Question

Sun Wu Save this response. < Question 4 of 20 Saved Question 4 5 points If your starting material contained more than one functional group, how could you force the reaction to occur only at one group and not the other? For the toolbar, press ALT+F10 (PC) or ALT+FN+F10 (Mac)

Answer 1

Chemoselectivity is a term that describes the ability of a reagent or inter­mediate to react with one group or atom in a mole­cule in preference to another group or atom present in the same molecule.

A reaction that operates exclusively on one functional group in the presence of other functional groups.

Reduced O O NaBH4 HO CH3CH2OH OCH3 OCH Not reduced

This NaBH4 reduction is chemoselective because the ketone is reduced but the ester is unchanged.


Reduced O 1. LiAlH4 O HO 2. H30+ OCH3 OH Reduced

This LiAlH4 reduction is not chemoselective because the ketone and ester are both reduced.

So by selecting the appropriate reagent, we can selectively react one group while keeping the second group intact.

Protecting group: A temporary group added during organic synthesis to prevent a portion of a molecule from reacting (i.e., it assists chemoselectivity). Protection is usually considered an undesirable synthetic strategy because it adds two steps (protection and deprotection) to the length of the overall synthesis, and because the added steps usually cause a decrease in overall yield and reduces atom economy. A painter's drop cloth is a useful metaphor: It prevents paint from getting on undesired areas (such as the floor) but adds extra time, effort, and cost to the painting process because the drop cloth must be put down before painting, and picked up afterward, and because the drop cloth adds cost to the process, and increases waste.

H НО

CrO3 aq. H2SO4

О НО О

Without a protecting group, both the alcohol and aldehyde are oxidized by Jones reagent. This oxidation is not chemoselective.

H НО

tBDMSCl Imidazole Protection

H OtBDMS

CrO3 aq. H2SO4

О НО OtBDMS

aq. HF Deprotection

НО ОН

When the alcohol is protected with a tBDMS group, only the aldehyde is oxidized by Jones reagent. Thanks to the protecting group the oxidation is now chemoselective, but the synthesis is now two steps longer because of the protection and deprotection steps.

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