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Alcohols from Hydroboration-Oxidation of Alkenes

Hydroboration-Oxidation is a two step pathway used to produce alcohols. The reaction proceeds in an Anti-Markovnikov manner, where the hydrogen (from BH3 or BHR2) attaches to the more substituted carbon and the boron attaches to the least substituted carbon in the alkene bouble bond. Furthermore, the borane acts as a lewis acid by accepting two electrons in its empty p orbital from an alkene that is electron rich. This process allows boron to have an electron octet.  A very interesting characteristic of this process is that it does not require any activation by a catalyst. The Hydroboration mechanism has the elements of both hydrogenation and electrophilic addition and it is a stereospecific (syn addition), meaning that the hydroboration takes place on the same face of the double bond, this leads cis stereochemistry.

Introduction

Hydroboration-oxidation of alkenes has been a very valuable laboratory method for the stereoselectivity and regioselectivity of alkenes. An Additional feature of this reaction is that it occurs without rearrangement.

The Borane Complex

First off it is very imporatnt to understand little bit about the structure and the properties of the borane molecule. Borane exists naturally as a very toxic gas and it exists as dimer of the general formula B2H6 (diborane). Additionally, the dimer B2H6ignites spontaneously in air. Borane is commercially available in ether and tetrahydrofuran (THF), in these solutions the borane can exist as a lewis acid-base complex, which allows boron to have an electron octet.

\[ 2BH_3 \rightarrow B_2H_6\]

 

THF.bmp

The Mechanism

Step #1

  • Part #1: Hydroboration of the alkene. In this first step the addittion of the borane to the alkene is initiated and prceeds as a concerted reaction because bond breaking and bond formation occurs at the same time. This part consists of the vacant 2p orbital of the boron electrophile pairing with the electron pair of the ? bondof the nucleophile.

 concerted reaction 1.bmp

Transition state

TRANSITION STATE FINAL 2 (2).bmp

* Note that a carbocation is not formed. Therefore, no rearrangement takes place.

 

  • Part #2: The Anti Markovnikov addition of Boron. The boron adds to the less substituted carbon of the alkene, which then places the hydrogen on the more substituted carbon. Both, the boron and the hydrogen add simultaneously on the same face of the double bond (syn addition). 

borination step 2.gif
 

Oxidation of the Trialkylborane by Hydrogen Peroxide 

Step #2

  • Part #1: the first part of this mechanism deals with the donation of a pair of electrons from the hydrogen peroxide ion. the hydrogen peroxide is the nucleophile in this reaction because it is the electron donor to the newly formed trialkylborane that resulted from hydroboration.

 

 

peorxide 1.bmp

 

 oxidation part 1 (1).bmp

  • Part 2: In this second part of the mechanism, a rearrangement of an R group with its pair of bonding electrons to an adjacent oxygen results in the removal of a hydroxide ion.

1,2, SHIFT.bmp

Two more of these reactions with hydroperoxide will occur in order give a trialkylborate

 trialkylborate.bmp

  • Part 3: This is the final part of the Oxidation process. In this part the trialkylborate reacts with aqueous NaOH to give the alcohol and sodium borate. 

 

 Sodium Borate.bmp

If you need additional visuals to aid you in understanding the mechanism, click on the outside links provided here that will take you to other pages and media that are very helpful as well. 

References

  1. Vollhardt, Peter, and Neil Shore. Organic Chemistry: Structure and Function. 5th. New York: W.H. Freeman and Company, 2007.
  2. Foote, S. Christopher, and William H. Brown. Organic Chemistry. 5th. Belmont, CA: Brooks/Cole Cengage Learning, 2005.
  3. Bruice, Paula Yurkanis. Oragnic Chemistry. 5th. CA. Prentice Hall, 2006.
  4. Bergbreiter E. David , and David P. Rainville. Stereochemistry of hydroboration-oxidation of terminal alkenes. J. Org. Chem., 1976, 41 (18), pp 3031–3033
  5. Ilich, Predrag-Peter; Rickertsen, Lucas S., and Becker Erienne. Polar Addition to C=C Group: Why Is Anti-Markovnikov Hydroboration-Oxidation of Alkenes Not "Anti-"? Journal of Chemical Education., 2006, v83, n11, pg 1681-1685

Problems

What are the products of these following reactions?

 

#1. 

 PROBLEM 1.bmp

#2.

 PROBLEM #2.bmp

#3.

PROBELM #3.bmp

 

 

Draw the structural formulas for the alcohols that result from  hydroboration-oxidation of the alkenes shown.

#4.

problem 4.bmp

 

#5. (E)-3-methyl-2-pentene 

If you need clarification or a reminder on the nomenclature of alkenes refer to the link below on naming the alkenes.

Answers

#1.

answer #1.bmp

#2.

hydrobor prob2 soln.png

#3.

Answer #3.bmp

#4.

ANSWER #4.bmp

#5.

Answer 5.bmp

Contributors

  • Gilbert Torres (UCD)