Hydroboration Boron trifluoride

The successful labeling of the elusive 14a-position in cholestane represents a very important application of this reaction.It is known that hydroboration of the double bond in 5of-cholest-14-ene (174) occurs on the a-side. Consequently, by using deuteriodiborane (generated by the reaction of boron trifluoride etherate with lithium aluminum deuteride) and then propionic acid for hydrolysis of the alkylborane intermediate, 14a-d,-5a-cholestane (175) is obtained in 90% isotopic purity. This method also provides a facile route to the C-15 labeled analog (176) when the alkylborane derived from 5a-cholest-14-ene is hydrolyzed with propionic acid-OD. ... 

A 500-ml flask is equipped with a thermometer, a magnetic stirrer, and a dropping funnel, and all openings are protected by drying tubes. The system is flushed with nitrogen and a solution of 2.84 g (0.075 mole) of sodium borohydride in 150 ml of diglyme is introduced followed by 28.3 g (0.30 mole) of norbornene. The flask is immersed in an ice-water bath and the hydroboration is achieved by the dropwise addition of 27.4 ml (0.10 mole) of boron trifluoride diglymate. The solution is stirred... 

The addition of a gas to a reaction mixture (commonly the hydrogen halides, fluorine, chlorine, phosgene, boron trifluoride, carbon dioxide, ammonia, gaseous unsaturated hydrocarbons, ethylene oxide) requires the provision of safety precautions which may not be immediately apparent. Some of these gases may be generated in situ (e.g. diborane in hydroboration reactions), some may be commercially available in cylinders, and some may be generated by chemical or other means (e.g. carbon dioxide, ozone). An individual description of the convenient sources of these gases will be found under Section 4.2. 

This borane reagent may be prepared as follows.6 In a dry 500-ml flask equipped with a condenser, a thermometer and a pressure-equalising dropping funnel are placed 80 ml of diglyme, 23.1 g of 2-methylbut-2-ene (0.33 mol) in 20ml of diglyme and 4.7 g of sodium borohydride (0.125 mol). The flask is immersed in an ice bath and 23.5 g of boron trifluoride-etherate (0.165 mol) is added dropwise over a period of 30 minutes. The semi-solid reaction mixture containing 0.165 mol of disiamylborane is permitted to remain an additional 15 hours at 0-5 °C, and then used for hydroboration. Examples of the use of disiamylborane are described in Sections 5.4.3 p. 542 and 5.7.1, p. 588. 

Diborane can be prepared by a variety of methods, the most common being the reduction of boron trihalides with active metal hydrides - and the reaction of hydroborate salts with boron trifluoride, tin(II) chloride, sulfuric acid, methane-sulfonic acid, orthophosphoric acid, or polyphosphoric acid. Although diborane is commercially available in bulk quantities, we have found the reaction of potassium hydroborate, KBH4, with 85% orthophosphoric acid to be convenient for the rapid preparation in a vacuum line of small quantities of this material. ... 

Reaction with trialkylboranes. Trialkylboranes when treated with silver nitrate in the presence of either sodium hydroxide or potassium hydroxide undergo coupling.20 For example, 1-hexene is hydroborated with sodium borohydride and boron trifluoride in diglyme. Aqueous potassium hydroxide is then added followed by aqueous silver nitrate. The major product is n-dodecane (66%). Other products are 5-methylundecanc (5%) and a mixture of 1-hexene and n-hexane. Yields of coupled products are somewhat lower in the case of internal olefins. Coupling of mixed trialkylboranes is also possible.21... 

The reduction of epoxides withborane is noteworthy as it gives rise to the less substituted alcohol as the major product (7.96), in contrast to reduction with complex hydrides (compare with Scheme 7.71). The reaction is catalysed by small amounts of sodium or lithium borohydride and high yields of the alcohol are obtained. With 1-alkylcycloalkene epoxides, the 2-alkylcycloalkanols produced are entirely cis, and this reaction thus complements the hydroboration-oxidation of cycloalkenes described in Section 5.1, which leads to trans products. Reaction with borane in the presence of boron trifluoride has also been used for the reduction of epoxides and for the conversion of lactones and some esters into ethers. 

Table 5.12 Hydroboration of propenylpyridine-boron trifluoride etherate complex [38]... 

A dry 250-ml flask equipped with a dropping funnel, condenser, and magnetic stirrer is flushed with nitrogen. A solution of 0.78 gm (20.6 mmoles) of sodium borohydride and 20 ml of diglyme is introduced, followed by 4.8 gm (50 mmoles) of 1-methylcyclohexene. The flask is immersed in an ice-water bath and hydroboration is carried out by the dropwise addition of 3.90 gm (27.5 mmoles) of boron trifluoride etherate. The solution is then stirred at room temperature for 3 hr. Then a solution of 6.22 gm (55 mmoles) of hydroxylamine-O-sulfonic acid in 25 ml of diglyme is added, and the solution is heated to 100°C for 3 hr. The reaction mixture is cooled, cautiously treated with 20 ml of concentrated hydrochloric acid, and then poured into 200 ml of water. The acidic phase is extracted with ether to remove diglyme and the residual boronic acid. The aqueous solution is then made strongly alkaline with sodium hydroxide and the amine is extracted with ether. The ether extract is dried over potassium hydroxide and the dried product solution is fractionally distilled. After removal of the ether, 5 gm (45 % of theory) of /ra/w-2-methylcyclohexylamine is isolated, bp 148°C (750 mm). 

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