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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.
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.
Hydroboration of trisubstituted alkenes places boron on the less substituted carbon. [8] Hydroboration of 1,2-disubstituted alkenes, such as a cis or trans olefin, produces generally a mixture of the two organoboranes of comparable amounts, even if the steric properties of the substituents are very different. For such 1,2-disubstituted olefins ...
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).
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 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 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.
In the hydroboration reaction, diborane also reacts readily with alkenes to form trialkylboranes. This reaction pattern is rather general and the resulting alkyl borates can be readily derivatized, e.g. to alcohols. Although early work on hydroboration relied on diborane, it has been replaced by borane dimethylsulfide, which is more safely handled.