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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.
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.
Regiospecific hydroboration with borane Hydroboration-oxidation of (E)-prop-1-en-1-ylbenzene Hydroboration-oxidation of 1-methyl-cyclohex-1-ene The net reaction is hydration. Because the addition of H-B to olefins is stereospecific, this oxidation reaction will be diastereoselective when the alkene is trisubstituted. [ 10 ]
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]
Many alternative routes are available for producing alcohols, including the hydroboration–oxidation reaction, the oxymercuration–reduction reaction, the Mukaiyama hydration, the reduction of ketones and aldehydes and as a biological method fermentation.
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The concerted mechanism of this step is similar to the mechanisms of the Baeyer–Villiger oxidation [6] and Criegee rearrangement reactions, and also the oxidation step of the hydroboration–oxidation process. [7] In 2009, an acidified bentonite clay was proven to be a more economical catalyst than sulfuric acid as the acid medium.