Localized Antiaromaticity Hotspot Drives Reductive Dehydrogenative Cyclizations in Bis- and Mono-Helicenes

We
describe reductive dehydrogenative cyclizations that form hepta-,
nona-, and decacyclic anionic graphene subunits from mono- and bis-helicenes
with an embedded five-membered ring. The reaction of bis-helicenes
can either proceed to the full double annulation or be interrupted
by addition of molecular oxygen at an intermediate stage. The regioselectivity
of the interrupted cyclization cascade for bis-helicenes confirms
that relief of antiaromaticity is a dominant force for these facile
ring closures. Computational analysis reveals the unique role of the
preexisting negatively charged cyclopentadienyl moiety in directing
the second negative charge at a specific remote location and, thus,
creating a localized antiaromatic region. This region is the hotspot
that promotes the initial cyclization. Computational studies, including
MO analysis, molecular electrostatic potential maps, and NICS(1.7)ZZ calculations, evaluate the interplay of the various effects
including charge delocalization, helicene strain release, and antiaromaticity.
The role of antiaromaticity relief is further supported by efficient
reductive closure of the less strained monohelicenes where the relief
of antiaromaticity promotes the cyclization even when the strain is
substantially reduced. The latter finding significantly expands the
scope of this reductive alternative to the Scholl ring closure.