Boric acid boranes

Most reported boric acid esters are trialkoxy or triaryloxy boranes. The esters range from colorless low boiling Hquids to soHds that possess high melting points. Boric acid esters usually have an odor similar to the hydroxy compound from which they are derived. A more complete description of the physical... 

The triaLkoxy(aryloxy)boranes are typically monomeric, soluble in most organic solvents, and dissolve in water with hydrolysis to form boric acid and the corresponding alcohol and phenol. Although the rate of hydrolysis is usually very fast, it is dependent on the bulk of the alkyl or aryl substituent groups bonded to the boron atom. Secondary and tertiary alkyl esters are generally more stable than the primary alkyl esters. The boron atom in these compounds is in a trigonal coplanar state with bond hybridization. A vacantp orbital exists along the threefold axis perpendicular to the BO plane. 

Alkyl boric acid esters derived from straight-chain alcohols and aryl boric acid esters are stable to relatively high temperatures. Methyl borate is stable to 470°C (11). Trialkoxyboranes from branched-chain alcohols are much less stable, and boranes from tertiary alcohols can even decompose at 100°C (12). Decomposition of branched-chain esters leads to mixtures of olefins, alcohols, and other derivatives. 

The main chemical products produced from these minerals are (a) boron oxides, boric acid and borates, (b) esters of boric acid, (c) refractory boron compounds (borides, eu .), (d) boron halides, (e) boranes and carbaboranes and (f) organoboranes. The main industrial and domestic uses of boron compounds in Europe (USA in parentheses) are ... 

Diborane also has a useful pattern of selectivity. It reduces carboxylic acids to primary alcohols under mild conditions that leave esters unchanged.77 Nitro and cyano groups are relatively unreactive toward diborane. The rapid reaction between carboxylic acids and diborane is the result of formation of a triacyloxyborane intermediate by protonolysis of the B-H bonds. The resulting compound is essentially a mixed anhydride of the carboxylic acid and boric acid in which the carbonyl groups have enhanced reactivity toward borane or acetoxyborane. 

Decomposition of the reaction mixtures with water followed by dilute acids applies also to the reductions with boranes and alanes. Modifications are occasionally needed, for example hydrolysis of esters of boric acid and the alcohols formed in the reduction. Heating of the mixture with dilute mineral acid or dilute alkali is sometimes necessary. 

Toxicosis in animals has resulted from ingestion of boric acid or borax solutions, from topical applications of boric acid solutions to damaged skin, and from inhalation of boranes the exact mechanisms of action are not understood. Boron and its compounds are potent teratogens when applied directly to the embryo, bnt there is no evidence of mutagenicity or carcinogenicity. Boron s unique affinity for cancerons tissnes has been exploited in neutron capture radiation therapy of malignant hnman brain tnmors. 

Volatile boron compounds, especially boranes, are usually more toxic than boric acid or soluble borates (Table 29.9) (NAS 1980). However, there is little commercial production of synthetic boranes, except for sodium borohydride — one of the least toxic boranes (Sprague 1972). Boron trifluoride is a gas used as a catalyst in several industrial systems, but on exposure to moisture in air, it reacts to form a stable dihydride (Rusch etal. 1986). Eor boric oxide dusts, occupational exposures to 4.1 mg/m (range 1.2 to 8.5) are associated with eye irritation dryness of mouth, nose and throat sore throat and cough (Garabrant et al. 1984). 

All the teabags were put into a resin kettle under nitrogen. Fifteen equivalents of boric acid and 15 equiv. of trimethylborate were added followed by the slow addition of 45 equiv. of borane (1 M in THF). After hydrogen production ceased, the reaction was heated at 65° for 72 h. The reaction solution was decanted and quenched by the slow addition of methanol. The resin was washed with methanol, THF, and piperidine. The polyamine-borane complex was disproportionated by overnight treatment (16 h) with piperidine at 65° followed by washes with DMF, DCM, and methanol. 

