Boron effects

Pinanediol (Dichloromethyl)boronate Effects of Lack of C2 Symmetry... 

Howe, P.D. 1998. A review of boron effects in the environment. Biol. Trace Elem. Res. 66 153-166. 

The partial hydrogenation of benzene to cyclohexene is of great industrial importance. A possible application of the product cyclohexene is its transformation to cyclohexanone. It was shown that ruthenium may give rise to increased selectivity toward cyclohexene.426 In addition, studies with various Ru-B catalysts demonstrated that boron effectively promotes the activity and selectivity of ruthenium for cyclohexene.427 428... 

This dramatic ductilization effect of boron has been and still is the subject of elaborate experimental and theoretical studies which, however, have not yet led to an agreement on the physical reasons for the boron effect, i.e. the mechanistic understanding of this ductilization effect is still unclear. This has been the subject of a set of papers recently, and the present state of knowledge and the controversial issues have been overviewed (Liu, 1991b). The important factors are briefly summarized in the following. 

Boron effectively counteracts symptoms of fluoride intoxication in humans and in rabbits poisoned experimentally. Humans suffering from skeletal fluorosis experienced 50-80% improvement after drinking solutions containing 300.0-1100.0 mg of borax per liter daily, 3 weeks a month for 3 months. Boron enhances sequestration of fluoride from bone and excretion through kidneys and possibly the intestinal tract. 

Minimum concentrations of dietary boron needed to maintain animal health are not known with certainty. However, diets containing <0.4 mg B/kg IW may adversely affect metabolism of rats and chicks accordingly, animal diets should contain >0.3 mg B/kg FW until necessary feeding data become available. Also, the defensible boron maximum for livestock drinking water may be considerably higher than 5.0mg/L (Table 4.2) because several safe water sources in Nevada exceeded this upper maximum and approached 80.0 mg B/L. Data are unavailable on boron effects on terrestrial wildlife. Until these data become available, it seems reasonable to apply the same criteria proposed for livestock protection (Table 4.2) to mammalian wildlife, that is, diets should contain more than 0.4 mg B/kg DW but less than 100.0 mg/kg, and drinking water less than 5.0 mg/L. 

Akkurt, L, A. Qalik, and H. Akyildirim. 2011. The boronizing effect on the radiation shielding and magnetization properties of AISI 316L austenitic stainless steel. Nucl. Eng. Des. 241 55-58. 

Odette G R, Lucas G E and Klingensmith D (1999), Irradiation hardening of pressnre vessel steels at 60°C The role of thermal nentrons and boron . Effects of Radiation on Materials 18th International Symposium Nanstad R K, Hamilton M L, Garner F A and Kumar A S, eds, ASTM STP1325, American Society for Testing and Materials, West Conshohocken, PA, 3. 

One of the initial theories used to explain why the boron effect was observed only in Ni-rich compositions was that boron segregation was increased in the Ni-rich compounds (Liu et al., 1985). The data used to make this point are shown in Figure l(Ka)- If one plots these... 

Other catalysts which may be used in the Friedel - Crafts alkylation reaction include ferric chloride, antimony pentachloride, zirconium tetrachloride, boron trifluoride, zinc chloride and hydrogen fluoride but these are generally not so effective in academic laboratories. The alkylating agents include alkyl halides, alcohols and olefines. 

The controlled thermal decomposition of dry aromatic diazonium fluoborates to yield an aromatic fluoride, boron trifluoride and nitrogen is known as the Schiemann reaction. Most diazonium fluoborates have definite decomposition temperatures and the rates of decomposition, with few exceptions, are easily controlled. Another procedure for preparing the diazonium fluoborate is to diazotise in the presence of the fluoborate ion. Fluoboric acid may be the only acid present, thus acting as acid and source of fluoborate ion. The insoluble fluoborate separates as it is formed side reactions, such as phenol formation and coupling, are held at a minimum temperature control is not usually critical and the temperature may rise to about 20° without ill effect efficient stirring is, however, necessary since a continuously thickening precipitate is formed as the reaction proceeds. The modified procedure is illustrated by the preparation of -fluoroanisole ... 

The acylation of ketones with acid anhydrides may be effected by means of the acid reagent boron trifluoride, for example ... 

