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Hydroboration–oxidation reaction

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Hydroboration–oxidation reaction

In organic chemistry, the hydroboration–oxidation reaction is a two-step organic reaction that converts an alkene into a neutral alcohol by the net addition of water across the double bond. The hydrogen and hydroxyl group are added in a syn addition leading to cis stereochemistry. Hydroboration–oxidation is an anti-Markovnikov reaction, with the hydroxyl group attaching to the less-substituted carbon. The reaction was first reported by Herbert C. Brown in the late 1950s and it was recognized in his receiving the Nobel Prize in Chemistry in 1979.

Contents

The general form of the reaction is as follows:

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

Hydroboration step

In the first step, borane (BH3) adds to the double bond, transferring one of the hydrogen atoms to the carbon adjacent to the one that becomes bonded to the boron. This hydroboration is repeated two additional times, successively reacting each B–H bond so that three alkenes add to each BH3. The resulting trialkylborane is treated with hydrogen peroxide in the second step. This process replaces the B-C bonds with HO-C bonds. The boron reagent is converted to boric acid. The reaction was originally described by H.C. Brown in 1957 for the conversion of 1-hexene into 1-hexanol.

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 antimarkovnikov manner. The reaction sequence is also stereoselective, giving 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-methylcyclpentane—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 hydroperoxide 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).

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.

Oxymercuration

Oxymercuration–reduction is another reaction that converts an alkene into an alcohol. hydroboration–oxidation, oxymercuration-reduction is not stereospecific. Further, oxymercuration–reduction is a Markovnikov reaction. Therefore, oxymercuration–reduction and hydroboration–oxidation are complementary because they add with opposite regiochemistry.

Other boron-mediated C-X bond forming reactions

Amines can be obtained from the intermediate organoborane by action of chloramine. Reaction with iodine or bromine afford the corresponding alkyl halides. A carboxylic acid simply replaces the borane group by a proton.

References

Hydroboration–oxidation reaction Wikipedia


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