Thioketens and Selenoketens

Synthesis.—Owing to their reactivity and instability, few thioketens were isolated until very recently. A remarkable method for the synthesis of thioketens (48) in good yields by the flash thermolysis of 1,2,3-thiadiazoles (47) has now 

Schaumann, S. Harto, and G. Adiwidjaja, Angew. Chem. Internat. Edn., 1976, 15, 40. 

Reactions.—Three interesting papers deal with the reactivity of thioketens. A noteworthy paper by Schaumann and Walter reports that thiophilic addition of organometallic compounds is observed with thioketens. At - 78 °C, dialkyl-thioketens (53) react with phenyl-lithium or methyl-lithium by 1,2 thiophilic addition to give enesulphides (55). At room temperature the main products 

The reaction of a ketone with a thioalkynolate gives a -thionolactone. The intermediacy of a thioketen has been suggested.  

Reactions.—Thioketen (83) reacts with azomethines to give /ff-thiolactams. It has been suggested that the dipolar species (84) is formed, instead of the classical dipolar species (85), in the rate-determining step of the cycloaddition. Bis-(trifluoromethyl)thioketen (86) adds to azomethines, isothiocyanates, and azides to form 1,3,5-dithiazines and thiazetidines, 1,3-dithietans, and A -1,2,3,4-thia-triazolines, respectively. The thioketen (86) also undergoes [4 + 2]cycloadditions with dienes.  

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Thioketenes (R2C=C=S), selenoketenes (R2C=C=Se) andtelluroketenes (R2C=C=Te) are less stable than ketenes. The latter have not as yet been synthesized, but their metal complexes are known. A theoretical comparison of ketenes, thioketenes and selenoketenes indicates that thioketenes and selenoketenes are more reactive than ketenes and that thioketenes resemble selenoketenes more than ketenes. Selenoketenes are often encountered as intermediates in chemical reactions, but highly sterically hindered derivatives are also known. 

The bonding in thioketene and selenoketene is expected to be qualitatively similar to that in ketene, although few detailed calculations have been reported. Some idea of the similarities can be gleaned from a comparison of the photoelectron spectra of ketene and thioketene. The interpretation of the two spectra, from the recent results of Bock et al. is shown in Fig. 38. The expected close resemblance is found, although the 2bj-2b2 energy separation is substantially reduced in the thioketene molecule. 

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 . 

Although a number of other electronic transitions have been reported for ketene, UV spectra are not available for thioketene and none of the electronic transitions of selenoketene have been identified. A similar lack of data on the transitions in substituted species makes further comparisons of their spectroscopy unrewarding. 

The structural assignment of selenoketene was also based on an ab initio SCF calculation of its PE spectrum as well as on a radical cation state comparison with ketene and thioketene by mass spectrometry (80CB3187). 

Photochemical elimination of nitrogen from a bicyclic 1,2,3-thiadiazole results in the formation of a thioketen, e.g. (79), instead of a thiiren (see Scheme 4), which is in contrast to the reaction of monocyclic thiadiazoles. When 1,2,3-selenadiazoles are subjected to thermolysis at 500—600 C, they give the selenoketens (80)— (82). The selenoketens have been trapped and characterized at —196 C. 

The unstable thioketenes were obtained by thermolysis of 1,2,3-thiadiazole, and the unstable selenoketenes were similarly generated and characterized in the vapor phase The highly reactive fulvene substituted selenoketene 2 was also obtained in the thermolysis of benzo-l,2,3-selenadiazole 1, and the red precipitate collected at -196 °C polymerizes violently on warming". ... 

CHjCSe The preparation of gas phase selenoketene was first reported by a group of microwave spectroscopists in 1978. Pyrolysis of 1,2,3-selenodiazole gave a species whose microwave spectrum in the 18 to 40 GHz region, including transitions of the Se and Se species in natural abundance, was consistent with that expected for CHjCSe. Further observations of deuterium substituted species confirmed the identification and established the structure of the molecule. Selenoketene is a transient species, like thioketene, which dimerizes and polymerizes rapidly at room temperature. Few other seienoketenes have been prepared. 

The gas phase infrared spectra of ketene and thioketene have been reported and a few bands of selenoketene have been observed in matrix isolation studies These molecules have C2V symmetry with their rotational axes oriented as shown in Fig. 36. The nine fundamental vibrations factorize such that the totally symmetric vibrations Vj-v belong to the a species, the out-of-plane vibrations Vj and to the bj species, and the in-plane vibrations to the bj species. The a fundamentals give type-A parallel bands, while type-C and type-B perpendicular bands are found for the b, and bj fundamentals, respectively. Since ketene and thioketene are near-prolate symmetric tops, the rotational structure of the IR bands is only minimally complicated by asymmetry splittings. 

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