Thiiranes intermediate

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 preparation of thiiranes is most conveniently performed in solution. However, there are also protocols reported for reaction in the gas and solid phase. By using diazo and thiocarbonyl compounds in ether as solvent, both alkyl and aryl substituted thiiranes are accessible. As indicated earlier, aryl substituents destabilize the initially formed 2,5-dihydro-1,3,4-thiadiazole ring and, in general, thiiranes are readily obtained at low temperature (13,15,35). On the other hand, alkyl substituents, especially bulky ones, enhance the stability of the initial cycloadduct, and the formation of thiiranes requires elevated temperatures (36 1,88). Some examples of sterically crowded thiiranes prepared from thioketones and a macro-cyclic diazo compound have been published by Atzmiiller and Vbgtle (106). Diphenyldiazomethane reacts with (arylsulfonyl)isothiocyanates and this is followed by spontaneous N2 elimination to give thiirane-2-imines (60) (107,108). Under similar conditions, acyl-substituted isothiocyanates afforded 2 1-adducts 61 (109) (Scheme 5.23). It seems likely that the formation of 61 involves a thiirane intermediate analogous to 60, which subsequently reacts with a second equivalent... 

Similar reactions were carried out on 17,29 18,30 and 19.30 The last gave thiochromans and 2,3-dihydrothiophenes from an apparent cyclization at the ortho methyl substituent, but in fact involving a variety of hydrogen and methyl shifts. The route to the two main products [Eq. (7)] of the thio-Claisen rearrangement has been suggested to be via the thiirane intermediate (20).31... 

Thiiran intermediates were also obtained from carbonyl compounds by the use of thiomethylene transfer reagents. Sulfur extrusion yielded olefins. Meyers s methylthio-2-oxazoIine is representative of such reagents [305],... 

Evidence has been presented591 that favours the involvement of thiirane intermediates in the formation of ( , )-divinyl sulfides (455) from the reaction of thiocarbonyl ylides (454) with activated alkynes, while the reaction of l-alkynyl-2,3-epithio alcohols (456) with a catalytic amount of Hg(II) has been shown to afford592 substituted thiophenes (457) (see Scheme 117). 

Benzenesulfenyl chloride alkene adducts may be transformed to many useful molecules. Intermediates, such as (3), can be treated with base to pi uce vinyl or allyl sulfi s (equation 2). Alternatively, the adducts can be oxidized and treated with base to yield vinyl sulfones in high overall yield (equation 3). The thiirane intermediates or adducts, i.e. (3), may be alkylated with alkyl-titanium and -aluminum reagents which replace the chloride substituent with retention of configuration. 

An unusal /3-chlorodisulfide product 55a was formed in the reaction between norbornene and triphenylmethylsul-fenyl chloride (Scheme 20) <1993TL4289>. The structure was verified by X-ray crystallography. A similar result was obtained with bicyclo[2.2.2]octene. These products were probably formed by electrophilic attack of sulfur atom on the thiirane intermediate 54a and 54b. 

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... 

Thioamides of two cyclopropanecarboxylic acids undergo coupling reactions with diazomethane derivatives in refluxing benzene containing rhodium(II) acetate. The reaction conceivably takes place via a thiirane intermediate which undergoes reductive elimination by reaction with excess diazo compound. An example of an intramolecular reaction is the formation of indolizine 2. If the diazo compound is not in excess a thiol is formed, this can be removed by Raney nickel desulfurization. ... 

Dialkyl sulphones may be converted to sulphonic acids by reaction with carbon tetrachloride and base at 80 °C259. This reaction proceeds by initial formation of a-chloro sulphones that are then converted to a thiirane intermediate, which decomposes to give a sulphonic acid (equation 48). 

It is required that this ring-distorted, triplet state thiirane intermediate possess a substantial energy barrier with respect to rotation about the CC bond to maintain the stereochemical information content of the reaction. Calculations yielded a value of 23.6 kcal for the rotational energy barrier, which explains the experimental results. 

S( P). The kinetics are consistent with a reaction mechanism involving an electronically excited thiirane intermediate, and the experimental ac-... 

Intermediates in Reactions.— A family of excited-state structures for thiophens in which a two-atom fragment is wrenched 90° out of the plane formed by the remaining three atoms is suggested as being preferable to a previously proposed family of structures involving thiirans to account for the photochemically induced valence-bond isomerizations observed. Thiiran intermediates (26) and (27) have been suggested to account for photochemical isomerizations of phenylthiazoles (25) and thiiran intermediates may be involved in the extrusion of sulphur from thiepins, e.g. 

Thiapyran (or thiabenzene) anions (33) expel sulphur, possibly via a thiiran intermediate (34). Diphenylthiiran (35) has been considered as an intermediate in the photochemical decomposition of stilbene trithiocarbo-nate to phenanthrene. Atomic sulphur in a triplet state may be involved in... 

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