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.