By James Ashenhurst

Protecting Groups In Grignard Reactions

Last updated: February 28th, 2023 |

Using Protecting Groups In The Formation of Grignard Reagents

Now that we’ve gone over the most useful reactions of Grignard reagents – addition to epoxides, aldehydes, ketones, and esters – let’s go back to the topic of how to make Grignard reagents, albeit with a twist.

Here’s the summary for today’s post:

summary of protecting groups in grignard reactions - using silyl ethers to protect alcohols and acetals for carbonyls

Table of Contents

  1. Yet Another Way To Screw Up The Formation of Grignard Reagents
  2. “Protecting Groups” Mask A Group From Attack
  3. Acetals As A Protecting Group For Aldehydes And Ketones
  4. How To Use Grignards With Protecting Groups: An Example
  5. Summary: Grignards And Protecting Groups
  6. Notes

1. Introducing Yet Another Way To Royally Screw Up Making A Grignard Reagent

In a previous post we said that there are cases where making Grignard reagents can fail due to the presence of an acidic proton. Like this example.

failure of grignard reagent formation due to presence of an acidic proton

The problem here is that Grignard reagents are strong bases, and will react with even weak acids (like alcohols). If we try to make a Grignard on a molecule with an acidic functional group, we’ll end up destroying our Grignard instead.

We saw that one way around this problem was to protect alcohols as some kind of inert functional group (like an ether) which doesn’t react with our Grignard.

Similarly, there are other cases of molecules where making a Grignard reagent will fail for similar reasons. For example: why does this reaction not give the desired Grignard reagent?

failure of grignard reagent synthesis due to the presence of a ketone

The problem here, as you might have guessed if you read the last post, is that this Grignard reagent reacts with itself! Once formed, the Grignard would react with the ketone from the starting material. This could then react with Mg to give a new Grignard, which would react with more ketone… and so on. The result is a mess.

a grignard reagent that would react with itself since it has a ketone

So how might we get around this?

2. “Protecting Groups” Mask A Functional Group From Attack

If we were able to find some way to “mask” the ketone in this case, possibly as some unreactive functional group that is completely inert to Grignard reagents, then we could then make the Grignard reagent without causing any problems of self-reactivity.  Then, once we’re done, we could then “unmask” the protecting or masking group, revealing our ketone again.

Like I said in this post (See article: Protecting Groups for Alcohols), you use protecting groups a bit like how you use painter’s tape: when you’re painting a room, for instance, you’ll cover up your electrical outlet with painter’s tape, paint the room, and then remove the painter’s tape when you’re done.

how do we get around the problem of a grignard reagent reacting with itself

3. Acetals As A Protecting Group For Aldehydes And Ketones

As you might have suspected, there’s a decent solution for this. It turns out that different varieties of ethers are great protecting groups because they’re unreactive towards strong bases and nucleophiles. Specifically in the case of ketones and aldehydes, a functional group called an acetal (sometimes called “ketal”) is great for the job. Acetals resemble ethers, except that we have a carbon attached to two O-R groups instead of one.

Acetals can be made from aldehydes and ketones by treating them with an alcohol and acid. [See post: Hydrates, Hemiacetals, and Acetals] . When the acetal protecting group isn’t needed anymore, the ketone or aldehyde can be restored by adding aqueous acid (H3O+).

one solution to avoid grignards reacting with themselves is to protect the aldehdes and ketones as acetals

OK, you might ask. So what? How does this work in real life?

4. How To Use Protecting Groups In Grignard Reagent Synthesis: An Example

To answer this question, dear reader, let’s show a specific example.

The idea is that if  want to make a Grignard reagent on a molecule that contains an aldehyde or ketone, we need to protect that aldehyde or ketone beforehand so that it doesn’t react with itself.

example of applying protecting groups in the synthesis of grignard reagents

Let’s walk through the example above. Note that we have an aryl halide (a halogen attached to a benzene group) that we’d like to turn into a Grignard reagent. However, we also have a ketone. In order to make the Grignard and use it to form a bond with a different ketone, we’d need to protect the ketone on our first molecule.

  • In step 1, we form the acetal, through use of an alcohol (CH3OH in this case) and catalytic acid (such as H2SO4).
  • In step 2, we make our Grignard reagent with magnesium (Mg) metal. “Ether” here is just the solvent – not essential to write out, but often mentioned. The Grignard reagent will not react with the acetal.
  • In step 3, we do the actual Grignard reaction, by adding a ketone (cyclopentanone in this case). We form a new C-C bond. This forms a new alkoxide. We then quench the reaction, when it’s done, by adding a mild acid in the workup. Note that it’s very possible to quench the reaction with mild acid without cleaving the acetal (for instance, with the mild acid NH4Cl. )
  • Finally, in step 4 we add strong aqueous acid, and this cleaves our acetal to restore our ketone.

Note that in this case we could combine steps 3 and 4 by quenching with H3O+ and heat, but I wanted to draw out each step separately so that it’s clear.

5. Summary: Reactions of Grignard Reagents With Alcohols

The bottom line for today is to watch out for reactive functional groups when making Grignard reagents.

In the next post, let’s start “thinking backwards” when it comes to using Grignards in synthesis.

Next Post: Synthesis Using Grignard Reagents (1)


Note 1: What about the other functional groups we covered, like epoxides and esters? Do we bother protecting them too?

With epoxides, we usually don’t employ a protecting group. Epoxides tend to be pretty reactive species, so it’s generally best to design our synthesis in a way to put the epoxide in at the end.

Esters are also a functional group we try to install after making a Grignard reagent, but for slightly different reasons. For our purposes, there aren’t a lot of great protecting groups for esters that are simple to use and will quickly block the group from attack. Instead, we usually have to employ a long sequence which involves 1) reducing the ester to an alcohol 2) protecting the alcohol 3) making the Grignard and then 4) doing the Grignard reaction, then 5) deprotecting the alcohol, 6) oxidizing the alcohol to a carboxylic acid, and then 7) make the ester from the carboxylic acid via Fischer esterification. Phew! In organic synthesis, there’s a phrase we use for a sequence like this. We call it a land war. 

Like epoxides, the best way to solve the problem of protecting an ester is to avoid it altogether, by designing our synthesis in an intelligent way.

do we need protecting groups with grignards and epoxides or esters - generally not if you plan well



Comment section

5 thoughts on “Protecting Groups In Grignard Reactions

  1. The first drawing contains an error in the “actual product” – it should be
    BrCH2CH2CH(O-)CH2CH2CHO + MgBr+.

  2. Acetals As A Protecting Group For Alcohols And Ketones

    should be

    Acetals As A Protecting Group For Aldehydes And Ketones

  3. how can we reduce just the carboxylic group on a molecule that contains both carboxylic acid and a ketone. Can we protect the ketone with glycol and use LiAlH4 to reduce just the carboxylic acid?
    Thanks James!

    1. Yeah! We can! An acetal is not attacked by a base and is inert in basic medium.So you put some LiAlH4 and when the carboxylic acid is converted to alcohol, you add some mild, weakly-nucleophilic acid(otherwise it can dehydrate the alcohol or have a SN reaction with the alcohol!)–which hydrolyzes the acetal and returns back the ketone.Thus, you get the desired product.

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