Hydrogen peroxide, reaction with organoboranes

Oxidation. The oxidation reactions of organoboranes have been reviewed (5,7,215). Hydroboration—oxidation is an anti-Markovnikov cis-hydration of carbon—carbon multiple bonds. The standard oxidation procedure employs 30% hydrogen peroxide and 3 M sodium hydroxide. The reaction proceeds with retention of configuration (216). 

In addition to the oxymercuration method, which yields the Markovnikov product, a complementary method that yields the non-Markovnikov product is also useful. Discovered in 1959 by H. C. Brown and cailed hydroboration, the reaction involves addition of a B-H bond of borane, BH3, to an alkene to yield an organoborane intermediate, RBH2. Oxidation of the organoborane by reaction with basic hydrogen peroxide, H2O2, then gives an alcohol. For example ... 

To a stirred suspension of diisopinocampheylborane (50 mmol) (1) in tetra-hydrofuran (18 ml) is added 4.5 ml of (Z)-but-2-ene. The reaction mixture is stirred at 25 °C for 4.5 hours. The solid diisopinocampheylborane disappears and the formation of the trialkylborane is complete. The organoborane is treated with 4 ml of methanol, followed by 18.3 ml of 3 m sodium hydroxide and the careful addition of 20 ml of 30 per cent hydrogen peroxide, maintaining the temperature of the reaction below 40 °C. The reaction mixture is further stirred at 55 °C for 1 hour, cooled, and extracted with ether (3 x 50 ml). The extract is washed successively with water (2 x 25 ml) and brine (3 ml) and dried over magnesium sulphate. The organic layer is carefully fractionated to provide butan-2-ol, b.p. 96-98 °C, 2.9 g (73%), purity > 95 per cent. The last traces of impurities are removed by preparative g.l.c. (2) to yield (R)-butan-2-ol, [a] 3 —13.23° (neat), ee 98.1 per cent. 

Organoboranes may be oxidized by hydrogen peroxide in an acidic medium. The mechanism of oxidation of alkyldihydroxyboranes in such conditions has been studied and the protonated transition state (1) proposed, as compared with transition state (2) for the reaction in an alkaline medium. ... 

Though oxidation with acidic hydrogen peroxide is rarely used, alkyl- and aryl-boronic acids are readily attacked with a migratory order of Bu PhCH2 > Bu > Bu" > Ph > vinyl Me. It tqqiears, therefore, that the reaction might be useful for selective oxidations as well as for oxidations of base lal e organoboranes. ... 

The synthetic value of the reaction lies in the modification of these organoboranes. The commonest reaction involves the decomposition of the borane by alkaline hydrogen peroxide. The highly nucleophilic hydroperoxide anion attacks the electron-deficient boron with the formation of an ate complex. Rearrangement of this leads to the formation of a borate ester which then undergoes hydrolysis to an alcohol in which an oxygen atom has replaced the boron (Scheme 3.15). The overall outcome of this reaction is the anti-Markownikoff hydration of the double bond. The regiochemistry is the reverse of the acid-catalysed hydration of an alkene. The overall addition of water takes place in a cis manner on the less-hindered face of the double bond. 

The reduction is bimolecular and thus the rate is dependent on concentration. Running the reaction neat provides the fastest rates. Usually an excess of Alpine-Borane is used to insure that the reaction does not become excessively slow at the end of the reduction. The excess organoborane may be destroyed by addition of an aldehyde such as Acetaldehyde. The resulting alkoxy-9-BBN may be treated with Ethanolamine to liberate the alcohol and precipitate the majority of the 9-BBN. Any remaining borane impurities may be removed by oxidation with basic Hydrogen Peroxide. 

TriaikykarUnols from trialkylhoraites. Trialkylcarbinols have been prepared by the reaction of trialkylboranes with carbon monoxide in diglyme followed by oxidation with hydrogen peroxide (equation 1). See 2. 60. The method has the disadvantage that at the temperature required isomerization of organoboranes can be significant. 

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