also formed. This fact raises the question of the mechanistic pathway of this reaction. In general, the thiirane dioxide is treated with a large excess of the base in an appropriate solvent for several hours at room temperature or below. Bases commonly used are 2n NaOH (in water), NaOCH3 (in methanol), t-BuO-K + (in f-BuOH) and BuLi (in tetrahydrofuran) or KOH-CCU (in t-BuOH)16-19"112 113. 

Similarly, the stereospecific formation of cis-2-butene from cis-2,3-dimethylthiirane dioxide19 may be rationalized in terms of a stereospecific ring opening to give the threo-sulfinate 120 which, in turn, decomposes stereospecifically to yield the cis-alkene, hydroxide ion and sulfur dioxide73. The parent thiirane dioxide fragments analogously to ethylene, hydroxide ion and sulfur dioxide (equation 49). 

Of trialkyl phosphites the most frequently used is triethyl phosphite (EtO)3P (M.W. 166.16, b.p. 156°, density 0.969) which combines with sulfur in thiiranes [291, 294] and gives alkenes in respectable yields. In addition, it can extrude sulfur from sulfides [295], convert a-diketones to acyloins [296], convert a-keto acids to a-hydroxy acids [297], and reduce nitroso compounds to hydroxylamines [298] Procedure 47, p. 111). 

Cyclic sulfides treated with triethyl phosphite eject sulfur and form a new ring less one member [577], From compounds containing sulfur in three-membered rings (thiiranes) alkenes are formed in high yields [291, 294. The same reaction can be achieved with triphenyl phosphine [291]. 

Thiiranes that are obtained from the reaction of diazo dipoles with C=S bonds can be transformed into alkenes by desulfurization. This reaction sometimes occurs spontaneously, but more often is achieved by treatment with phosphanes (225). This important methodology represents an alternative for the W ittig reaction and has high merit for the preparation of stericaUy hindered (226-229) and uncommonly functionalized alkenes (214,216,217,230,231). Some examples are given in... 

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