Hydroboration-oxidation reaction

In organic chemistry, the hydroboration-oxidation reaction is a two-step organic chemical reaction that converts an alkene into a neutral alcohol by the net addition of water across the double bond. [cite book
author = Loudon, Marc G.
year = 2002
title = Organic Chemistry
chapter = Addition Reactions of Alkenes.
edition = Fourth Edition
pages = 168-172
publisher = Oxford University Press
location = New York
id = ISBN 0-19-511999-1] [Organic Chemistry Help. Hydroboration of alkenes. [http://www.chemhelper.com/hydroboration.html http://www.chemhelper.com/hydroboration.html] ] [Organic Chemistry Portal. Hydroboration (including recent literature). [http://www.organic-chemistry.org/namedreactions/brown-hydroboration.shtm http://www.organic-chemistry.org/namedreactions/brown-hydroboration.shtm] ] 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 general form of the reaction is as follows:

where THF is tetrahydrofuran, the archetypal solvent used for this reaction. In the first step, borane (BH₃) adds to the double bond, transferring one hydrogen from itself to the adjacent carbon. The second step substitutes the boron group BH₂ with the hydroxyl group, creating the final product.

Hydroboration mechanism

Borane exists as a toxic, colorless gas called diborane (B₂H₆). In diborane, two hydrogen atoms are each bonded to both boron atoms by single pairs of electrons ("three-center two-electron bonds"). This delocalization satisfies the octet around each boron and reduces the electrophilicity. That said, even diborane is intensely Lewis acidic, because of its vacant p orbitals. Because dimerization happens instantaneously, it is not possible to isolate pure borane. However, when diborane is treated with an ether or amine, a stable complex is formed, as the lone pair from the Lewis basic oxygen or nitrogen atom is donated to the borane. These complexes act chemically like borane. Solutions of BH₃ complexes in THF or diethyl ether are commercially available and more easily handled than diborane gas, and so are the more common form found in laboratories. For simplicity in illustration, borane will be used instead of the borane-ether complex in this article.

The addition of BH₃ to the alkene is a concerted reaction, with multiple bond formation and breaking occurring simultaneously. The intermediate step can be visualized more clearly by a theoretical transition state.

Knowing that the group containing the boron will be replaced by a hydroxyl group, it can be seen that the first step is the stereospecific-determining step. The hydroborane will add to the alkene so that the boron always ends up on the lesser substituted carbon. In the transition state, the more substituted carbon bears a partial positive charge (a partial carbocation). As a general rule, carbocations that are more substituted tolerate positive charge better than those that aren't. Had the hydroborane attacked with the opposite orientation, the lesser substituted carbon will bear the positive charge, which is electronically unfavorable.

Until all hydrogens attached to boron have been transferred away, the boron group BH₂ will continue adding to more alkenes. This means that one equivalent of hydroborane will conduct the reaction with three equivalents of alkene. Furthermore, it is not necessary for the hydroborane to have more than one hydrogen. Therefore, BH₃ can be better represented as R-BH, where R can represents the remainder of the molecule. A widely used hydroboration reagent is 9-BBN which has just one hydrogen at boron and the same applies for catecholborane.

Hydroborations also take place stereoselective in a syn mode, that is on the same face of the alkene. Thus 1-methylcyclopentene reacts with diborane predominantly to the trans-alkane ["Hydroboration. IX. The Hydroboration of Cyclic and Bicyclic Olefins-Stereochemistry of the Hydroboration Reaction " Herbert C. Brown, George Zweifel; J. Am. Chem. Soc.; 1961; 83(11); 2544-2551. DOI|10.1021/ja01472a027] .

Hydroboration-oxidation

In the second hydroboration-oxidation step, 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 monoalkly borinic ester BH₂OR).

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. ["Catecholborane (1,3,2-benzodioxaorole) as a new, general monohydroboration reagent for alkynes. Convenient synthesis of alkeneboronic esters and acids from alkynes via hydroboration" Herbert C. Brown, S. K. Gupta; J. Am. Chem. Soc.; 1972; 94(12); 4370-4371. DOI|10.1021/ja00767a072 ] Amines can be obtained by action of chloramine. [ "The Reaction of Organoboranes with Chloramine and with Hydroxylamine-O-sulfonic Acid. A Convenient Synthesis of Amines from Olefins via Hydroboration"Herbert C. Brown, Wolfgang R. Heydkamp, Eli Breuer, and William S. Murphy J. Am. Chem. Soc.; 1964; 86(17) pp 3565 - 3566; DOI|10.1021/ja01071a036] Reaction with iodine or bromine afford the corresponding alkyl halides. A carboxylic acid simply replaces the borane group by a proton.

ee also

* Oxymercuration-reduction is another reaction that converts an alkene into an alcohol. Unlike 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.

References