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.