Fluoride Anion Recognition by a Multifunctional Urea Derivative: An Experimental and Theoretical Study

Abstract

In this work we demonstrate the ability of a multifaceted N,N′-disubstituted urea to selectively recognize fluoride anion (F−) among other halides. This additional function is now added to its already reported organocatalytic and organogelator properties. The signaling mechanism relies on the formation of a charge-transfer (CT) complex between the urea-based sensor and F¯ in the ground state with a high association constant as demonstrated by absorption and fluorescence spectroscopy. The nature of the hydrogen bonding interaction between the sensor and F¯ was established by 1H-NMR studies and theoretical calculations. Moreover, the recovery of the sensor was achieved by addition of methanol.

Expanding the Functional Scope of the Fmoc‐Diphenylalanine Hydrogelator by Introducing a Rigidifying and Chemically Active Urea Backbone Modification

“…First, the anion binding of Fmoc‐FuF was explored. Urea‐based compounds are prominent anion receptors, which bind anions in a specific manner via two directional hydrogen bonds . In the case of Fmoc‐FuF, specific interaction with fluoride ions (F − ) generated blue fluorescence, attributed to the formation of a charge‐transfer complex ( Figure a).…”

“…Urea‐based compounds are prominent anion receptors, which bind anions in a specific manner via two directional hydrogen bonds . In the case of Fmoc‐FuF, specific interaction with fluoride ions (F − ) generated blue fluorescence, attributed to the formation of a charge‐transfer complex ( Figure a). Blue fluorescence upon UV illumination was observable to the naked eye when an Fmoc‐FuF DMSO solution was supplemented with F − , but not with other anions (Figure b, a 100 × 10 −3 m pH of ≈7 stock solution of tetrabutylammonium fluoride, TBAF, was used as the F − source).…”

Urea Activation by an External Brønsted Acid: Breaking Self-Association and Tuning Catalytic Performance

“…We have previously studied both (thio)urea-catalyzed Friedel-Crafts-type alkylation reactions ( Figure 2) [43][44][45][46][47][48][49][50][51]. However, the poorer results obtained with the ureas in comparison with thiourea analogues made us discard this family of structures in successive studies.…”

“…Within this context, more efficient catalysts were prepared, on introducing internal acidic elements (Figure 1). For instance, taking our catalyst 1a [43][44][45][46][47][48][49][50][51] as a model, subsequent analogues 2 [52], 3 [53] and 4 [54] were designed and have been proved to increase catalytic activity. Using this approach, the authors changed the most In this respect, Sigman and co-workers demonstrated that both the reaction rate and the enantioselectivity could be correlated to catalyst acidity and it could be efficiently used for the tuning and design of new catalyst structures for hydrogen-bond-catalyzed enantioselective reactions [57].…”

“…Among the most appealing organocatalyzed reactions, the Friedel-Crafts alkylation reaction has received great attention and is a very efficient strategy for C-C bond formation [58][59][60][61]. Based on our previous experience using (thio)urea catalysts [43][44][45][46][47][48][49][50][51][62][63][64][65][66], we reported a pioneering concept regarding the cooperative effect between a Brønsted acid additive and a chiral thiourea organocatalyst [45]. In this work, we hypothesize that similar beneficial effects could take place with urea organocatalysts in combination with a Brønsted acid, which could activate the urea by diminishing significantly its self-association tendency and, therefore, increasing its acidity.…”