Diazoalkanes with thiocarbonyl compounds

Cycloaddition of diazoalkanes with thiocarbonyl compounds occurs usually in the regiochemistry derived from a thiophilic addition of the carbon end of the dipole to sulfur, yielding a 1,3,4-thiadiazoline (1). 

Reaction of diazoalkanes with thiocarbonyl compounds, which may be considered an extension of Pechmann s synthesis, forms 1,2,3-thiadiazole derivatives (75SST(3)67o). For example, methyl dithioacetate reacts with diazomethane to form a mixture of thiadiazole isomers and thiirane the latter often is the main product of this type of reaction (equation 26). O-Ethyl thionoacetate reacts with ethyl diazoacetate to form a substituted thiadiazole (equation 27). However, an aryl thionoester reacts with diazomethane to yield a 1,2,3-thiadiazole derivative which is only formed as an intermediate (31) and then rapidly decomposes to alkene (Scheme 5). 

The action of diazoalkanes on thiocarbonyl compounds provides a versatile route to thiirans 1,2,3-thiadiazole derivatives, which function as intermediates, are isolable in suitable cases. Thus, diazomethane reacts with thionesters (1) yielding 5-alkoxy-5-methyl-A -l,2,3-thiadiazolines (2) (ca. 60%) together with the next higher homologous thionester (3) (ca. 30%). Aromatic members (2 R = Ar) decompose readily to the alkenes (5). Ethyl thioformate (1 R = H, R = Et) gives rise, by methylation and aromatiza-tion, to (4), and to linear products. Comparable observations involving diazoethane and diazopropane are also on record. The formulation of the products as 1,2,3-thiadiazolines (2) in preference to the possible A -1,3,4-thiadiazoline structures (6) is supported by the results of n.m.r. studies. However, both isomers, (8) (20%) and (9) (45%), are obtained when diazomethane acts on methyl dithioacetate (7) at —70 C (compare also these Reports, Vol. 1. p. 450). ... 

The reaction of thiocarbonyl compounds with diazoalkanes (alkyl, aryl substituted) frequently gives good to excellent yields of thiiranes. The mechanism may involve addition of a carbene across the thiocarbonyl group, especially in the presence of rhodium(II) acetate... 

The chemistry outlined in Schemes 8.32 and 8.34 illustrates the complexity of reactions that occur between thiocarbonyl compounds and diazo compounds. Heimgartner and co-workers (214-217) observed a similar reactivity pattern when they combined l,3-thiazol-5(477)-thiones (153) with diazoalkanes. When ethyl diazoacetate was used, additional reaction pathways occurred giving rise to a complex mixture of products (218). An interesting aspect of this chemistry involves... 

Thiocarbonyl compounds react readily with diazoalkanes resulting in the formation of products containing a C-S bond such as thiadiazoles. dithiolanes and thiiranes49. The degree of stereoselectivity in such reactions strongly depends on structural factors. This can be best illustrated by the reaction of diazoethane with various methyl alkanedithiocarboxylates 1. With the exception of methyl 2,2-dimethylpropanedithiocarboxylate, this reaction affords almost quantitatively the diastereomeric thiiranes 250. 

Another example which illustrates the importance of the structure of the diazoalkane in reactions of thiocarbonyl compounds and derivatives thereof is the reaction of trithiocarbonate S,S-dioxide 6 with diazomethane and phenyldiazomethane52. Whereas the former gives the dithiolane derivative 7, the latter affords the thiirane 8. Both reactions are, however, fully stereoselective affording single diastereomeric compounds the most probable structures are depicted in the scheme. 

In a study of the interaction of electron-depleted thiocarbonyl compounds with diazoalkanes, the C-sulphonylthioformamide derivative flO) and ethyl chlorodithioformate (11) were found to yield mixtures of products that have been provisionally formulated as 1,2,3- and 1,3,4-thiadiazoles. ... 

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