Modulating the reactivity of chromone and its derivatives through encapsulation in a self-assembled phenylethynylene bis-urea host

Abstract

A B S T R A C TThis manuscript reports on the modulation of the photoreactivity of a series of chromones, also known as benzo-g-pyrones, by absorption into a porous self-assembled host formed from phenylethynylene bisurea macrocycles. Chromone and four derivatives namely 6-fluorochromone, 6-bromochromone, 7-hydroxy-4-chromone, and 3-cyanochromone are unreactive in the solid-state. Each of these derivatives was loaded into the nanochannels of self-assembled phenylethynylene bis-urea macrocycles to form solid hostÁguest complexes, which were subsequently UV-irradiated at room temperature under argon atmosphere. We observed that chromone and 6-fluorochromone underwent selective [2 + 2] photodimerization reactions to produce anti-HT dimers in high selectivity and conversion. The 6-bromochromone also reacted in high selectivity and conversion to afford an aryl coupling adduct. In each case, the products were extracted, and the crystalline host recovered. In comparison, 7-hydroxy-4-chromone, and 3-cyanochromone were unreactive within the complex. Simple GCMC simulation studies suggest that chromone, 6-fluorochromone, and 6-bromochromone were loaded in orientations that facilitate photoreaction, and correctly predicted that the anti-HT dimer would be favored in the chromone case. In contrast, syn-HH dimers were predicted by GCMC simulations for the halogen containing derivatives but were not observed. The simulations with 7-hydroxy-4-chromone were in agreement with the observed reactivity. We compare these computational and experimental findings and suggest future methods for optimizing simulation parameters. Our goal is to expand the scope and accuracy of the simulations to be able to predict the reactivity of guests encapsulated within columnar nanotubes.

Fluorescence Polarization Measurements to Probe Alignment of a Bithiophene Dye in One-Dimensional Channels of Self-Assembled Phenylethynylene Bis-Urea Macrocycle Crystals

“…The average host:guest ratio for 3 loading experiments was 10.8:1. The loading for 3 was lower than that for previously reported polar guest 3-cyano chromone (2:1 host:guest) . Dye 3 is significantly larger than the chromone (281.1 vs 197.5 Å 3 ) and, based on the structural model, the loading ratio appears to represent about 50% of the available space.…”

“…Crystalline materials with simple one-dimensional (1D) channels provide excellent model systems for studying complex processes, including adsorption, diffusion, and molecular recognition and for probing the effects of confinement on the encapsulated guest’s electronic properties and reactivity. − A wide variety of porous solids have been reported with 1D channels that can uptake solvents, gases, and guests. Such solids include porous zeolites, − tris­( o -phenylenedioxy)­cyclotriphosphazene (TPP), , and perhydrotriphenylene (PHTP) , inclusion complexes.…”

“…X-ray diffraction demonstrates that the macrocycles are stacked in a columnar fashion, each column enclosing a 1D channel. After activation by removal of the included DMSO solvent, the channels can be used as a container for stereoselective photodimerization of encapsulated guests such as coumarin, chromone, and acenaphthalene. , The channels in the activated fibers have a round cross section with a diameter of ∼9.0 Å and are amenable to molecular transport studies on micrometer scales, for example, revealing Fickian diffusion dynamics using hyperpolarized 129 Xe NMR . Larger molecules can be expected to experience stronger interactions with the host and constraints on diffusion.…”

Supramolecular Photochemistry as a Potential Synthetic Tool: Photocycloaddition

“…The pore size of the channel depended on the aromatic tether that linked the urea functionality in the bis-urea scaffold. For example, in the crystalline channel, UC1 and UC2 (Scheme ) with an ether and a carbonyl linker, respectively, had a pore size of ∼7 Å, while UC4 (Scheme ) with a phenylethynylene linker had a pore size of ∼9 Å. ,, The self-assembled bis-urea macrocycle as an organic nanotube was effective in controlling the photodimerization of enones, , coumarins, , acenaphthylene, and chromones in the crystalline state. Irradiation of crystals of cycloalkenones 23 within the channels of UC1 gave the anti dimer exclusively (Table ) with excellent conversions (80–90%).…”

“…Channels of bis-urea macrocycle with phenylethynylene linkers were able to encapsulate larger guest molecules like coumarins, , acenaphthylene, and chromones (Schemes and ). Irradiation of crystals of coumarins 29a–c encapsulated within the channels of UC4 resulted in the formation of anti- HH dimer 31 (Table ) as the major isomer (84–97% selectivity) with conversions varying between 46% and 55%.…”