By James Ashenhurst
Hydroboration of Alkenes
Last updated: March 26th, 2019
So far in this series on alkenes, we’ve gone through two families of mechanism pathways. In the carbocation pathway, we saw reactions that proceed with “Markovnikov” regioselectivity, a mixture of “syn” and “anti” stereochemistry, and can be accompanied by rearrangements. In the 3 membered ring pathway, the regiochemistry is also “Markovnikov”, the stereochemistry is trans (anti), and the reaction proceeds through a 3 membered ring intermediate.
We’ve met at least a dozen different alkene reactions that can fit into these families so far.
But then along comes a reaction that doesn’t fit. In the mid 1950’s, H.C. Brown and B. Subba Rao were investigating the use of boron hydrides as reducing agents. When performing the reduction of an unsaturated ester (ethyl oleate) with NaBH4 and catalytic AlCl3 , Subba Rao observed that an excess number of mole equivalents of boron hydride were consumed: 2.37 ( vs. 2.00 for the reaction of saturated ethyl stearate) . Upon further investigation it was found that B-H was in fact adding to the alkene, in a reaction that subsequently became known as “hydroboration”.
It was subsequently determined that borane (which exists natively as B2H6, simplified here as “BH3“) adds to alkenes with the following pattern:
Note that the hydrogen is adding to the more substituted end of the carbon (“anti-Markovnikov”) and the stereochemistry is syn.
This doesn’t fit with any of the patterns we’ve seen before!
That means it’s likely going through a different mechanism!
Of interest is the observation that no rearrangements are observed, even in strained molecules such as pinene:
Furthermore, trapping by solvent is not observed (actually protic solvents are generally a bad idea for hydroboration, as they lead to formation of hydrogen gas and destruction of borane itself).
Interestingly if we add an excess of alkene, we can observe multiple hydroborations. If presented with sufficient alkene, for example, BH3 can perform three additions to alkenes.
Here’s another interesting observation. Selectivity for the “anti-Markovnikov” product is very high for propene (94:6). However, when electron withdrawing substituents are added to the alkene, the selectivity for the “anti-Markovnikov” product drops to 74:26. Whatever is responsible for the anti-Markovnikov selectivity must also explain this observation! [Ref]
Finally, it is of interest that organoborane compounds are not particularly stable under atmospheric conditions (they tend to react with oxygen, burning with a beautiful green flame). However, they can be easily converted to alcohols, which are extremely valuable compounds. The transformation of organoboranes to alcohols can be performed by treating them with basic hydrogen peroxide.
This process is called “hydroboration-oxidation”. Note how the stereochemistry of the C-B bond is preserved in the C-O bond.
How might we explain all these observations? Chew on them for a bit and we’ll go through a proposed mechanism next time.
NEXT POST: Hydroboration of Alkenes – The Mechanism
* I recall one of my undergraduate instructors, (Prof. Walter Szarek) telling a story about how Brown ended up as a boron chemist due to the fact that his wife gave him a book on boron compounds. I was delighted to find the full story in Brown’s Nobel Lecture:
I received the Assoc. Sci. degree from Wright Junior College (Chicago) in 1935 and the B.S. degree from the University of Chicago in 1936. Why did I decide to undertake my doctorate research in the exotic field of boron hydrides? As it happened, my girl friend, Sarah Baylen, soon to become my wife, presented me with a graduation gift, Alfred Stock’s book, The Hydrides of Boron and Silicon. I read this book and became interested in the subject. How did it happen that she selected this particular book? This was the time of the Depression. none of us had much money. It appears that she selected as her gift the most economical chemistry book ($2.06) available in the University of Chicago bookstore. Such are the developments that can shape a career!
** Another neat feature of this reaction, not treated in detail here, is that hydroboration is reversible; heating of the organoborane can result in reversion to borane and alkene, and subsequent hydroboration. In such a way can organoborane compounds isomerize, provided there is a sufficient driving force.