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We have an alpha beta unsaturated ether, along with a keto group in the same compound, which is being treated with a grignard reagent shown (assume the grignard reagent is in excess). Will it only attack the carbonyl group, or are things different here?

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This reaction is one of a special kind. The one and onlyend product is 3-hydroxy,3- methylcyclohexan-1-one. As the Grignard reagent is in excess, and it is an $\alpha$- $\beta$ unsaturated ketone, there is a possibility of undergoing two different type of reaction mechanisms. But Surprisingly, both will give the same ultimate end product after two steps.
The major mechanism through which the reaction should undergo, is the direct nucleophilic attack of the grignard reagent at the electrophilic carbonyl carbon. As, Grignard reagent is a hard base , it's attack to hard acid centre of carbonyl carbon is more preffered. So, first it will undergo this addition after which it acidic hydrolysis is done, which cleaves the ether and forms an enol which tautomerises to give the final end product ketone. The reaction scheme is shown below,
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Now, the minor mechanism is the process of conjugated addition of the methyl group to that $\alpha$-$\beta$ unsaturated ketone. But again after tautomerisation and hydrolysis it actually forms the same compound, which is 3-hydroxy,3-methyl cyclohexan-1-one. The reaction scheme is shown below,
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So, it is seen that however the presence of excess reagent may interfere with the reaction mechanism but the end product is $100 $$\%$ $3$-hydroxy,$3$-methylcyclohexan-$1$-one.

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  • $\begingroup$ With the minor reaction scheme you would not normally expect cleavage of the ether under the sort of mild conditions that you normally use to workup a Grignard (that do hydrolyse an enol ether). Is this using forcing conditions? $\endgroup$ – Waylander May 11 '18 at 21:12
  • $\begingroup$ @Waylander, I am assuming that the hydrolysis is done in such acidic conditions which can also protonate the oxygen of the ether and the there will be a $S_N1$ type attack of water molecule at the tertiary carbocation formed after $\ce{MeOH}$ leaves the system. Actually as there is a possibility of tertiary carbocation formation, I think slightly acidic conditions can also facilitate the leaving of $\ce{MeOH}$ and perform the hydrolysis. $\endgroup$ – Soumik Das May 12 '18 at 5:32
  • $\begingroup$ This system is essentially tBu-OMe and that usually requires strong acid (HBr or HI) in anhydrous conditions to cleave. I do not think you will get such a hydrolysis in work up. Do you have a reference for this reaction? $\endgroup$ – Waylander May 12 '18 at 6:29
  • $\begingroup$ The acid is not given, so I assumed that it is a strong enough to cleave the ether. $\endgroup$ – Soumik Das May 12 '18 at 6:31
  • $\begingroup$ I do not think that is a good assumption. $\endgroup$ – Waylander May 12 '18 at 7:10

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