Hydroboration–oxidation reaction

Source: Wikipedia, the free encyclopedia.
(Redirected from
Hydroboration-oxidation
)

Hydroboration–oxidation reaction is a two-step

regiochemical alternative to other hydration reactions such as acid-catalyzed addition and the oxymercuration–reduction process. The reaction was first reported by Herbert C. Brown in the late 1950s[2] and it was recognized in his receiving the Nobel Prize in Chemistry
in 1979.

The general form of the reaction is as follows:

Tetrahydrofuran (THF) is the archetypal solvent used for hydroboration.

Mechanism and scope

Hydroboration step

Main article: hydroboration

In the first step,

H.C. Brown in 1957 for the conversion of 1-hexene into 1-hexanol.[3]

Hexanol synthesis

Knowing that the group containing the boron will be replaced by a hydroxyl group, it can be seen that the initial hydroboration step determines the regioselectivity. Hydroboration proceeds in an

syn addition (on the same face of the alkene): the hydroboration is syn-selective and the oxidation replaces the boron with hydroxyl having the same geometric position. Thus 1-methylcyclopentene reacts with diborane predominantly to give trans-1-hydroxy-2-methylcyclopentane[4]
—the newly added H and OH are cis to each other.

Until all hydrogens attached to boron have been transferred away, the boron group BH2 will continue adding to more alkenes. This means that one mole of hydroborane will undergo the reaction with three moles of alkene. Furthermore, it is not necessary for the hydroborane to have more than one hydrogen. For example, reagents of the type R2BH are commonly used, where R can represents the remainder of the molecule. Such modified hydroboration reagents include 9-BBN, catecholborane, and disiamylborane.

Oxidation step

In the second step of the reaction sequence, the nucleophilic

anion attacks the boron atom. Alkyl migration to oxygen gives the alkyl borane with retention of stereochemistry (in reality, the reaction occurs via the trialkyl borate
B(OR)3, rather than the monoalkyl borinic ester BH2OR).

Hydroboration–oxidation mechanism

The 'H' atom in the reaction comes from B2H6, the 'O' atom comes from hydrogen peroxide (H2O2) whereas the O attached 'H' atom comes from the solvent (refer mechanism).

Alkyne hydroboration

A hydroboration reaction also takes place on alkynes. Again the mode of action is syn and secondary reaction products are aldehydes from terminal alkynes and ketones from internal alkynes. In order to prevent hydroboration across both the pi-bonds, a bulky borane like disiamyl (di-sec-iso-amyl) borane is used.[5]

Hydroboration–oxidation of terminal alkyne

Alternative oxidations

Use of other oxidants instead of hydrogen peroxide can lead to carbonyl products rather than alcohols from alkenes.

dichromates or related chromium(VI) reagents give ketones as well, but give carboxylic acids instead of aldehydes for terminal alkenes.[7]

References

  1. ISBN 0-19-511999-1
    .
  2. doi:10.1021/ja01510a059
    .
  3. doi:10.1021/jo01360a626
    .
  4. doi:10.1021/ja01472a027
    .
  5. doi:10.1021/ja00767a072
    .
  6. doi:10.1016/S0040-4039(97)00476-0
    .
  7. doi:10.1021/jo00049a024
    .

External links
Retrieved from "https://en.wikipedia.org/w/index.php?title=Hydroboration–oxidation_reaction&oldid=1210662237"