Hydration of Alkenes
Hydroboration of Alkenes
- Hydroboration, defined as the addition of borane (BH3) to an alkene, is an alternative to oxymercuration if the opposite regioselectivity is desired. The reaction sequence of hydroboration, oxidation and hydrolysis yields an alcohol with anti-Markovnikov orientation.
BH3 is an unstable reagent because the boron atom has only an electron sextet. Therefore, it dimerizes, particularly in the gas phase, to the flammable toxic gas diborane B2H6. Though the structure of diborane could be considered analogous to that of ethane, this is not the case. Formally, dimerization allows both boron atoms in diborane to obtain an electron octet. However, six hydrogen and two boron atoms only provide 12 valence electrons as compared to ethane with 14 valence electrons.
A solution of diborane in ether forms an ether-borane complex stable enough even for distillation. The solution of this complex in tetrahydrofuran (THF) is the most frequently used hydroborating reagent.
It could be assumed that the strong Lewis acid BH3 forms an intermediate carbenium ion by adding to an alkene analogous to the addition of the Brønsted acid H3O+ to an alkene. Experimentally, however, only syn additions are observed in hydroborations. Therefore, unlike the formation of a carbenium intermediate during the addition of hydrogen halides to alkenes, a similar mechanism for hydroborations can be excluded. Otherwise, syn as well as anti products would be formed and rearrangement products would be observed. Based on these results, hydroboration obviously proceeds in a concerted mechanism via a four-membered cyclic transition state.
Regioselectivity of hydroboration
Potentially, two regiochemically different products could be formed by hydroboration of an asymmetrically substituted (alkylated) alkene.
Boron preferentially adds to the lower-substituted carbon atom of the double bond to avoid stronger steric hindrance between borane and the alkyl substituents at the higher-substituted carbon atom. Additionally, the high regioselectivity of hydroboration can be explained by looking at the mechanism. It starts with the interaction between the π electron pair of the alkene (HOMO) and boron which is the most electron-deficient atom of borane (LUMO). Though being a concerted reaction, in the transition state the formation of the carbon-boron bond is already more advanced than the formation of the new carbon-hydrogen bond as schematically shown below by a shorter carbon-boron bond. A carbocation intermediate is not formed. The transition state with boron at the lower-substituted carbon atom is energetically favored because the partial positive charge located at the higher-substituted carbon atom is more stabilized by hyperconjugation.
The overall reaction sequence of hydroboration, oxidation and hydrolysis yields an alcohol with anti-Markovnikov orientation. While the initial hydroboration step is a Markovnikov addition with the partial positive charge located at the higher-substituted carbon in the transition state, the subsequent steps of oxidation and hydrolysis yield an alcohol with anti-Markovnikov orientation.
Since the boron atom of the hydroboration product, a monoalkylborane, still carries two hydrogen atoms, additional hydroborations can occur. The final product of a triple hydroboration is a trialkylborane. The second and/or third hydroboration can be prevented if steric hindrance of the alkyl substituents at the boron atom is large enough, .
Trialkylborane, the product of triple hydroboration, can be used as starting material for various reactions which were extensively investigated by H.C. Brown, honored for his work with the Nobel Prize in 1979 .