Self-Assembled Benzophenone Bis-urea Macrocycles Facilitate Selective Oxidations by Singlet Oxygen

Abstract

This manuscript investigates how incorporation of benzophenone, a well-known triplet sensitizer, within a bis-urea macrocycle, which self-assembles into a columnar host, influences its photophysical properties and affects the reactivity of bound guest molecules. We further report the generation of a remarkably stable organic radical. As expected, UV irradiation of the host suspended in oxygenated solvents efficiently generates singlet oxygen similar to the parent benzophenone. In addition, this host can bind guests such as 2-methyl-2-butene and cumene to form stable solid host-guest complexes. Subsequent UV irradiation of these complexes facilitated the selective oxidation of 2-methyl-2-butene into the allylic alcohol, 3-methyl-2-buten-1-ol, at 90% selectivity as well as the selective reaction of cumene to the tertiary alcohol, α,α'-dimethyl benzyl alcohol, at 63% selectivity. However, these products usually arise through radical pathways and are not observed in the presence of benzophenone in solution. In contrast, typical reactions with benzophenone result in the formation of the reactive singlet oxygen that reacts with alkenes to form endoperoxides, diooxetanes, or hydroperoxides, which are not observed in our system. Our results suggest that the confinement, the formation of a stable radical species, and the singlet oxygen photoproduction are responsible for the selective oxidation processes. A greater understanding of the mechanism of this selective oxidation could lead to development of greener oxidants.

Hydrogen Bonding in Supramolecular Crystal Engineering

“…Photodimerization of the above enones could not happen within 33c-tube. Instead, the nanotube was able to act as a triplet sensitizer for the cis-trans photoisomerization of trans-β-methylstyrene and selective oxidation of 2-methyl-2-butene to a primary allylic alcohol (Scheme 3.14) [70,109]. As well, an enlarged nanotube 33d-tube was shown to facilitate the selective photodimerization of coumarin to the anti-head-to-head product [71].…”

Regioselective Photooxidation of Citronellol: A Way to Monomers for Functionalized Bio-Polyesters

“…Regioselectivity in photooxygenation reactions has been successfully achieved for simple trisubstituted alkenes by performing them in a confined space, using molecular containers such as self-assembled molecular hosts (Natarajan et al, 2007;Geer et al, 2013), zeolites (Shailaja et al, 2000;Stratakis and Froudakis, 2000;Kaanumalle et al, 2002;Chen et al, 2005), or microemulsions (Nardello et al, 2004). The confined space allows to better control the conformation of alkene and thus the accessibility of the allylic hydrogen(s) with respect to the attacking singlet oxygen during the hydroperoxide formation.…”

Persistent Radicals of Self‐assembled Benzophenone bis‐Urea Macrocycles: Characterization and Application as a Polarizing Agent for Solid‐state DNP MAS Spectroscopy

“…The photogenerated radical was first noted when investigating the applications of benzophenone bis -urea macrocycle 1 to facilitate selective photooxidations. 6, 7 The prolonged stability of these endogenous radicals at room temperature appears to be a consequence of the columnar assembly and crystal packing of the porous organic crystals, where as no evidence of radical formation is observed in solution. 6 Herein, we probe the structure of the radical, estimate its quantity, and evaluate its lifetime by EPR spectroscopy.…”

“…6, 7 The prolonged stability of these endogenous radicals at room temperature appears to be a consequence of the columnar assembly and crystal packing of the porous organic crystals, where as no evidence of radical formation is observed in solution. 6 Herein, we probe the structure of the radical, estimate its quantity, and evaluate its lifetime by EPR spectroscopy. Finally, we demonstrate that the low levels of endogenous radicals in 1 can be applied to hyperpolarize nuclei and enhance the NMR signals of both the host and its encapsulated DMSO guest (Figure 1).…”

“…6 Molecular self-assembly and crystal packing in 1 dramatically quenches the phosphorescence lifetime from µs to < 1 ns, likely through a non-radiative pathway. 6 Mechanistic investigations suggest that columnar assembly and packing stabilizes some type of photogenerated radical that is stable for weeks in the dark at room temperature. No evidence of radical formation is observed in solution where the molecule only exists in monomer form.…”