Ch_322a_11.09 - Alcohols through Oxidation of Alkylboranes...

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Alcohols through Oxidation of Alkylboranes Reaction of an alkylborane with hydrogen peroxide (H 2 O 2 ) and base (NaOH) leads to replacement of the borane group with a hydroxyl group. CH 3 CH 2 CH 2 -B H H (R) (R) H 2 O 2 NaOH, H 2 O CH 3 CH 2 CH 2 OH + Na 3 BO 3 sodium borate The sequence of hydroboration-oxidation of an alkene yields an alcohol with anti-Markovnikov orientation . CH 3 CH=CH 2 CH 3 CH 2 CH 2 OH anti-Markovnikov product
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Stereochemistry of the Oxidation Reaction The hydroxyl group replaces the borane group with retention . CH 3 H H BH 2 H 2 O 2 NaOH, H 2 O retention CH 3 H H OH overall syn-addition to the alkene
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Mechanism of the Oxidation Reaction In the aqueous basic solution used, hydrogen peroxide (pK a = 11.6) is converted into its anion: HOOH + HO - HOO - + HOH peroxy anion The peroxy anion adds to the electron-deficient alkylborane producing an unstable peroxyborate intermediate: C B * alkylborane + - OOH C B * OOH - peroxyborate unstable
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A Molecular Rearrangement The peroxyborate intermediate reacts by
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