Reappraising the formation of Jaffé's base: studies of the treatment of imidazolidine-2-thione with mild oxidising agents.

Throughout reports detailing the preparation of Jaffé's Base, which date as far back as 1894, its identity and the nature of an intermediate prior to its formation have been constant points of conjecture. This report presents firm evidence that the oxidation of imidazolidine-2-thione, SCN(H)C2H4N(H), with potassium triiodide yields the ‘thiol’ condensation product 2,2′-bis(4,5-dihydro-1H-imidazolidine)disulfide {NC2H4N(H)CS}2 (1), which under mild conditions undergoes partial self condensation to yield 1-(4,5-dihydro-1H-imidazolidin-2-yl)imidazolidine-2-thione, Jaffé's Base – SCN(CNC2H4N(H))C2H4NH, (2). Crystallisation of 1 from acetone results in the unexpected formation of heterobicyclic 3-methyl-5,6-dihydroimidazolidin[2,1-b]thiazole, 3. The conversion of 1 to 3 has been studied by 1H NMR spectroscopy, which suggests the concurrent formation of 2 as an unreactive by-product. The solid state structures of 2, its HBr salt (2·HBr) and 3·(HI·I2), the latter being the isolated form of 3 under the conditions employed, have been determined using XRD methods.