Thiirenes generation

To summarize under favorable conditions the acidity of a-hydrogens facilitates the generation of a-sulfoxy and a-sulfonyl carbanions in thiirane and thiirene oxides and dioxides as in acyclic sulfoxides and sulfones. However, the particular structural constraints of three-membered ring systems may lead not only to different chemical consequences following the formation of the carbanions, but may also provide alternative pathways not available in the case of the acyclic counterparts for hydrogen abstraction in the reaction of bases. 

Similar cycloadditions between thiirene dioxides and 1,3-dipoles generated in situ give heterocycles which result from either loss of sulfur dioxide or from the three-membered ring opening of the initially formed adduct (e.g. 174). Such cycloadditions with nitrilium imides (173a) and nitrile ylids (173b) are illustrated in equation 69175. 

The suggestion that three-membered heterocycles (362) are involved is illustrated by the postulated generation of derivatives of IH-diazirine (315 and 345), thiiren (328), thiazirin (337), and lif-triazirine (358). These three-membered heterocycles belong to the class of 4n-antiaromatic heterocycles their possible role as reaction intermediates is of general interest. There is, however, an interesting difference between the photochemistry of sydnones (Fig. 4) and meso-ionic l,2,4-triazol-3-ones (Fig. 6). In both cases IH- azirines (315 and... 

Support for another intermediate, the thiirene (10), has been generated by experiments in which one thiadiazole (11) forms two isomers as illustrated in Scheme 3 (77TL2643). Others have also proposed the existence of a thiirene intermediate (79CB1769). Attempts to trap the thiirene as an iron complex (12) failed instead, reaction of nonacarbonyldiiron with thiadiazoles affords red crystalline carbene complexes (13) (75SST(3)670). 

Reaction of S( D2) and S( Pi) atoms (generated by photolysis of COS in the gas phase) with acetylene yields carbon disulphide, benzene, and thiophene. With hexafluorobut-2-yne, however, only perfluorotetramethylthiophene is formed, although on prolonged irradiation, more complex products such as thiophenes (161) and (162) can be isolated. Highly reactive thiirenes and thio-formylmethylenes have been proposed as intermediates. 

A formation of thiirene by secondary photolysis of thioketene, which is generated by photolysis of 1,2,3-thiadiazole, has been reported <92ACS482>. Irradiation of [4-13C]-l,2,3-thiadiazole together with diethylamine in an EPA glass (diethyl ether-isopentane-ethanol (5 5 2)) at 77 K gave N,N-diethylthioacetamide with 37% carbon randomization in the thioketene (Scheme 9). Experiments with [4-13C]-l,2,3-thiadiazole at room temperature demonstrate lack of carbon randomization in the thioacetyl group of the trapping product. Therefore, the reaction mechanism of photolysis of... 

The thiirene 1,1-dioxide (31a) gives two products when it is treated with the nitrile ylide (41) in situ generated in benzene at room temperature. The major product is 3-(p-nitrophenyl)-2,5,6-triphenyl- 1,4-thiazine 1,1 -dioxide (44) the minor product is 2-(/>-nitrophenyl)-3,4,5-triphenylpyrrole (43a) (Scheme 19) <84JOC1300>. 

Intermediates of type 42 may be generated by cycloaddition reactions. Thus, when the thiirene 1,1-dioxide 45 was treated with the mesoionic compound 46 at ambient temperature, the sulfone 44b was produced in high yield the thiazine dioxide 44a was prepared in a similar manner. It is noteworthy that the intermediate cycloadducts 47a and 47b lost carbon dioxide in preference to sulfur dioxide. 

Two further papers in a series on the generation of nitrilimines (RC=NNR ) by irradiation of sydnones suggest that the reactive excited state is a (n, n ) triplet state,and demonstrate again the use of the reactive nitrilimines in the synthesis of heterocyclic compounds. In continuing studies on the formation of thiirens from 1,2,3-thiadiazoles (23), the parent thiiren has been characterized in a low-temperature matrix after photolysis of (23 R = R = H). Product analysis in experiments with C-substituted substrates suggests that thiiren is formed to a considerable extent thermally but to a lesser extent photochemically from (23 R = H, Ri = Ph). 

Only a few photoeliminations yielding three-membered ring products have been investigated by matrix-isolation techniques. Nonetheless, one of these — the generation of thiirene and its selenium analogue — was probably one of the most significant contributions made by matrix isolation to the study of organic reactive intermediates. 

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