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Formally, the reaction is an example of a group transfer reaction. However, an analysis of the orbitals involved reveals that the reaction is 'pseudopericyclic' and not subject to the Woodward–Hoffmann rules for pericyclic reactivity. Hydroboration of a terminal alkene to a trialkylborane, showing idealized image of the cyclic transition state.
Due to the experimental ease of its use, BMS has become common in hydroboration reactions. [5] In hydroborations with BMS, the dimethylsulfide dissociates in situ, liberating diborane, which rapidly adds to the unsaturated bonds. The resulting organoborane compounds are useful intermediates in organic synthesis.
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 rhodium-catalyzed hydroboration reaction is thought to be initiated with the dissociation of a triphenylphosphine from the Rh(I) centre. Oxidative addition of the B-H bond of the borane reagent to this 14 e − species is then followed by coordination of the alkene to the 16e − Rh(III) hydride complex.
Disiamylborane is prepared by hydroboration of trimethylethylene with diborane. [1] The reaction stops at the secondary borane due to steric hindrance. Disiamylborane is relatively selective for terminal alkynes and alkenes vs internal alkynes and alkenes. Like most hydroboration, the addition proceeds in an anti-Markovnikov manner. [1]
These solutions, which are colorless, are used for reductions and hydroboration, reactions that are useful in synthesis of organic compounds. A common alternative to BHF•THF is borane–dimethylsulfide, which has a longer shelf life and effects similar transformations. [1]
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.
Diisopinocampheylborane was originally prepared by hydroboration of excess α-pinene with borane, [2] but it is now more commonly generated from borane-methyl sulfide (BMS). [3] The compound can be isolated as a solid, but because it is quite sensitive to water and air, it is often generated in situ and used as a solution.