Stepwise Formation of 1,3-Diazolium-4-thiolates by Münchnone Cycloadditions: Promising Candidates for Nonlinear Optics

An improved preparation of mesoionic
heterocycles 1,3-diazolium-4-thiolates
by [3 + 2] cycloadditions of münchnones with aryl isothiocyanates
is reported. The process takes place with high or complete regioselectivity,
and fast and clean transformations are observed under microwave heating
in DMF. DFT calculations support that this cycloaddition proceeds
preferably through a stepwise mechanism. Given the pattern substitution
around the mesoionic ring resulting in a push–pull system,
theoretical estimations predict large hyperpolarizabilities in some
cases, which is typical of molecules exhibiting nonlinear optical
responses.