Group 2 Catalysis for the atom-efficient synthesis of imidazolidine and thiazolidine derivatives

A wide variety of functionalised imidazolidine-2-ones and -thiones, 2-imino-imidazolidines and thiazolidine-2-thiones have been synthesised under very mild reaction conditions by using simple and cost-effective alkaline earth bis(amide) precatalysts, [Ae{N(SiMe<inf>3</inf>)<inf>2</inf>}<inf>2</inf>(THF)<inf>2</inf>] (Ae=Mg, Ca, Sr). The reactions ensue with 100 % atom efficiency as one-pot cascades from simple, commercially available terminal alkyne and heterocumulene reagents. The reactions take place through the initial assembly of propargylamidines, which are utilised in subsequent cyclisation reactions through addition of the isocyanate, isothiocyanate and, in one case, carbon disulfide reagents. This reactivity is deduced to take place through a well-defined sequence of heterocumulene hydroacetylenation and alkyne hydroamidation steps, which are all mediated at the alkaline earth centre. The rate and regioselectivity of the cyclisation reactions are, thus, found to be heavily dependent upon the identity of the catalytic alkaline earth centre employed. Similarly, the selectivity of the reactions was observed to be profoundly affected by stereoelectronic variations in the individual substrates, albeit by a similar Group 2-centred reaction mechanism in all cases studied.