Synthesis of Phototrappable Shape-Shifting Molecules
A R T I C L E S
Scheme 1. Degenerate Strain-Assisted Cope Rearrangements of
Bullvalene and Oligosubstituted Bullvalenes Synthesized in This
Work
interconverting population. A considerable number of chemical
systems that equilibrate between two or more different structural
isomers are known, including epimerizable stereocenters,7
metastable atropisomers,8 ring-chain tautomers,9 and dynamic
foldamers.10 The majority of these, however, would not make
suitable platforms for dynamic combinatorial chemistry because
of either the very small number of structurally distinct isomers
or the presence of highly reactive chemical functionality.
Bullvalene11 is a 10-carbon bridged tris(vinyl)cyclopropane
whose unique bonding pattern endows it with the intriguing
ability to alter the bonding pattern and spatial orientation of its
atoms spontaneously through degenerate strain-assisted Cope
rearrangements (Scheme 1). The dynamic properties of bullvalene
were first conceived by Doering and Roth11a,b during their work
on strain-assisted Cope rearrangements. They hypothesized that
a structure such as bullvalene would give rise to a rapid fluxional
tautomerism of ∼1.2 million degenerate structures resulting in
1
studies of the nature of this spontaneous rearrangement.14 While
isomerization occurs rapidly at ambient temperature, this dy-
namic molecular skeleton is remarkably stable even at high
temperatures. Although the rapidity and reagentless nature of
this rearrangement could provide a powerful reaction to mediate
the interconversion of configurationally distinct isomers of an
appropriately functionalized bullvalene,1f the lack of a versatile
synthesis to provide these intricately functionalized bullvalenes
has precluded its successful application as a dynamic shape-
shifting molecule.
Herein, we report a versatile stepwise synthesis of novel
highly functionalized bullvalene derivatives, including photo-
labile o-nitroveratryloxycarbonate (NVOC) bullvalenes 1 and
2. This synthetic challenge required not only an efficient method
to access the fluxional tetracyclic bullvalene scaffold but also
methods for the installation of recognition motifs such as
porphyrin units, a means of inducing the dynamic behavior at
a late stage in the synthetic sequence, and the design and
incorporation of a mechanism to halt the dynamic isomerization
of the bullvalene core. NVOC bisporphyrin bullvalene 1 con-
stitutes a dynamic combinatorial library that rapidly equilibrates
and can be immobilized without the need for additional chemical
reagents. Using 1H NMR and UV binding studies, we were able
to demonstrate that this molecule can adapt its shape to form
favorable binding interactions with C60 (Figure 1). Furthermore,
these structural conformations could be frozen and isolated,
establishing the utility of 1 as a virtual library in which all of
the components can be accessed by a single molecule. The self-
contained nature of this system offers advantages over the more
complex multicomponent assemblies and opens up new pos-
sibilities for the development of self-contained dynamic com-
binatorial libraries.
a single H NMR resonance. Indeed, the subsequent photo-
chemical synthesis of bullvalene by Schroeder11c as well as other
syntheses of bullvalene11d,e,12 and its fundamental derivatives13
have validated this hypothesis and allowed for informative
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Results and Discussion
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