6,6-Dicyanopentafulvenes: Electronic Structure and Regioselectivity in [2 + 2] Cycloaddition–Retroelectrocyclization Reactions

We present an investigation of the electronic properties
and reactivity
behavior of electron-accepting 6,6-dicyanopentafulvenes (DCFs). The
electron paramagnetic resonance (EPR) spectra of the radical anion
of a tetrakis­(silylalkynyl) DCF, generated by Na metal reduction,
show delocalization of both the charge and unpaired electron to the
nitrogens of the cyano moieties and also, notably, to the silicon
atoms of the four alkynyl moieties. By contrast, in the radical anion
of the previously reported tetraphenyl DCF, coupling to the four phenyl
rings is strongly attenuated. The data provide physical evidence for
the different conjugation between the DCF core and the substituents
in both systems. We also report the preparation of new fulvene-based
push–pull chromophores via formal [2 + 2] cycloaddition–retroelectrocyclization
reaction of DCFs with electron-rich alkynes. Alkynylated and phenylated
DCFs show opposite regioselectivity of the cycloaddition, which can
be explained by the differences in electronic communication between
substituents and the DCF core as revealed in the EPR spectra of the
radical anions.