Ureas symmetrical

The first are the electron-transport inhibitors such as ureas, symmetric triazines, triazinones, and certain phenylpyridazinones also included are phenol compounds, bentazon or some diphenylethers (for chemical names of herbicides or formulas see legend of Figure 2). All have been shown to affect the B/plasto-... 

Melamine resins were introduced about ten years after the Beetle molding compound. They were very similar to those based on urea but had superior quaHties. Henkel in Germany was issued a patent for a melamine resin in 1936 (7). Melamine resins rapidly supplanted urea resins and were soon used in molding, laminating, and bonding formulations, as well as for textile and paper treatments. The remarkable stabiHty of the symmetrical triazine ring made these products resistant to chemical change once the resin had been cured to the insoluble, cross-linked state. 

Fluorophenyl isocyanate [1195-45-5] M 137.1, b 55°/8mm, n 1.514. Purify by repeated fractionation through an efficient column. If IR indicated that there is too much urea (in the presence of moisture the symmetrical urea is formed) then dissolve in dry EtOH-free CHCI3, filter, evaporate and distil. It is a pungent LACHRYMATORY liquid, [see Hardy J Chem Soc 2011 1934-, and Hickinbottom Reactions of Organic Compounds Longmans p. 493 1957.]... 

Use of the valine derived (4S )-3-acetyl-4-isopropyl-1,3-oxazolidine (8)92, the C2-symmetric reagents (2.5,55)-l-acetyl-2,5-bissubstituted pyrrolidine 994, or the doubly deprotonated acetyl urea /V-acetyl- V..V -bis[(.S)-l-phcnylethyl]urea (10), also does not lead to sufficient induced stereoselectivity combined with acceptable chemical yield. When the acetyl urea enolate is reacted with aliphatic and aromatic aldehydes, the diastereomeric adducts (ratios ranging from 1 1 to 3 1) may be separated by column chromatography to give ultimately both enantiomers of the 3-hydroxy acids in 99% ee110. 

For symmetrical and unsymmetrical diphenyl-urea we expect some extra resonance energy in addition to that for two benzene rings and urea because of conjugation of these groups the values found are 0.55 v.e. and 0.50 v.e., respectively. 

Salts of dithiocarbamic acid can be prepared by the addition of primary or secondary amines to carbon disulfide. This reaction is similar to 16-9. Hydrogen sulfide can be eliminated from the product, directly or indirectly, to give isothiocyanates (RNCS). Isothiocyanates can be obtained directly by the reaction of primary amines and CS2 in pyridine in the presence of DCC. ° In the presence of diphenyl phosphite and pyridine, primary amines add to CO2 and to CS2 to give, respectively, symmetrically substituted ureas and thioureas ... 

Chiral amines and diamines are readily available substrates for the synthesis of ligands for transition metal-catalysed reactions since they can easily be transformed into chiral ureas and thioureas. Therefore, several groups have prepared chiral symmetrical ureas and thioureas, dissymmetrical ureas and thioureas, amino-urea and thiourea derivatives. Finally polyureas and non-soluble polythioureas were also prepared and tested as ligands for asymmetric catalysis. 

Treatment of a chiral amine with phosgene is the cheapest way to prepare symmetrical ureas [29]. Nevertheless, due to the toxicity and reactivity of that reagent, it can advantageously be replaced by triphosgene [30] or l,l -carbonyldiimidazole [31-34] or other derivatives such as l,l -carbonyldi-2(lH)-pyridinone [35]. This procedure can be extended to thiophosgene (Scheme 1) and its thio-analogues, such as l,l -thiocarbonyldi-2(lH)-pyridinone to produce thioureas [36] chiral diamines can thus be transformed into the corresponding monoureas or monothioureas. 

Symmetrical disubstituted ureas in which the two subsequent groups are alike can dearrange in only one manner. 

Au Amines (oxidative carbonylation with CO/O2 to carbamates and symmetric ureas)... 

