Addition of Grignard reagents to esters to give tertiary alcohols

by James

Description: Grignard reagents add twice to esters, giving tertiary alcohols (after addition of acid)
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Notes: 

  • It’s possible to show the “acid” step in many ways, such as “H+”, “H3O(+)”, etc. They all amount to the same thing.
  • Note that X in this case is a halide (e.g. Cl, Br, I) and the identity of it does not affect the reaction.
  • Finally it’s important to note that it’s not possible to get the reaction to stop after addition of just one equivalent of Grignard reagent.

Examples:

Notes:

Note that there are many ways to show the “acid workup” step. They all amount to the same thing. Also be sure to understand example #3 – many students have a hard time with this.

Mechanism:

Addition of Grignard reagent to the ester (Step 1, arrows A and B) followed by 1,2-elimination of alkoxide (Step 2, arrows C and D) provides a ketone. A second 1,2-addition of Grignard reagent to the ketone (Step 3, arrows E and F) furnishes the tertiary alkoxide, which is then protonated upon addition of acid (Step 4, arrows G and H).

Again note that addition of only 1 equivalent of Grignard reagent will lead to 0.,5 equivalent tertiary alcohol and 0.5 equivalent of starting ester – the Grignard is more reactive with the ketone formed after step 2 than it is towards the starting ester!


Notes:

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{ 1 comment… read it below or add one }

Rajarshi Banerjee

In the third example shouldn’t the diol go through dehydration and become a cyclic ether

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