Why are reactions of 2- and 8-thioquinoline derivatives with iodine different?

ABSTRACT

The crystal structures of 1,2-dihydro-1,1'-bi[thiazolo[3,2-a]quinoline]-10a,10a'-diium diiodide hemihydrate, C22H16N2S22+·2I-·0.5H2O, and 1,2-dihydro-1,1'-bi[thiazolo[3,2-a]quinoline]-10a,10a'-diium iodide triiodide, C22H16N2S22+·I-·I3-, obtained during the reaction of 1,4-bis(quinolin-2-ylsulfanyl)but-2-yne (2TQB) with iodine, have been determined at 120 K. The crystalline products contain the dication as a result of the reaction proceeding along the iodocyclization pathway. This is fundamentally different from the previously observed reaction of 1,4-bis(quinolin-8-ylsulfanyl)but-2-yne (8TQB) with iodine under similar conditions. A comparative analysis of the possible conformational states indicates differences in the relative stabilities and free rotation for the 2- and 8-thioquinoline derivatives which lead to a disparity in the convergence of the potential reaction centres for 2TQB and 8TQB.