Photolysis of Tetraalkyl‐1‐pyrazoline‐4‐thiones. – Diastereoselective Formation of (E)/(Z)‐Alkylidenethiiranes from cis‐ and trans‐Tetraalkyl‐1‐pyrazoline‐4‐thiones

The tetraalkyl‐1‐pyrazolin‐4‐ones 5 react with hydrazine to afford the hydrazones 6 which are transformed into the thiones 7 in high yields by treatment with disulphur dichloride in the presence of triethylamine. Selective excitation of the azo chromophor of 7a with 350‐nm light gives rise to the isopropylidenethiirane 8, besides molecular nitrogen, in a very clean reaction; at almost quantitative conversions, less than 2% of byproducts are observed, and about 10% of 8 have isomerized to the pentadienethiol 9. Photolysis of the stereoisomers cis‐ and trans‐7b yields mixtures of the alkylidenethiiranes (E)‐and (Z)‐10 with (E)/(Z) ratios of 35:65 and 49:51. The results are interpreted in terms of diastereomeric bis‐orthogonal (cis‐ and trans‐21) and mono‐orthogonal thioxyallyl diradicals (E)‐ and (Z)‐23 which cyclize to furnish (E)‐ and (Z)‐10. There seems to be a qualitative resemblance between the photochemical and thermal stabilities in the series of 4‐substituted tetramethyl‐1‐pyrazolines, viz. 7a < 12 < 5a, 13, similar to that suggested by Engel for 2,3‐diazabicyclo[2.2.2]oct‐2‐ene derivatives.