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 H NMR spectroscopy.
Carbon nanobelts are molecules of
high fundamental and technological
interest due to their structural similarity to carbon nanotubes, of
which they are molecular cutouts. Despite this attention, synthetic
accessibility is a major obstacle, such that the few known strategies
offer limited structural diversity, functionality, and scalability.
To address this bottleneck, we have developed a new strategy that
utilizes highly fused monomer units constructed via a site-selective
[2 + 2 + 2] cycloaddition and a high-yielding zirconocene-mediated
macrocyclization to achieve the synthesis of a new carbon nanobelt
on large scale with the introduction of functional handles in the
penultimate step. This nanobelt represents a diagonal cross section
of an armchair carbon nanotube and consequently has a longitudinally
extended structure with an aspect ratio of 1.6, the highest of any
reported nanobelt. This elongated structure promotes solid-state packing
into aligned columns that mimic the parent carbon nanotube and facilitates
unprecedented host–guest chemistry with oligo-arylene guests
in nonpolar solvents.
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.
A general synthetic strategy for the construction of large, nitrogen-containing polycyclic aromatic hydrocarbons (PAHs) is reported. The strategy involves two key steps: (1) a titanocene-mediated reductive cyclization of an oligo(dinitrile) precursor to form a PAH appended with di(aza)titanacyclopentadiene functionality; (2) a divergent titanocene transfer reaction, which allows final-step installation of one or more o-quinone, diazole, or pyrazine units into the PAH framework. The new methodology enables rational, late-stage control of HOMO and LUMO energy levels and thus photophysical and electrochemical properties, as revealed by UV/vis and fluorescence spectroscopy, cyclic voltammetry, and DFT calculations. More generally, this contribution presents the first productive use of di(aza)metallacyclopentadiene intermediates in organic synthesis, including the first formal [2 + 2 + 2] reaction to form a pyrazine ring.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.