Selective Photodimerization in a Cyclodextrin Metal–Organic Framework

Abstract

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-cyclodextrincontaining 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 highyielding 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 coconformations 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.

Action Plots in Action: In-Depth Insights into Photochemical Reactivity

“…Photocycloadditions are among the oldest photoreactions in the field of synthetic chemistry. The [4 + 4] photocycloaddition of anthracene was first described by Fritzsche in 1867 and still finds ample application in chemistry due to its selectivity and catalyst-free nature. − Despite a plethora of research exploring photocycloadditions, almost all applications in photoligations utilize harsh UV light to initiate anthracene photodimerization. Only recently, we began to investigate the wavelength (λ) dependence of anthracene photocycloaddition (Figure , reaction C6 ) using action plots. , Surprisingly, the reaction is initiated just as efficiently with visible light (λ = 410 nm) as with UV light (λ < 400 nm), challenging a paradigm of more than 100 years.…”

Controllable multiple-step configuration transformations in a thermal/photoinduced reaction

“…However, cyclobutanes have four stereocenters, and their control suffers from relatively poor regio- and stereoselectivity 3 – 9 . Various supramolecular templates 10 – 12 , host-guest assemblies 13 , 14 , quantum dots 15 , chiral molecular catalysts 16 – 18 , and Lewis acid cocatalysts 19 , 20 have been developed to control the enantioselectivity and diastereoselectivity of [2 + 2] photocycloaddition transformations. Although it is possible to capture cyclic intermediates at low temperature 21 , the control of their stereochemistry during the reaction has not yet been documented.…”

Enantioselective Construction of Modular and Asymmetric Baskets

“…[49] In fact, we measured NOEs between the methyl ester and bridge protons in the product (Figure 3B; Figure S45) to confirm the formation of anti-[15] 2 ;t hese nuclei reside in ac lose proximity in anti- [15] 2 (3.8 ,F igure 3B)but not syn- [15] 2 .The principal formation of the anticycloadduct could be driven by both steric and electronic complementarity of two anthracenes meeting in ahead-to-tail manner during the cycloaddition. [50] By exposing asolution of anti- [15] 2 to 254 nm light, the cycloreversion took place with rapid and quantitative reversal to 15 (Figure S51). The switchable conversion of ab asket-shaped cavitand [51] into a"dual-claw host" is unprecedented [52] and we aim to use the concept for examining allosteric sequestration of small molecules [53] or delivery of targeted pharmaceuticals.…”