Many methods have been proposed and are used to study the thermal stability of propellants and to ensure the absence of possible autocatalysed decompositions during storage. None are sufficiently reliable to merit individual description. In practice, stabilisers are added, the usual being diphenylamine for nitrocellulose powders and symmetrical diethyl diphenyl urea (carbamate or centralite) for double base propellants. Provided a reasonable proportion of stabiliser remains, the propellant can be assumed to be free from the possibility of autocatalytic decomposition. The best test of stability is therefore a chemical determination of the stabiliser present. 

Bottom Pictorial representation of (a) the achiral, S8 symmetric homodimer 16-16 and of (b) the two enantiomeric forms of a calixarene capsule heterodimer. The head-to-tail directionality of the urea groups (arrow) defines the chirality. 

The reviewer did the reaction on loose resin (because MicroTubes are no longer available) and washed it extensively in the isocyanate reaction with DMF to remove the unwanted symmetrical urea. 

The symmetrical anhydride is prepared using dicyclohexylcarbodiimide in dichloromethane, the urea and solvent are removed, and the anhydride is dissolved in dimethylformamide and added to the peptide-resin (see Section 2.5). The anhydride is a more selective acylating agent than the 0-acylisourea and, thus, gives cleaner reactions than do carbodiimides, but twice as much amino-acid derivative is required, so the method is wasteful. It avoids the acid-catalyzed cyclization of terminal glutaminyl to the pyroglutamate (see Section 6.16) and is particularly effective for acylating secondary amines (see Section 8.15). 

Optimal conditions for the synthesis of symmetrically disubstituted or tetra-substituted ureas as well as of unsymmetrically disubstituted or trisubstituted ureas have, however, been developed (see also next Section). 

The versatility of the PCH2/KI catalytic system is further demonstrated by its ability to catalyze the oxidative carbonylation of primary amines to symmetrically substituted ureas (Eq. 51), still under mild conditions (100 °C, 16 atm of CO, 4 atm of air in DME as the solvent) and with unprecedented catalytic efficiencies for this kind of reaction (up to ca. 2500 mol of product per mol of Pd) [274,275]. In some cases, working in the presence of an excess of CO2 (40 atm) had a beneficial effect on the reaction rate and product selectivity. 

Primary amines RNH2 (e.g. R = Bu) are converted into symmetrical ureas RNHCONHR at room temperature and atmospheric pressure by the action of... 

Symmetrical dialkyl and diaryl thioureas have been prepared in generally good yield (60-90%) from the reaction of the amine with thiophosgene, formed in situ from the catalysed reaction of carbon tetrachloride with sodium sulphide [5], Reaction with diamines yields cyclic ureas. 

Hydrolysis of the monomer also furnishes symmetrically disubstituted urea, in constrast to that of CF3NCO (7) ... 

The additional sulfur atom seems to reduce the reactivity of the S—Cl bond to nucleophilic attack cmly to a slight extent. Hydrolysis yields, here too, the symmetrically disubstituted iV,A -bis(trifluoromethylmer-captosulfenyl) urea ... 

A unique opportunity was created for the carbonylation of alkylamines or aromatic amines with CO2 in CsOH-treated [BMIM]C1, as both reactants have sufficient solubility in the ionic liquid. The products are symmetric urea derivatives. Yields >90% were obtained from the carbonylation of alkylamines, but lower yields were obtained with aromatic amines. The products precipitated when water was added 193). 

A catalytic reaction also took place under similar conditions with primary aliphatic amines but it led to the formation of symmetrical ureas and the best results were... 

Symmetrical ureas can also be prepared by heating isocyanates with water [36, 37] (Eq. 24). 

In addition, symmetrical diphenylurea may be metalated to give the dianion which then may be condensed with active halogen compounds as shown in Eq. (48). This metallation does not work when applied to urea, phenylurea,... 

General Procedure for Preparation of Symmetrical and Unsymmetrical Ureas by the Oxidative Carbamoylation Method [79]... 

If isocyanates are available, they may be allowed to react with water to yield symmetrical ureas (Eq. 2). 

---