stereochemistry of the hydroboration-oxidation
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stereochemistry of the hydroboration-oxidation

January 5, 2020

Good day from Chem Help ASAP. The stereochemistry of an addition reaction
is classified as either syn, anti, or neither. Let’s see what the stereochemistry of the
hydroboration is. [turn page]
Before we talk about the reactions on this slide, let’s talk about the starting alkene
in both reactions. The alkene is a bit special. One carbon of the alkene has a deuterium atom
– D. Deuterium is just an isotope of hydrogen. Normally, for hydrogen, we write H. We don’t
specify which isotope, but most people would assume that H stands for the 1H isotope (properly
called *protium*). Deuterium is specifically the 2H isotope. It’s still hydrogen, but it has an extra
neutron in the nucleus. I’m using this deuterium label to help us
keep track of the stereochemistry of the addition. Chemists will often use a *deuterium label*
to help determine the stereochemistry or mechanism of a reaction. OK, now to the reactions. The top reaction is an acid-catalyzed hydration. It starts with protonation of the alkene. Protonation can occur on the top or bottom
face of the alkene. I’ve only shown the top face protonation
(so deuterium is “down” and the new hydrogen is “up”). The resulting carbocation can be attacked
from either the top or bottom face. Following a deprotonation, the product is
the Markovnikov alcohol. Notice that we have added water to the alkene
(H and OH). In the product, we get about 50% of the product
with the H and OH cis on the ring (this is the syn product) and 50% of the product with
the H and OH trans on the ring (this is the anti product). The acid-catalyzed hydration of an alkene
has no stereocontrol. It is neither syn nor anti. Great, but we already knew that. How about the hydroboration? On the bottom half of the screen, we first
add BH3 across the alkene. BH3 can add from the top or bottom face of
the alkene. I’m just showing the top face addition. That gives both the added H and BH2 cis This
is the syn product. BH3 adds through a syn addition. In the oxidation step, the C-B bond is replaced
with a C-O bond, and the stereochemistry of the bond is unchanged. This is a type of substitution, and this particular
substitution involves *retention* of stereochemical configuration. Therefore, in the overall hydration product
(the anti-Markovnikov alcohol), the H and OH are cis in the ring. The hydroboration-oxidation is a syn addition. This is kind of interesting. When we first learned alkene hydration with
acid and water, it was neither a syn nor anti addition. You might think that all hydrations therefore
are not stereoselective. That’s not true though. The acid-catalyzed hydration is not stereoselective,
but the hydroboration-oxidation (another hydration) is selective. If the mechanism is different, then the stereochemical
outcome may also be different.

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