Carbonyl Groups as Molecular Valves to Regulate Chloride Binding to Squaramides

Ramalingam, Vijayakumar; Domaradzki, Maciej E.; Jang, Seogjoo; Muthyala, Rajeev S.

doi:10.1021/ol801204s

Cited by 51 publications

(42 citation statements)

References 43 publications

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“…[11] Recently, a squaramide-based chloride ion receptor, the anion binding cavity of which can be opened and closed by using carbonyl groups as valves, has been described. [12] In this context, we intend to investigate in detail anionrecognition tendencies of the squaramide subunit, to be compared with those of the urea subunit. Anion binding of the urea derivative, equipped with the most powerful electron-withdrawing substituent (1,3-bis(4-nitrophenyl)-urea, 1), towards anions in acetonitrile has been previously investigated by this group.…”

Section: Introductionmentioning

confidence: 99%

The Squaramide versus Urea Contest for Anion Recognition

Amendola

Bergamaschi

Boiocchi

et al. 2010

Chemistry A European J

178

139

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The interaction of a neutral squaramide-based receptor, equipped with two 4-nitrophenyl substituents (R(sq)), with halides and oxoanions has been studied in MeCN. UV/Vis and (1)H NMR spectroscopy titration experiments clearly indicated the formation of 1:1 hydrogen bonding [R(sq)X](+) complexes with all the investigated anions. X-ray diffraction studies on the chloride and bromide complex salts confirmed the 1:1 stoichiometry and indicated the establishment of bifurcated hydrogen-bond interactions between the squaramide-based receptor and the halide anion that involved both 1) amide N-H and 2) aryl proximate C-H fragments, for a total of four bonds. Probably due to the contribution of C-H fragments, complexes of R(sq) with halides are 1 to 2 orders of magnitude more stable than the corresponding ones with the analogous urea-based receptor that contains two 4-nitrophenyl substituents (R(ur)). In the case of oxoanions, R(sq) forms complexes, the stability of which decreases with the decreasing basicity of the anion (H(2)PO(4) (-)>NO(2) (-) approximately HSO(4) (-)>NO(3) (-)), and is comparable to that of complexes of the urea-based receptor R(ur). Such a behaviour is ascribed to the predominance of different contributions: electrostatic interaction for halides, acid-to-base 'frozen' proton transfer for oxoanions. Finally, with the strongly basic anions F(-) and CH(3)COO(-), R(sq) first gives genuine hydrogen-bond complexes of 1:1 stoichiometry; then, upon addition of a second anion equivalent, it undergoes deprotonation of one N--H fragment, with the simultaneous formation of the dianion hydrogen-bond complexes, [HF(2)](-) and [CH(3)COOHCH(3)COO](-), respectively. In the case of the urea-based derivative R(ur), deprotonation takes place with fluoride but not with acetate. The apparently higher Brønsted acidity of R(sq) with respect to R(ur) reflects the capability of the squaramide receptor to delocalise the negative charge formed on N--H deprotonation over the cyclobutene-1,2-dione ring and the entire molecular framework.

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Section: Introductionmentioning

confidence: 99%

The Squaramide versus Urea Contest for Anion Recognition

Amendola

Bergamaschi

Boiocchi

et al. 2010

Chemistry A European J

178

139

View full text Add to dashboard Cite

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“…17 Hydrogen bonding is the most general and important interaction underpinning chemosensors designed to detect anions. 15,16,[18][19][20][21][22][23][24] A few chemosensors of them containing XÀ ÀH moieties (where X is an electronegative atom such as N or O) moieties enable selective anion recognition in aqueous solution. 18,19 It is noted that a p-system bearing an XÀ ÀH moiety may be an appropriate candidate for designing chemosensors for anions.…”

Section: Introductionmentioning

confidence: 99%

A TD‐DFT study on the cyanide‐chemosensing mechanism of 8‐formyl‐7‐hydroxycoumarin

et al. 2010

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Proton transfer (PT) and excited-state PT process are proposed to account for the fluorescent sensing mechanism of a cyanide chemosensor, 8-formyl-7-hydroxycoumarin. The time-dependent density functional theory method has been applied to investigate the ground and the first singlet excited electronic states of this chemosensor as well as its nucleophilic addition product with cyanide, with a view to monitoring their geometries and spectrophotometrical properties. The present theoretical study indicates that phenol proton of the chemosensor transfers to the formyl group along the intramolecular hydrogen bond in the first singlet excited state. Correspondingly, the nucleophilic addition product undergoes a PT process in the ground state, and shows a similar structure in the first singlet excited state. This could explain the observed strong fluorescence upon the addition of the cyanide anion in the relevant fluorescent sensing mechanism.

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“…[3] Squaramide derivatives have been intensively investigated within the area of molecular recognition because of their strong hydrogen-bonding activity. [4] However, applications of squaramide derivatives for electrophilic activation by hydrogen bonding in enantioselective reactions appear scarce compared with their corresponding thioureas, which were recognized as a "privileged" platform for dual hydrogenbonding catalysis over the past decade. [5,6] Very recently, the utility of chiral squaramides as organocatalysts was disclosed by Rawals group for the conjugate-addition reaction of 1,3-dicarbonyl compounds to nitroolefins, which benefit from squaramides characteristic dual hydrogen-bonding catalysis, in which the two hydrogen atoms are further apart than those in thioureas.…”

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confidence: 99%