Boronate-urea

The boronate-urea (132)-catalysed formal 3 + 3-cycloaddition reactions of nitrones (130) and nitrocyclopropane carboxylates (131) produced highly functionalized... 

Most of the synthetic approaches toward this ring system utilize N-amino pyridinium salts functionalized at the a-position with a carbonyl group. Thus, the amination of 2-(l,3-dioxolan-2-yl)pyridine with tosyl-hydroxylamine gave 78, whose reaction with urea in the presence of boron trifluoride-acetic acid gave 79, which gave the thermally unstable... 

C-nucleophile (X = active H-borate, boronate) N-nucleophile (amine, NaN3, tosyl amide, amide, lactam, imine, carbamate, urea) O-nucleophile (alcohol, acid, carbonate) S-nucleophile (PhS02Na)... 

The c-BN phase was first obtained in 1957 [525] by exposing hexagonal boron nitride phase (h-BN) to high pressures and low temperatures. A pressure of more than 11 GPa is necessary to induce the hexagonal to cubic transformation, and these experimental conditions prevent any practical application for industrial purposes. Subsequently, it has been found that the transition pressure can be reduced to approximately 5 GPa at very high temperature (1300-1800°C) by using catalysts such as alkali metals, alkali metal nitrides, and Fe-Al or Ag-Cd alloys [526-528]. In addition, water, urea, and boric acid have been successfully used for synthesis of cubic boron nitride from hexagonal phase at 5-6 GPa and temperature above 800-1000°C [529]. It has been... 

Amino guanidine bicarbonate, Amm acetate, Amm phosphate, Amm sulfate, apatite, asbestos, Ba sulfate, boron nitride, cerium oxide, cryolite, dicyandiamide, dimethyl oxamide, dimethyl urea, d sodium phosphate, mercurous nitrate, methylene urea,... 

The traditional method for the preparation of boron nitride is by the fusion of urea with boric acid in an atmosphere of ammonia at 750 °C.54 The product from these reactions is hexagonal boron nitride with a layer structure like that of graphite. Unlike graphite, it is colorless and is not an electronic conductor. Conversion of the hexagonal form to a cubic modification requires heating at 1,800 °C at 85,000 atmospheres pressure. 

Unusual flux precursors like urea, ammonium nitrate, boric acid, or ammonium boron-hydride have also been studied [173], In all these cases derivatives of amino borate form the liquid phase. 

Wood Composites—these are resin-bonded composite boards where the particles are wood shavings, flakes, chips, or fibers bonded with thermosetting adhesives that can be urea formaldehyde, melamine formaldehyde, phenol formaldehyde, or diisocyanate. In recent years, the markets for OSB and MDF board have been rapidly increasing. Most particle board production uses urea-formaldehyde as a binder that is acid setting. Hence, sodium borates (alkaline) can interfere with the setting. As a result, boric acid has been the major boron compound used as the flame retardant in particle board.28 29 Typically, a loading of 12%-15% of boric acid in MDF is required to meet the ASTM E-84 Class A rating. If sodium borate is used as a flame retardant, phenol-formaldehyde binder, that is compatible with alkaline chemicals, is commonly used. 

Boron nitride (BN) can normally be prepared from the reaction of boric acid and urea or melamine. For example, the pyrolysis of MB can yield hexagonal BN. It is commonly referred to as white graphite because of its platy hexagonal structure similar to graphite. Under high pressure and at 1600°C, the hexagonal BN is converted to cubic BN, which has a diamond-like structure. 

The cyclic boron esters (99) resulted from the three-component reaction between benzil (96), urea (97), and boric acid (Scheme 21). The stereoisomeric products arose as a consequence of boric acid addition to the intermediate cyclic amides (98) <89PHA110>. Carbon-13 NMR spectroscopy and mass spectrometry are consistent with structure (99). 

A mixture of boric acid (1 g) and urea (11.8 g) was taken in 40 ml distilled water and heated at 70 °C until the solution became viscous the a-CNTs were soaked in it for nearly 2 h. They were later separated physically and dried in air at 40 C overnight. The dried sample was thermally treated at 970 °C for 3 h for 40 nm nanotubes in a N2 atmosphere, and for 12 h in the case of the larger diameter (170 nm) nanotubes, and then cooled down to room temperature. The product was subsequently heated in an NHt atmosphere at 1050 °C in case of 170 nm nanotubes and 900 C in case of 40 nm nanotubes for three hours to give black-coloured boron-carbon-nitride nanotube brushes. The products were investigated by transmission electron microscopy and other physical techniques. 

Tightly adsorbed protein on boronate resins are desorbed by 6 M urea or 30% ethylene glycol prior reuse of the columns. 

A selection of A//p gp values has already been given in Table 2-4 in Section 2.2.6. This new Lewis basicity scale is more comprehensive and seems to be more reliable than the donor number scale. Analogously, a Lewis basicity scale for 88 carbonyl compounds (esters, carbonates, aldehydes, ketones, amides, ureas, carbamates) has been derived from their standard molar enthalpies of complexation with gaseous boron trifluoride in dichloromethane solution [143]. The corresponding Aff Q gp values range from 33 kJ mol for di-t-butyl ketone to 135 kJ mol for 3-diethylamino-5,5-dimethyl-cyclohexen-2-one. 

The reaction is acid or base catalyzed. Many catalysts have been tried, including potassium acetate and sodium acetate (27), dimeth-ylformamide (DMF) (28-30), urea ammonium sulfate (29), magnesium perchlorate (31-33), trifluoroacetic acid (32), boron trifluoride (30), sodium acetate (31), potassium hydrogen phosphate (34), and y-rays (35). The best acetylation condition, however, is uncatalyzed acetic anhydride in xylene at 100-130 C (36). 

Urea-formaldehyde Foams. While urea-formaldehyde (UF) foams can be rated as difficult to bum, blending of UF with another polymer can decrease the resistance of the foam to burning. Fire retardants, including phosphorus and boron compounds, have been added to decrease the flammability of UF foams (42). According to Frisch (42) phosphonates, furfuryl alcohol and ethylene glycol have been used as fire retardants. 

High purity submicrometer (100-150 A) powders of BN have been synthesized in the range —75 to -t-750°C, by means of reactions of borax and carbamide (urea) in ammonia. Metal borohydrides and boron halides in NH3 and ammonium salts with benzene solutions of BCI3 were also studied. BN ceramic aerogels were also described. ... 

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