Urea complexation

Bis(carbamoyl) peroxides of the foUowiag structure were prepared by reaction of the corresponding carbamoyl chlorides and hydrogen peroxide—urea complex in the presence of pyridine (211). 

Separation Techniques. Current methods for separating fatty acids are by solvent crystaUi2ation or by the hydrophili2ation process. Other methods that have been used in the past, or perhaps could be used in the future, are panning and pressing, solvent extraction, supercritical fluid extraction, the use of metal salts in assisting in separation, separations using urea complexes, and adsorption/desorption. 

Figure 15 Conjugation of a carboxylic add and an amine using the carbodiimide method. The carbodiimide activates the carboxyhc acid to speed up the reaction to the amine. Carbodiimides can be used with nonpolar or polar solvents, including water. Undesirable urea complexes may form as by-products. Details of the reaction are given in Table 3...

Incorporation of the position of water molecules that are firmly bound to the protein can impart affinity and novelty to the designed ligand. A prime example is the design of a class of HIV protease cyclic urea inhibitors by DuPont scientists that incorporated a water molecule bound to both flaps of the enzyme into their ligand [32]. The crystal structure of the HIV protease-cyclic urea complex [32] shows the urea carbonyl oxygen substituting for the position of the water molecule. 

Mak, J.Y., D. Heaven, D. Kubek, C. Sharp, and M. Clark, Synthesis Gas Purification in Gasification to Ammonia/Urea Complex, Gasification Technology Conference, October 2004. 

A number of water-soluble calcium salts provide convenient vehicles for the administration of therapeutic anions. Probably the most widely encountered is the acetylsalicylate of soluble aspirin (patented 1935) the urea complex of calcium acetylsalicylate (water-solubility 231 gdm-3 at 310K, pH 4.8 (717)) is also widely used. Other examples include calcium bromide and bromolactobionate (sedatives), calcium 2-hydroxy-3-mercapto-l-propanesulfonateaurate(III) (chrysanol, antiarthritic), and calcium Af-carbamoylaspartate (tranquillizer). Calcium iron(II) citrate has been used to remedy iron deficiency - it has the advantage of being stable to air oxidation of the iron(II). The cyclamate anion is certainly not therapeutic, but is relevant here. 

The refractive index will vary with the purity of the oleic acid. Oleic acid purified by low-temperature crystallization and by conversion into the oleic acid-urea complex (95.3% oleic, 0.7% linoleic) yielded a product with wf 1.4613. 

Urea complex with hydrogen peroxide (UHP) seems to be a very useful and convenient oxidation system in nonaqueous medium. The UHP reaction proceeds in methanol and is catalyzed by Mo (VI), W (VI) salts, or SeC>2 (83). Catalytic oxidation in a water-free medium can be carried out with alkylhydroxyperoxides, the catalysts being titanium alkoxides (84) or selenium compounds (85). The reaction appears to proceed quickly and with good selectivity. 

Thus -alkanes (C10-C14) separated from the kerosene fraction of petroleum (by urea complexation or absorption with molecular sieves) are now used as one source of the alkyl group. Chlorination takes place anywhere along the chain at any secondary carbon. Friedel-Crafts alkylation followed by sulfonation and caustic treatment gives a more linear alkylbenzenesulfonate (LAS) which is soft or biodegradable. The chlorination process is now the source of about 40% of the alkyl group used for the manufacture of LAS detergent. 

The simple cyclic silyl peroxide, l,l,4,4-teteramethyl-l,4-disila-2,3-dioxane, 75, was prepared by classical synthetic methodology from its corresponding cyclic disilazane 74 and the urea complex of H2O2 (equation 85) . 

In a second historical paper30a) on cyclic oligoethers, Pedersen mentioned several crystalline dibenzo[l8]crown-6 adducts with thiourea in which the thiourea to crown ratio varied from 12 3 to 6 1 30b). The urea complex was first prepared in 1981 by the addition of ether to a homogeneous solution of [18]crown-6 and urea in a 1 2 methanol/chloroform mixture31). Recrystallization from methanol/ethylacetate yielded the analytically pure 5 1 complex with a melting point of 148-150 °C. An X-ray analysis at — 125 °C 31) revealed a crystalline solid with a 5 1 urea/host ratio containing 2 1 complexes and uncomplexed urea. 

Lam et al. (1994) have also used this trick of both mimicking and displacing the water molecule. They designed a series of 7-membered ring cyclic ureas, including 68, which is a potent enzyme = 0.27 nM) and virus inhibitor, and also has significant oral bioavailability. This compound is currently under clinical investigation. Crystal structures of 10 of these cyclic urea complexes with protease have been determined, and show that, as the... 

Torisawa [17] developed an alternative oxidative amidation of aldehydes using palladium chloride (PdCl2)-xantphos complex as a catalyst. The use of hydrogen peroxide (H2O2)-urea complex as oxidant prevents the formation of imine from the carbinolamine intermediate and minimizes the level of benzoic acid side... 

NaOH or BaO, and fractionally distd. The distd lutidine was converted to its urea complex by stirring lOOg with... 

Chromium is unusual in that it forms a stable hexakis O-bonded urea complex. The complex was first prepared as the chloride salt by Pfeiffer804 and a crystal structure of the complex salt [Cr OC(NH2)2 6][Cr(CN)6]-2DMSO>2EtOH has recently been reported.805 Coordination at chromium(IH) is octahedral r(Cr—O) is in the range 1.96-1.98 A. The reduction of the perchlorate salt of this complex to a chromium(II) species has been studied polarographically.806 Detailed studies of the luminescence spectra of several salts of the chromium urea complex have been reported.807,808... 

A large number of urea and substituted urea complexes with actinide(IV) compounds are known in these the ligands are bonded to the metal via the carbonyl oxygen atom, and these complexes are therefore described in the section dealing with carboxylic add amide complexes (p. 1164). 

The IR spectra of all the complexes indicate that the ligands are bonded to the metal atom via the carbonyl oxygen atom. From the IR spectra of Th(N03)4 2urea-(l or 6)H20 it is clear that ionic nitrate is absent, but the spectra of most of the other urea complexes of thorium tetranitrate show that both covalent and ionic nitrate are present. 

The structures of three urea complexes of dioxouranium(VI) sulfate have also been reported. In the bis complex, UO2S04-2urea, the structure consists of infinite ribbons of OUO groups... 

Table 79 Some Urea Complexes of Dioxouranium(VI) Compounds...

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