Synthesis, isolation, and dimerization and trimerization of monosubstituted thiophene 1,1-dioxides

Monosubstituted thiophenes were oxidized with dimethyldioxirane in acetone at -20 degrees C and then the solvent and volatile materials were removed b elow -40 degrees C. This allowed the isolation of kinetically labile 2-meth yl-, 3-methyl-, 2-ethyl-, and 2-bromothiophene 1,1-dioxides (3a, 3b, 3c, an d 3e, respectively) in practically pure form. These were characterized by H -1- and C-13-NMR, IR, UV/Vis, and MS spectroscopies. The half-lives of the parent thiophene 1,1-dioxide (1), 3b, 3a, and 3c were determined to be 14, 47, 68, and 76 min, respectively, at 313 K (40 degrees C) in 0.32 M CDCl3 s olutions. The 1,1-dioxide (3a) underwent a [4+2] cyclodimerization in which one molecule of 3a acted as a diene and the other as a dienophile to provi de two isomeric products (4a and 5a). In addition, the trimeric product (16 a) was formed owing to a further [4+2] cycloaddition of the major isomer (4 a) with 3a which took place in the endo and head-to-head mode. Dimeric and trimeric products formation was also observed on decomposition of 3b and 3c , whereas 3e underwent only dimerization.