Increasing Scope of Clickable Fluorophores: Electrophilic Substitution of Ylidenemalononitriles

Recently,
we demonstrated that ylidenemalononitriles (YMs) react
with amines to form cyclic amidines and that the starting linear YMs
are nonemissive in solution and the cyclic amidines are fluorescent.
These turn-on systems were of interest to us because of their potential
as biosensors and synthons for accessing functionalized pyridines.
While our original method was promising, several limitations persisted,
including access to more functionalized and polar-solvent-soluble
structures as well as increased control over the rate of cyclization.
Herein, we report a new approach that allows the electrophilic substitution
of YMs. These substituted YMs exhibit faster turn-on rates, color
tunability, access to polar-solvent-soluble species, and increased
control over cyclization rate. This allowed us to significantly expand
the fluorophore’s chemical space.