1.3- Bis urea

The tetranitrosamine (12) and the tetranitramine (13) are also synthesized from the bis-urea (8), although these are less energetic and have less favourable oxygen balances than (9), (10) and(ll).2... 

The authors proposed that the Brpnsted base interaction on the catalyst is imperative for reactivity. Catalysts lacking a basic amine moiety, specifically mono- and bis-ureas, did not promote the asymmetric catalytic addition well, if at all. In screening a variety of amine bases and bis-ureas, it became apparent that presence of a Brpnsted base was necessary for catalytic activity (Scheme 61) [113]. The reactivity was extremely low in absence of Brpnsted base (Table 2, entry 2), but slightly improved with presence of NEtj (Table 2, entry 1). Combined, a chiral Brpnsted acid and Brpnsted base increase conversion and showed some enantiose-lectivity (Fig. 8). 

On the basis of this concept, bis-urea (13) and bis-thioureas (14-18) were synthesized and their binding with dihydrogenphosphate (H2PO4) and various other anions in dimethyl sulfoxide (DMSO) was investigated by H NMR spectroscopy. Stability constants, /r,, for the 1 1 complexes are given in Table 1, together with those for the 1 1 complexes with )V,A/ -dimethylurea (19) and NJ -dimethylthiourea (20). ... 

Figure 5. Suggested structure of 1 1 complex between dihydrogenphosphate (H2PO4) and bis-urea (X = O) and bis-thiourea (X = S) receptors.

Related mixed urea-amidopyridyl or tetraamidic hosts have been reported by the same group Vicent C, Fan E, Hamilton AD (1992) Tetrahedron Lett 33 4269 Albert JS, Hamilton AD (1993) Tetrahedron Lett 34 7363. For a bis(urea) host compound, see Hamann BC, Branda NR, Rebek J Jr (1993) Tetrahedron Lett 34 6837... 

Interestingly, the bis-urea pocket represents also a good host for anions and, indeed, upon the addition of ammonium chloride the pocket is occupied with the chloride anion. The binding pocket can change the reactivity ofthe metal center as it facilitates substitution processes at the palladium center. Upon the introduction of... 

Dewal, M. B., Lufaso, M. W., Hughes, A. D., Samuel, S. A., Pellechia, P., Shimizu, L. S., Absorption properties of a porous organic crystalline apohost formed by a self-assembled bis-urea macrocycle. Chem. Mat. 2006, 18, 4855-4864. 

J.-M. Lehn et al. designed helical architectures via the pre-programming of molecular self-assembly through specific non-bonding interactions [116, 117]. The formation of chiral fiber-like architectures had been also observed from non-chiral monomers such as in the gels of bis-urea building blocks [118]. 

The X-ray crystal structures of pyrazine V.JV -dioxide (134) <02AX(E)1253>, the P-polymorph of phenazine (135) <02AX(C)181>, cobalt(III) complexes of pyrazine-2,6- and pyridine-2,6-dicarboxylic acids <02JIC458>, and bis-urea-substituted phenazines <02ZN(B)937> were reported. Fluorescent pyrido[l,2-a]quinoxalines 136 prepared as pH indicators were examined by X-ray crystallography <02JCS(P2)181>, as were macrocyclic quinoxaline-bridged porphyrinoids obtained from the condensation of dipyrrolylquinoxalines 137 and 1,8-diaminoanthracene... 

The reaction of macrocyclic dicarbodiimides with water affords the expected bis-ureas. With ammonia and primary and secondary alkylamines macrocyclic bis-guanidines are obtained in high yield. ... 

Figure 6 shows the pseudo-phase diagram of bis-urea 3 in toluene [40]. It is likely that other (not necessarily dynamic) supramolecular structures exist at... 

Fig. 9 Tentative supramolecular structures proposed for bis-urea 3 thin filament (a) and tubular arrangements b side-view, c top-view...

The bis-urea thin filaments can be very long in non-polar solvents such as 1,3,5-trimethylbenzene. Consequently, these solutions show a high viscosity r]/r]Q = 8 at a concentration C = 0.04 molL and at T = 20 °C) and a high concentration dependence of the viscosity (ri/rio C ) [43]. As in the case of UPy based supramolecular polymers, the value of this exponent is in agreement with Cates s model for reversibly breakable polymers [26,27]. However, the solutions are not viscoelastic, even at concentrations well above the overlap concentration [43]. Consequently, the relaxation of entanglements, probably by chain scission, must be fast (r < 0.01 s). 

In the non-hnear regime, the bis-urea solutions display stress-strain curves typical of shear-banding (Ducouret et al., unpublished results). 

In summary, the rheological properties of these bis-urea solutions can be switched from a viscoelastic behavior (at low temperatures) to a purely viscous behavior (at high temperatures). Moreover, the transition has been shown to be fast, reversible (without hysteresis) and extremely cooperative the conversion of tubes into thin filaments occurs within a temperature range of 5 °C only [40]. This transition can be triggered by temperature, but also by a change in the solvent composition or by a change of the monomer composition. 

Because of its electrostatic component, the hydrogen bond is affected by the solvent polarity the lower the polarity of the solvent, the stronger the association [91]. Quantitatively, this effect can be quite significant for HBSPs for instance the self-association constant of bis-urea 3 is two orders of magnitude larger in toluene than in chloroform [92], This translates into a tenfold effect on the degree of polymerization. 

---