Hydroboration of Alkenes
Description: Hydroboration-oxidation transforms alkenes into alcohols. It performs the net addition of water across an alkene.
Notes: Note that the oxygen is always attached at the less substituted carbon (anti-Markovnikoff). Furthermore the stereochemistry is always syn (H and OH add to same side of the alkene).
The boron byproduct will depend on the # of equivalents of BH3 used reative to the alkene. Here their molar ratio is 1:1. One equivalent of BH3 can hydroborate up to 3 equivalents of alkene.
BH3-THF is the same as BH3. Tetrahydrofuran (THF) is merely a solvent. Sometimes B2H6 is written, which is another form of BH3. It behaves in exactly the same way as BH3.
Notes: Example 1 just shows a simple anti-Markovnikov addition of BH3 to an alkene. Examples 2 and 3 show the hydroboration of a cyclic alkene. Note the syn addition; the C-H bond and C-OH bond are formed on the same side of the ring (this results in a mixture of enantiomers in this case). Example 4 has an existing chiral center (the methyl group) which is unaffected by hydroboration, so the result will be a mixture of diastereomers.
The reaction begins with the concerted syn addition of B and H across the double bond, with the boron adding to the less substituted carbon (Step 1, arrows A and B). In the second step, hydrogen peroxide and a base such as NaOH are added. the NaOH deprotonates the hydrogen peroxide (Step 2, arrows C and D) which makes the conjugate base of hydrogen peroxide (a better nucleophile than H2O2 itself). The resulting NaOOH then attacks the boron (Step 3, arrow E). This sets up the key migration step, where the carbon-boron bond migrates to the oxygen bound to boron, breaking the weak oxygen-oxygen bond (Step 4, arrows F and G). The OH expelled then comes back to form a bond on the boron (Step 5, arrows H and I) resulting in the deprotonated alcohol (alkoxide). The alkoxide is then protonated by water or some other comparably acidic species (Step 6, arrows J and K).
Notes: In step 1 the addition is syn and the reaction is concerted.
If excess alkene is present the two remaining B-H bonds can do subsequent hydroborations.
Note that only one enantiomer is shown here (but the product will be racemic)
The migration step (Step 4, arrows F and G) occurs with retention of stereochemistry at the carbon.
For the last step it’s reasonable to use water, HOOH or any other comparably acidic species as the acid.
(Advanced) References and Further Reading
- First example
CONVENIENT NEW PROCEDURES FOR THE HYDROBORATION OF OLEFINS
Herbert C. Brown and George Zweifel
Journal of the American Chemical Society 1959 81 (15), 4106-4107
Early paper by Nobel Laureate H. C. Brown, describing variants of the classic hydroboration reaction.
- Mechanistic studies
XXIV. Directive Effects in the Hydroboration of Some Substituted Styrenes
Herbert C. Brown and Richard L. Sharp
Journal of the American Chemical Society 1966 88 (24), 5851-5854
A very nice Physical Organic study of the hydroboration of styrenes, involving a Hammett plot (a classic tool in physical organic chemistry) to determine a relationship between the stereochemistry of the reaction and the electron density of the alkene.
- References from Organic Syntheses
A SIMPLE AND CONVENIENT METHOD FOR THE OXIDATION OF ORGANOBORANES USING SODIUM PERBORATE: (+)-ISOPINOCAMPHEOL
George W. Kabalka, John T. Maddox, Timothy Shoup, and Karla R. Bowers
Organic Syntheses, Coll. Vol. 9, p.522 (1998); Vol. 73, p.116 (1996)
DOI : 15227/orgsyn.073.0116
A Hydroboration-oxidation procedure where the oxidizing agent is sodium perborate, a cheap, safe, easily handled oxidant commonly used in laundry detergents.
- Modern Example
Hydroboration with Pyridine Borane at Room Temperature
Julia M. Clay and and Edwin Vedejs
Journal of the American Chemical Society 2005 127 (16), 5766-5767
A modern method for doing hydroboration at room temperature using pyridine-borane, which usually requires heating to 75-100 °C to liberate the borane.