New polymorphic hydrogen bonding donor–acceptor system with two temperature coincident solid–solid transitions

Portell, Anna; Barbas, Rafael; Braga, Dario; Polito, Marco; Prohens, Rafel

doi:10.1039/b813086h

Cited by 29 publications

(25 citation statements)

References 18 publications

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“…of its uncomplexed squaramide counterpart (3,4-bis-(benzylamino)cyclobut-3-ene-1,2-dione). 80 Both molecules seem nicely preorganised for anion binding. In the solid state, urea and squaramide exhibit significant geometrical differences, as far as H-bond donor sites are concerned (see Fig.…”

Section: Methodsmentioning

confidence: 99%

Anion recognition by hydrogen bonding: urea-based receptors

2010

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Since 1992 a variety of urea-based anion receptors have been synthesised, of varying complexity and sophistication. This critical review will focus on some distinctive aspects of anion recognition by urea derivatives, with a special reference to: (i) design and synthesis, (ii) methodologies for the investigation of the receptor-anion interaction in solution, (iii) the interpretation of the solution behaviour on the basis of the structural interplay between the receptor and the anion. It will be shown that the efficiency of urea as a receptor subunit depends on the presence of two proximate polarised N-H fragments, capable (i) of chelating a spherical anion or (ii) of donating two parallel H-bonds to the oxygen atoms of a carboxylate or of an inorganic oxoanion, a property which is shared with other diamides, e.g. squaramide. The wide use of urea in the design of neutral anion receptors seems to depends on the ease of its synthesis, in particular through the reaction of a primary amine group with an isocyanate, which allows the high-yield preparation of symmetrically and unsymmetrically substituted derivatives (83 references).

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Section: Methodsmentioning

confidence: 99%

Anion recognition by hydrogen bonding: urea-based receptors

2010

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“…After the reaction was complete, the precipitate was filtered, washed with a small amount of ice-cold MeOH and dried. Further purification was not necessary; yield: 644 mg (91%); white crystals; mp 234-237 °C (dec.); R f = 0.84 (silica gel TLC, DCM/MeOH 10:1); [] D 25…”

Section: Tert-butyl (S)-(-)-2-({[2-(benzylamino)-34-dioxocyclobut-1-en-1yl]amino}methyl)pyrrolidine-1-carboxylate (10)mentioning

confidence: 99%

Application of Proline-Derived (Thio)squaramide Organocatalysts in Asymmetric Diels–Alder and Conjugate Addition Reactions

et al. 2022

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The synthesis of chiral proline-derived squaramide and thiosquaramide organocatalysts, which are capable of the dual activation in asymmetric reactions is reported. The (thio)squaramide moiety can form hydrogen bonds to activate the substrates and to stereocontrol the reaction, while the pyrrolidine unit can form enamines to activate carbonyl compounds via aminocatalysis. Comparing the performance of thiosquaramide to squaramide, the Diels–Alder reaction of (anthracen-9-yl)acetaldehyde and trans-β-nitrostyrene was examined, which has been investigated in the literature using quantum chemical calculations. Both squaramide and thiosquaramide gave excellent yields (up to 99%) and enantiomeric excess values (up to 98%). Moreover, their catalytic performance was compared in conjugate addition of lawsone to β,γ-unsaturated α-keto ester.

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“…In one report, three polymorphs of dibenzylsquaramide 6 were examined in different solvents and at varying temperatures (Figure 3). 19 Synthesized through reaction between diethylsquarate and benzylamine in ethanol, 6 was subjected to polymorphic screening in different combinations of DMF and DMSO with polar and non-polar solvents. DSC analysis revealed three distinct polymorphs and showed that forms 6B and 6C transform into form 6A at 257 C. Crystal structures of metastable forms 6A and 6C were studied by single-crystal X-ray diffraction (XRD) and demonstrated welldefined head-to-tail hydrogen bonding (N-H$$$O 2.834 and 2.779 Å , respectively) between the intermolecular squaramide units with different packing directions.…”

Section: Squaramides As Supramolecular Self-assembly Motifsmentioning

confidence: 99%