Heterocyclic Tautomerism. V. X-Ray Crystal Structures of Two 5-Substituted 1,2,4-Thiadiazol-3-ones

Abstract

Contrary to a recent report, 3-hydroxy-5-phenyl-1,2,4-thiadiazole is shown to exist as the OH tautomer (2a). At 130 K crystals of (2a) are monoclinic, P2₁/n, a 5.453(3), b 4.956(2), c 27.28(2) Ǻ, β 94.12(5)°, Z = 4; the structure was refined to a conventional R[I > 2σ(I)] 0.051. 5-Benzoylamino-1,2,4-thiadiazol-3-one is shown to exist as the N(2)H tautomer (3b) but with a contribution to the structure from a bicyclic or zwitterioni resonance contributor containing an S-O bond. At 130 K crystals of (3b) are monoclinic, Pn , a 3.771(1), b 15.465(4), c 8.105(1) Ǻ, β 98.72(2)°, Z = 2; the structure was refined to a conventional R[I > 2σ(I)] 0.038.