Methyl borate

P-Hydroxy acids lose water, especially in the presence of an acid catalyst, to give a,P-unsaturated acids, and frequendy P,y-unsaturated acids. P-Hydroxy acids do not form lactones readily because of the difficulty of four-membered ring formation. The simplest P-lactone, P-propiolactone, can be made from ketene and formaldehyde in the presence of methyl borate but not from P-hydroxypropionic acid. P-Propiolactone [57-57-8] is a usehil intermediate for organic synthesis but caution should be exercised when handling this lactone because it is a known carcinogen. 

The general formula for boric acid esters is B(OR)2. The lower molecular weight esters such as methyl, ethyl, and phenyl are most commonly referred to as methyl borate [121 -43-7] ethyl borate [130-46-9J, and phenyl borate [1095-03-0] respectively. Some of the most common boric acid esters used in industrial appHcations are Hsted in Table 1. The nomenclature in the boric acid ester series can be confusing. The lUPAC committee on boron chemistry has suggested using trialkoxy- and triaryloxyboranes (5) for compounds usually referred to as boric acid esters, trialkyl (or aryl) borates, trialkyl (or aryl) orthoborates, alkyl (or aryl) borates, alkyl (or aryl) orthoborates, and in the older Hterature as boron alkoxides and aryloxides. CycHc boric acid esters, which are trimeric derivatives of metaboric acid (HBO2), are known as boroxines (1). 

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 equihbrium shown in equation 3 normally ties far to the left. Usually the water formed is removed by azeotropic distillation with excess alcohol or a suitable azeotroping solvent such as benzene, toluene, or various petroleum distillate fractions. The procedure used depends on the specific ester desired. Preparation of methyl borate and ethyl borate is compHcated by the formation of low boiling azeotropes (Table 1) which are the lowest boiling constituents in these systems. Consequently, the ester—alcohol azeotrope must be prepared and then separated in another step. Some of the methods that have been used to separate methyl borate from the azeotrope are extraction with sulfuric acid and distillation of the enriched phase (18), treatment with calcium chloride or lithium chloride (19,20), washing with a hydrocarbon and distillation (21), fractional distillation at 709 kPa (7 atmospheres) (22), and addition of a third component that will form a low boiling methanol azeotrope (23). 

From Boric Oxide and Alcohol. To avoid removing water, boric oxide, B2O3, can be used in place of boric acid. The water of reaction (eq. 4) is consumed by the oxide (eq. 5). Because boric acid reacts with borates at high temperatures, it is necessary to filter the reaction mixture prior to distillation of the product. Only 50% of the boron can be converted to ester by this method. In cases where this loss can be tolerated, the boric oxide method is convenient. This is particularly tme for methyl borate and ethyl borate preparation because formation of the undesirable azeotrope is avoided. 

Methyl borate is beheved to be the boric acid ester produced in the largest quantity, approximately 8600 metric tons per year (28). Most methyl borate is produced by Morton International and used captively to manufacture sodium borohydride [16940-66-2]. Methyl borate production was studied in detail during the 1950s and 1960s when this compound was proposed as a key intermediate for production of high energy fuels. Methyl borate is sold as either the pure compound or as the methanol azeotrope that consists of approximately a 1 1 molar ratio of methanol to methyl borate. 

The toxicity of a few boric acid esters has been summarized (30). In general the toxicities are directiy related to the toxicity of the alcohol or phenol produced on hydrolysis. Methyl borate has an oral rat LD q of 6.14 mL/kg in a range finding test (31) and the percutaneous LD q for the rabbit of 1.98 mL/kg. In eadier work (32), the oral LD q for the rat was 2.82 mL/kg the intraperitoneal LD q was 3.2 mL/kg. It has been shown that the mouse is more susceptible to these compounds than the rat. Methyl borate was found to be moderately irritating in an ocular toxicity test using rabbits (31,32) but only mildly irritating to skin (31). 

Production of Sodium Borohydride. In the pulp and paper industry, sodium borohydride is used to generate sodium hydrosulfite (sodium dithionite), a bleaching agent, from sodium bisulfite. Methyl borate is used as an intermediate in the production of sodium borohydride (33). 