Ebelman and Bouquet prepared the first examples of boric acid esters in 1846 from boron trichloride and alcohols. Literature reviews of this subject are available. B The general class of boric acid esters includes the more common orthoboric acid based trialkoxy- and triaryloxyboranes, B(0R)3 (1), and also the cyclic boroxins, (ROBO)3, which are based on metaboric acid (2). The boranes can be simple trialkoxyboranes, cyclic diol derivatives, or more complex trigonal and tetrahedral derivatives of polyhydric alcohols. Nomenclature is confusing in boric acid ester chemistry. Many trialkoxy- and triaryloxyboranes such as methyl, ethyl, and phenyl are commonly referred to simply as methyl, ethyl, and phenyl borates. The lUPAC boron nomenclature committee has recommended the use of trialkoxy- and triaryloxyboranes for these compounds, but they are referred to in the literature as boric acid esters, trialkoxy and triaryloxy borates, trialkyl and triaryl borates or orthoborates, and boron alkoxides and aryloxides. The lUPAC nomenclature will be used in this review except for relatively common compounds such as methyl borate. Boroxins are also referred to as metaborates and more commonly as boroxines. Boroxin is preferred by the lUPAC nomenclature committee and will be used in this review. 

Boric acid may be similarly reduced to BjH under basic conditions on a large scale. Hydrogen in the presence of Al reduces borates to boranes quantitatively. These bo-ranes are trapped as the amine borane, e.g., phenylborate is dissolved in triethylamine and to the solution is added activated Al powder and small amounts of AICI3 catalyst. The mixture is agitated at 180°C for 1 h at 14.2 MPa of Hj. Quantitative yields of the (CH3)3NBH3 species are available from this process with a lower yield of C5N5N(CH3)2BH3 ... 

The reaction of alkenes with borane, monoalkyl and dialkylboranes leads to a new organoborane (see 15-16). Treatment of organoboranes with alkaline H2O2 oxidizes trialkylboranes to esters of boric acid." This reaction does not affect double or triple bonds, aldehydes, ketones, halides, or nitriles that may be present elsewhere in the molecule. There is no rearrangement of the R group itself, and this reaction is a step in the hydroboration method of converting alkenes to alcohols (15-16). The mechanism has been formulated as involving initial formation of an ate complex when the hydroperoxide anion attacks the electrophilic boron... 

Similar migration of the other two R groups and hydrolysis of the B-0 bonds leads to the alcohol and boric acid. Retention of configuration is observed in R. Boranes can also be oxidized to borates in good yields with oxygen," with sodium perborate NaBOs," " and with trimethylamine oxide, either anhydrous" or in the form of the dibydrate." The reaction with oxygen is free radical in nature." ... 

The oxidation of carbon-boron bond converts boranes into alkyl or aryl borates, which may be hydrolyzed subsequently to alcohols and boric acid [991], The oxidation is carried out with hydrogen peroxide [183,1201, 1202] or trimethyiamine oxide [991, 992]. Phenylboronic acids are oxidized to phenols biochemically [1034]. 

These studies have thus shown that amineboranes (tcrt-butylamineborane and borane-ammonia complexes) are bleaching chemicals more efficient than hydro-sulfite. They are noncorrosive, and their bleaching degradation products are boric acid and an amine. However, their main disadvantage is their high cost, even if their optimal bleaching conditions can be obtained at about 10°C lower than what is used for hydrosulfite. 

Liquid, mp - 123. bp 63° bpM 0. Unstable. When heated or allowed to stand for long periods of time, it produces diborane, tetraborane, hydrogen, pentaborane, deca-borane and brown nonvolatile liqs and solids. Ignites spontaneously in air. Hydrolyzes in water to boric acid and hydrogen. Reacts with aptmonia to form a tetraammoniate. 

CHEMICAL PROPERTIES thermally unstable water reactive hydrolyzes in water to hydrogen and boric acid reacts with ammonia to form diborane diammoniate reacts slowly with bromine to form boron bromides reacts with hydrocarbons or organoboron compounds to give alkyl- or aryl-boron compounds reacts with metal alkyls to form metal borohydrides reacts with strong electron pair donors to form borane addition compounds FP (-90°C, -130°F) LFL/UFL (0.9%, 98%) AT (40-50°C, 104-122°F) HF (35.6 kJ/mol gas at 25°C). 

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