Sulfur (³P) Reaction with Conjugated Dienes Gives Cyclization to Thiophenes under Single Collision Conditions

We combine crossed-beam velocity map imaging with high-level ab initio/transition state theory modeling of the reaction of S(³P) with 1,3-butadiene and isoprene under single collision conditions. For the butadiene reaction, we detect both H and H₂ loss from the initial adduct, and from reaction with isoprene, we see both H loss and methyl loss. Theoretical calculations confirm these arise following intersystem crossing to the singlet surface forming long-lived intermediates. For the butadiene reaction, these lose H₂ to form thiophene as the dominant channel, H to form the detected 2H-thiophenyl radical, or ethene, giving thioketene. For isoprene, additional reaction products are suggested by theory, including the observed H and methyl loss radicals, but also methyl thiophene, thioformaldehyde, and thioketene. The results for S(³P) + 1,3-butadiene, showing direct cyclization to the aromatic product and yielding few bimolecular product channels, are in striking contrast to those for the analogous O(³P) reaction.