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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.
One example of an isolable (bulky) primary borane is thexylborane (ThxBH 2), produced by the hydroboration of tetramethylethylene: [6] A chiral example is monoisopinocampheylborane, obtained by hydroboration of (−)‐α‐pinene with borane dimethyl sulfide. Although often written as IpcBH 2, it is a dimer, [IpcBH 2] 2. [7]
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.
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]
9-Borabicyclo[3.3.1]nonane or 9-BBN is an organoborane compound. This colourless solid is used in organic chemistry as a hydroboration reagent.The compound exists as a hydride-bridged dimer, which easily cleaves in the presence of reducible substrates.
Alkene hydroboration-oxidation: Stereospecific: Can only be syn addition – hydrogen and hydroxyl (-OH) are added to the same face. The reaction is anti-Markovnikov. Hydroxyl attaches to the less substituted carbon. Halogenation: Stereospecific: Can only be anti-addition – both halogen molecules are on different planes.
Wilkinson's catalyst is usually obtained by treating rhodium(III) chloride hydrate with an excess of triphenylphosphine in refluxing ethanol. [9] [10] [1] Triphenylphosphine serves as both a ligand and a two-electron reducing agent that oxidizes itself from oxidation state (III) to (V).
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).