All this this versatility comes with a drawback, however. Sometimes alcohol functional groups can get in the way of other reactions we might like to do. Let me show you what I mean. What gives? So why does this reaction not lead to formation of a C-C bond? Same reason! Our acetylide ion is a strong base, and deprotonates the O-H group, which then participates in an SN2 reaction with the alkyl halide 4 bonds away forming a five membered ring.
This is a textbook example of what we saw in our last post — an intramolecular SN2 reaction. Acid-base reactions are fast , relative to substitution reactions]. So how could we have prevented this from occurring? Then you come to one of those annoying wall outlets.
You could paint over it of course. So what do you do? THEN you can plug in the typewriter. That would allow us to perform a synthesis of our desired molecule second scheme above. Well, you might have guessed by now that enterprising chemists have developed a solution for this problem. The only important reaction of ethers you cover in Org 1 is how to cleave them with very strong acid e.
Other than that, ethers are inert to pretty much any other reaction condition you can name. Instead of making a typical ether e. The Si-F bond is unusually strong — even stronger than Si-O. Addition of a source of fluoride ion F- will lead to cleavage of Si-O bonds without affecting the rest of the molecule.
A typical source of fluoride ion is the salt tetrabutylammonium fluoride TBAF. Are there other protecting groups for alcohols? You betcha. For more information, see Note 2. How could we get this sequence to work? There you have it. All we needed to get our desired reaction to work was a way of masking the OH until we were done performing our surgery on the other half of the molecule. Summary: Protecting Groups For Alcohols This post barely scratches the surface of protecting groups for alcohols.
Protecting groups are used for alcohols in a variety of different situations, far beyond the SN2 examples we covered here. Another example might be if you wanted to selectively oxidize one of two different alcohols in a molecule. We can add more posts on this topic as we go along. Learning how to deal with molecules that have more than one key functional group is, in my opinion, where Org 2 ends and Org 3 begins.
Even if that acetylide formed, it would be quickly protonated by any spare alcohol R-OH swimming around, giving rise to the alkoxide. Acid-catalyzed cleavage of trimethylsilyl ethers. The silylated alcohol 0. In a transesterification approach a 0. The lower boiling trimethylmethoxysilane is removed by distillation. Base-catalyzed cleavage of trimethylsilyl ethers. The mildest conditions for the base-catalyzed deprotection of trimethylsilyl ethers is the treatment of a methanol solution of the silylated alcohol with an excess of potassium carbonate for 1 to 2 h.
Acid-catalyzed cleavage of triethylsilyl ethers. Alternatively, a THF solution of the triethylsilyl ether is treated with an aqueous solution of trifluoromethanesulfonic acid. Cleavage of a tert-butyldimethylsilyl ether with tetra-n-butylammonium fluoride — representative procedure for the deprotection of silyl ethers with TBAF.
This depends on the environment of the TBS ether, but usually requires from 2 to 16 h.
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