The Urea Renaissance

Abstract

Over the last decade the use of urea derivatives as useful reagents, catalysts, and structural features in organic chemistry has increased rapidly. They now find utility as hydrogen-bond donors in organocatalysts and anion transporters, as important scaffolds in supramolecular chemistry, as lithiation directors, amination substrates, and promoters of metalation, and as substrates for novel rearrangement reactions. Highlighted herein is the remarkably rapid and recent development of the chemistry of ureas, which for many years had been considered unreactive, intractable, and of little value.

Orthohalogen substituents dramatically enhance hydrogen bonding of aromatic ureas in solution

“…1). In fact, at this concentration, the free N-H vibration band (3420-3450 cm -1 ) 8 is small in the case of bH 3 and bMe 3 and is barely detectable in the case of bF 3 , bCl 3 and bBr 3 , which qualitatively hints at the positive effect of the halogens on the strength of intermolecular association. The structure of the assemblies was examined by Small Angle Neutron Scattering (SANS).…”

“…At low concentration (2x10 -5 mol/L) in chloroform, FTIR spectra show that all these compounds are monomeric and that an intramolecular interaction between the N-H α (Scheme 1) and the halogens in ortho position is present in the case of the chlorinated bCl 3 and brominated bBr 3 bis-ureas (see ESI †). Ab initio calculations on model mono-ureas support these results and highlight the significant influence of the substituents on the dihedral angle (φ) between the urea and aromatic groups (see ESI †).…”

“…The endothermic signal results from the breaking of hydrogen bonds consequent to the dilution. Qualitatively, it is possible to see that the halogenated bis-ureas are more strongly self-associated than bH 3 and bMe 3 because in their case the concentration of the experiment had to be reduced to induce full dissociation. To quantify this effect, the experiments were reproduced with optimized initial concentrations and injection volumes, and the heat signal was integrated and plotted versus the concentration in the calorimetric cell (Fig.…”

“…3). From these curves, it is possible to determine graphically the critical concentration (c*) 18 below which the assemblies dissociate into monomers: the stability of the assemblies increases very strongly from the reference bis-ureas bH 3 (c* = 1.6mM) and bMe 3 (c* = 0.6mM) to the halogenated bis-ureas bF 3 (c* = 0.1mM), bBr 3 (c* = 0.056mM) and bCl 3 (c* = 0.041mM). Moreover, the exact shape of the titration curves can be accounted for by a simple mass action law model describing the evolution of the concentration of monomers (M) and filaments (F n ) of any degree of polymerisation (n).…”

Conformation‐Determined Through‐Bond versus Through‐Space Electronic Communication in Mixed‐Valence Systems with a Cross‐Conjugated Urea Bridge

“…Depending on the substituents at the nitrogen atom, arylureas display significantly different conformations. [18] When the substituents are H atoms, diarylurea usually adopts a linear trans,trans-conformation (the bottom panel in Figure 1), and they are very important building blocks in supramolecular chemistry. [19] However, a simple N-methylation results in a folded cis,cis-conformation.…”