Selective Photodimerization in a Cyclodextrin Metal–Organic Framework

For the most part, enzymes contain
one active site wherein they
catalyze in a serial manner chemical reactions between substrates
both efficiently and rapidly. Imagine if a situation could be created
within a chiral porous crystal containing trillions of active sites
where substrates can reside in vast numbers before being converted
in parallel into products. Here, we report how it is possible to incorporate
1-anthracenecarboxylate (1-AC–) as a substrate into a γ-cyclodextrin-containing metal–organic
framework (CD-MOF-1), where the metals are K+ cations, prior to carrying out [4+4] photodimerizations between
pairs of substrate molecules, affording selectively one of four possible
regioisomers. One of the high-yielding regioisomers exhibits optical
activity as a result of the presence of an 8:1 ratio of the two enantiomers
following separation by high-performance liquid chromatography. The
solid-state superstructure of 1-anthracenecarboxylate potassium salt
(1-ACK), which is co-crystallized with γ-cyclodextrin,
reveals that pairs of substrate molecules are not only packed inside
tunnels between spherical cavities present in CD-MOF-1, but also stabilizedin addition to hydrogen-bonding to the
C-2 and C-3 hydroxyl groups on the d-glucopyranosyl residues
present in the γ-cyclodextrin toriby combinations of
hydrophobic and electrostatic interactions between the carboxyl groups
in 1-AC– and four K+ cations on the waistline between the two γ-cyclodextrin
tori in the tunnels. These non-covalent bonding interactions result
in preferred co-conformations that account for the highly regio- and
enantioselective [4+4] cycloaddition during photoirradiation. Theoretical
calculations, in conjunction with crystallography, support the regio-
and stereochemical outcome of the photodimerization.