Thioketenes dimerization reactions

Dithiocarboxylic acids (118) can be converted into 1,3-dithietans (119) by acid chlorides,iodine, HCl, DCCI, or upon standing for a long time, and they are formally thioketen dimers. The cycloaddition of two C=S groups yields thioketen dimers and (120) from methyl isothiocyanate. Derivatives (121) are prepared by the reaction of dithiocarboxylic acids (118) with phosgene. ... 

The reaction of diphenylmethylene phosphorane with carbon disulfide affords polymeric diphenylthioketenes . In a similar reaction fiuorenylidene triphenylphosphorane affords the corresponding thioketene dimer . An exchange reaction is observed in the reaction of... 

In the absence of an alkylating agent, a dithiadiazine 163 is formed on protonation <1984BSB405>. Since desaurine is a dimer of a thioketene, we can classify this reaction in this section. Alternatively, this could be seen as a ring transformation. 

The 4-methylene-l,3-dithietane-2-ones lose carbon oxysulfide to give thio-ketenes that dimerize to 2,4-dimethylene-l,3-dithietanes, as exemplified by the flash-vacuum thermolysis of 551. Reaction of several methylene 1,3-dithietane-2-ones with secondary amines also appears to proceed via loss of carbon oxysulfide followed by addition of the amine to the thioketene. An exception to the loss of carbon oxysulfide is the reaction of a guanidine derivative with 551 to give the ring-opened product 552. ... 

Thioketenes are stable isolable compounds only if the heterocumulene system is protected against dimerization by bulky substituents or perfluoroalkyl groups. 4 in these cases they are suitable educts for addition of hydroxylic compounds. 4. 4.ii5 reaction is also possible for the ylid (SO). The resulting thioxoester (51) does not however react with p-nitrobenzaldehyde in a Wittig reaction. ... 

The alkene-l,l-dithiolates (111), on phosgenation, give labile dithietanones (112), which give reactions typical of thioketens. The structure of the dimer of the lachrymatory factor (113) of onion has been re-assigned as (114), i.e. the first stable 1,2-dithietan derivative. Procedures for the oxidation of tetrafluoro- and tetrachloro-l,3-dithietans to the corresponding disulphones have been developed.Infrared and Raman spectral studies indicate that 1,3-dithietan is puckered (C21, symmetry) when free but planar (D2,-,) in the solid state. 

The diazomethane-keten reaction gave no 1,3-dipolar adducts, such as those obtained readily enough from the corresponding thioketen (see Vol. 1, p. 44), even at low temperatures. The 2 1 adduct obtained (165) cannot arise by prior dimerization of the keten, whose jS-lactone dimer (166) reacts to give a y-lactone (167) by methylene insertion at the C—O bond, so that quite different hydrolysis products are obtained (see Scheme 48). ... 

Reactions of 1,3-Dithietans.—Monothiobenzil can be generated by photolysis of its dithietan dimer.Dithietanones (121) can be used as thioketen equivalents. Tetrafluoro-l,3-dithietan reacts with oxygen atoms to give CFj, SO, and CFjS as primary products, and with trifluoromethyl hypochlorite to give (122). With tris(trifluoromethyl)methyl hypochlorite it gives (123). The 1,1-dioxides and the 1,1,3,3-tetroxides of tetrafluoro-, tetrachloro-, and tetrabromo-l,3-dithietans have been reported. ... 

As in the case of extrusion of dimethylsilanone, Mc2Si=0 (10), in the thermolysis of certain silaketenes , a similar type of silanethione (Me2Si=S 112) extrusion was postulated in the flash vacuum pyrolysis of bis(trimethylsilyl)thioketene (113) and (dimethylsilyl)(trimethylsilyl)thioketene (121) as shown in Schemes 37 and 38. In both cases, the formation of all the reaction products (compounds 115-120 for the pyrolysis of 113 shown in Scheme 37 and compounds 118, 120,123 and 124 for the pyrolysis of 121 shown in Scheme 38) can be mechanistically rationalized by processes each initiated by isomerization of the starting thioketenes via a 1,2-shift of a trimethylsilyl group to the corresponding a-thioketocarbenes 114 and 122. Under the pyrolytic reaction conditions used (700 or 768 °C) the intermediate silanethione 112 underwent ready oligomerization to give its dimer 120 and/or trimer 124. 

As a general rule ketenes undergo non-catalyzed [2+2] cycloaddition reactions across their C=C bonds, with the exception of ketene itself. In contrast, disubstituted thioketenes undergo cyclodimerization across their C=S bonds. Mono substituted thioketenes undergo dimerization via a [3+2] cycloaddition reaction, also involving the C=S bonds. 

The cycloaddition reactions of thio- and selenoketenes are similar to ketenes, but some exceptions are observed. For example, the dimerization of thioketenes occurs across the C=S double bonds, while in ketenes, dimers resulting from addition across the C=C bonds and unsymmetrical dimers, resulting from addition across both the C=C and the C=0 bonds are obtained. The [2- -2] cycloaddition reactions of thioketenes can involve the C=S or the C=C bonds, and additions across either one of these groups occurs. In their additions across C=N bonds both types of additions are also encountered. The progress of these reactions is monitored by the disappearance of their intensive color. Diarylthioketenes are blue, dialkyl derivatives are purple and monosilylthioketenes are red . 

In the attempted thioketene trapping experiments, sometimes dimerization of the generated thioketenes is observed. The dimerization occurs by a [2-1-2] cycloaddition reaction across the C=S bond to give the 1,3-dithietane derivatives 3. A similar dimer is isolated in the thermal generation of MeCOCH=C=S... 

The Diels-Alder reaction of thioketenes with cyclopentadiene and 2,3-dimethylbutadiene proceeds across the C=S bond of the thioketene to give the adducts in high yields. For example, trapping of dichlorothioketene, obtained in the flash vacuum pyrolysis of its dimer, with cyclopentadiene in an argon matrix at -196 °C, affords the [4-1-2] cycloadduct ". Also, pyrolysis of 5-isopropyl-4-phenyl-1,2,3-thiadiazole at 530 C and trapping of the thioketene 83 with cyclopentadiene gives a quantitative yield of the [4-1-2] cycloadduct 84 . ... 

The majority of the papers that during the past two years have been dealing with thioketens report on these compounds as transient species or postulated intermediates. However, one of two exceptional papers describes the successful synthesis of the hitherto unknown dimethylthioketen (138). This deep red compound, which actually had been postulated earlier to be an intermediate in the thermolysis of tetramethylcyclobutane-l,3-dithione (82), appeared as the main product by flash thermolysis of (82). In order to avert dimerization, (138) was trapped at -196 °C together with added trichlorofluoromethane, and was thus characterized spectroscopically as a solute, as well as by its reaction with dimethylamine to afford the thioamide (139). The thioketen (138) decomposed immediately at room temperature, whereas the half-life of its —0.3 molar trichlorofluoromethane solution at -70 °C was found to be 60 minutes. ... 

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