Hydroboration
Hydroboration -
Anti-Markovnikov Hydration
In 1979, Purdue University professor Herbert C. (H.C.) Brown won the Nobel Prize in chemistry for his work on the hydroboration of alkenes. Brown discovered that hydroboration is a way to make anti-Markovnikov alcohols. The simplest hydroborane, BH3, has six valence electrons on the boron atom. Remembering the octet rule, we should realize this is electron deficient and therefore an electrophile. It just so happens that there is a nice, juicy pair of negative electrons in an alkene which is more than happy to attack the BH3!
BH3 is available in a couple of forms. One possibility is to buy diborane, B2H6. Diborane breaks apart to make BH3.
But, diborane is a toxic, flammable, explosive gas. So, this is not the preferred method. More commonly, BH3 is purchased dissolved in a solvent like THF. This is a much safer way to use BH3. Since it is an adduct with the solvent, it is a liquid that can be measured out.
The mechanism for the hydroboration reaction begins when a B-H bond of BH3 adds across the C=C in one step to make a square transition state. The boron atom gains a little negative electron density and therefore becomes partially negative. The carbon atom opposite the boron in the square loses some negative electron density and becomes partially positive. When the two transition states are compared, the partially positive carbon atom that is more substituted is more stable. It is the pathway that is followed more. When this carborane is oxidized with H2O2 and NaOH, the BH2 is replaced with OH making an anti-Markovnikov alcohol. This is an extremely important reaction. Now we can make either Markovnikov or anti-Markovnikov alcohols. And, we’ll learn later in organic chemistry, that we can turn alcohols into many different types of functional groups.
Hydroboration of alkyne
The hydroboration of an alkyne can lead to an anti-Markovnikov enol. Since alkynes contain two pi-bonds, two molecules of borane could add to it. A big, bulky, sterically hindered borane must be used in order to prevent this addition of two boranes. Usually, the borane used is di(sec-isoamyl)borane, commonly called disiamylborane. Instead of the small BH3 molecule, disiamylborane has two big, large, deer antler-like alkyl groups attached to it. There are five carbon atoms in each alkyl branch. Amyl is an old, common name for pentyl.
Di(sec-isoamyl)borane or disiamylborane
Disiamylborane reacts with an alkyne to make an anti-Markovnikov borane. This borane can be oxidized with peroxide and sodium hydroxide to make an anti-Markovnikov enol. This enol can tautomerize to the keto-form. In the case of a terminal alkyne, this keto-form is an aldehyde with a hydrogen attached to the carbonyl group.
The mechanism of the base catalyzed keto-enol tautomerization is different than the mechanism was in acid. Bases deprotonate. So, the first step of this mechanism is the deprotonation of the enol at the alcohol position. This is pretty easy to remember because we are turning the alcohol portion of the enol into a carbonyl group. Once deprotonated, we draw resonance forms to get us closer to the keto product, then clean it up with some water to make the final aldehyde.
Base-catalyzed keto-enol tautomerization
4. Draw the products of the following reactions.
a)
b)
c)
d)
Answers
4.
a)
b)
c)
d)
