Expanded Helicenes: A General Synthetic Strategy and Remarkable Supramolecular and Solid-State Behavior

A divergent synthetic strategy allowed
access to several members
of a new class of helicenes, the “expanded helicenes”,
which are composed of alternating linearly and angularly fused rings.
The strategy is based on a three-fold, partially intermolecular [2+2+n] (n = 1 or 2) cycloaddition with substrates
containing three diyne units. Investigation of aggregation behavior,
both in solution and in the solid state, revealed that one of these
compounds forms an unusual homochiral, π-stacked dimer via an
equilibrium that is slow on the NMR time scale. The versatility of
the method was harnessed to access a selenophene-annulated expanded
helicene that, in contrast to its benzannulated analogue, exhibits
long-range π-stacking in the solid state. The new helicenes
possess low racemization barriers, as demonstrated by dynamic 1H NMR spectroscopy.