Urea compounds, complex

Such reactions are also possible in vitro, as several mild oxidizing agents are at hand nowadays. Thus, the Dess-Martin periodinane (DMP) [50] has been proven to be a versatile and powerful reagent for the mild oxidation of alcohols to the corresponding carbonyl compounds. In this way, a series of new iodine(V)-mediated reactions has been developed which go far beyond simple alcohol oxidation [51], Ni-colaou and coworkers have developed an effective DM P-mediated domino polycy-clization reaction for converting simple aryl amides, urethanes and ureas to complex phenoxazine-containing polycycles. For example, reaction of the o-hydroxy anilide 7-101 with DMP (2 equiv.) in refluxing benzene under exposure to air led to polycycle 7-103 via 7-102 in a yield of 35 % (Scheme 7.28) [52]. 

Quinuronium is a complex urea compound (Fig. 5.7) widely used in Babesia infections in horses, cattle, sheep, and swine. The drug is administered only subcutaneously at dosages of 0.3-0.5 mg/kg bw in horses and 0.5 mg/kg bw in cattle, sheep and swine. 

Figure 1. (a) End-view cross section of the urea-hydrocarbon complex, (b) View of the hexagonal PHTP inclusion compound in the ab plane. PHTP inclusion compounds are composed of infinite stacks of host molecules, repeating at about 4.78 A, parallel to the molecular threefold axes. The regular packing of the stacks gives rise to parallel channels. Channel cross section in both cases is 5 A. 

Substituted urea compounds are often used as accelerators for DICY cure, specifically 3-(4-chlorophenyl)-l,l-dimethylurea (Monuron) and the 3-(3,4-dichlorophenyl) derivative (Diuron). This introduces a further level of complexity to the curing mechanism. Son and Weber 62) showed that Monuron could dissociate to form dimethyl-amine and 4-chlorophenylisocyanate and that this is facilitated by the reaction of DICY with the isocyanate to form a carbanilinoguanidine ... 

The XH NMR study of receptors 70a-c revealed [90] that these compounds complex simultaneously sodium cation (lower rim amidic cavity) and anions (upper rim urea/thiourea), which markedly improves the solubility of the cor-... 

Comparison of the urea-anion inclusion compounds with the thiourea-anion inclusion compounds indicates that urea molecules more readily combine with other anionic moieties to generate composite dimers or ribbons. Composite dimers or ribbons exist in seven complexes among the nine urea-anion inclusion compounds studied here. For instance, both urea-bicarbonate complexes 1.6 and 1.7 (Figure 36 and Figure 37) contain a hydrogen-bonded [((NH2)2CO)2(HC03)2] ribbon built of alternating urea dimers and cyclic dimeric bicarbonate moieties (LB... 

Although uronium behaves very similarly to guanidinium in the com-plexation discussed, the binding and extraction of urea are a challenging point. Actually, the conversion of urea to uronium requires acidification with, e.g., perchloric acid, which is inconvenient for practical applications. The problem with urea itself is that this neutral molecule forms only weak complexes with various hosts [132,133]. Some initial attempts to overcome the difficulty involved crowns with intra-anular acidic groups and (proton-ated) pyridino crowns [126,134,135], designed to protonate urea upon complexation. However, it was found that these hosts are not efficient in extraction and membrane transport the pyridine compounds failed to protonate urea and have an unfavorable tendency toward self-complexation [136]. Acidic functionalities help to bind urea strongly, but their nature simultaneously manifested itself in unfavorably low host lipophilicity. 

Urea has the remarkable property of forming crystalline complexes or adducts with straight-chain organic compounds. These crystalline complexes consist of a hoUow channel, formed by the crystallized urea molecules, in which the hydrocarbon is completely occluded. Such compounds are known as clathrates. The type of hydrocarbon occluded, on the basis of its chain length, is determined by the temperature at which the clathrate is formed. This property of urea clathrates is widely used in the petroleum-refining industry for the production of jet aviation fuels (see Aviation and other gas-TURBINE fuels) and for dewaxing of lubricant oils (see also Petroleum, refinery processes). The clathrates are broken down by simply dissolving urea in water or in alcohol. 

Association Complexes. The unshared electron pairs of the ether oxygens, which give the polymer strong hydrogen bonding affinity, can also take part in association reactions with a variety of monomeric and polymeric electron acceptors (40,41). These include poly(acryhc acid), poly(methacryhc acid), copolymers of maleic and acryflc acids, tannic acid, naphthoHc and phenoHc compounds, as well as urea and thiourea (42—47). 

Complexing agents, which act as buffers to help control the pH and maintain control over the free metal—salt ions available to the solution and hence the ion concentration, include citric acid, sodium citrate, and sodium acetate potassium tartrate ammonium chloride. Stabilizers, which act as catalytic inhibitors that retard the spontaneous decomposition of the bath, include fluoride compounds thiourea, sodium cyanide, and urea. Stabilizers are typically not present in amounts exceeding 10 ppm. The pH of the bath is adjusted. 

The first of these types is most familiarly represented by the hexaaquo ion which is present in acidic aqueous solutions and, in the solid state, in the alum CsTi(S04)2.12H20. In fact few other neutral ligands besides water form a [TiLg] + complex. Urea is one of these few and [Ti(OCN2H4)g]l3, in which the urea ligands coordinate to the titanium via their oxygen atoms, is one of the compounds of titanium(III) most resistant to oxidation. 

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