Acylation may also be effected with the acetic acid - boron trifluoride complexes BF3.CH3COOH and BF3.2CH3COOH. 

Nitration has also been effected with the complexes from dinitrogen tetroxide and Lewis acidsin the case of boron trifluoride the complex appears to be a mixture of nitronium and nitrosonium tetrafluoroborates. ... 

The relative basicities of aromatic hydrocarbons, as represented by the equilibrium constants for their protonation in mixtures of hydrogen fluoride and boron trifluoride, have been measured. The effects of substituents upon these basicities resemble their effects upon the rates of electrophilic substitutions a linear relationship exists between the logarithms of the relative basicities and the logarithms of the relative rate constants for various substitutions, such as chlorination and... 

Boron trichloride, usually in conjunction with an additional Lewis acid, effects o-chloroacetylation of anilines. The resulting products are converted to indoles by reduction with NaBH4.[l], The strength of the Lewis acid required depends upon the substitution pattern on the ring. With ER substituents no additional... 

There is a pronounced tendency for boron to become bonded to the less substituted car bon of the double bond Thus the hydrogen atoms of diborane add to C 2 of 1 decene and boron to C 1 This is believed to be mainly a steric effect but the regioselectivity of addition does correspond to Markovmkov s rule m the sense that hydrogen is the neg atively polarized atom m a B—H bond and boron the positively polarized one... 

Steric effects may be an even more important factor m controlling the regioselec tivity of addition Boron with its attached substituents is much larger than a hydrogen atom and becomes bonded to the less crowded carbon of the double bond whereas hydrogen becomes bonded to the more crowded carbon... 

Boron. The principal materials used are borax [1303-96-4] sodium pentaborate, sodium tetraborate, partially dehydrated borates, boric acid [10043-35-3] and boron frits. Soil appHcation rates of boron for vegetable crops and alfalfa are usually in the range of 0.5—3 kg/hm. Lower rates are used for more sensitive crops. Both soil and foHar appHcation are practiced but soil appHcations remain effective longer. Boron toxicity is not often observed in field appHcations (see Boron compounds). 

Health and Safety Factors. Boron trifluoride is primarily a pulmonary irritant. The toxicity of the gas to humans has not been reported (58), but laboratory tests on animals gave results ranging from an increased pneumonitis to death. The TLV is 1 ppm (59,60). Inhalation toxicity studies in rats have shown that exposure to BF at 17 mg/m resulted in renal toxicity, whereas exposure at 6 mg/m did not result in a toxic response (61). Prolonged inhalation produced dental fluorosis (62). High concentrations bum the skin similarly to acids such as HBF and, if the skin is subject to prolonged exposure, the treatment should be the same as for fluoride exposure and hypocalcemia. No chronic effects have been observed in workers exposed to small quantities of the gas at frequent intervals over a period of years. 

Table 1 Hsts some of the physical properties of duoroboric acid. It is a strong acid in water, equal to most mineral acids in strength and has a p p o of —4.9 as compared to —4.3 for nitric acid (9). The duoroborate ion contains a neady tetrahedral boron atom with almost equidistant B—F bonds in the sohd state. Although lattice effects and hydrogen bonding distort the ion, the average B—F distance is 0.138 nm the F—B—F angles are neady the theoretical 109° (10,11). Raman spectra on molten, ie, Hquid NaBF agree with the symmetrical tetrahedral stmcture (12).

In addition, boron, aluminum, and gallium tris(triduoromethanesulfonates) (tridates), M(OTf)2 and related perduoroalkanesulfonates were found effective for Friedel-Crafts alkylations under mild conditions (200). These Lewis acids behave as pseudo haUdes. Boron tris(tridate) shows the highest catalytic activity among these catalysts. A systematic study of these catalysts in the alkylation of aromatics such as benzene and toluene has been reported (201). 

Metal Alibis and Alkoxides. Metal alkyls (eg, aluminum boron, sine alkyls) are fairly active catalysts. Hyperconjugation with the electron-deficient metal atom, however, tends to decrease the electron deficiency. The effect is even stronger in alkoxides which are, therefore, fairly weak Lewis acids. The present discussion does not encompass catalyst systems of the Ziegler-Natta type (such as AIR. -H TiCl, although certain similarities with Friedel-Crafts systems are apparent. 

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