Sodium borohydride hydroboration

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... 

Acetyl-5//-dibenz[/>,/]azepine-10-car bon itrile (17, R = CN) when treated with sodium borohydride undergoes reduction (73 % yield) at the CIO - Cl 1 double bond without reduction of the acetyl or cyano groups.212 However, hydroboration of 5-acetyl-5//-dibenz[/y/]azepine (17, R = H) with diborane in tetrahydrofuran under standard conditions is accompanied by reduction of the acyl function to yield 5-ethyi-10,l l-dihydrodibenz[6,/]azepin-10-ol (18).72... 

The search for solvents led to the discovery that sodium borohydride is an excellent reducing agent for aldehydes and ketones. The search for catalysts to enhance the reducing power of sodium borohydride led to an anomalous result in the reduction of ethyl oleate. Investigation of this anomalous result led to the discovery of hydroboration. 

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. 

Reduction of tertiary kaiides. Tertiary halides are reduced to hydrocarbons by. sodium borohydride in tetramethyicne sulfone (sulfolane, 1, 1144-1145 2, 402 -403). The reaction proceeds by way of elimination, hydroboration, and protonolysis. Halides... 

Sodium borohydride and trifluoroacetic acid (TFA) reduce 2-aryl-l,3-dioxoIanes to hydroxy ethers. Zinc borohydride, with TMS-Cl as its acidic partner, has proven to be effective in the reductive cleavage of a variety of acetals. MOM ethers are reduced to methyl ethers in good yield (equation 13). Cyclohexene is hydroborated under these conditions, indicating that diborane may be present. 

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... 

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