Thiophene 1,1-Dioxide: Synthesis, Isolation, and Properties

Abstract

Abstract Oxidation of thiophene with dimethyldioxirane at −20 °C and removal of the solvent and volatile materials below −40 °C allowed the isolation of the parent thiophene 1,1-dioxide (1) as colorless crystals in pure form. The 1,1-dioxide 1, which had eluded isolation for a long time despite many efforts, melted at about 6 °C and the melt solidified at room temperature because of the dimeric and trimeric products formation. The structure of 1 was fully characterized by 1H- and 13C NMR, IR, Raman, UV/vis, and Mass spectroscopies. The decomposition of 1 in solution provided the dimer (7) as the principal product along with the trimer (9), whereas the decomposition of a neat sample furnished 7 and 9 in comparable amounts. The half-life of 1 is dependent upon the concentration of 1; 137 and 747 min at 25 °C in 0.12 and 0.025 M CDCl3 solutions (1 M = 1 mol dm−3), respectively. Kinetics of the dimerization of 1 in a dilute solution provided the activation parameters of Ea = 64.4 (±0.3) kJ mol−1, ΔH≠ = 62.0 (±0.3) kJ mol−1, and ΔS≠ = −59.8 (±1.0) J K−1 mol−1. Attempted reactions of 1 with a series of dienophiles and dienes all failed because the dimerization of 1 is much faster than reactions with these additives. An exception is the reaction with cyclopentadiene which gave the Diels–Alder adduct in good yield. The adduct of the dimer 7 with dimethyl acetylenedicarboxylate was found to undergo a retro Diels–Alder reaction to regenerate 1.