Desulfurization, of thiiranes

Oxygen nucleophiles usually attack a ring carbon atom rather than the sulfur atom of a thiirane, and those cases in which desulfurization is observed on treatment of a thiirane with oxygen bases probably involve the extrusion of sulfur by mechanisms other than a nucleophilic attack on sulfur, e.g. thermal. Desulfurization of thiirane intermediate (43)... 

The P(m) reagents have an even greater affinity for sulfur than they do for oxygen and there are a number of useful synthetic procedures based on desulfurization.9 In contrast to the deoxygenation of epoxides, the desulfurization of thiiranes is stereospecific and represents a potentially useful synthetic route to alkenes. Unfortunately, however, thiiranes were not easily obtained until the... 

The kinetics and mechanism of rhenium-catalyzed desulfurization of thiiranes was studied experimentally and by means of computation <2006IC5351>. The oxorhenium dimer MeReO(edt)2 (X), where edt= 1,2-ethanedithiolate, was found to catalyze the transfer of S from thiiranes to triarylphosphines and triarylarsines. In the case of phosphines, catalysis was less effective because they converted the dimer catalyst rapidly to MeReO(edt)PAr3, a less effective catalyst. The rate law for AsArs, which did not transform the catalyst, was v = [thiirane][X] with k = 5.581 mol s at 25°C in CDCI3, for X = cyclohexene sulfide. The value of was 10.0 0.9kcalmoP, while AY was —21 3calK moP. The proposed mechanism is shown in Scheme 6. 

Very rapid desulfurization of thiiranes by a bimetallic complex was reported late in 1995. Bergman and co-workers found that Cp2Zr(/t-A -f-Bu)IrCp, where Cp is cyclopentadienyl and Cp is pentamethylcyclopenta-dienyl, desulfurizes thiiranes via a bridging sulfido complex 121, for which a crystal structure was obtained <1995JA10041>. Small substituent effects in the thiiranes and retention of substituent stereochemistry suggested that ring-opened intermediates are not involved and that the reaction rate might approach the diffusion-controlled limit. 

Reaction of various thiiranes 208, catalyzed by aminium salts (tris-(4-bromophenyl)aminium hexachloroantimonate 209 or tris-(2,4-dibromophenyl)aminium hexachloroantimonate 210) in freshly distilled dichloromethane, led to desulfurization of starting thiiranes to alkenes 211 in good yield (typically 80%) (Scheme 53) <1995T8935>. The mechanism of desulfurization of thiiranes catalyzed by aminium salts is presented in Scheme 54. 

Other fused thiiranes are more thermally stable. For example, the compound 45 desulfurizes at 160 °C (Scheme 11) <1987JA3801, 1990JA3029>. The reaction provided alkenes with retention of stereochemistry. In the presence of a desulfurizing agent such as phosphines or phosphites, the desulfurization temperature may be decreased to room temperature (Section 1.06.6.6) <1984CHEC(7)131>. The desulfurization of thiiranes is also subjected to transition metal catalysis. Since the initial step involves coordination of the metal to thiirane sulfur atom, the topic is discussed further in Section 1.06.6.4. 

Transition metal complexes catalyze desulfurization of thiiranes. Complexes of Rh <1986TL3573>, Re <1997TL7701, 1999CC1003>, and Mo <2003JA3871> have been successfully employed in catalytic quantities. The presence of a stoichiometric sulfur acceptor such as carbon monoxide or an alkene, phosphine, or arsine is required. In Re-catalyzed desulfurization, the formation of an intermediate Re =S species via initial coordination of the thiirene ligand has been suggested (Schemes 12 and 13) <1997TL7701>. Kinetics and mechanism of sulfur... 

Desulfuration of thiiranes by metallic lithium <1984IJB879> or sodium <1998TL2671> in refluxing hydrocarbon solvents probably proceeds via a radical mechanism. The method is also effective for desulfurization of other organic sulfides but both monocyclic and fused thiiranes are particularly reactive. 

The exo-allenic thietane dioxide 407 is obtained by cycloaddition of an allenic sulfene to an enamine. Desulfurization of thiirane 406 by triphenylphosphine... 

The reductive desulfurization of thiiranes has been accomplished with Raney nickel, Li in EtNHa, Zn in HOAc, BusSnH, and P2I4 in DMF but the reaction has seldom been applied to organic synthesis. A noteworthy procedure for preparing moderately hindered tetrasubstituted alkenes, which proceeds via a thiirane intermediate, was developed independently by Barton and Kellogg. The procedure, illustrated in Scheme 14 for the preparation of (34), involved reaction of thiocamphor (30) with diazoalkane... 

Metallic reagents, e.g. w-butyllithium, also bring about a stereospecific desulfurization of thiiranes. 

Rapid desulfurization of thiiranes hy heterodinuclear complexes is achieved by using an early-late dinuclear transition-metal complex. Cp2Zr(/r-N Bu)IrCp reacts with thiiranes in almost diffusion controlled rates to give /r-sulfido complex and corresponding olefin with retention of configuration at the alkenyl carbon center (Scheme 3.68) [132]. Two mechaiusms such as completely concerted sulfur transfer reaction and insertion of the C-S bond into the Zr-Ir bond followed by concerted four-center ehmination are proposed. 

A series of heterodinuclear late organotransition metal complexes (dppe)RPt-M(C0)4 (M = Mn, Re) show the unexpected regio- and seteroselective desulfurization of thiiranes controlled by the ancillary alkyl ligand (Scheme 3.69) [133]. Reaction of the methyl complex, (dppe)MePt-M(CO)4 with cis- and trans-2-butene sulfide results in the insertion of the thiirane into the Pt-M bond followed by the CO insertion into the carbon-M bond and coordination of the S atom to the... 

There has not been much research in the area of metal-catalyzed reactions of thiiranes, which are sulfur analogues of epoxides. It has been shown that chlorodicarbonylrhodium(I) dimer can catalyze the homogeneous desulfurization of thiiranes to olefins [35]. This reaction is stereospecific and proceeds for thiiranes having aliphatic, aromatic, ether, and ester substituents (Equation (6.8)). This chemistry contrasts with that observed with cobalt. At room temperature, when 2-phenylthiirane, was treated with CO (latm), CH3I, KOH, CgHg, tetrabutylam-monium bromide, and a catalytic amount of cobalt carbonyl, fl-mercapto acid, was... 

The stereospecific desulfurization of thiiranes may be effected by carbenes (70). Epoxides are deoxygenated with much lower efficiency (71). 

Reagent for deoxygenation of epoxides and peroxides, and desulfurizations of thiiranes, and polysulfides. Ligand for metals of Groups IB, IIB, IVB, VB, VIB, VIII. Used as C H soln. for extraction-separation of In (as ion-associate with InBr4 , diisopropyl ether). Liq. with garlic-like odour. Misc. org. solvs. Fp 40°. Bp 240°, Bp2o 130°. 1.4635. 

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