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Hydroboration–oxidation is an anti-Markovnikov reaction, with the hydroxyl group attaching to the less-substituted carbon. The reaction thus provides a more stereospecific and complementary regiochemical alternative to other hydration reactions such as acid-catalyzed addition and the oxymercuration–reduction process.
The stoichiometry and idealized regiochemistry of hydroboration of terminal alkenes follows: BH 3 + 3 RCH=CH 2 → B(CH 2 −CH 2 R) 3. In reality, each hydroboration step follows 1,2-addition but ca. 4% gives the 2,1 addition (affording the B(CH(CH3)R isomer). [1] In extreme cases, such as risubstituted alkenes, hydroboration affords.
One example is thexylborane (ThxBH 2), produced by the hydroboration of tetramethylethylene: [6] A chiral example is monoisopinocampheylborane. Although often written as IpcBH 2, it is a dimer [IpcBH 2] 2. It is obtained by hydroboration of (−)‐α‐pinene with borane dimethyl sulfide. [7] Dialkylboranes are also rare for small alkyl groups.
Catalyzed hydroboration-oxidation of substituted alkenes can be rendered enantioselective. In 1990, Brown and co-workers achieved asymmetric hydroboration using an achiral catalyst and chiral borane sources derived from ephedrine and pseudoephedrine. In most cases, the regioselectivity was poor although the ee values can be close to 90%. [16]
In practice, the mercury adduct product created by the oxymercuration reaction is almost always treated with sodium borohydride (NaBH 4) in aqueous base in a reaction called demercuration. In demercuration, the acetoxymercury group is replaced with a hydrogen in a stereochemically insensitive reaction [ 6 ] known as reductive elimination.
In hydroboration-oxidation, the OH group adds to the less-substituted carbon in the double bond. While researching these reducing agents, Brown's coworker, Dr. B. C. Subba Rao, discovered an unusual reaction between sodium borohydride and ethyl oleate. The borohydride added hydrogen and boron to the carbon-carbon double bond in the ethyl oleate.
The general chemical equation for the hydration of alkenes is the following: . RRC=CH 2 + H 2 O → RRC(OH)-CH 3. A hydroxyl group (OH −) attaches to one carbon of the double bond, and a proton (H +) adds to the other.
The complex is commercially available but can also be generated by the dissolution of diborane in THF. Alternatively, it can be prepared by the oxidation of sodium borohydride with iodine in THF. [2] The complex can reduce carboxylic acids to alcohols and is a common route for the reduction of amino acids to amino alcohols [3] (e.g. valinol).