Interplay between anti - anti and syn - anti 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 anti - anti and syn - anti conformations by adding and removing Cu + ions, as evidenced by the NMR and crystallographic study. The anti - anti form was found to be catalytically active in the Michael addition reaction, whereas the syn - anti form was catalytically inactive.