Gas Fluxing. The methyl borate azeotrope is used as a gaseous flux for welding and brazing. The Gas Flux Co., Elyria, Ohio, manufactures the methyl borate azeotrope for their own use. The azeotrope acts as a volatile source of boric oxide and is introduced directly into the gas stream as a flux for the surfaces to be joined in the welding process. The European automobile industry is the primary user of this process, though there may be some usage for this purpose in the United States. 

Methyl Borate (Trimethyl borate or Trimethoxy borine), (CH3.0)3B, mw 103.92, OB to C02 — 115.47, colorl liq, mp —29°, bp68°,d 0.92 g/cc at 20° vap d 3.59. Prepn other properties are given in Beil (Ref l). Sax (Ref 2) lists this compd as a dangerous fire hazard when exposed to heat or flame. It reacts with w or steam to produce toxic flammable vapors, and vigorously with oxid materials Refs 1) Beil 1, 287, (143) [275] 2) Sax... 

Fortunately, the Signal Corps was persuaded to support research to find improved methods of preparing sodium borohydride and catalysts to facilitate its solvolysis. We soon discovered that sodium borohydride could readily be prepared by treating sodium hydride with methyl borate at 250° C. 

Riordan and co-workers have examined zinc complexes of pyrazolyl-bis[(methylthio)methyl]-borate ligands as models for methionine synthase.547, 69 The ligand coordinates in a face-capping fashion with the desired NS2 donor set. 

Methanol, ethanol and 3-methylbutanol [1], acetaldehyde, trichloroacetaldehyde [2] and acetone [3] all ignite in contact with gaseous fluorine. Lactic acid, benzoic acid and salicylic acid ignite, while gallic acid becomes incandescent. Ethyl acetate and methyl borate ignite in fluorine [2],... 

Furan-2-boronic acid (89) and furan-3-boronic acid are readily prepared by interaction of the furyllithium with methyl borate (MeO)3B followed by acid hydrolysis.233 234 Like most boronic acids they owe their acidity more to coordination with a water molecule than to simple proton transfer they... 

Sodium borohydride was discovered by H I. Schlesinger and H.C. Brown in 1943 and was prepared by heating methyl borate with sodium hydride. 

Boron (B/W core) W2B5, WB4 (inner core), B (outer core) B2O3 as methyl borate B-OH, B O B... 

Also, potassium borohydride can be made by reacting potassium hydride with methyl borate at high temperature ... 

Anaiysis. If a borate is treated with H2SO4 and methanol H3COH, methyl borate is produced. If a small amount of the mixture is introduced to a flame, a green flame color is produced. The test is sensitive to 20 ppm of B. Ba and Cu interfere, both of these also giving a green color. A spot test with curcumin will detect 5 ppm of B. Titration of H3BO3 in a mannitol solution... 

Three hundred thirty-six milliliters (312 g., 3.00 moles) of methyl borate is distilled directly into the 500-ml. dropping funnel shortly before starting the reaction (Note 4). One liter (511 g., 3.0 moles) of a 34/ ethereal solution of phenylmagnesium bromide is pressure-transferred with dry nitrogen into the 1-1. 

Methyl borate (b.p. 68°) forms a 1 1 azeotrope (b.p. 54.6°) with methanol (b.p. 64°).5 Since the presence of even a small amount of methanol reduces the yield considerably more than would be expected from the stoichiometry,4 6 methyl borate stocks should be freshly distilled through a good column to remove as fore-run any methyl borate-methanol azeotrope which may have been formed by hydrolysis during storage. 

Mallinckrodt analytical reagent grade ether, dried over sodium, was used. The methyl borate was the commercial product of American Potash and Chemical Corporation containing 99% ester as received. The phenylmagnesium bromide was purchased as a 3.0M solution in ether from Arapahoe Special Products, Inc., Boulder, Colorado. 

POLY[(METHYLTHIO)METHYL]BORATES AND REPRESENTATIVE METAL DERIVATIVES... 

Poly[(methylthio)methyl]borates and Representative Metal Derivatives 109... 

---