Boronic atmospheric oxidation

Allylic boronates are more stable to atmospheric oxidation and are thns mnch easier to handle than the corresponding allylic boranes. The stability of the boronate reagents arises from the partial donation of the lone pairs of electrons on the oxygen atoms into the empty p-orbital of boron. This mesomelic effect is responsible for the npfield shift of the boron atom in NMR compared to that of allylic boranes (compare allylboronate 31 and allylborane 32). ... 

Furthermore, pyrazol-3-ones have been oxidized by a variety of oxidizing agents such as ozone in oxygen, hydrogen peroxide solution, 3-chloroperbenzoic acid, aqueous sodium periodate, lead(IV) acetate with boron trifluoride etherate, or atmospheric oxidation. The reactions lead mainly to epoxidation of an alkene or imine functionality and cleavage of the pyrazol-3-one ring. 

Using the simple combination of alloys and elements in baU miUs, Cu-Co, Fe-Mo and Mn-Al aUoys have been produced. Elemental combination has been used to prepare boron-containing alloys in the Ni-Nb-B and Ti-Al-B systems [4]. Due to the risk of atmospheric oxidation of the metals, the reactions are often carried out under an inert gas, typically argon. They generaUy require relatively long milling times (24-300h). 

The use of oxidant such as molecular oxygen [60] or di-terf-bntylperoxide [61] with catalytic amount of copper led to eliminate the use of base in the boronic acid oxidative coupling (Scheme 20.28). Fu and coworkers used atmospheric oxygen as oxidant with Cu O for the selective synthesis of primary amines at room temperature using aqueous ammonia as an amine source [62]. Yamamoto and coworkers developed air- and water-stable cyclic triolborate complex to increase the nucleophilicity of the attached group in boronic acid derivative [63]. Copper-mediated selective coupling of methylboronic acids with primary amines has been subsequently developed [64]. 

A description of this has been developed based on gas phase diffusion of the H ByO (g) species outward and boron-enhanced oxidation of the SiC to form a bor-osilicate plug (Jacobson et al., 1999b). This is shown schematically in Fig. 7-56. Exposures of the SiC fiber with BN fiber coatings in a SiC matrix were conducted at 700 and 800 °C in 1 and 10% H2O/O2 atmospheres with one end of the test sample ground off to expose the fiber ends and coatings. Then recession distances were measured. As illustrated in Fig. 7-55, the exact distance was difficult to determine as interfaces were not always sharp. In order to apply the model to the experiments an adjustable parameter of boron-influenced SiC oxidation rates had to be introduced. Using substantially enhanced oxidation rates, reasonable agreement between the model and experimentally measured recession distances were obtained. 

Preparation. Boron carbide is most commonly produced by the reduction of boric oxide with carbon in an electric furnace between 1400 and 2300°C. In the presence of carbon, magnesium reduces boric oxide to boron carbide at 1400—1800°C. The reaction is best carried out in a hydrogen atmosphere in a carbon tube furnace. By-product magnesium compounds are removed by acid treatment. 

Preparation. Hexagonal boron nitride can be prepared by heating boric oxide with ammonia, or by heating boric oxide, boric acid, or its salts with ammonium chloride, alkaU cyanides, or calcium cyanamide at atmospheric pressure. Elemental nitrogen does not react with boric oxide even in the presence of carbon, though it does react with elemental boron at high temperatures. Boron nitride obtained from the reaction of boron trichloride or boron trifluoride with ammonia is easily purified. 

Oxidation of C12-C14 n-paraffms using boron trioxide catalysts was extensively studied for the production of fatty alcohols.Typical reaction conditions are 120-130°C at atmospheric pressure. ter-Butyl hydroperoxide (0.5 %) was used to initiate the reaction. The yield of the alcohols was 76.2 wt% at 30.5% conversion. Fatty acids (8.9 wt%) were also obtained. Product alcohols were essentially secondary with the same number of carbons and the same structure per molecule as the parent paraffin hydrocarbon. This shows that no cracking has occurred under the conditions used. The oxidation reaction could be represented as ... 

In certain respects, the combustion of boron is different from that of carbon because, under normal temperature and pressure conditions, the product oxide, B203, is not a gas. Thus, a boron particle normally has an oxide coat that thickens as the particle is heated in an oxidizing atmosphere. This condition is characteristic of most metals, even those that will bum in the vapor phase. For the efficient combustion of the boron particle, the oxide coat must be removed. The practical means for removing the coat is to undertake the oxidation at temperatures greater than the saturation temperature of the boron oxide B203. This temperature is about 2300 K at 1 atm. 

In a high-temperature atmosphere created by the combustion of a host hydrocarbon fuel, there will be an abundance of hydroxyl radicals. Thus, boron monoxide reacts with hydroxyl radicals to form gaseous metaboric oxide HOBO. 

In a hydrogen-free oxidizing atmosphere, a slower step forms the boron dioxide,... 

Method A A solution of 0.5 mmol of (2/ /S ,3R/S )-2,3-dialkyl-1,4-diarylbutane and 0.125 mL (1.0 mmol) of boron trifluoride-diethyl ether complex in 2 mL of trifluoroacctic acid is added to a suspension of 0.12 g (0.26 mmol) of thallium(III) oxide in 2 mL of trifluoroacetic acid at — 40°C to +25CC under an argon atmosphere. The dark colored solution is stirred until the reaction is complete, diluted with ethyl acetate, then washed successively with water (twice) and sat. aq NaCl. Evaporation of the dried extract gives the crude product. In a variant of this method the boron trifluoride-diethyl ether complex can be omitted. 

Atmospheric boron is in the form of particulates or aerosols of borides, boron oxide, borates, boranes, organoboron compounds, trihalide boron compounds, or borazines. The half-time persistence of airborne boron particles is short, usually on the order of days (USPHS 1991). 

If desired, plasma oxide films can be doped much as the plasma nitride film we discussed earlier. In fact, doping with boron and phosphorus has been carried out as an alternative to the standard atmospheric-pressure thermal CVD process for BPSG.11 12 The latter process has the drawbacks of high defect density and poor thickness uniformity, so it was hoped that plasma BPSG would be an improvement. However, there are differences in the films in terms of H2 and N2 content, and their effect on reflow temperature, intrinsic stress and passivation effectiveness had to be examined. 

This basic system was designed to deposit Si02 from the SiH4 + 02 reaction at about 400°C and atmospheric pressure. It can also deposit doped oxides by introducing PH3 for phosphorus doping or B2H6 for boron doping. In order to protect personnel from these toxic dopants, the reactor is housed in a vented enclosure. 

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