Expanded Helicenes as Synthons for Chiral Macrocyclic Nanocarbons

Expanded helicenes are large, structurally
flexible π-frameworks
that can be viewed as building blocks for more complex chiral nanocarbons.
Here we report a gram-scale synthesis of an alkyne-functionalized
expanded [11]­helicene and its single-step transformation into two
structurally and functionally distinct types of macrocyclic derivatives:
(1) a figure-eight dimer via alkyne metathesis (also gram scale) and
(2) two arylene-bridged expanded helicenes via Zr-mediated, formal
[2+2+n] cycloadditions. The phenylene-bridged helicene
displays a substantially higher enantiomerization barrier (22.1 kcal/mol)
than its helicene precursor (<11.9 kcal/mol), which makes this
a promising strategy to access configurationally stable expanded helicenes.
In contrast, the topologically distinct figure-eight retains the configurational
lability of the helicene precursor. Despite its lability in solution,
this compound forms homochiral single crystals. Here, the configuration
is stabilized by an intricate network of two distinct yet interconnected
helical superstructures. The enantiomerization mechanisms for all
new compounds were probed using density functional theory, providing
insight into the flexibility of the figure-eight and guidance for
future synthetic modifications in pursuit of non-racemic macrocycles.