Thiophene 1,1-dioxide: Synthesis, isolation, and properties

Oxidation of thiophene with dimethyldioxirane at -20 degrees C and removal of the solvent and volatile materials below -40 degrees 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 degrees C and the melt solidified at room temperature because of the dimeric and trimeric products formation. The structure of 1 was fully characterized by H-1- and C-13 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 degrees 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 E-a = 64.4 (+/-0.3) kJ mol(-1), Delta H-not equal = 62.0 (+/-0.3) kJ mol(-1), and Delta S-not equal -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.