Interplay between - and - conformations of thiourea modulating ON-OFF catalysis.

The design, synthesis and operation of a readily accessible two-state switch are demonstrated. The switch initially exists in an intramolecularly hydrogen-bonded self-locked state, as evidenced by the solution-state NMR and solid-state structure. The switch can be reversibly altered between - and - conformations by adding and removing Cu ions, as evidenced by the NMR and crystallographic study. The - form was found to be catalytically active in the Michael addition reaction, whereas the - form was catalytically inactive.